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the bulking zone and high energy absorption characteristics swellex bolts are weak in shear and if they become locked in the hole the theoretical ductility may not be achieved split set bolts show high ductility but low energy adsorption and low load capacity figure 8 8 9 the bolts are weak in shear and if they become locked in the hole will rupture at low loads medium depth reinforcement design 3 15 m static reinforcement elements cable bolt support a conventional cable bolt is a flexible tendon consisting of a number of steel wires wound into a strand which is grouted into a borehole cable bolts are normally installed in regularly spaced boreholes to provide reinforcement and support for the walls roof and floor of underground or surface openings cable bolting is a versatile form of support because strands can bend around fairly tight radii making installation of long bolts from confined openings possible and because they can be fabricated with a number of configurations of the steel wires e g plain bulb nut case a variety of performance characteristics can be achieved in north american hard rock mining cable bolts are made from 15 6 mm seven wire reinforcing strand manufactured to astm 416 standard having a yield strength of about 20 t at 0 8 strain and an ultimate strength of 24 t at 3 5 4 strain in theory cable bolts can be placed to any depth but for pragmatic reasons are seldom installed to depths exceeding about 20 m multiple cables can be placed in the same hole to increase tensile capacity if the borehole diameter is large enough in addition face restraint can be attached in the form of plates straps and mesh cable bolts are commonly used in combination with other support systems such as shotcrete rock bolts and mesh figure 8 8 10 shows twin strand bulb cable bolts coiled to about 1 m in diameter plain strand and bulb cable are the configurations most commonly used in north american hard rock mining alternately in the australian coal industry the standard long cable tendon has around 63 t capacity and the heaviest just over 80 t capacity quality control issues with cable bolts deal with grout quality and cable cleanliness cables can be grouted either toe to collar using 0 35 w c grout or collar to toe i e breather tube method using 0 4 w c grout high w c grout reduces cement strength and stiffness and results in significant reduction of cable bond capacity commonly resulting in stripping failure the use of bulb cable helps overcome grout quality control to some degree plates are attached using a barrel and wedge assembly the barrel and wedge are susceptible to rust which causes the wedges to lock up and the assembly will slide off at the seating load nominally 5 t copper coat grease or equivalent should be applied to the inner surface of the barrel and the outer surface of the wedges to prevent corrosion a detailed coverage of all aspects of cable bolt practice is gi

ven in hutchinson and diederichs 1996 connectible super swellex the pm24c swellex bolt can be used to achieve deep anchorage by connecting two or more threaded sections the bolt has a 200 kn breaking load and achieves 10 strain swellex is commonly used in combination with other support systems such as shotcrete rock bolts mesh and so forth qa qc issues are the same as for ordinary swellex under static support conditions these support elements may be too soft i e may allow too much ground movement static support design for cable bolt support in hardrock mining static support design is generally done empirically using the stability graph method hoek et al 1995 which is widely used in north american and australian hard rock mines numerical modeling can be used to help establish required support depth connectible swellex can be used to replace cable bolts but the swellex density must be adjusted to achieve equivalent system capacity dynamic reinforcement elements dynamic support elements suitable for medium depth conditions include debonded cable bolts and connectible super swellex bolts debonded cable bolts are created by placing a plastic sheath over a plain strand section of the cable bolt when the cable bolt is grouted in the borehole the plastic sheath acts to debond the cable strand from the grout the debonded section is then allowed to stretch freely achieving 0 8 strain at yield and 3 5 strain at rupture each meter of debonded cable therefore permits 8 mm of stretch at yield and 35 mm of stretch at rupture normally at least 2 m of cable is exposed at the toe beyond the debond section the cable may be secured at the collar by attaching a plate and barrel and wedge assembly alternately a section of bulbed cable can also be left exposed at the collar the use of debonded bulbed cable bolts under dynamic loading is discussed by bawden and jones 2002 qa qc issues are the same as for static applications pm24c swellex bolts can be used to provide deep tendon support under dynamic loadingdynamic support design dynamic support design effectively implies rock burst resistant support design and is a function of the size of the seismic event and the location of the event hypocenter relative to the mine infrastructure in question figure 8 8 11 local site factors such as the orientation of the infrastructure can also play a significant role in support and ground performance support ductility is required to achieve displacement capacity and energy absorption capacity figure 8 8 12 tables 8 8 6 and 8 8 7 provide displacement and energy absorption criteria for most common support elements further coverage on this topic is beyond the scope of this chapter a thorough coverage is provided in the canadian rockburst support handbook kaiser et al 1996 mesh is discussed in more detail later in this chapter deep seated ground anchors 15 m these long anchors tend to be arranged in larger crosssectional are

as to handle the greater volumes of unstable material used extensively in civil engineering and less so in mining they are usually composed of a large number of individual elements that act together as a composite reinforcing element to enable prestressing they are usually discretely coupled over a fairly long anchorage and may be used to impart considerable load to the surface of the rock mass by way of large rigid built in stressing blocks situated at the free surface these devices often play a critical role in maintaining stability and are subject to stringent installation quality assurance by proof testing programs high shear capacity elements shear keys may be in the form of universal steel sections large diameter tubes or commonly railway lines cast in concrete these elements are most commonly used as pre reinforcement in surface excavations the large cross sectional area of the steel elements provides high shear resistance to transverse displacements windsor and thompson 1993 ground support screen or mesh screen or mesh is used to form a surface retention support system to cover the rock mass between the reinforcing tendons that are usually spaced at 1 to 2 m centers the mesh is attached to the reinforcing tendons by the plate assembly or by push plates installed after the reinforcing tendons have been installed there are several types of mesh construction including welded wire mesh chain link mesh and expanded metal mesh the wire gauge and construction technique control the load capacity and stiffness of the mesh the mechanical properties of several common mesh types are provided in table 8 8 7 mesh serves the additional purpose of providing reinforcement and ductility for plain shotcrete welded wire mesh with a minimum 100 mm hole size is recommended to ensure that the shotcrete can penetrate and bond to the rock straps steel straps 6 35 100 mm are sometimes attached between reinforcing tendons to increase the footprint of the plates and to help secure specific wedges hole locations are preformed in the straps and it is often difficult to install bolts at the required location and to make the straps conform to irregular rock surfaces this style of strap has largely been replaced with zero gauge welded wire straps figure 8 8 13 which are easy to install and pull tight to the rock surface and have proven very effective shotcrete shotcrete is the generic name for cement sand and fine aggregate concretes which are applied pneumatically and compacted dynamically under high velocity shotcrete can be applied in either a dry or wet form in the dry application shotcrete components which can be slightly predampened to reduce dust are fed into a hopper with continuous agitation compressed air is introduced through a rotating barrel or feed bowl to convey the materials in a continuous stream through the delivery hose water is added to the mix at the nozzle in underground mining dry shotcrete componen

ts are normally purchased premixed in 1 000 kg bags for ease of material handling hoek et al 1995 in the case of wet mix the shotcrete components and the water are mixed before delivery into a positive displacement pumping unit low profile agitator trucks have been developed for underground mining applications shotcrete is usually mixed in a surface batch plant and delivered underground using either agitator trucks on a ramp system or by slick line down the shaft shotcrete is normally applied in layers from about 50 mm to 100 mm thick because plain shotcrete is very brittle and will crack and spall off with small movement of the rock mass it should always be reinforced welded wire mesh attached to the reinforcing tendons will provide reinforcement if plain shotcrete is placed over it alternately fibers steel or plastic can be added to the mix to create what is normally called fibercrete the welded wire mesh and or fibers provide tensile strength and ductility to the shotcrete to withstand movement of the rock mass shotcrete may be placed as an entire ring or only over selected sections of the excavation profile a typical shotcrete application should achieve a uniaxial compressive strength of about 35 mpa after 28 days wet mix shotcrete is the most popular form because drymix shotcrete tends to have high rebound volumes increasing unit costs and usually results in inaccessibility of the area due to dust however wet mix shotcrete has higher capital cost qa qc issues with shotcrete revolve primarily around preparation of the surface to be sprayed the surface must be washed down clean in order to achieve proper bonding of the shotcrete and control of the sprayed layer s thickness areas of active research with shotcrete today include the use of rapid curing shotcrete via the addition of accelerators etc deciding when it is safe to work under the shotcrete and balancing the improvement in curing time against the inevitable loss of longer term properties additional discussion of shotcrete design is provided by hoek et al 1995 concrete liners the three main types of concrete liners are poured sprayed and segmental 1 poured concrete liners are constructed by pumping concrete behind moveable forms usually some type of slip from assembly the poured concrete liner trails the advancing face by a discrete time distance increment and is usually considered to be a secondary support system poured concrete liners are common in modern shaftsinking operations and in some civil tunneling operations 2 sprayed concrete liners are created through the application of one or more shotcrete layers using a remote application arm sprayed liners can be applied right up to the advancing face 3 segmental concrete liners are normally erected mechanically behind advancing tunnel boring machines and usually form the permanent liner for the excavation this method is common in civil applications but is seldom used in mining rigid st

eel sets steel arches or steel sets are used where high load carrying elements are required to support tunnels or roadways a wide range of rolled steel sections are available for this application where the rock is well jointed or becomes well fractured after the excavation is made the spaces between the sets may be filled with steel mesh steel or timber lagging figure 8 8 14 steel sets cannot be preloaded against the rock and their efficacy depends on the quality of the blocking provided to transmit loads to the steel set improper blocking may lead to point loading of the set often resulting in buckling failure of the legs today steel sets are sometimes shotcreted in place providing a more uniform loading and much stiffer response brady and brown 2004 while steel sets give the impression of high load carrying capacity due to their bulk in terms of carrying capacity per weight of steel they are often not that efficient in most mining situations they are an absolute last resort due to the difficulties involved in their erection particularly under difficult ground conditions yielding steel sets steel arches were widely used historically to support roadways in coal mines where they are often required to sustain large deformations these deformations can be accommodated using yielding arches containing elements designed to slip at predetermined loads figure 8 8 15 brady and brown 2004 the modern u s south african u k and australian coal industries primarily use roof bolts and tendons and this trend is continuing into other developing countries such as india and china steel sets present an impediment to efficient coal mining and are generally not considered as effective for ground control as reinforcing bolts and tendons hence they have been phased out over the past 30 years other than in arduous conditions around localized structures shotcrete pillar support shotcrete pillars can be constructed by first forming a square cage of rebar anchored in boreholes in both the floor and the back of the opening each rebar should be 3 m long with 1 m protruding from the hole additional rebar can be tie wrapped to this rebar to complete the reinforcing cage steel halfculverts can be used to form the pillars with angle iron tack welded so the half culverts can be bolted together shotcrete should be tremmied into the form which should then be vibrated to remove air voids finally the gap at the back is sprayed full of shotcrete a 0 9 m diameter shotcrete pillar using a 24 hour compressive strength of 15 mpa would carry a load of 1 200 t timber and steel support timber support is no longer widely used in north america the simplest form of timber support is the post and cap or prop and lid consisting of a single upright with a plate above the post may be made of timber or steel or may be a hydraulic prop timber posts sometimes called sticks are widely used to assist in supporting the mined out area behind the stope

face in the mining of narrow reef like metalliferous ore bodies these sticks are usually 100 to 200 mm in diameter and are designed to support or help support the dead weight of the first 1 to 2 m of rock in the hanging wall a typical 200 mm diameter stick can have a short term loadcarrying capacity of 60 t korf 1978 1979 but for longterm use and low rates of load application lower design capacities should be used supports that are stiffer and have higher load carrying capacities than timber posts or sticks of the same diameters are provided by pipe sticks in which a stick is press fitted into a steel pipe typically 150 mm in diameter with a 4 mm thick wall steinhobel and klokow 1978 1979 sticks are passive support and require significant deformation to develop the full load bearing capacity they are also difficult to set properly commonly leading to local falls of ground and increased loosening of the back their use is restricted to narrow openings or else they will act as slender columns and fail in buckling brady and brown 2004 in larger openings timber cribs can be installed for local back support figure 8 8 16 such timber cribs must be blocked against the back and their effectiveness is highly dependent on the quality of the blocking timber ribs can also be backfilled with waste rock to increase their load capacity timber cribs are a passive support and require significant ground movement in order to mobilize full loadbearing capacity an issue not often discussed with timber support is shrinkage if green timber is used timber cribs can shrink from the roof by 100 mm over a relatively short period hydraulic props hydraulic props are set with a pre load to provide active support and suffer from none of the disadvantages of sticks although they are obviously much more expensive the load bearing capacities of individual props may vary from as little as 5 t for a very light prop to more than 100 t for a 0 3 m diameter prop in the deep level gold mines of south africa rapid yielding hydraulic props are widely used to provide concentrated active support of the hanging wall close to the face their rapid yielding capability allows the energy released by rock bursts to be absorbed rapidly and safely thereby minimizing the damage caused brady and brown 1985 a major risk with hydraulic props are leaks in the system particularly longwall hydraulic supports as mentioned earlier safety the majority of ground support is installed to ensure the safety of mine operating personnel and where properly utilized rock fall related injuries and fatalities have been dramatically reduced over the past 50 years the statistics now indicate that most ground control related injuries occur during the installation of ground support where workers are often in close proximity to unsupported ground and are not due to subsequent ground instability today it is common practice in many mines to use mechanized equipment to install

primary support in australia it is common practice in development headings to install split set bolts and screen using the jumbo on advance in north america it is more common to use mechanized bolters in either case the operator is protected by a steel canopy in addition mechanized scaling units can be used to scale the fresh heading prior to support installation another approach is to apply a flash coat of shotcrete about 50 mm thick and then to bolt through the shotcrete fiber reinforced shotcrete can be used and may in certain conditions negate the need for screen work force training at the end of the day once all of the geotechnical design work is done and state of the art products procured for use it is all handed over to a group of miners to install the quality of the final product is dependent on how well the work force is trained how well the design is communicated and on the quality of equipment provided the importance of proper operator training cannot be overstated and in many areas the need for operator training in ground control principles is enshrined in the legislation the phenomenon of subsidence is an inevitable consequence of the extraction from underground of any resource be it solid liquid or gas during recent years with the trend toward larger mines and increasing requirements for environmental protection it is no longer possible to ignore it worldwide great attention is devoted to the subject and its methodical study and strict regulations for its control have been introduced by government agencies in many regions to protect the public interest the problems associated with subsidence have been recognized since antiquity agricola s de re metallica of 1556 talks about a mountain or hill subsiding by its weight as a result of mining agricola 1556 the early to mid 20th century saw many developments in the understanding and prediction of subsidence motivated by legal action resulting from severe damage to surface structures communications and agricultural resources caused by underground mining it was the defense against unjustified claims that required improved understanding of subsidence phenomena thus the major objectives of subsidence engineering are prediction of ground movements determining the effects of such movements on structures and renewable resources and minimizing damage due to subsidence consequently subsidence engineering entails much more than the prediction of ground movements it requires knowledge of geomechanics both soil and rock mechanics structural geology hydrology and hydrogeology mining methods and techniques construction procedures agriculture socioeconomics and mining and property law although the effects of fluid extraction on subsidence have been widely investigated they are beyond the scope of this chapter nevertheless mining may lower the local water table and this can induce ground movements that cause surface damage this must no

t be overlooked finally the term subsidence as used in this chapter encompasses the complete range of surface effects associated with the mining of minerals and not just the vertical displacement of the surface as is sometimes implied in the literature principles of subsidence development of subsidence the creation of any subsurface opening perturbs the stress state in the surrounding material this perturbation produces deformations and displacements of the material the magnitudes of which depend on the degree of the stress change the spatial extent over which it occurs and the nature of any rock support or reinforcement if sufficiently large these changes can cause the rock around a mine excavation to collapse into the mined void figure 8 9 1 the ground movements associated with such collapse tend to propagate to the ground surface with the deformations and displacements experienced there being termed subsidence surface subsidence generally entails both vertical and lateral movements and may be discontinuous steps cracks or cavities form at the surface or continuous the surface deforms smoothly discontinuous subsidence is generally of limited areal extent and is characterized by large vertical displacements it occurs when material overlying an extraction zone collapses into the void and its form depends on the mining method the geometry of the extraction zone and the geomechanical properties of the rock above the extraction zone figure 8 9 2 crown holes may form because of roof or pillar collapse of shallow room and pillar r p mines a similar phenomenon is chimney caving also known as piping or funneling which occurs when a collapse at the mine level migrates upward sometimes over distances of many hundreds of meters through weak overlying materials both crown holes and chimney caving can occur above solution cavities which themselves may be either a deliberate or inadvertent effect of solution mining the presence of weak structural features e g faults or boundaries between different geological materials may lead to plug subsidence in which a large plug of material falls suddenly and instantly downward into the mine void the speed and suddenness of the process means this is particularly dangerous mining methods such as block caving and sublevel caving also lead to discontinuous subsidence but in these operations use of and access to the surface area affected by the subsidence is generally prohibited in the case of continuous subsidence above laterally extensive extraction zones such as longwall coal mining operations observations of subsidence profiles or troughs above the mined areas have shown they can be characterized on the basis of shape in particular the absence or presence of an essentially horizontal central region this leads to the concepts of subcritical critical and supercritical conditions figure 8 9 3 with the critical width being the minimum necessary to cause maximum surf

ace subsidence the position where the vertical displacement is effectively zero is known as the limit of subsidence and the angle between a line joining this to the edge of the extraction zone and the vertical is termed the angle of draw or limit angle the geometry shown in figure 8 9 3 indicates that critical width is uniquely defined in terms of the mining depth and the angle of draw and that the surface area affected by extraction is greater than the area of the workings themselves the angle of draw has been found to be site specific as it depends on the geomechanical properties of the overlying materials table 8 9 1 gives various examples in some areas particularly those where the overlying rocks are particularly strong a minimum extraction width has been found below which no significant subsidence occurs sheorey et al 2000 this is generally termed the noneffective width the minimum amount of mining required to lead to maximum subsidence of a point p is the extraction of a circular area whose diameter is equal to the critical width and this circular region is known as the area of influence as every part of this area contributes in some way to the subsidence extraction of material at a single point produces an elemental subsidence profile figure 8 9 4 that is circular in shape the subsidence developed above any mining operation can be thought of as being formed by the summation of many such elemental profiles as subsidence occurs points on the ground surface move horizontally toward the center of the mined area as well as vertically downward vertical displacement is greatest at the center of the mined area and zero at the limit of subsidence with horizontal movement varying from a maximum almost vertically above the edge of the mined area to a minimum essentially zero at both the center of the mined area and the limit of subsidence as the horizontal displacement is not constant horizontal strain is induced at the ground surface these strains are extensile i e dilatant outside of the boundaries of the mined area and contractile within them figure 8 9 5 although across the central region of the subsidence profile above critical and supercritical width workings the contractile strains are zero note that the terms tensile and compressive strictly refer to forces and stresses and the terms extensile and contractile refer to strains the inclination to the vertical of the line connecting the edge of the mined area to the surface point exhibiting the maximum extensile strain is called the angle of break or angle of fracture not to be confused with the angle of draw defined earlier as a mine working is progressively extended it begins at a subcritical width and depending on the extent of the extraction may pass into critical and supercritical widths as the working face moves so the horizontal extensile and contractile strain regions also move meaning that a given point on the ground surface may experienc

e different strain states as mining progresses this is of critical concern for damage of surface structures finally as figure 8 9 6 indicates the zone of subsidence reflects the shape of the extraction zone factors affecting mine subsidence experience has revealed that many geological and mining parameters besides the width of the extraction zone can affect the magnitude and extent of subsidence the numberand interrelation of these factors means that predicting in an accurate quantitative manner the magnitude and time to subsidence onset is generally not straightforward extraction thickness the thicker the material extracted the larger the amount of possible surface subsidence note that it is the actual extracted thickness not the in situ thickness that must be considered where mining takes place in several overlying mining horizons subsidence is related to both the total extracted thickness and the sequence of horizon extraction mining depth for supercritical longwall operations the maximum subsidence amount is unrelated to depth but for other operations both the magnitude and time to onset of subsidence are dependent on depth inclination of extraction horizon asymmetric subsidence occurs when the zone being mined is inclined the subsidence profile is translated in a downdip direction with both the limit angle and the horizontal strains increased downdip and reduced updip degree of extraction reducing the amount of material extracted will reduce the amount of subsidence thus lower extraction ratios tend to both reduce and delay the onset of subsidence mined area the critical width of a mined void must be exceeded in all directions if maximum subsidence is to develop this is especially important if overlying competent materials are present as these may tend to form bridges in the direction of subcritical width and thus decrease subsidence method of working the amount of subsidence is largely controlled by the degree of caving induced by the mining method e g complete subsidence for block caving and longwall mining and zero for r p together with the amount of support offered by any backfilling nearly immediate but predictable subsidence occurs with longwall mining whereas with r p operations both the magnitude and onset of subsidence are largely unpredictable extraction rate surface subsidence follows the face as it progresses and so to minimize the effect of strain and tilt on surface structures a fairly rapid constant face advance rate should be adopted legget 1972 competence of surrounding materials since subsidence propagates from the mine level the mechanical behavior of the rock adjacent to the mined void directly affects the initiation of subsidence weak roofs and floors accentuate subsidence whereas strong materials can delay or even prevent collapse and hence subsidence strong massive materials above the mine level are able to withstand the effects of extraction for a prolonged period and henc

e defer the occurrence of subsidence in situ stress state high horizontal stresses may foster formation of an arch in the material overlying a mined void thereby attenuating subsidence however arch formation is a complex phenomenon depending on many geomechanical parameters it cannot be guaranteed and arches may fail suddenly and catastrophically geological discontinuities the existence of faults folds and the like may increase and localize subsidence potential so strongly that in areas of adverse geological conditions the effects of the other parameters can be discounted as mining perturbs the stress state in the ground fault planes may become critically stressed and hence undergo shearing the associated release of strain energy can be rapid and dramatic producing seismic events or can be more benign resulting simply in the ground surface being displaced into a series of steps displacements will be concentrated toward faults although horizontal strains may become negligible in their immediate vicinity structures that straddle faults may be severely damaged while nearby buildings remain relatively intact figure 8 9 7 joints and fissures in the strata affect subsidence behavior in a manner similar to faults but on a smaller scale near surface geology and surface topography the nature of any near surface soils and unconsolidated rocks affects subsidence development with both the thickness and mechanical characteristics of these materials being important for example cracks and fissures may form in stiff clays whereas soft clays may deform plastically and cohesionless sands may flow down into fractures in the underlying rocks specialist geotechnical advice may be needed in order to determine the behavior of structures in or on these materials sloping ground tends to emphasize downward movements because of gravity and extensile strains may increase on hilltops and decrease in valleys although there is evidence that the effect of topography on strain is unpredictable ewy and hood 1984 deeply incised valleys particularly those in regions of high horizontal in situ stress may display valley bottom vertical subsidence that is markedly less than predicted for a horizontal ground surface and horizontal closure greater than predictions of horizontal strain would suggest waddington and kay 2003 these phenomena seem to be analogous to natural periglacial cambering and valley bulging in weak rocks hydrogeology deformation of the strata around mined areas may alter hydraulic gradients resulting in either the flooding or draining of surface areas and the formation or draining in aquifers of underground reservoirs booth 2002 rocks may be weakened by changes in saturation and in carbonate e g limestone areas caverns or karst may develop over a period of time if surface runoff from precipitation or water from leaking pipes is allowed to percolate into ground fractured by mining the increased groundwater pressu

re can reduce the effective stress thereby inducing shear on faults elapsed time subsidence does not occur instantaneously but over a period of time in r p operations subsidence may only develop a long time possibly centuries after the mining is complete when pillar degradation leads to roof collapse in caving operations surface displacements may occur almost immediately after the face passes below an area however as noted the presence of strong competent layers overlying an opening can delay this discontinuous subsidence the wide range of circumstances leading to discontinuous subsidence means that no generic method is available for predicting or analyzing the phenomenon instead the principles of geomechanics must be applied on a case by case basis as all discontinuous subsidence involves the movement of clearly defined bodies of rock or soil it is often straightforward to develop a free body diagram of the system and examine its equilibrium continuous subsidence subsidence measurement and monitoring the differential settlement and horizontal strain developed during subsidence tend to be critical in terms of structural damage as these are related to vertical displacement measuring and monitoring subsidence displacements is of prime importance to be of maximum benefit monitoring should commence before any mining activity begins and be continued for as long as either ground movement is likely to occur or surface structures are liable to be affected by movement traditionally subsidence measurement and monitoring has been undertaken using customary precise leveling techniques measurements must be made on structures or markers that respond directly to the subsidence of the rock surface and it is essential that all survey points are unaffected by surface movements e g fluctuations in elevation due to freeze thaw action or changes in saturation although many forms of survey marker have been devised their essential characteristics are a long steel tube or bar topped with a disk or hemispherical dome of rust resistant metal grouted into the rock at a depth appropriate to the local conditions and surrounded by concrete for protection although the markers need to be sufficiently close to allow changes in the local subsidence gradient to be detected economy of cost generally will require the spacing to be as large as possible the u k national coal board recommended a spacing of 0 05d where d is the depth to the mine workings national coal board 1975 in the united states a spacing of 0 05d to 0 1d has been used the accuracy of the measurements should be such as to detect horizontal strains of 1 10 4 which is about one tenth of the strain that causes structural damage the measurement procedures may be obtained from any text on surveying but attention needs to be paid to details such as the area to be covered surface topography labor requirements and survey frequency vertical displacements may be measured di

rectly by trigonometric leveling precision optical or laser differential leveling or tilt measurement when using a theodolite vertical angles must be measured correctly to one half second of arc with precise leveling a micrometer direct reading to about 1 5 mm should be employed an inclinometer with a sensitivity of 10 seconds of arc is generally adequate for subsidence measurements the increasing availability of high resolution satellite imagery means that indirect measurement of vertical displacement using satellite interferometry is becoming more widespread as a subsidence monitoring tool wright and stow 1999 carnec and delacourt 2000 although displacements obtained in this manner currently have a resolution limited to the order of a few millimeters ge et al 2007 the technique has many benefits including removal of the need for surface surveys regular automatic acquisition of data and measurements not limited to specific profiles for new mines it is also possible to use imagery obtained before mining in order to obtain baseline conditions with the continual deployment of ever more powerful remote sensing satellites it is reasonable to assume that interferometric techniques will become the future standard method of subsidence monitoring subsidence prediction subsidence prediction techniques may be characterized as either phenomenological or empirical with phenomenological techniques using computer based numerical modeling to reproduce subsidence by simulating the behavior both continuous and discontinuous of the rock around a mine and empirical methods using various models and formulas based on observations and experience of actual mining subsidence empirical methods include influence functions profile functions and graphical techniques although empirical techniques have formed the basis of most subsidence predictions to date continuing developments in numerical modeling and subsurface characterization using geophysics suggest that phenomenological techniques will become preeminent phenomenological methods phenomenological methods are computational methods based on the principles of mechanics and assume some particular material model to be applicable to the rock initial work concentrated on elastic viscoelastic plastic and elastoplastic continuum models e g marshall and berry 1967 crouch 1973 but recent work has moved toward discontinuum and hybrid continuum discontinuum approaches e g wu et al 2004 zangerl et al 2008 these are likely to develop further and become more prevalent because they more closely capture the fragmentation that occurs during subsidence than do continuum approaches application of phenomenological methods requires sophisticated software the complexity of the physical processes involved in subsidence means that simple numerical techniques are seldom appropriate and suitably skilled analysts although powerful and comprehensive they cannot be used in an impromptu mann

er long term effects the duration of subsidence resulting from mining is composed of two distinct phases active and residual active subsidence consists of those movements occurring contemporaneously with mining operations while residual subsidence consists of those that occur later than this either following the cessation of mining or the passing of a zone of influence the magnitude and duration of residual subsidence has particular relevance to structural damage and legal obligations but both of these vary markedly with the mining method and the geomechanical characteristics of the ground subsidence induced environmental effects the surface displacements and deformations characteristic of subsidence will affect any use made of the ground surface this includes the ground as a foundation for buildings and structures as well as a resource in its own right subsidence affects both the built and the natural environment as previously outlined subsidence comprises five major components 1 vertical displacement 2 horizontal displacement 3 slope or tilt the derivative of the vertical displacement with respect to the horizontal displacement 4 horizontal strain the derivative of the horizontal displacement with respect to location 5 vertical curvature or flexure given approximately by either derivative of the slope vertical displacements alone cause little structural damage although lowering of the land may adversely affect both urban and rural drainage systems flow through transport pipes groundwater regimes and vertical alignment of roads and railways similarly uniform horizontal movements of the ground surface also cause little damage to structures however spatial variation in vertical displacement causes formation of slopes that can interfere with drainage systems and the vertical alignment of roads and railways and can cause unacceptable tilt in tall structures such as chimneys surface horizontal strains cause most of the damage to structures located above mined areas extensile strains can lead to breaks in pipes electricity or communications lines and roads and cracks in masonry or similar brittle structures contractile strains lead to distortion and buckling of steel structures pipes and railway tracks damage to walls and failure of the weaker parts of masonry structures e g around door and window openings in most cases it is a serviceability rather than an ultimate limit state that governs whether or not subsidence damage is tolerable wahls 1994 quoting other work wahls suggests that movement is not tolerable if damage requires costly maintenance and or repairs and a more expensive construction to avoid this would have been preferable the concept of tolerable damage is important in general it is related to the consequences of any damage and the ease and economics of repair but in the context of mining it is also related to the cost and economics of changed or abandoned mining plans if ne

eded to reduce the effects of subsidence work continues worldwide to codify the effects of deformation on structures for example european standard en 1997 1 2004 i e part 1 geotechnical design of eurocode 7 often referred to simply as ec7 cen 2004 presents guidelines for limiting values of structural deformation and foundation movement this code requires designers to consider overall settlement relative or differential settlement rotation tilt relative deflection relative rotation and horizontal displacement of structures see figure 8 9 12 for definitions and gives guidelines for critical values for many of these parameters thus the maximum acceptable relative rotations for open framed structures infilled frames and loadbearing or continuous brick walls are likely to lie in the range from about 1 2 000 to about 1 300 with a maximum relative rotation of 1 500 being acceptable for many structures as these ratios apply to sagging mode deformations the code suggests that the values be halved for hogging modes i e vertical displacement of the edges more than that of the parts between similarly it is suggested that for normal structures with isolated foundations total settlements up to 50 mm may be tolerable with larger settlements being acceptable provided they do not cause excessive relative rotations tilting or other problems such as with the services entering the structure finally the code stresses that these guidelines apply to normal routine structures for other structures specialist advice should be sought buildings and structures although ec7 requires the effects of differential settlement horizontal strain angular distortion and tilt to be recognized it is the complex interaction between the magnitudes of these deformations and the characteristics of the structures themselves that determines the nature and amount of damage extensile horizontal ground strain tends to produce vertical and step like cracks in brick walls generally of uniform aperture extension cracks in floor slabs usually occur at right angles to the direction of the extensile strain with the sides of the cracks showing little or no shear offset contractile strains lead to bulging of walls and buckling and heaving of floor slabs and rigid coverings such as floor tiles angular distortion or relative rotation is a measure of shearing and leads to diagonal cracks in masonry and brick walls and their plaster coverings and binding of doors and windows the concept of tolerable damage has led to the development of classification schemes such as that shown in table 8 9 4 the information in this table has a history stretching back to the middle 20th century and is widely accepted however the tolerable damage concept means that this table and others like it should be considered as guidelines for example distorted window frames may be tolerable for some buildings whereas even slight cracking may be intolerable for others

these damage criteria are often embodied in various charts as examples figure 8 9 13 shows how horizontal strain and angular distortion combine and figure 8 9 14 shows how horizontal strain and length of structure affect the damage classification damage may be caused to bridges by horizontal ground strain resulting in the movement of the supports of piers either toward or away from one another differential vertical settlement or distortions in the horizontal plane may bring about complex and often serious effects on the decking and arches national coal board 1975 it is generally difficult to determine the location and nature of the ground movements based on visual observation of damage often compressive damage leads to crushing and spalling of concrete decks with combined compression and extension due to bending causing opening and closing of construction joints in abutments more substantial damage is characterized by distress in the superstructure inward horizontal movement of abutments jamming of beams and girders against the back wall of the abutments and serious damage to the bearings moulton et al 1985 these findings are summarized in table 8 9 5 across a wide range of other structures consensus seems to be converging on a value of 1 0 10 3 for tolerable horizontal strain see for example nishida and goto 1970 lackington and robinson 1973 lee 1977 institution of civil engineers 1977 kratzsch 1983 however specialist advice should be sought for nonroutine structures public utilities and communications roads and airport runways subsidence principally affects roads and roadways through formation of cracks and undulations in the surface distortions to both horizontal and vertical alignment damage to ancillary structures such as drains fences and curbs deterioration of base courses and subgrade and ponding of surface water structures used for high speed operations e g runways and motorways are particularly vulnerable to such damage the formation of tensile cracks on the pavement surface usually coinciding with the position of the rib sides in the mine workings and compression ridges near panel centers donnelly and melton 1995 are the most common forms of damage as highway bridges are generally more susceptible to subsidence than the highways themselves bridge damage criteria may be used to give conservative limits for highways railways railway authorities are generally protective of railway infrastructure and will set quite prescriptive limits on subsidence effects one of the first effects of subsidence on railways is rider discomfort which may require the reduction of maximum permissible speeds changes in ground slope may adversely affect track performance by formation of localized depressions or creation of gradients greater than permissible for a given type of traffic the reversals of surface horizontal strain i e from zero to extensile strain initially followed by contraction observ

ed over longwall panels as the working face progresses are particularly damaging institution of civil engineers 1977 at higher levels of contractile strain rail tracks have a tendency to snake or bend and in more extreme cases the rails themselves may be forced from the track the extent to which a railway line is affected by ground movements is related to types of traffic involved speed limits types and construction of track preventive and remedial works and nature and magnitude of ground movements serviceability limits of 2 0 10 3 and 10 0 10 3 for horizontal strain and slope respectively have been suggested kratzsch 1983 pipelines generally pipelines are laid below the ground surface and so deform in response to ground movements if the magnitudes of these movements are such that either the pipeline or its joints or couplings are unable to accommodate the strains and rotations developed in them the pipeline will fail in the case of thin wall welded steel pipe this failure may be one of serviceability in that the pipe undergoes local buckling for other cases it is likely to lead to either interruption or loss of service the consequences of these depend on the contents of the pipeline and the nature of the service it provides the resistance a pipeline offers to ground movements is dependent upon such factors as the mechanical properties of the pipe material the rotation and pull out capacity of the couplings connections to other structural elements corrosion resistance of pipe and joints state of repair and installation technique trautmann and o rourke 1985 abdoun et al 2009 for pipelines constructed with flexible joints the joints constitute the weakest component in a pipeline thus the location of joints with respect to the subsidence profile together with the rigidity of the pipeline will significantly influence its susceptibility to damage which is focused on whether subsidence induced displacements can cause leakage at the joints luo et al 1998 for pipelines constructed with welded heat fusion or rigid bolted joints the joints are as strong as the pipeline itself and induced stress becomes the critical factor curvature and horizontal strain at the ground surface resulting from subsidence induce stress in a pipeline and this must be added to the normal pipeline operating stresses to minimize subsidence effects buried pipelines can be uncovered and actively releveled while subsidence is taking place in all cases where substantial ground deformation is anticipated suitable pipe layouts should be used to accommodate these pipeline manufacturers supply standard details for such layouts subsurface structures the deformations associated with mining occur at all depths within the rock above an extraction zone as indicated in figure 8 9 15 and so subsidence damage is liable to affect subsurface structures in addition to those at the ground surface as figure 8 9 15 suggests the deformati

ons and hence damage experienced by subsurface structures depend on thei locations relative to the extraction zone of course mining operations may mean that such a zone will extend laterally and in this case a subsurface structure may experience a range of strain regimes shafts are the structures most critically affected by subsurface deformations although it is possible to locate shafts outside the area of influence of any workings that is within a protective shaft pillar this may lead to uneconomic sterilization of resources for deep mines in laterally extensive deposits such as coal seams in those cases where shafts cannot be protected within a pillar it is important that they are designed and constructed to be serviceable under all of the vertical and horizontal deformations that the mining will induce the tolerable deformations will depend on factors such as type of shaft lining shaft use i e ventilation or hoisting and the presence of water bearing zones the effect of subsidence on smaller mine structures such as access drives and chambers for machinery and services depends primarily on their position relative to the extraction zones below and at the level of extraction the increased stress due to excavation rather than deformation is the most problematic with appropriate design all such structures should remain serviceable throughout mining structures within the caving material immediately above an extraction zone are unlikely to survive regardless of their form of construction at greater distances above the extraction zone where the fracturing and deformation of the rock are more limited it may be possible to design and construct structures that remain serviceable the same holds true for structures in overlying soils agricultural resources essentially subsidence damage caused to agricultural resources is characterized by loss of use or reduced productivity subsidence generally affects agriculture through interference with the established hydrology for example formation of surface fissures in areas of extensile strain leading to erosion and reduced water retention changes to ground slope leading to either increased runoff and erosion or reduced runoff and waterlogging disruption of surface drainage patterns and systems and contamination of groundwater with deleterious minerals such as sulfides and chlorides singh and bhattacharya 1987 subsidence features such as sinkholes can also affect the hydrology and in extreme cases can lead to sterilization of the resource or loss of livestock and equipment although forests and grazing lands are generally less susceptible to subsidence damage than are cultivated lands the types of damage to which they are vulnerable are broadly similar hydrological resources if zones of extensile strain form beneath a body of surface water there is the risk that fissures may develop in the underlying ground leading to partial or total loss of water if the body of water

is extensive e g a lake or the sea then catastrophic inundation of the mine workings can occur to prevent this latter occurrence strict conditions are placed on working beneath water and these can be used to offer conservative guidelines for prevention of subsidence damage for example the u s bureau of mines recommends a safety zone around and beneath any body of surface water as shown in figure 8 9 16 and gives guidelines for mining beneath such a safety zone babcock and hooker 1977 for the case of aquifers subsidence induced fissuring may significantly affect the hydrogeological and geochemical regime singh and bhattacharya 1987 booth 2002 water table depression is common because all underground openings act as sinks but the presence of impermeable materials in the overburden may inhibit this structural features such as faults and folds and the nature of the rocks themselves may have a significant influence on subsidence effects flow rates in surface streams may change and the general loss of water can lead to reduced plant growth the increased fracturing and changes in water flow paths can lead to oxidation of a number of minerals leading to reductions in water quality and hence usability this can be particularly important for potable water supplies the limits set for undersea mining will probably lead to conservative conditions for mining beneath aquifers pseudo mining damage inhabitants of mining areas may blame mining activity for any damage observed in local structures or lands especially if compensation is available for mining related damage because damage may be due to effects other than mining it is important that any other causes are recognized and the true source of the damage properly determined differential settlement can occur in various ways all foundations settle when first loaded and so differential settlement can happen when old and new foundations are combined this commonly occurs when old structures are extended laterally foundation settlement is particularly pronounced on clayey soils and laterally varying soil properties can lead to differential settlement likewise constructing a foundation on fill material especially when the fill is improperly compacted or the fill only partially underlies the structure can give the same effect some clayey soils change volume dramatically as their water content changes shrinking as they dry and swelling as they become wet such changes can occur seasonally can be the result of leaking water pipes and sewers or can be due to planting or removal of nearby vegetation similar effects occur in fine grained soils from freezing and thawing effects as soil volume change by itself leads only to vertical strain the absence of horizontal strain in a structure can be used to exclude mining as the cause of damage changes to the hydrological regime can similarly lead to damage building activity irrigation or drainage associated with major excavation

s can all remove water from the ground inadequate or faulty drainage can lead to rising groundwater levels and saturation of the ground seepage into subsurface structures can result as can deterioration of materials such as wood and plaster however hydrological changes resulting from mining activity are subsidence damage and must be recognized as such poor quality construction and building materials can increase the likelihood of damage to structures as can natural wear and tear construction and maintenance records together with an assessment of anticipated mining damage can help identify these effects poor detailing whereby materials of widely different coefficients of thermal expansion are affixed to one another can lead to either cracking or buckling as the ambient temperature changes buckling in hot weather is particularly troublesome for poorly detailed linearly extensive steel structures such as pipelines and railways many components of buildings and structures are susceptible to chemical attack in the case of ferrous components embedded in masonry or concrete the volumetric expansion associated with the formation of rust can lead to tensile splitting and spalling of the host material with steel window and door frames cracked glazing may occur concrete is susceptible to a number of adverse chemical reactions the presence of siliceous or carbonate aggregates in concrete can cause alkali silica or alkali carbonate reactions whereby the reactive compounds swell due to absorption of water and the swelling leads to formation of random patterns of cracking sulfate attack from either minerals in the ground or the aggregate or ions dissolved in groundwater can lead to similar swelling and cracking dynamic loads from seismic events machinery in industrial plants or road and railway traffic can lead to cracking in brittle construction materials and differential settlement due to consolidation of foundation materials substantial surface movements on slopes can also be generated by various natural processes such as freeze thaw and solifluction flow of saturated soil down a slope extraction of fluids particularly from aquifers and petroleum reservoirs can also lead to substantial subsidence and damage such activities are usually licensed and highly regulated and as such it will be straightforward to determine the degree of damage due to such operations control and prevention of damage the measures that can be implemented to control and minimize subsidence damage fall into the categories of adoption of particular mining techniques postmining stabilization architectural and structural design and comprehensive planning singh 1984 each of these comprises a number of devices in all of these comprehensive calculations are required to identify the economically optimal approach or combination of approaches adoption of particular mining techniques the principal measures to consider are partial mining change

s to the mine layout harmonic mining backfilling and changing the extraction rate partial mining generally involves such measures as leaving protective pillars beneath critical surface infrastructure as noted previously with deep mines this is liable to lead to sterilization of substantial resources and hence be rendered uneconomic any structure supported by a protective zone will remain at its original elevation and thus end up at a higher level than the surrounding area once subsidence has taken place this may lead to problems with transport links and utilities in coal mining partial mining can also involve extracting less than the full thickness of a seam apart from subsidence control this can also be beneficial if either the immediate roof or floor is weak or susceptible to weathering other partial mining techniques such as electing to use an r p operation or mining subcritical widths in order to reduce the maximum subsidence are in some respects changes to the mining layout such changes involve locating and orienting extraction zones so as to limit deformations in surface structures to tolerable levels for example aligning the center line of a supercritical extraction zone with a surface structure will reduce the effects of changes in horizontal strain it is possible to adopt a mining layout whereby sized pillars separate adjacent and parallel extraction zones so as to generate essentially uniform subsidence across the complete mining area figure 8 9 17 clearly for optimal benefit this requires extraction of adjacent zones to proceed simultaneously this is an example of harmonic mining whereby extraction of adjacent zones is scheduled so as minimize changes or magnitudes of horizontal strain as figure 8 9 5 shows regions of extensile and contractile strain exist above the rib side of an extraction zone and so by simultaneously mining two appropriately separated zones it is possible to substantially minimize the horizontal strain however harmonic mining severely constrains the mine operation and in a mechanized environment requires substantial investment in mining equipment as a result it is generally only applied where protection of surface facilities is paramount and mining costs can be disregarded backfilling a mined void is well known to reduce subsidence however in mining operations that generate little waste material the material for backfilling will need to be imported and the economics of this will need careful assessment where suitable material exists separate infrastructure from that required for extraction may be needed for its emplacement the reduction in subsidence damage brought about by backfilling benefits all surface facilities and may be particularly appropriate in areas where maintenance of surface drainage regimes is required finally subsidence development depends to some extent on extraction rate goulty and al rawahy 1996 thus it is worthwhile to select an extraction rate t

hat minimizes subsidence damage generally higher rates are appropriate for unfractured rocks and lower rates for fractured rocks however after the equipment for a mechanized mine has been selected and commissioned it is difficult to increase the extraction rate should that be found necessarypostmining stabilization stabilization of complete mine sites extending over many hectares may be achieved by backfilling as previously outlined grouting or in the case of shallow voids beneath derelict or unused land complete excavation and backfilling grouting which has found widespread application involves pumping or injecting a cementitious material into the mine void using an array of boreholes drilled from the surface high pressure injection may be necessary if the grout is required to penetrate joints or fissures in order to stabilize a fragmented rock mass stabilization of individual structures generally involves either localized grouting of the mined void or in the case of shallow workings underpinning with new foundation structures such as deep piles architectural and structural considerations when structures are to be built in areas of known or future mining activity designs should be adopted that will tolerate the anticipated ground movements this may include locating and orientating structures so they expose their shortest dimension to a subsidence trough figure 8 9 14 shows how damage decreases with decreasing length of structure and avoiding the surface outcrop of any faults figure 8 9 7 many design techniques are available to produce structures tolerant of subsidence for structures that are small in plan it may be appropriate to adopt a rigid foundation and superstructure that is capable of withstanding differential settlement and horizontal strain such structures will tilt as a rigid body which needs to be fully taken into consideration in other cases flexible structures are designed to ride over the subsidence wave associated with a mining face and do so by permitting free ground movement below the foundation ensuring sufficient superstructure support and withstanding ground deformations without loss of structural stability concrete floor slabs no longer than about 18 m which contain both top and bottom reinforcement and are founded on granular material are characteristic of these designs trenches around the structures backfilled with granular material can be used to absorb horizontal strain with utility buildings such as low rise industrial units and warehouses some minor damage may be tolerable in these cases it may be most economic to adopt a design that is a hybrid of the rigid and flexible approaches and undertake minor repairs as required for structures where avoidance of tilt is crucial jacks or similar leveling devices may be necessary between the foundation and the superstructure specific design details that are appropriate for domestic buildings include provision of movement joints and u

se of structural connections capable of withstanding both extensile and contractile strains window and door frames positioned so as not to weaken the overall structure internal walls finished with plasterboard rather than plaster floors and roof secured to the walls omission of bay windows porches and unreinforced masonry outbuildings detached from main buildings provision of increased falls to all gravity drainage systems including gutters no paving immediately adjacent to buildings flexible paving materials and light boundary fences rather than walls in the case of existing structures overall repair expenses may sometimes be reduced if the structures are suitably modified prior to the commencement of subsidence kratzsch 1983 measures that can be taken include provision of temporary supports and or strengthening to parts susceptible to damage supporting screens partitions and the like independent of the walls and floor installing wall ties or temporary corbels to roof trusses likely to be pulled from their seats taping windows especially those with metal frames to prevent flying glass removing stained glass windows to safe storage temporarily installing pretensioned steel mesh around exterior walls excavating trenches around buildings to below foundation level without disturbing the foundations backfilling with compressible material and if required covering with flexible paving cutting slots in rigid pavements floors and superstructures to leave units about 18 m long removing parts of structures e g connecting corridors and removing complete units from within rows of buildings in all cases where structures are damaged by subsidence no repair work should take place until subsidence is complete as figure 8 9 5 shows an approaching working face is preceded by a zone of extensile strain that quickly becomes one of contraction as a result cracks should not be filled until all movements have stopped and any debris that has fallen into cracks should be removed before the contractile strain develops comprehensive planning successful implementation of any surface land use or mine plans requires extensive knowledge of the requirements of each and complete collaboration between the various interested parties it is essential that everyone i e not only the general public but also mine personnel developers government and utility officials and public interest groups likely to be affected by subsidence is completely aware of all the issues principles to be applied in the planning of development include the following avoid construction across faults construct only specially designed structures over shallow workings locate structures in areas where little strain is anticipated locate significant individual structures over areas that have either completely subsided or will not be mined locate linear structures such as canals motorways and railways so they can be uniformly lowered along subst

antial lengths avoid locating important structures near mine boundaries because coordination with several mine operators and surface landowners is onerous it may be necessary to bar construction from particularly risky areas which should then be used for open spaces development of areas likely to undergo subsidence must be socially and culturally acceptable as well as economically justifiable this implies that the associated development plans must not only be agreed to by the mine owners and landowners but also be open to public comment prior to adoption any changes to such plans require the same public input through a wide range of regulations including environmental impact assessments environmental protection building regulations safety requirements and mining regulations national regional and local government authorities exert considerable control over both subsidence and the development of land that is vulnerable to subsidence damage in various places worldwide mine owners have statutory obligations to prepare and gain approval for subsidence management plans that are circulated to all interested parties for comment and suggestion before being approved it is likely that in the future this requirement will become universal the components of tailings facility design generally include siting studies to select the most appropriate location and configuration site geotechnical and geological investigations to characterize subsurface conditions and identify potential construction material borrow sources geochemical studies and modeling to estimate the tailings solids and water quality to assist in developing the environmental management plan hydrology and hydrogeologic studies to understand surface and groundwater flows directions quantities and qualities to assist in developing the environmental management plan water balance calculations to define the logic for managing water within to and from the tailings facility and to estimate the associated flows and volumes environmental and social studies to define the prefacility or baseline conditions and estimate the changes or impacts positive and negative that may be expected when operations begin field and laboratory testing to develop engineering parameters to be used in analyses and engineering analyses to select the various components of the tailings facility as well as their configurations and specifications often including the following embankments including zonation filters and drains basin area lining system and underdrainage tailings slurry rheology and dewatering to select an appropriate consistency for deposition tailings delivery and deposition systems fresh or nonimpacted water management systems including perimeter diversions and process water management systems including tailings water recovery and reuse often the design of each of these components is iterative with data collection and progresses in steps from c

onceptual development to prefeasibility studies to feasibility design and final design in addition most tailings facilities are developed in stages and expanded throughout the life of the mine and thus the design does not occur as a single step but is an ongoing process these concepts are discussed in greater detail in the following sections tailings management principles the consistent application of good practice and continuous improvement in tailings management over the life of a tailings facility in some cases in perpetuity requires the use of a comprehensive information and resources management system such a system often referred to as a framework for tailings management addresses the generation organization dissemination documentation implementation and continuous upgrading of information procedures and policy matters related to a tailings facility over its life cycle the life cycle of a tailings facility typically includes initial site selection planning and design construction of stage 1 or the starter facility operation and periodic construction of subsequent stages and decommissioning closure and postclosure a framework for tailings management not only defines the engineering operation maintenance and surveillance activities for the facility it defines the roles and responsibilities for the personnel involved this includes management in terms of policy and procedural reviews and the provision of appropriate resources for completing the necessary tasks the framework also addresses procedures for managing change in personnel design and operating criteria and emphasizes proper and consistent documentation through all stages of the life cycle in addition the framework discusses engagement and open communication with all identified stakeholders external to the mine a good overview of the need for a tailings management framework is provided with examples in the document by the international council on metals and the environment icme 1998 the introductory chapter titled a management perspective begins with the following statement the challenge to the mining industry is to assure itself governments local communities and other stakeholders that it is capable of meeting its responsibilities to manage tailings deposition in a manner which achieves the highest standards it is not enough to rely on the fact that the technology is available to ensure all environmental objectives are met the mining industry must go beyond technical excellence and demonstrate that it has the commitment and the management systems and skills to be able to manage tailings in keeping with the expectations of its stakeholders an excellent guideline for developing a tailings management system is provided by the mining association of canada mac 1998 which describes the five principal elements of the mac management system that are continuously evaluated throughout the life cycle of a tailings facility the five el

ements are 1 policy and commitment of the company 2 planning 3 implementing the plan 4 checking and corrective action and 5 management review for continuous improvement in keeping with the icme statement it is pertinent to note that two of the five components of the mac guide are policy and commitment of the company and management reviews the mac guide contains checklists of activities for each component under each life cycle stage to serve as a starting point in developing a framework another guide for developing a tailings management system is provided by the australian government s department of industry tourism and resources ditr 2007 section 4 0 of the guide describes the components of a tailings management system and section 5 0 describes the guide s use in leading practice types of tailings facilities and considerations in their selection because tailings characteristics vary from mine to mine and each site is different tailings facilities can take a variety of configurations with different operating principles methods of delivery and deposition solids management and facility development solution management systems and closure plans the large majority consists of on land impoundments developed behind berms embankments or dams figure 8 10 1 gives some examples in many facilities the tailings are deposited in slurry form onto sloping beaches developed from previous deposition surface water drainage consisting of tailings process water and precipitation falling on the tailings facility runs down the slope to a low point on the beach where a surface water pond is maintained typically at a safe prescribed distance from the embankment water is often reclaimed from this pond for reuse in the mill and if the climate is wet excess water may need to be released after any necessary treatment has been completed control of the beach including the slope angle length and direction and thus control of the pond location is accomplished in part by the slurry deposition system such systems may consist of a perimeter distribution pipe with numerous valve controlled offtakes for a high level of control or a few single point outlets if less control is required some facilities using thickened tailings to produce a slightly steeper beach slope may employ a central discharge point or series of points whereby the beach develops radially outward and storage is provided in a conical deposit contained behind a low height perimeter berm in some cases where the tailings contain sulfide minerals that may turn acidic if oxidized common for example in porphyry type ore deposits the tailings facility must be designed to reduce the risk of acid generation in such a case there are two common approaches 1 develop the facility as a water impoundment and place the tailings under water which has a low oxygen diffusion coefficient and thus reduces the risk of oxidizing the tailings this can only be done in areas where ra

infall runoff or other water supply is sufficient to maintain a water cover 2 utilize a multiple offtake or spigot type of tailings deposition system that enables the points of slurry discharge to uniformly blanket each area of the beach and frequently rotate the points around the facility such that each area becomes covered with fresh tailings before the underlying tailings can turn acidic usually the tailings at the time of discharge from the mill contain excess alkalinity from the milling process and the key is to cover the previous tailings layer with the next layer before this alkalinity is consumed other more unique disposal methods may also be adopted depending on site specific circumstances some of these are described later in the unique tailings management schemes section materials for construction many types of structures are built to contain tailings such as embankments and to protect the surrounding environment such as liners and drains often the embankments are classified by the materials used to construct them and by the shape of the cross section embankments may be earth fill or rock fill or both and in some cases may be concrete or membrane faced roller compacted concrete dams have also been used the use of waste products from the mining process including waste rock from the mine and the tailings from the milling process can be economically and environmentally beneficial in certain cases for example waste rock can often be directly hauled from the mine to the embankment at a relatively low overhaul cost tailings can be processed using hydrocyclones or controlled deposition to separate the tailings into coarse and fine fractions and the coarse fraction can be used for embankment construction the use of mine waste products can reduce the need for excavating natural materials and thereby reduce the overall area of land disturbance it is however necessary to fully characterize the waste materials before using them in embankment construction mineralized waste rock may be subject to oxidation and release of metals that could cause environmental impacts oxidation can also lead to mechanical breakdown of rock particles which can result in a loss of strength the coarse fraction of tailings can also be subject to oxidation which would result in acid drainage and or can be subject to water and wind erosion the alternative to using waste materials is the use of borrowed natural materials the availability of different types of materials for example low permeability soils sand and gravel and durable rock may drive the design of the containment structures however excavation of these materials from within the facility s storage basin can have the added advantage of increasing the facility s storage capacity staged development because tailings are generated and deposited progressively over the life of a mine the development of the storage facility can be sequenced and built in stages these stages may inc

lude dam construction basin raises or lateral expansions the advantages of staged development include deferral of some of the capital costs over the life of the mine allowance for design adjustments in the latter stages as experience and knowledge are gained from the early stages allowance for adjustments to account for changing conditions including hazard consequence if the land use around the facility changes increased use of waste rock or drained or cycloned tailings produced over the life of the mine and facilitation of a slow loading rate to maintain stability in staged development at sites where soft soils form the embankment foundation staged development also has some disadvantages over time unless properly managed some of the original design objectives and supporting information may become lost or unclear as personnel change the quality of the construction may vary from stage to stage if different contractors materials and or quality assurance and control procedures are used these disadvantages can be countered with good documentation adequate resources at all times through operations and a commitment by management and the operating staff to continuous improvement these are all aspects of a sound tailings management system the staging sequence and schedule for the facility s operation should be defined at the time of the initial design of the facility they are usually connected to the mine plan because that determines the schedule of ore production and thus by association the schedule of tailings production the mine plan also determines the waste rock production over the life of mine in case the waste rock is used for facility construction however most mine plans change frequently and thus any change that could affect the tailings facility should trigger a review of the staged development and tailings deposition plans however care must be taken when making significant changes because changing the construction materials schedule may affect the following elevations and alignments of tailings delivery and water reclaim systems dam foundation stability e g if the dam is on a soft clay foundation and its rate of rise increases an undrained shear failure in the foundation may be triggered construction material properties for embankments drainage layers and so forth by virtue of the reduced availability of the most suitable materials which may result in performance changes that could detrimentally affect impoundment behavior tailings deposit stability e g if the tailings are being relied upon for structural support of a perimeter slope and the rate of rise increases without adequate drainage an undrained shear failure in the slope may occur tailings deposit density and compressibility e g if the rate of rise of the tailings increases without adequate drainage the degree of consolidation of the deposit could decrease as more pore water is trapped in the voids resultin

g in a lower density and a longer period for the new rapidly placed tailings to consolidate basin liner systems the requirements for lining a tailings facility basin in part depend on the geochemical characterization of the tailings to be stored the groundwater characteristics below the basin and regulatory requirements regulatory requirements can be either performance driven or prescriptive in nature performance driven regulations usually set limits on seepage rates or levels of impact on nearby groundwater whereas prescriptive regulations mandate a specific liner design on the basis of the waste to be contained in either case the chemistry and rates of potential seepage from the tailings facility are the motivating criteria in liner design sites that may not require a liner if the chemistry of the fluid is adequate and the regulations permit will generally meet one or more of the following criteria low permeability ground conditions cause the seepage rate into the ground to be low hydraulic containment is provided by the groundwater regime tailings are drained with minor or no pore pressures i e filter stack or subaerially deposited tailings in a dry climate such that the potential is low for seepage outflows groundwater is deep in the bedrock such that the seepage path length between the tailings deposit and groundwater is long enough to allow for attenuation of flow and elemental constituents groundwater is of naturally poor quality such that the seepage inflows will not significantly degrade existing conditions the difficulty in adopting either the first or second of these criteria is in conclusively demonstrating during the site investigation phase of the design that no discrete leakage pathways will exist brown 2002 for protection of groundwater where tailings impoundments cannot meet one or more of these criteria it is necessary to design and construct a liner over the impoundment base liners may be as simple as the selective placement of low permeability soil to cover outcrops of pervious bedrock or granular soils alternatively liners may consist of a multilayered system of low permeability soil and or geomembrane layers in some cases with a drain between certain layers for seepage removal in many cases it is conventional practice to incorporate a drainage layer above the liner which functions as an underdrain beneath the tailings to reduce the pressure head on the liner reducing seepage through the liner another benefit of such underdrainage is that it provides bottom drainage for the tailings promoting accelerated consolidation and ultimately a higher strength in a shorter period of time however in some cases the tailings immediately above the underdrain may consolidate to a low permeability thereby restricting further drainage from the tailings mass in this case the benefit of the underdrain is to remove head on the liner only the drainage layer typically consists of at least a 300 mm

12 in thickness of granular material with perforated pipes at selected intervals within the drainage layer the pipes are installed to increase the hydraulic flow carrying capacity of the system a filter layer usually consisting of nonwoven filter fabric is also used to prevent the ingress of tailings solids into the drain low permeability soil liners have some distinct advantages over geomembrane liners because of their thickness the initial seepage front through the liner has significant time to travel concentration of solutes in the seepage water may be reduced as a result of dispersion diffusion and adsorption by the soil the liner will consolidate under loading by the tailings deposit resulting in further reductions in its permeability a disadvantage of soil liners is that seepage takes place through the entire area of the liner for large facilities even if the permeability of the soil liner is low the overall seepage rate can still be significant other disadvantages are that soil liners are erodible and subject to desiccation cracking frost heave and osmotic consolidation brown 2002 geomembrane liners are formed of thin plastic membranes typical plastics used for these liners include high density polyethylene hdpe linear low density polyethylene and polyvinyl chloride the liners are placed in sheets and the seams are welded or fused in the field to create a continuous membrane layer although the plastic materials are effectively impermeable the liners invariably have a finite permeability due to leakage through pin holes seaming defects and water vapor permeation typical leakage rates for well installed geomembrane liners are approximately 0 3 to 3 m3 d ha 30 to 300 gal d acre of liner brown 2002 a composite liner comprises a geomembrane liner placed in direct contact with an underlying soil liner this combines the best of both liner types a small unit leakage rate over a small area and results in leakage rates substantially lower than either liner used individually a main factor controlling the leakage rate through a liner is the hydraulic head on the liner for a single liner without an underdrain between the tailings and liner the head can be significant the head on the overall liner system can be controlled by the installation of an inner liner separated from the outer or lower liner by a drainage layer the drainage layer is designed to have sufficient capacity to remove leakage from the inner liner without significant buildup of hydraulic head on the outer liner the drainage layer between liners may be used as a leak detection system to measure leakage through the upper liner brown 2002 in this application the drainage layer is often termed a leakage collection and recovery system embankment types four basic configurations of embankments are used in tailings facilities downstream upstream centerline and modified centerline downstream embankment the downstream embankment

is constructed in stages so the centerline of the embankment crest moves downstream with each stage because this embankment type uses the largest amount of construction material it is often the most expensive option its performance is completely independent of the physical properties of the tailings deposit and for this reason in some jurisdictions such as chile this is the only option available for a tailings embankment upstream embankment the upstream embankment is constructed in stages with each stage constructed as a berm on the tailings beach immediately upstream of the previous stage the centerline of the embankment crest therefore moves upstream with each stage because this embankment type uses the least amount of construction material it is often the lowest cost option upstream dams rely entirely on the strength of the tailings deposited upstream of the berm for support a large proportion of tailings facilities constructed using the upstream method have utilized a spigoting method of deposition to produce a coarse fraction on the upper beach for strength the spigoting method consists of numerous closely spaced and independently controlled offtakes that allow the tailings slurry to be deposited in a low velocity manner over the beach for maximum segregation so that a free draining coarse sand deposit forms in the structural zone major failures of embankments constructed by the upstream method for the most part have been due to a lack of drainage in the structural zones and have led to the ban of upstream construction methods in some jurisdictions interestingly no upstream dam has failed that has been rigorously designed using modern engineering principles to ensure that the embankment is adequately drained and the phreatic surface is controlled centerline embankment the centerline embankment is constructed in stages so the location of the centerline of the embankment crest does not change with each stage the upstream toe of each embankment stage is constructed slightly over the tailings beach but the majority of each new stage is founded on the previous embankment stage this method relies on some strength and structural support of the tailings for the upstream slope but does not rely on the tailings strength for overall stability liquefaction of the tailings as a result of earthquake loading could result in some localized instability of the upstream slope of the most recent stage but this would not result in significant damage to the structure the centerline method is a compromise between the higher cost downstream embankment and the higher risk upstream embankment modified centerline embankment a variation of the centerline embankment is the modified centerline embankment in this method the embankment crest centerline moves slightly upstream thus reducing the quantity of construction materials required in the downstream shell zone of the embankment modern analytical techniques are used to determine how far t

he embankment crest centerline can be moved upstream with staged construction and still keep the embankment independent of the tailings strength for overall stability these techniques have resulted in significant cost savings without compromising embankment stability many modified centerline embankments have been designed permitted and constructed in canada south america and the united states liquid solid separation in the tailings facility a key factor in the selection and design of many tailings facilities over the last 20 to 30 years is liquid solid separation which has the benefits of increasing the density of the deposit thus increasing the storage efficiency and reducing the potential for liquefaction under static conditions and dynamic loading increasing the stability of the deposit because the shear strength of deposit tailings is derived from particle toparticle contact of the solids the liquid has no shear strength reducing the amount of long term drainage from the deposit which requires collection monitoring and release possibly continuing well after the facility s closure possibly requiring water treatment reducing the amount of long term consolidation of the deposit thus reducing the amount of settlement that the final surface may undergo which can undermine the integrity of the cover and recovering as much of the liquid as possible from the tailings so that it can be recycled to the mill for reuse in the process thus reducing the amount of water required to be added from an external supply this is often the main factor in dry climates where outside water sources may be scarce in some cases enhanced liquid solid separation can also result in a slightly steeper slope angle on the deposit which may improve storage efficiency but caution should be exercised because the actual slope angle is affected by a number of factors and may be less steep than expected and a steeper slope may be more susceptible to liquefaction enhanced liquid solid separation can be achieved in a few ways one is a deposition method that discharges the slurry at a low velocity over the deposit allowing the solid particles to drop out of suspension into the next layer while the liquid flows down the slope in this approach the tailings slurry is introduced at the top of the deposit through a number of adjacent but concurrently operated offtake points often termed spigots such that each point discharges a relatively small flow rate onto the deposit by strategically relocating the points of active deposition each newly deposited layer can be allowed to consolidate and in dry climates to air dry further enhancing liquid solid separation while deposition is occurring elsewhere this approach is sometimes referred to as the subaerial method if a significant amount of clay is in the tailings the addition of a flocculant to the slurry just before subaerial deposition may further enhance liquid solid separation mechani

cal dewatering by thickeners often in concert with flocculant addition is another method in recent years significant advancements have been made in thickening technology and product development such that mechanically thickened tailings are becoming more common maximizing liquid solid separation may not always be the objective of a tailings facility design for example in wet environments where precipitation or run on into the facility together with the separated liquid from the tailings produces an amount in excess of that needed for recycle less emphasis may be placed on liquid solid separation and more on securely storing the higher moistures content tailings in this case the objective is to reduce the amount of excess water generated in the facility that requires releasing tailings continuum the consistency of a tailings slurry has often been defined in terms of the weight of solids in the slurry as a percentage of the total weight of the slurry which is termed the slurry density and expressed as percent solids this convention is being amended however because the weight of solids is a function of the specific gravity of the parent material and two slurries containing the same amount of water will have different solids contents if the specific gravities of the solids are different the convention now being adopted is to use yield stress a strength parameter of the slurry as the indicator of its consistency in rheological terms yield stress can be defined as the shear stress that must be applied to the slurry in order to initiate its movement which is applicable to the design of a transport system in geotechnical terms yield stress can be defined as the undrained shear strength of the slurry at rest which is relevant to the slope of a beach on which a fresh layer of tailings will cease flowing in any tailings slurry as water is removed progressively and the percent solids increases the yield stress will also increase in a nonlinear relationship tailings in the higher water content region consist of conventional low density thickened moderate density and heavily thickened high density slurries and have yielded stresses that are generally between 0 and 200 pa as the degree of thickening increases progressively across this region the propensity for sedimentation and segregation of the solid particles from the slurry reduces these tailings can typically be transported by centrifugal type slurry pumps although at the top end of the range positive displacement pumps may be necessary tailings with higher solids concentrations have a paste consistency which is sometimes defined as nonsegregating typical yield stresses for paste are often up to several hundred pascals and these tailings typically require positive displacement type pumps for transport the liquid limit of the tailings is often in the paste region tailings with consistencies greater than heavily thickened consist of filter cake material and are

typically placed and compacted into the tailings facility by mechanical instead of hydraulic methods dewatering to a cake consistency requires filters typically vacuum or press type filters the broad spectrum of tailings consistencies from low to high density is termed the continuum because the boundaries are not well defined historically tailings have been produced and disposed at the low density end of the continuum however over the last 10 to 20 years there has been an increasing move toward thickened and paste disposal in concert with a significant advancement in the technology to dewater tailings the key advantage of thickenin is increased water recovery for reuse although reduced long term consolidation of the deposit and or higher storage densities may also occur but on a case by case basis these factors need to be weighed against the cost of thickening transporting a higher yield stress material and managing additional recovered water thus the selection of any tailings methodology on the tailings continuum is the subject of trade offs one challenge in the design of a tailings facility and delivery system is that the rheological properties of the tailings can change between the plant and disposal facility thus it is important to begin the design at the point of deposition into the tailings facility and design the tailings facility using criteria consistent with the desired point on the continuum and then work upstream to the plant the effects of shear thinning of the slurry or paste in the pipe and pumps must then be accounted for when designing the thickener site characterization climate and hydrology the climate and hydrology of a region are influenced by patterns of temperature wind and atmospheric moisture and patterns of surface runoff respectively the total amount and timing of runoff dictate many aspects of a tailings facility ranging from environmental concerns such as what level of impact the facility might have on water quality to engineering concerns such as how much storage is required for temporary storage of seasonal or storm runoff or whether stormwater diversions or a spillway are needed and if so how large they should be typically little or no site specific climatic and hydrologic data are available for most development sites at the time of initial design in most instances a data collection program is initiated during project feasibility and environmental impact studies but to a large degree these data are for short term periods and of limited use at the time of project design as a result the climatic characterization of a project area is generally conducted on the basis of regional data which may be extrapolated to the project site according to known or suspected weather patterns similarities of watershed characteristics and an understanding of the fundamentals of hydrometeorologic systems including lapse rates orographic effects and rainfall runoff mechanisms a correla

tion of the regional data to the short term site specific data is often attempted to support the extrapolation the hydrometeorologic characterization of a site typically involves estimates of temperature evaporation precipitation rain and snow and runoff on both average monthly and annual bases including some measure of variation also required are values of extreme precipitation and flow usually on a daily basis and usually presented in terms of likelihood of occurrence in addition in colder climates patterns of snow accumulation and melt are needed often with depths of frost penetration and ice accumulation geology the regional geology typically defines bedrock types lithology and alteration and structure faults and locations unconformities and fracture types and their frequencies orientations and strengths in the site vicinity specific site investigations should then be carried out to investigate and accurately map these features on a more concentrated scale their location and characteristics can be important to the site characterization and type and configuration of tailings facility to be built regional geology maps can usually be obtained from various government agencies generally smaller scale maps in the order of 1 1 000 000 are easily obtained the availability of larger scale maps i e more highly detailed maps such as 1 250 000 or 1 100 000 is more variable because of the economical political and geological characteristics of the area many industrialized countries have government geology departments at the federal and provincial or state level although many developing countries only have limited geological data at the federal level in addition the size and population of the country state or province may dictate the map scale another source useful for gathering regional information is the internet many countries have maps in digital form for viewing and printing after regional maps have been obtained it is recommended that a qualified person carry out a field reconnaissance to verify the mapping larger scale mapping should be carried out if these maps do not exist or the geological or structural setting at the proposed site is complicated it is important at this stage to identify weak or otherwise unfavorable bedrock units or active faults these identifications will aid designers and form the framework for detailed site investigations terrain analysis terrain analysis assists in characterizing the origins of and landforms associated with the surficial or near surficial materials soils and rocks in the vicinity of the tailings facility this analysis is designed to identify and classify material types potentially for construction and to identify potential natural hazards that could impact the tailings facility the terrain analysis will not necessarily determine accurate characteristics of the materials but is a useful tool for defining where detailed site investigations are required th

ere are two levels of terrain analysis the first is at a desktop level and comprises aerial photography the second involves ground reconnaissance and mapping to verify or modify the observations made from the air photos the objectives of these two levels are to define as many of the following attributes as possible bedrock geology quaternary geology landforms and geomorphology soil types weathering erosion and deposition modes slope instability climate effects vegetation hydrogeology geotechnics volcanic activity seismicity natural dams human activity and land use this list is quite exhaustive and many of these attributes will not be present at a given tailings facility site however it is prudent to initially check for all of them in areas of steep wet terrain it is recommended that 1 15 000 scale air photographs be used for the first level of reconnaissance steep terrain is generally characterized as having significant portions of the area with slopes greater than 50 or 27 if drier flatter terrain is present in the site vicinity 1 40 000 scale air photographs are adequate if reconnaissance flights and air photo production can be acquired this exercise can be completed in conjunction with topographic surveys the second level of terrain analysis comprises ground verification of the air photo interpretation the requirements for ground reconnaissance are based on the terrain attributes identified in the first stage generally areas where the slopes are greater than 50 require foot traverses while areas of flatter terrain can be completed by ground checks supported by helicopter or vehicle unstable or potentially unstable areas identified during the air photo interpretation must be checked in detail by foot traverses and 1 5 000 scale mapping the results of the terrain assessment should be made into a terrain hazard map outlining areas of potential natural hazards or geological risks construction materials and targeted dam sites geotechnical site investigation geotechnical site investigations for a tailings facility need to be carefully planned with an initial design concept in mind and should be conducted to an increasing level of detail as the facility moves from scoping level to preliminary and on to detailed design further site investigations should investigate the geologic interpretation of the site developed during the previous site characterization exercises the key objectives of site investigations for tailings facilities are to confirm potential natural hazards identified during the terrain analysis phase characterize foundation materials through sampling and laboratory index tests such as particle size distribution and plasticity characterize existing groundwater conditions through drilling investigations including in situ hydrogeologic properties of the foundation soils such as permeability determine geotechnical properties of the foundation soils such as shear strength compres

sibility and permeability and confirm availability and characteristics of the earth fill or rock fill materials required to construct the facility according to the proposed design concept including mine waste materials from open pit development typically test pits drill holes and seismic refraction in some cases electrical conductivity or gravity surveys are sufficient tools for site investigations for detailed design of a tailings facility test pits are used to investigate potential construction material borrow sources and the shallow foundation conditions for the tailings dam or basin test pits alone may be sufficient for scoping level designs to excavate test pits hydraulic excavators with minimum 5 m 16 5 ft depth of excavation are typically used drilling investigations of the foundation are required for the feasibility and detailed design of a tailings dam a properly planned and executed site investigation includes careful selection of the drilling method s to ensure the necessary data are collected it is strongly recommended that a qualified geotechnical engineer supervises all drilling activities to ensure sampling and testing are completed at appropriate locations and following appropriate methods and that appropriate information is being collected electronic cone penetrometer testing cpt is now commonly used to gain continuous profiles of in situ soil properties in sands silts clays and tailings deposits often these tests are carried out using a tip fitted with a water pressure transducer to record pore pressures cptu and in some cases the cptu can be equipped to carry out in situ seismic tests in which the cone is equipped with a geophone to record the arrival of shear waves from a surface source cptu is an excellent method to extend the data and fill in gaps from more traditional drilling sampling and standard penetration testing spt for gravelly materials the becker drill is a common method that provides samples and penetration data that can be correlated to the spt samples collected from the investigations are tested in a soils laboratory for index strength compressibility and permeability characteristics for the latter two types of tests undisturbed samples of foundation materials are recommended for materials from potential borrow areas index tests will likely include moisture density or compaction testing to confirm their suitability for placement in the tailings dam basin liners or drainage systems strength and compressibility tests are also required for fill materials and should be performed on remolded samples compacted to the density criteria that will be required by the construction specifications permeability tests are required for materials to be used in high or low flow applications such as filter drains or core and liner zones respectively seismicity in regions of high or even moderate seismicity often the seismic loading controls the stability and thus design of a ta

ilings dam consequently a seismicity review of the region where the project is located should be carried out during the initial stages of project planning initial studies at the conceptual or prefeasibility level may be limited to a review of existing information or maps regarding the regional seismicity and preliminary seismic design parameters may be obtained from seismic hazard maps for the region if available in some areas of the world these are available through the u s geological survey and or the global seismic hazard assessment program both of which have web sites however for feasibility and final design stages of a tailings facility more sophisticated methods of analysis are typically required including both deterministic and probabilistic methods of seismic risk analysis a probabilistic analysis defines a unique probability of occurrence for each possible level of ground acceleration experienced at a site using methodology based on cornell s probabilistic analysis cornell 1968 the likelihood of earthquake occurrence within defined seismic source zones is determined by examining seismicity data using historical earthquake records for the region magnitude frequency recurrence relationships are established for each potential earthquake source or fault zone unlike the probabilistic analysis the deterministic method does not account for the likelihood of a predicted ground acceleration occurrence seismic source zones or fault systems in the region are defined and maximum earthquake magnitudes are assigned to each source based on the characteristics of the fault including type and length expected ground motions including acceleration are then transmitted to the site using attenuation relationships the maximum acceleration produced by this procedure is referred to as the maximum credible acceleration and the corresponding earthquake as the maximum credible earthquake mce the mce is defined as the largest reasonably conceivable earthquake that appears possible along a recognized fault or within a geographically defined tectonic province under the presently known or presumed tectonic framework icold 1989 the seismic design parameters selected for the design of a tailings facility are dependent on the seismicity level in the region and the geologic and tectonic conditions at and in the vicinity of the site they are also dependent on the hazard consequence classification of the dam or facility for high or very high consequence facilities the mce may be adopted as the maximum design earthquake mde which is often defined as the earthquake that a facility must be able to withstand without catastrophic failure for lower consequence facilities a lesser earthquake from the probabilistic analysis may be selected for the mde another design earthquake is the operational basis earthquake which the facility must be able to withstand without impeding its operation and is usually selected from the probabilistic ana

lysis alternatives analysis and selection the selection of a site configuration and in some cases the design basis for a tailings facility requires a systematic and defensible method to properly account for the pertinent factors that apply these often span a wide range of subjects and in some cases may conflict with each other usually a few steps are involved the first is the identification of potential facility sites and configurations on topographical maps preconcept level sizing is based on the anticipated tonnage of tailings to be stored and an estimate of the in storage density the second step is often a fatal flaw assessment of the identified options to eliminate those that for one reason or another could not be developed fatal flaws may include a site on land that cannot be acquired or developed possibly because of land status or ownership restricted areas an ecosystem or water resource that cannot be affected a community on or adjacent to the site that cannot be relocated or utilities that cannot be moved and a site at risk of being affected by a natural hazard such as a major active fault unstable ground or a major avalanche or flow debris chute the third step in an alternatives analysis is to compare and rank the options remaining after the fatal flaw assessment by scoring them against appropriate comparative criteria in a structured matrix depending on the level of detail required the matrix may be relatively simple or complex in the early stage of planning a simple approach may be warranted if the list of comparative criteria is short due to limited information on the project or if the initial number of options is large a more detailed comparison of a few selected options can then follow a more detailed comparison uses a tiered multipleaccounts matrix method often utilizing a three tiered structure for grouping similar or related criteria together robertson 2004 kerr et al 2003 the first tier consists of the major account headings such as technical economic environmental or social the second tier consists of subaccount headings which are grouped under each of the account headings for example capital cost of the initial stage of the tailings facility which would be positioned under the economics account the third tier drills down further under the subaccounts level and contains the specific comparative criteria that will be used to score and rank the options for example capital cost of the initial stage dam under the capital cost of the initial stage of the tailings facility subaccount numerical weights are applied at the three levels of the matrix to reflect the relative importance of one criterion subaccount or account to another criterion subaccount or account at the same level but not across levels individual scores are then assigned to each option at the criteria level and the results are calculated as weighted output scores for each option at the subaccounts accounts

and overall levels at the overall or upper tier level a comparison of the weighted output scores gives the overall ranking of the options at the accounts and subaccounts levels the comparison gives the ranking of facilities for each account or subaccount understanding which options are considered better or worse at these levels may be just as important as at the overall level the multiple accounts matrix approach also allows for the calculation of discriminatory values at the subaccounts and accounts levels which define the accounts or subaccounts that had a greater or lesser influence on the weighted scores at these levels one of the greatest advantages of the multiple accounts approach is that by applying the weights in the manner described experts in different fields can provide input to only their appropriate accounts or subaccounts with others providing input elsewhere while not biasing the overall results in many regulatory environments it is now often mandatory to demonstrate that a program of options identification analysis and appropriate ranking was adopted to support the selection of the site and design basis for a tailings facility the multiple accounts matrix approach if used properly can serve as a thorough transparent and defensible tool for this characteristics of tailings tailings are often composed of solid liquid usually water and gas usually air phases in various relative volumes or masses the liquid and air phases occupy voids between the solid particles if air is absent the voids consist only of liquid and the tailings are said to be saturated some useful terms and definitions related to these phases are shown in table 8 10 1 the total weight or mass of tailings is composed of the ws and ww components while the total volume of tailings is made up of vs vw and va components plasticity and related issues the tailings continuum described previously was based on rheology from a geotechnical point of view tailings are generally considered to exist in four states of consistency from high to low moisture content viscous fluid plastic semisolid and solid if the tailings contain clay minerals the plastic range is significant a property associated with the clay minerals plasticity can be defined as the ability of the clay to deform without cracking plasticity results from the electrochemical behavior of the clay particles and the attractive bonds that remain between them even after relatively large deformations it is often quantified by the plasticity index pi which is the difference between the boundaries of the following consistency states liquid limit ll is the moisture content of the clay at the high end of the plastic range at the boundary between the plastic and viscous liquid states remolded clayey materials at this boundary have a low undrained shear strength often in the 1 to 2 kpa range plastic limit pl is the moisture content of the clay at the low end of

the plastic range below which the clay changes from a plastic consistency to a moist semisolid material or cake remolded clayey materials at this boundary often have an undrained shear strength in the order of 100 kpa or possibly higher the pl and ll values are termed the atterberg limits and are determined from tests in which samples of the material are remolded deformed plastically under defined procedures given the correlations with remolded strength the atterberg limits define the range of moisture contents over which the strength of the material increases by approximately 100 times most tailings of hard rock origin have a low clay content and thus low plasticity in terms of moisture content change the difference between the higher and lower end of the plasticity range is low in some cases the difference between the pl and ll is zero and these tailings are termed nonplastic from a tailings engineering perspective the important considerations related to pl ll and pi are tailings with a low pi e g of hard rock or low clay content can gain significant strength with only a minor reduction in moisture content and tailings with a high pi require a significant amount of water to be removed usually under load or evaporation the process of consolidation to gain significant strength the boundary between the semisolid and solid states is defined as the shrinkage limit which is important because it defines the moisture content below which no further volume reduction or shrinkage will occur with additional drying or moisture loss at this point the particles have reached such a dense packing that any further moisture loss results without a further reduction in void ratio the result is a reduction in saturation as air enters the voids to replace the ongoing water loss seismic and liquefaction considerations tailings liquefaction is a major challenge in the design of tailings facilities and can be triggered by both cyclic and static shear stresses liquefaction occurs when a soil loses a significant portion of its strength or stiffness for a relatively short time but long enough for slope stability or bearing capacity failures to occur because it is unpredictable and occurs under various conditions liquefaction has been viewed as a difficult problem there are many recorded cases of liquefaction failure of mine waste and quite likely many more undocumented cases been and li 2009 liquefaction including occurrences due to earthquake loading or from static causes requires close assessment typically by specialist practitioners various methodologies assess the liquefaction potential of mine tailings in situ methods are often employed for such an assessment using applications such as the spt cptu seismic cptu field vane shear test becker hammer test or self boring pressure meter various laboratory approaches also assess the liquefaction potential of soils such as cyclic simple shear and cyclic triaxial testing

typically a tailings facility designer will assess the likelihood of liquefaction due to earthquake shaking and static loading for earthquake induced liquefaction laboratory or in situ testing results are typically required with in situ testing often the preferred method an example approach for assessing earthquake induced liquefaction is the use of the cpt piezocone a tapered electronic probe that is advanced into a soil deposit typically using a hydraulic ram such as a geotechnical drill rig the piezocone is equipped with load cells and transducers to measure the device s penetration resistance at its tip friction along its side shaft and pore pressures that develop during penetration this pore pressure has been termed dynamic pore pressure by some practitioners but the authors suggest it should be termed quasi static pore pressure because the piezocone is a quasi static test by pausing the penetration the quasi static pore pressure is allowed to dissipate to reestablish the equilibrium pore pressure which represents the in situ pore pressure at that point in the deposit by conducting several of these dissipation tests it is possible to develop a pore pressure profile throughout the deposit s depth this profile is useful in understanding the effective stresses in the vicinity of the probe and helps assist to establish whether the material is contractive or dilative data from the piezocone tests are combined with parameters pertaining to the design earthquake event i e earthquake magnitude and ground acceleration to complete the analysis procedures for completing an earthquake induced liquefaction analysis use piezocone seismic cptu spt and becker hammer harder and boulanger 1997 the art and science of liquefaction assessment is continually developing and the national center for earthquake engineering research nceer report has been updated several times robertson and wride 1998 robertson 2004 arango 1996 once the likelihood of the occurrence of earthquakeinduced liquefaction is established it is necessary to develop strength properties for the soil zones that are predicted to liquefy yoshimine et al 1999 olsen and stark 2002 the yoshimine method produces a more conservative analysis robertson 2004 geochemistry characterization of both the solid and solution portion of the mill tailings is an essential step in the development of the tailings management plan the acid generating potential and metal leaching characteristics of the solid tailings mass and the chemistry of the liquid effluent will affect the design of the tailings containment facility and be important considerations in solution management and reclamation planning acid rock drainage ard metal leaching and contaminant metal release from mine tailings facilities are recognized environmental concerns to ensure that natural aquatic systems are not significantly degraded or that human or other receptors are not impacted it is importan

t to fully understand the tailings material by conducting a thorough waste characterization program a phased approach to waste characterization is often a prudent way to proceed the relatively inexpensive static tests can be performed on a large number of tailings solids samples using the results of the initial testing to plan for additional more detailed testing requirements this stage of the characterization program will include acid base accounting aba or equivalent to determine the relative balance of potentially acid generating and potentially acid consuming minerals leach extraction testing to measure the soluble components of the samples and other testing to determine trace element content of the waste whole rock analysis and mineralogical descriptions may also be conducted trace element testing will usually consist of a full suite of metal analyses inductively coupled plasma mass spectrometry icp ms inductively coupled plasma emission spectrometer trace element concentrations will indicate which constituents are naturally high in the waste and may be a concern for future leaching a comprehensive list of testing methods can be found in the canadian mine environment neutral drainage documentation mend 1991 aba or the determination of the relative amounts of acidgenerating and acid neutralizing minerals in a sample can be accomplished through a number of test procedures with the sobek acid base account test and the modified acid base account test being the most common these tests measure the acid potential ap also called the maximum potential acidity of the sample based on its sulfur or sulfide content the sample s neutralizing potential np is determined by titrating a pulverized sample of the material with an acid with the resulting np representing the acid neutralizing capacity of the sample the sample s net neutralizing potential nnp np ap ratio of np to ap and paste ph are also determined the results of the aba testing are compared to general guidelines to assess whether the samples are likely to generate acid guidelines may vary for different areas but are generally based on the nnp and the ratio between np and ap np ap static testing provides valuable insight into the acidgenerating and metal leaching potential of the waste it does not however provide information about the rate of acid generation or neutralization and should not be used to predict drainage water quality in the field in addition to static testing simple leaching tests such as synthetic precipitation leaching procedure meteoric water mobility procedure or net acidgenerating leachate analyses will provide preliminary data on potential contaminants of concern the next step in the characterization process for the tailings solids is kinetic testing if static testing results show high variability between samples a relatively high number of samples should be considered for kinetic testing if the static testing results a

re largely consistent fewer samples are needed for the kinetic testing phase because similar results indicate less variability in the samples kinetic testing is used to confirm the samples acid generating or acid neutralizing characteristics while determining the reaction rates for acid generation and neutralization kinetic tests are most often conducted in a laboratory where chemical weathering is simulated over time in cells or columns the composition of the leachate collected from kinetic testing can be used to predict drainage water quality in the field if concentrations are corrected for the effects of climate and particle size the leachate should be analyzed for total and dissolved metals icp ms conductivity total dissolved solids tds a full set of anions and cations and ph kinetic testing procedures include humidity cell humidity column column leach soxhlet extraction and field plot tests it is equally important to characterize the tailings solution or the aqueous portion of the tailings slurry a sample of the tailings solution from the pilot test work should be analyzed for a range of parameters including total and dissolved metals icp ms nutrients and reagents and reagent by products used in the process tds and turbidity for example if cyanide is used in the process the effluent should be tested for the full range of cyanide species total cyanide free cyanide weak acid dissociable cyanide strong acid dissociable cyanide cyanate and thiocyanate if warranted speciation of metals of concern should be determined because the species of metal present significantly affects the availability and toxicity of the metal effluent concentrations are compared to in country effluent guidelines or the world bank effluent guidelines a toxicity test of the effluent is also often required to ensure that the effluent is not acutely toxic to aquatic life if discharge to nearby waterways is considered effluent concentrations coupled with the site hydrology can also be used to perform water quality modeling exercises predicted water quality concentrations are then compared to in country international water quality guidelines or criteria for aquatic life protection brown 2002 water management a key part of the tailings facility operations is the management of water in the facility water enters the tailings facility as process water in the tailings slurry direct precipitation and runoff from surrounding undiverted catchments it is lost in the tailings deposit as pore water in the voids and to evaporation and seepage in almost all cases water is recycled to the mill for reuse in the process generally if the total quantity of the water lost to permanent storage in the voids evaporation and seepage is greater than the quantity of water entering the facility from precipitation and runoff the facility is considered to be in deficit and makeup water is required to sustain the milling operation if the oppo

site applies the facility is in surplus and excess water may need to be treated and released or adequate storage provided to contain it precipitation and evaporation usually vary seasonally which results in water surpluses some months and required makeup water in other months water can be stored in the surface water pond in the tailings facility during the wetter months and used as makeup water in the drier months a detailed water balance that accounts for the water inflows and losses on a monthly basis is required for a tailings facility it should consider climatic variations to ensure that makeup capacity for dry periods and treatment or storage capacity for wet periods are sufficient the water balance is used to predict the variation in size of the surface water pond in the tailings facility pond on a seasonal basis as the facility is filled a further aspect of water management is to ensure that the tailings facility has adequate capacity at all times to store route or otherwise handle runoff from extreme precipitation events overtopping as a result of storm runoff is one of the most common causes of tailings facility failure it is imperative that adequate storage is provided to safely contain the surface water pond within the tailings facility at all times particular focus should be on the wet season with the addition of the design storm plus required freeboard for wave run up if sufficient temporary storage cannot be provided it is often prudent to include an emergency spillway although in a stage developed facility constructing a spillway for each stage may be complex water storage the average and maximum monthly design sizes of the tailings surface water pond throughout the mine life can be determined by various calculations the authors prefer the following two methods 1 average monthly pond volume plus a design storm event and 2 monthly percentage chance of exceedance analysis for providing adequate pond storage capacity the larger result of the two should be considered however the largest monthly pond size from either method may vary from year to year throughout the operating life of the mine and this should be accounted for in the staged expansion plans for the tailings storage facility in the first method the averages of the monthly tailings pond volumes are calculated for each of the 12 months of the year and combined with the volume contributed by the selected design storm event the design storm volume may vary throughout the months of the year so the maximum of these two components is sought in the second method a frequency analysis is performed on the various january to december pond volumes often to define the 0 001 representing a 100 000 year storm event 0 01 a 10 000 year storm event 10 a 10 year storm event and 50 a 2 year storm event chance of exceeding pond volumes water removal systems the control of the supernatant pond is probably one of the most important procedures in

managing a tailings system inadequate pond control can result in overtopping increase in pore pressures reduction of freeboard high seepage rates and embankment settlement engels and dixon hardy 2009 these conditions can lead to instability and a high risk of problematic situations for a conventional impoundment particularly upstream and centerline embankments it is essential that the ponded water be kept to a minimum volume well back from the embankment crest and the freeboard be sufficiently high all along the tailings embankment suitable monitoring and management of the supernatant pond are required to safely manage the tailings decant systems for tailings management should be designed for daily management of the supernatant pond as well as storm event surges the design of the decant system should allow for a high surge capacity of stormwater to compensate for near future storm events if the pond cannot drain fast enough typically within weeks then the freeboard of the embankment may be lost if a near future storm occurs the three most common methods of water control within a tailings management facility are 1 drains associated with subaerial deposition 2 free standing or sidehill riser decant towers and 3 decant barges each of these water removal systems is briefly discussed in the following subsections drains subaerial deposition in subaerial tailings deposition the slurry typically discharges out of drop bar just ahead of a rising tailings beach this leads to some dissipation of energy as the tailings slurry exits the drop bar onto the beach the tailings then flow at a relatively low velocity over the beach which allows for liquid solid separation the points of active discharge are frequently alternated around the sides of the tailings management facility to form a thin layered drained and stable tailings deposit against the embankment for this type of facility the beach is sloped into the central area of the facility so that surface water draining from the tailings and runoff from precipitation are displaced away from the embankment a small surface water pond is maintained in the central area directly against a blanket underdrain that covers the lined base of the facility which slopes down to a low point in the facility the underdrain comprises a layer of freedraining gravel within which a network of perforated corrugated polyethylene tubing pipes is installed and this layer is sometimes covered with a geotextile to filter the tailings solids the geotextile is covered with another layer of gravel and cobbles for erosion and protection of the geotextile from ultraviolet radiation exposure a reclaim pond or return water pond stores the water being decanted from the facility the pond is situated outside the confining walls of the tailings storage area a short distance from the embankment toe the water in the reclaim pond is either sent to treatment polishing ponds for discharge to the env

ironment or pumped back to the plant for use in the processing operation if the climate is suitable some reclaim water can be sent to evaporation ponds or sprays decant tower systems a decant tower is an intake structure consisting of a hollow tower riser that is connected to a horizontal conduit or pipe that normally travels beneath the impoundment and through under the tailings facility embankment the riser is extended as the level of tailings in the impoundment rises the decant tower skims off the clear water from the surface of the supernatant pond and carries it away by gravity through the underlying conduit european commission 2004 decant towers can be effective at removing ponded water from a tailings facility but can be difficult to operate as more tailings are disposed of in an impoundment decant tower systems come under increasing stress the ever increasing weight of the tailings can crack and damage a decant conduit that flows underneath and through an impoundment facility engels and dixon hardy 2009 sidehill decant systems utilizing stop logs are less likely to have operational problems because the decant tower is always accessible from shore and the placement of stop logs is relatively easy one major disadvantage is that the water has to be continuously positioned around the decant tower unlike a decant barge a tower cannot be relocated if a decant tower becomes inoperative emergency pumping or spillways need to be implemented any tailings facility operating plan should have precise contingency plans documented in case a decant tower becomes inoperative either by isolation blockage or failure decant barges a decant barge consists of a floating platform that houses pumps used to reclaim water from the supernatant pond to the processing plant or reclaim pond unlike decant towers that are gravity fed a decant barge requires electrical power to operate the pumps that decant the water from the supernatant pond this increases operating costs because a constant and reliable power source is required to ensure the pumps operate as a power failure results in no water being decanted it is good practice to have standby pumps and diesel generators to use in an emergency or when a decant barge cannot cope with rapid inflows during storm conditions or when process water is required in large quantities before power or equipment failures occur there should be emergency procedures and response plans that can be implemented in both normal and storm conditions in order to rapidly mitigate any decant problems engels and dixon hardy 2009 the capacity of the decant barge should be adequate to remove day to day decant demands as well as stormwater accumulation the barge should also be situated in an easily accessible location for maintenance and inspection purposes ideally against the side of a valley wall for a valley impoundment or against the side of a jetty wall in an impoundment area where ponded water is the de

epest the water depth below the barge can influence the clarity of the decant water being extracted by the barge as the supernatant pond location changes and or the tailings volume increases a decant barge or submersible pump can be moved for valley impoundments or in pit disposal the decant barge or pump is generally retracted to keep the equipment close to the valley or pit walls this makes it easier to access and prevents the use of heavy anchoring to control varied movement which can be expected the farther away the equipment is from surface anchoring points each time a barge is moved to other cells the plant water demand should be recalculated to ensure the barge is capable of meeting the water demand in the new cell engels and dixon hardy 2009 tailings delivery and deposition tailings are transported from the mill to the tailings management facility using any of the following methods pump and pipeline systems gravity flow in open channels and pipelines widely used in chile and peru for large tonnage systems belt conveyor and trucks limited to low tonnage filtered tailings operations the following points should be considered for the design of a pump and pipeline tailings delivery system ideally the tailings facility should be located in relatively close proximity to the mill to minimize capital and operating costs the elevation difference between the mill and tailings facility has an important bearing on the operating complexity and cost of the delivery system if the tailings are transported uphill the additional elevation change increases the pumping requirements and may lead to greater capital and operating costs for tailings facilities located downslope of the mill the pumping requirements can be considerably reduced however if the elevation difference exceeds the pipeline friction losses choke stations or drop boxes are required to dissipate the excess energy which can complicate the system operation for thickened tailings systems where the tailings facility is some distance from the mill and or at a higher elevation locating the thickeners adjacent to the tailings facility rather than at the mill may have cost advantages centrifugal pumps are suitable for most tailings applications high concentration non newtonian tailings applications may require the use of positive displacement pumps typically the capital costs of the systems will be greater than comparable centrifugal pump systems although the operating costs may be reduced because of the increased pump efficiency and lower maintenance costs tailings delivery pipelines are generally constructed aboveground using hdpe low pressure applications or unlined carbon steel piping for especially corrosive or abrasive tailings lining steel pipe with hdpe rubber or polyurethane has cost benefits in certain applications cementitious type pipes are sometimes used provision must be made along the tailings pipeline corridor for the conta

inment of possible spillages associated with blockage clearance or pipeline failure this is especially important near water features such as lakes and rivers some applications may require the use of double contained piping to minimize the likelihood of pipeline failures the pipeline condition must be regularly monitored in cold climates provision must be made for draining the pipeline to prevent freezing during pipeline or pumping system shutdowns construction considerations detailed designs for construction of tailings management facilities comprise technical specifications for the work and detailed drawings issued for construction these documents can be included in a contract for facility construction by a third party contractor or can be used by the mine to carry out the construction using its own equipment and staff the design of a tailings facility like that of most large civil structures is based on the available topographic and geologic mapping and information generated by the site investigation into soil and rock groundwater and surface water conditions it is inevitable that conditions encountered in the field during construction will differ to some degree from those assumed or estimated in the design and thus it is good practice that a representative of the design engineer be on site to provide technical direction for construction this will ensure that variations in site conditions are recognized and that the design is modified as required to meet the design intent this service can be and often is integrated with the quality assurance quality control qa qc role qa qc is required during the construction of a tailings facility so that conformance of the work with the technical specifications and construction drawings is measured and recorded this continuity from the design through construction is also important from a liability perspective a designer that has no involvement with the construction can take the position that any shortcoming is a result of the construction or changed conditions that were evident only during construction while the constructor conversely can take the position that the design had shortcomings continuity of qualified outside consultants over the life of a facility can also be important as by its nature the mine may have a high staff turnover often the design engineer provides the only continuous presence on a tailings facility over the mine life an important but sometimes overlooked aspect tailings facility construction is the preparation of detailed as built drawings and a record of construction for the completed facility the preparation of these important items takes place at the end of the project when budgets may be nearly exhausted they are invaluable however as the basis for ongoing staged construction and if any problems arise with the facility s performance monitoring operation maintenance and surveillance operation maintenance and surveillance oms are critical aspec

ts of any successful tailings management facility the procedures should be documented in an oms manual and regular checks or audits made to confirm they are being followed the procedures should be modified and the oms manual revised as necessary an oms manual should be in place upon commissioning and maintained through closure it should provide a clear documented framework for actions to be taken for a wide variety of occurrences the oms manual should also provide a reliable basis for measuring the facility s performance and for demonstrating due diligence of the operating team the detail level of an oms manual should reflect site specific requirements the need for revision may be triggered by changes in such matters as hazard classification of the dam operational performance of the facility project personnel or the company organizational structure regulatory or social considerations life cycle and or design philosophy mac 2002 geology and climate an excellent guideline on tailings facilities oms is available through mac mac 2002 an oms manual should provide the user with information in a clear logical format that is easy to read and implement and supporting documents should be clearly identified and referenced the user should be able to easily identify what is required and how to access the needed information the manual should allow the facility s performance to be compared to expectations design criteria and operating intent particularly in the event of significant incidents confirmation that design objectives are being met initial designs for tailings facilities are established on the basis of engineering estimates about the behavior of the embankments the deposited tailings seepage and groundwater conditions and overall water management it is essential that sufficient instrumentation is installed and that a comprehensive monitoring and surveillance program is implemented to measure actual performance against the design objectives the monitoring program should include a formal inspection carried out at least annually by a senior engineer with full knowledge of the design and operational criteria for the facility the results from the monitoring program should be reviewed on a regular basis preferably by the design engineer to assess whether the design requirements are being met a regular review of monitoring data can provide early warning of developing problems and allow remedial action to be taken before problems develop into a major concern or worse a failure many regulatory jurisdictions require that monitoring reports be prepared on a regular basis which fully describe the tailings facility operation over the reporting period and record the volumes and types of tailings that have been deposited the formal inspections findings and resulting recommendations and the data from instrumentation and their interpretations brown 2002 monitoring may consist of visual observation of the emba

nkment and an assessment of data from piezometers and other instrumentation preventive maintenance based on the timely observation of potential problems can help maintain the stability of the structure control seepage and contain costs distress signals such as cracking sloughing saturated areas on the downstream face unusual plant growth and settlement indicate potential deficiencies in the structure but without proper instrumentation it may be difficult to accurately interpret the extent of the problem piezometers pressure gauges survey monuments and borehole slope inclinometers can be used to indicate developing trends in the facility s behavior observations made from these instruments combined with daily operators logs which show dates and locations of deposition meteorological conditions and so forth can help in the analysis of the facility s condition vick 1990 instrumentation visual observations alone are an insufficient means of monitoring the safety of a tailings facility instrumentation should be installed in the embankment and or its foundation to monitor changes that may be critical to stability and to help predict unstable conditions instruments can be installed to measure pore water pressures seepage flows embankment movements and earth pressures epa 1994 pore water pressures are measured with piezometers which encompass several types used to monitor pore pressures such as standpipes isolated tip and fully slotted or pneumatic vibrating wire or silicon strain gauge ceramic tip they may be installed during the construction of a tailings facility or later in a borehole each type of piezometer has various advantages and disadvantages a full discussion of which is beyond the scope of this chapter piezometers should be installed in the structural zone of embankments in drains and in the tailings deposit a well conceived installation of piezometers can play a crucial role in understanding the pore pressure distributions in a tailings facility and care should be taken to install this equipment to provide information on critical sections of the facility for use in slope stability analyses measuring slope movements can also play a critical role in facility monitoring simple methods for measuring embankment deformations can be easily employed markers can be installed on the surface aligned in a straight line of sight to permit rapid detection of movement during periodic observations survey methods may be used to enhance monitoring of tailings facilities especially if visual observations indicate possible slope instability the simplest example of a survey method consists of a line of survey points installed along the embankment slope or toe of the slope these markers are surveyed shortly following installation and periodically thereafter a more advanced device for assessing deformations within an embankment is a borehole inclinometer which consists of a cased borehole in which the linin

g is equipped with two sets of orthogonal longitudinal guiding tracks a slope indicator device is lowered into the borehole and records the verticality of the lining in two directions at right angles to each other and at constant depth increments these directions are usually parallel to and at right angles to the slope by integrating the recorded slopes of the borehole lining the deformation throughout the casing can be assessed slope cross sections and available freeboard for pond water levels may also be important aspects of the monitoring program especially for upstream or cycloned facilities and for traditional paddock dams flows from drains should also be measured and samples tested periodically for water quality safety audits the operator of a tailings facility should implement an annual audit and review of the facility to help assess whether the operations remain consistent with the design criteria the annual audit should be undertaken by a suitably qualified and experienced person preferably the original designer or an independent third party individual periodic operational audits provide a valuable status report of actual facility performance compared to the design parameters expectations and assumptions the records provide an ongoing history of the facility and can be vital for sites with frequent personnel changes an audit can assist with tailings management planning dam construction scheduling and improvements to the overall operation a comprehensive periodic audit and review report should include the following updated site plan updated survey plan of the facility including cross sections and contours of the embankment and tailings beach certification from a qualified and experienced person that any construction since the previous report meets appropriate engineering and safety standards data about the engineering properties of construction materials and comparisons of those properties with the design criteria updated data on the properties of the tailings stored in the facility reconciliation of the stored volume and densities of the tailings with the design values calculations of the deposition rate against capacity and of the remaining facility capacity in terms of time and volume water balance data climatic conditions over the period between reports data from daily inspections and periodic monitoring during facility operation review of the safety and stability monitoring results review of the environmental monitoring results information on the operation of diversion channels drains and freeboard information on the location and depth of boreholes and their proposed monitoring program inspection and maintenance schedules for tailings pipelines and other pertinent equipment information on the operations in terms of risk assessment planned operations for the next reconciliation period unique tailings management schemes owing to the wide variety of tailings types sites settings

and environments and operational requirements each tailings facility is unique this has prompted the development of some unique schemes to enhance the performance and or reduce a specific facility s risk and cost a few examples are given disposal of tailings within other mining facilities at the la quinua mine which forms part of the yanacocha complex in peru a new thickened tailings management facility has been commissioned within an active heap leach pad where the heap leach ore pile is used to provide full containment of the tailings kerr et al 2009 mud farming fine tailings placed in large paddocks in a dry environment often form significant surface crusts from evaporative forces which reduce subsequent evaporation and thus densification and stiffening of the underlying tailings the surface may be farmed using a modified bulldozer equipped with long floats and archimedes screws this practice is used at the wagerup alumina refinery in western australia pit tailings disposal tailings are placed in mined out open pits for cost effective storage and to limit the overall footprint of the mine in some cases to limit oxidation operations place potentially acid generating tailings in a reducing environment below the groundwater level care must be taken to determine that the tailings will not contaminate the groundwater which is often accomplished by establishing and maintaining a flow gradient into the pit this is practiced at various operations worldwide including some of the uranium mines in northern saskatchewan algae introduction into the tailings stream this method promotes algae growth on the surface of the tailings deposit to suppress dust generation it is being investigated at the sierrita mine in arizona united states research is also being carried out on a number of potential tailings improvement initiatives that have not yet made it to full scale implementation these include use of foam to transport filtered tailings in a pipeline instead of conveyors or truck haulage in an effort to reduce cost addition of fibers into a tailings stream that will become integrated into the deposited mass and provide added strength in the structural zone and mixed disposal of tailings and mine waste rock in a single mass such that the rock component provides strength and the tailings if maintained saturated reduce the oxygen diffusion into the mass thus reducing the potential for acidic conditions to develop if the rock or tailings have significant sulfides lessons learned although mining and tailings disposal have been part of human history for thousands of years the disposal of tailings into dedicated facilities has been practiced only since the early 1900s initially the construction of these facilities was by hand due to a lack of earthmoving equipment which prompted the use of upstream techniques for most dams by the 1940s and 1950s the availability of large earthmoving equipment and engineer

ing principles for water storage dams in terms of placing selected materials in certain zones in compacted horizontal lifts began to be applied to mine tailings facilities in the 1960s largely as a result of a few earthquake induced failures of tailings dams the phenomenon of liquefaction began to be understood and accounted for in design although it is only now that a related phenomenon static liquefaction is becoming well understood planning for the safe management and containment of water in tailings facilities is also relatively new having become a specific design and monitoring subject in the 1960s and 1970s more recent is the recognition that chemical stability is a crucial aspect of tailings management and over the last 20 years much research has been carried out to among other things control acid drainage from sulfide tailings deposits notwithstanding the relatively rapid advancement in tailings management engineering the mining industry has been saddled with a somewhat checkered history in part due to tailings facility failures and incidents like any other field of human endeavor learning from its mistakes is vital it has been reported that there have been no unexplained failure events and concluded that in all cases over the past thirty years the necessary knowledge existed to prevent the failure at both the design and operating stage but that knowledge was not used martin et al 2002 this position was updated in 1998 the mining industry has the technology to safely design build operate and decommission tailings facilities this technology must be consistently applied for the safe and environmentally responsible management of tailings mac 1998 with those statements as a backdrop it is worth investigating the lessons learned from past incidents the following trends are reported for tailings facility incidents martin et al 2002 active impoundments are more susceptible to tailings dam failure than inactive facilities the correlation between upstream constructed dams and the likelihood of incidents is strong although the large number of upstream dams in the database must be recognized seepage related phenomena are the main failure mode for non upstream tailings dams earthquakes are of little consequence for most nonupstream dams for inactive impoundments overtopping is the primary failure mode the international commission on large dams bulletin 121 icold 2001 provides a comprehensive report of these lessons drawing from a range of tailings management facility failures and incidents the main causes of failures and incidents identified were lack of water control in the facility leading to overtopping and or seepage erosion piping lack of construction control and general lack of understanding of the features that control safe operations tailings facility failures were due to in order of prevalence slope instability earthquake loading on upstream dams overtoppi

ng inadequate foundations and seepage successful planning and management of tailings facilities could benefit greatly from involvement of stakeholders thorough investigations and risk assessments comprehensive documentation and tailings management integrated into mine planning operations and closure icold 2001 it is also suggested that benefits can be realized by adoption of a comprehensive tailings management system and maintaining continuity of staff including consultants to the greatest extent possible specific examples of tailings facility failures where lessons can be learned include the stava tailings facility in italy ulrich 1996 and the merriespruit tailings facility in south africa fourie et al 2001 stava dam incident on july 19 1985 two tailings dams near stava italy catastrophically failed resulting in the destruction of two villages and causing extensive property damage two hundred and sixty nine lvives were lost genevois and tecca 1993 a total of 190 000 m3 154 acre ft of liquefied tailings flowed down the stava valley the flow reached the village of tesero a distance of 4 km 2 5 mi from the mine in only minutes the failures occurred without any warning berti et al 1988 and are not attributed to earthquake shaking or stormwater flooding these dams were never engineered at least not in the conventional use of the term however the initial construction of the upper dam was based on the successful operation of the lower dam the dams were constructed with minimal design effort site investigations were never carried out in any form until after the failure lab testing was never conducted and design drawings were never made surveying was never attempted at the site and no instrumentation of any kind was ever installed the failure of the upper dam was caused by the presence of a high phreatic surface within the sand shell leading to the initial failure which strained sufficiently to produce liquefaction of the sandy materials the critical factor was direct ingress of water into the sand shell of the embankment and subsequent saturation of a portion of it d appolonia and morgenstern 1988 when the saturated zone had incorporated a region of the sand shell approximately 7 5 m 24 6 ft wide failure was incited under drained conditions because the sand shell was in a loose state the straining that took place generated positive pore pressures sufficient to induce liquefaction under static loading conditions i e static liquefaction and the sand shell began to flow a condition of undrained loading of the slimes resulted from this loss of confinement and the slimes deformed in an undrained manner d appolonia and morgenstern 1988 the relationship between the stability of a dam and the location of the phreatic surface within a dam has been well recognized by the geotechnical community for a considerable time records of pond water level indicate that the pond was at its histori

c maximum elevation at the time of failure in addition there is ample evidence that appreciable seepage existed in the sand shell just prior to failure given such information it would appear that the dominating factor causing the failure was related to the location of the phreatic surface d appolonia and morgenstern 1988 he mining industry of the need for proper design operations and monitoring of tailings facilities and as an important reminder to the geotechnical community of the need to understand the possibility of failure mechanisms such as static liquefaction and how subtle changes in operation practices may lead to significant changes in important characteristics such as seepage patterns within a tailings facility merriespruit tailings failure failure of the 31 m 100 ft high gold tailings facility just outside of the village of merriespruit south africa in february 1994 resulted in the death of 17 people and widespread damage to the village and the surrounding environment the failure occurred shortly after a 50 mm 2 in rainfall event approximately 600 000 m3 21 2 million ft3 of liquid tailings flowed from the facility through the village of merriespruit stopping about 3 km 1 9 miles downgradient it is accepted that the primary cause of the failure was overtopping which resulted in large scale removal of tailings from the slope face wagener et al 1998 the postfailure investigations found that the dam overtopped at the breach location and water flowed over the crest of the dam for 1 to 2 hours removal of tailings from the outer slope would have exposed tailings inside the facility that had previously been confined conventionally however gold tailings produced from witwatersrand quartzites are considered to be strongly dilatant blight 1998 and thus even if confinement were removed the tailings should not have moved for any significant distance because for a strain hardening soil the resistance increases continuously during undrained loading zhang and garga 1997 in addition the outer zone of the tailings dam was considered to be well consolidated due to the thin layer deposition procedure wherein layers of tailings are placed sequentially around the perimeter and allowed to drain desiccate and consolidate blight and steffen 1979 undoubtedly it came as a surprise to the geotechnical community in south africa that 600 000 m3 21 3 million ft3 of gold tailings would liquefy and flow as far as 3 km 1 9 mi wagener et al 1998 the fact that no gold tailings dam in south africa had previously failed with such devastating consequences probably contributed to the complacent response to the excessive seepage and toe sloughing that occurred prior to the dam s catastrophic failure conventional stability analyses carried out a few months prior to the failure indicated a satisfactory factor of safety against slope instability based on a traditional geotechnical engineering approach to slope stabili

ty perhaps the perception that the facility was inherently safe can to some degree be excused many practicing geotechnical engineers today if presented with the information available at the beginning of february 1994 might also come to the conclusion that the tailings impoundment although not in a completely satisfactory state was not intrinsically unsafe it is only when the concepts of static liquefaction are invoked that incipient instability might have been predicted such a prediction would have been difficult if not impossible without either piezocone profiles or information regarding the in situ void ratios and knowledge of steady state soil mechanics the failure of the merriespruit tailings dam caused a major reconsideration of operating procedures of tailings dams in south africa and contributed in some measure to the development and publication of a code of practice for mine residue deposits south african bureau of standards 1998 although such improvements are commendable it is important not to lose sight of the fact that the merriespruit failure was a single isolated incident and that many dozens of gold tailings dams in south africa have been successfully and safely operated for nearly 100 years the stark lessons learned from merriespruit should not be forgotten poor pool control subaqueous tailings deposition and inadequate freeboard are all well known sources of operational difficulties on tailings dams the merriespruit failure simply emphasized this point with tragic consequences current and future trends the mining industry has made and continues to make many significant improvements to tailings management such changes come under the following categories martin et al 2002 management practices creation of guidance documents initiatives by the mining companies use of qualified engineering consultants response to regulatory trends tailings management technologies improved geotechnical designs designing for geochemical issues metallurgical improvements control of ard when applicable management of cyanide when applicable and other chemical reagents increasingly guidance documents are being prepared by the mining industry and other stakeholders including regulatory authorities as described previously in the tailings management principles section such documents provide guidelines for a wide variety of topics ranging from nontechnical issues such as the development of management frameworks and identification of management responsibilities to more technical matters such as oms of tailings facilities international standards are frequently mentioned yet they are not well defined in general they consist of a compendium of standards and guidelines by lending agencies such as the world bank and its international finance corporation ifc mining industry initiatives and regulatory agencies the ifc has recently produced an environment health and safety guidance document for min

ing ifc 2007 that includes tailings and water management the mining minerals and sustainable development mmsd project of the international institute for environment and development iied produced its report breaking new ground iied 2002 which includes a section on large volume wastes including tailings the united nations under its united nations environment programme unep the international council on metals and the environment icme and more recently the international council on mining and metals icmm have also produced case study and risk management guideline documents for tailings the broad objectives of these documents are generally to monitor interpret communicate and implement improvement trends in mining and the environment provide direction and influence to international and national government polices and company practices related to improving environmental management regulations and practice and seek cost effective solutions to environmental challenges in mining mining companies themselves are also producing several initiatives to improve mining operations globally in addition to the mac initiatives described earlier one other such initiative is the international cyanide management code that is administered by the international cyanide management institute icmi for the manufacture transport and use of cyanide in the production of gold icmi 2009 this code is typically referred to simply as the cyanide code the cyanide code is a voluntary program for the gold mining industry illustrating the commitment of mining companies to among other things improve tailings facility design and operation and promote responsible management of cyanide used in gold mining enhance the protection of human health and reduce the potential for environmental impacts companies that become signatories to the cyanide code must have their operations audited by an independent third party to demonstrate their compliance with the code the results of which are published and divulged to the public in addition many international mining companies have written their own in house guidelines much of which can be found on their company web sites several companies have promulgated such undertakings as internal audits of their procedures and management systems forming review boards with industry experts from internal and external sources especially for significant high profile undertakings probably the biggest current initiatives are improved designs of tailings facilities including the use of thickened paste and filtered tailings and incorporating closure and reclamation objectives into the design the practice of tailings management has come a long way in a relatively short period thanks to 1 its establishment as a dedicated topic of engineering and environmental science and 2 the commitment made by many mining companies industry associations and regulatory bodies for improvement however continuous i

mprovement must be sought the following topics warrant attention in the near future improved consistency of good tailings management practices across the industry it is now accepted that the technology exists to address all currently contemplated engineering and environmental science issues related to safe and secure tailings disposal although it is also accepted that technology improvements will continue to be made however it is also recognized that the technology is not always used or used adequately the industry is working to put tools in place that will help rectify these gaps but is not yet at the finish line widespread adoption of full life cycle tailings management systems as outlined in the tailings management principles section will help as will acceptance by all mine owners that a well developed and safely operating tailings facility has value and is not simply a cost good documentation is fundamental to this endeavor because over the life of many tailings facilities key people come and go and the current people must be aware of the thinking to date improved recognition of the uniqueness of each tailings facility and the need for appropriate expertise because each tailings facility is unique an off the shelf or cookie cutter approach to design is not acceptable this is why mining industry guidelines are just that guidelines they do not and must not replace sound project specific engineering and environmental science provided by appropriately qualified firms or individuals knowledge and experience are fundamental to take advantage of the lessons learned and to fully understand and appreciate the nuances associated with the site conditions tailings properties and the facility s operating principles improved recognition that tailings facilities are permanent features with an indefinite life tailings dams differ in many ways from water dams in that they are generally permanent structures unless the tailings deposit is subsequently removed which can happen if the tailings are reprocessed or the area is evacuated for mine expansion thus indefinite time frames may need to be contemplated when designing and operating a facility planning from the beginning for closure and a postclosure period is gaining acceptance but planning for indefinite closure may be more appropriate although peak deterministic equivalent to a very long return period earthquake and storm events are now being used the long term durability and potential degradation of materials for seepage barriers drains and structural zones are not always contemplated in addition the implications of future land use changes that could increase the hazard classification of the facility should be appreciated increased application of dewatered tailings in appropriate circumstances the technology associated with dewatering tailings for bulk surface disposal is improving rapidly in response to demand by the industry the most pressing need is in dry climate

s where the value of water is high and thus the benefit of recovering and recycling water is also high this is particularly true at the large copper mines in chile where significant volumes of water are lost to evaporation from the tailings facility the biggest challenge is associated with the large tonnages of tailings produced and thus the large dewatering capacity required a second challenge is the cost in recent years substantial advancements have been made to reduce capital equipment and operating power costs and as further experience is gained additional capacity expansions and unit cost reductions can be expected understanding the behavior of thickened paste or filtered tailings when deposited is also a key challenge particularly in terms of predicting shear strength and liquefaction potential and beach slope angles for highdensity tailings often the decision to use thickened or paste tailings in addition to increased water recovery is because development of more steeply sloped deposits reduces the height required for containment structures care must be taken however to avoid overpredicting the slope if the beach length is large because this may result in the need for subsequently constructing an unexpected rise to the structure dust control dust generation has become a problem for some tailings facilities with large exposed beaches in windy environments often the dust is generated from sandy tailings in the upper portion of a segregated beach because the sandy tailings are unable to retain the capillary moisture that finer tailings can which helps to hold the particles on the beach however in areas where freeze thaw cycles are significant both sandy and finer tailings can be dewatered and loosened from the beach creating a dust concern current methods for controlling dust include the installation of snow fences in strategic high impact locations application of water to moisten the dry and dust prone areas or the application of tackifiers to adhere the loose particles to the beach a more innovative approach involves the injection of algae into the tailings stream to produce a biological cover on the exposed beach surface thickened tailings can help in areas where freeze thaw is not an issue because the degree of particle segregation is reduced placing greater emphasis on integrating a tailings facility with other mining and mine waste facilities placement of tailings underground in mined out workings as backfill has been successfully done in numerous mines worldwide and is partly responsible for the origins of the tailings dewatering industry because dewatering was required to give the tailings substantial strength and stiffness however the disposal of tailings in combined surface facilities with other mine wastes such as waste rock from the pit or heap leach pads is less common the la quinua thickened tailings management facility previously described is unique because it is the only one in which th

e tailings are fully contained within an active heap leach ore pile combining these facilities requires careful consideration of the design and operating principles of each and achieving compatibility this may not always be possible however if compatibility can be reached the benefits are large and include cost land use and closure efficiencies extrapolation of this example to tailings within waste rock storage facilities may hold similar promise of successful applications the terms mine waste piles and dumps refer to piles of waste rock or leached ore that carry little or no economic value at the time they are placed as commodity values rise and process methods gain efficiency waste piles and dumps may be reclassified as ore and gain value also the waste material may be valuable at some future time as an aggregate source for use in riprap drain material or other process method that recovers the commodity at lower grades or has lower acceptable rates of return heaps are ore piles that are amenable to a leaching process both with and without the use of liners and share physical characteristics with piles and dumps types of waste piles dumps and heaps this section provides a description of waste piles waste dumps and heap leach pads both lined and unlined although these types of facilities are similar the liner aspect introduces an additional potential for failure along the liner as part of design on the other hand for unlined facilities it is important to consider the materials geochemistry both the ore as well as the resulting pregnant solutions and the site s hydrology to ensure that natural water resources are adequately protected configurations collectively waste pile dump stockpile or a leach heap can be referred to as waste structures as such their layout generally falls into the following categories depending on the type of waste the purpose of the waste structure and the physical constraints at the site each of the configurations is shown in figure 8 11 1 and discussed in further detail in the following paragraphs a valley fill waste structure as the name indicates fills a valley many of the lined valley fill leach pads require some type of stability berm at their toes construction of a lined valleyfill leach pad would begin at the toe berm and progress up the valley construction of a waste dump not a leach pad usually begins at the upstream end of the valley and dumping proceeds along the downstream face as shown in figure 8 11 1 for a heap leach facility stacking should begin at the toe and proceed up the valley to avoid slope stability problems the top surface is usually sloped to prevent water ponding stormwater run on can be controlled by constructing diversion channels up gradient of the facility in steep terrain where the facility is going to take a long time to fill it may be more economic to construct a rock drain below the facility to pass stormwater subdrains may also

be needed below the structure to control seepage from natural springs and material drainage a cross valley structure crosses the valley but the valley is not completely filled up gradient the structure is usually designed with a rock drain at the bottom of the valley to control the storage and or discharge of stormwater flows or a water diversion system must be installed up gradient to provide drainage around it this type of structure could also be used as a retention dam for fine coal or waste slurries in which case the design must conform to applicable regulations for dams and impoundments a sidehill structure lies along the side of a slope but does not cross the valley bottom this structure may be constructed to impound either water or mine waste slurries and therefore would need to conform to applicable dam regulations as with a cross valley structure a sidehill embankment should also be designed and constructed with either stormwater diversion channels or rock drains to control the storage and or discharge of flood flows in some cases the hillside may require benching and or a keyway at the toe to increase the stability of the facility a ridge embankment straddles the crest of a ridge and waste material is placed along both sides unlike the crossvalley or sidehill configurations this type of structure is typically not used to impound fine grained material or water in some cases one or both sides of the ridge may require benching and or a keyway at the toe to increase stability a diked embankment is constructed on nearly level terrain and can either impound fine grained or coarse grained mine waste by definition this type of embankment is composed of two parts a down gradient containment dike and the embankment or dump itself these two parts may or may not be isolated from one another by liners if fine wastes are impounded by coarser waste the structure is considered a dike if the embankment is homogeneous and coarse the embankment is termed a heap such as a heap leach pad leach dumps or heaps leach heaps consist of low grade ores spread or stacked on large platforms where the pile is irrigated with leaching solution to leach out the recoverable product of value although heap leaching has been used mostly for precious metal and copper ores in the past it is now also being used for other products such as uranium and nickel in recent times even municipal wastes have been leached using similar methods to accelerate the decomposition of waste and add capacity to the facility heaps are normally placed on impermeable liners of natural and synthetic materials discussed in more detail later in this chapter dumps usually refer to material piles created by end dumping run of mine ore is sometimes simply dumped instead of being stacked on a leach pad and leached for economic recovery of the contained commodity a process known as dump leaching the same procedure is often used for secondary recovery from leache

d ores dumps are generally placed on natural soil or rock subgrade surfaces that have been demonstrated to have some degree of natural solution containment and are normally located on sloping ground or in a valley to promote drainage to the toe stockpiles the term stockpile refers to any pile of material that is placed for future use this can include material with either proven or potential value material for structural fill or other materials obtained from borrow pits or removed from stripping projects waste rock or processed material to be used as backfill can also be categorized as a stockpile these materials which are stored for processing or future use appear much the same as waste rock except they are normally isolated from waste materials so they may be recovered at some later time as economically as possible and without being contaminated with waste stockpiled material such as the ore itself may be chemically unstable and the stockpile may require liners caps and or stormwater diversion structures to prevent water infiltrating the pile and causing water contamination placer waste and tailings deposits during placer mining for gold or aggregates the practice of washing sand and gravel to recover minerals can produce tailings with particle sizes ranging from coarse to fine 75 m and wash water which should be treated the coarse waste fraction can be disposed of using one of the methods previously described however the fines portion is similar to the tailings from a milling operation considerations for these types of wastes include the placement and storage of the tailings and treatment of the wash water to meet discharge requirements with physical constraints of space limitation and the rising cost of conventional impoundment methods for tailings storage the use of process items such as thickeners and filter presses to put tailings in piles or mounds has become more common with the removal of additional moisture alternative disposal methods such as thickened tailings paste backfill treated paste backfill and dry stacking become viable options which can add capacity to the facility impacts of waste dumps waste dumps and heaps have several actual and potential impacts on the environment which must be considered as part of their permitting and design these impacts include disturbance of the land water quality issues slope stability and visual effects in the past waste dump disasters have led to the contamination of surface and groundwater as well as massive slides which have buried communities planning waste disposal facilities requires evaluating the regulatory constraints identifying an appropriate site designing the structural and environmental integrity of the facility developing an operating and maintenance plan and developing a reclamation plan for future land use center and zlaten 1982 and ritcey 1989 as cited in zahl et al 1992 from the design point of view the specific issues to

be considered are the contamination potential of the waste slope stability the condition of the waste structure s formation under normal and seismic loading and ways to control water both internal and external to the dump water quality water quality impact issues associated with waste unlined dumps or poorly constructed heap leach facilities can be a major environmental concern waste rock should be thoroughly tested at the design stage for acid generating and metals leaching potential to ensure that water resources are adequately protected to provide background information on flows and water quality groundwater and surface water samples should be collected before construction begins these measurements and samples should be collected throughout the year so that seasonal fluctuations can also be monitored and effectively evaluated all drainages and aquifers in the vicinity of the project should be tested to ensure that water quality for the entire project area is well understood before the project begins initial testing of the water samples should include major cations and anions metals nitrates dissolved and suspended solids salts and organic compounds as well as other constituents that may emerge as relevant during the process and involve potential changes to water chemistry samples should be collected from dedicated monitoring wells and surface sampling locations both up and down gradient of the project site a water quality monitoring plan should be prepared to document sample locations sampling frequencies and protocol for collecting the samples at a minimum the plan should contain the following items identification of the surface and groundwater sources monitoring objectives description of water quality parameters sampling point descriptions and a map of their locations analytical procedures data quality control objectives data management and quality control details sampling equipment to be used sample preparation and handling procedures chain of custody and data sheets to be used reporting requirements land disturbance wherever mine waste is placed the natural environment is changed and the process is therefore classified as land disturbance the initial disturbance creates the potential for sedimentation of natural waterways caused by erosion and water quality degradation which are among the major potential impacts of waste dump construction although waste dumps can be designed to minimize the impacts of land disturbance and blend in with natural surroundings as part of reclamation in some locations these disturbances have been perceived by some as highly destructive to the environment specifically in california all metallic mines are now required to use waste rock to backfill all open pits as part of the state s mine reclamation requirements the u s office of surface mining requires restoration to approximate original contours for surface coal mining these requirements can add c

onsiderable cost to final closure since most waste structures are not compacted the volume of a pile or a dump can be much greater than the volume of the pit which adds further to the issues of how to hide or at least reduce their impact visual impacts visual impacts from mine waste dumps and leach pads can be a major concern for mines located in the vicinity of populated areas or where the facilities will be visible from roads and highways visual impact and viewshed studies are now performed routinely in many areas of the world as part of initial mine permitting in areas where the color of the rock blends with the natural color of the terrain visual impacts will be less than in areas with sharp color contrasts in flat areas hills develop and in mountainous terrain ridge tops appear and grow and drainages are filled by maintaining slope angles that are similar to natural slopes visual impacts may be reduced and many companies are now designing dump surfaces to simulate the original topography however contrasts in colors from the natural vegetation to rock and topsoil can take several years to blend together as the revegetated slopes take hold following reclamation a visual impact study may include the following components as described by the federal highway administration fha 1981 description of the project setting and the major viewsheds photographic study of the project from the major views description and analyses of the existing visual resources and responses from people in the area renderings of the project alternatives views assessment of the visual impacts of the project alternatives possible methods to mitigate the adverse visual impacts as part of the visual impact study maps are usually produced and show the areas from which the project would be visible according to different design options the design options typically include several different ultimate elevations and possible configurations of the waste dump or heap design of waste dumps this section provides an overview of waste dump design further details regarding the design of waste dumps may be obtained from the following recommended sme aime publications and from several other references cited throughout this chapter mccarter 1985a 1990 and hustrulid et al 2000 proper planning and design require a thorough understanding of the material properties of the waste rock or ore liner interface strengths in the case of a lined facility and foundation conditions in the case of a dump or heap leach groundwater and seepage properties of the ore must also be understood in order to properly design these types of facilities studies would include a field investigation consisting of mapping of soils and rock drilling boreholes monitoring well installation excavating a test pit sampling waste rock ore and foundation materials laboratory testing and analyses slope stability slope instability and failure are major issues for all types of

mine waste dumps and heap leach operations the risks and environmental impacts of waste dump instability are a major concern for both mine operators and regulators a slope failure in a waste structure could cause injuries and disruption of operations because of equipment burial or closure of an access or haul road slope failure in a heap leach pile can lead to a liner failure and the potential release of pregnant solution which may result in contamination of groundwater resources as well as a loss of revenue in either case there are clean up and remediation costs proper preplanning and design are imperative to avoid these types of costs numerous factors affect waste dump or pile stability including site topography dump geometry rate of stacking and lift thickness geotechnical properties method of construction equipment loads phreatic surface and seismic forces all of which must be considered in the evaluation of the waste structure s stability over its design life generally limit equilibrium analysis using one of the several prevalent approaches is considered adequate to evaluate slope stability of waste dumps failure modes the basic failure modes of waste dumps must be considered during the stability evaluation and design detailed descriptions of identifiable waste dump failure modes and appropriate analyses are described by many in the literature e g bcmdc 1991 caldwell and moss 1985 each of the main failure modes are shown in figure 8 11 2 surface or edge slumping the most common failure mode is edge slumping crest slumping where a thin wedge of material translates down the slope parallel to the dump face this shallow failure typically originates near the crest of the dump because of oversteepening cohesive or low permeability waste materials allow the development of oversteepened slopes end dumping the waste in thick lifts or pushing material over the dump crest also leads to a higher risk of over steepening and edge slumping edge slumping failures often occur after heavy precipitation which leads to increased pore pressures in the low permeability waste in coarse rock fill dumps oversteepening of the crest may develop due to initial interlocking of the blocks bcmdc 1991 plane failure similar to edge slumping may occur deeper within the waste dump materials in this case sliding occurs along a single plane of weakness within the dump which may have been created because of a zone of poor quality waste or from dumping waste on top of snow or ice the plane of weakness parallels the dump slope or daylights at the dump face shallow flow slides flow slides are shallow slumping failures of saturated or partially saturated waste typically triggered by rain or snowmelt they result in material flowing down the slopes due to shear failure or collapse of the soil structure rotational circular failures rotational circular failure mass failure along a curved failure surface may occur within the waste as

a result of excessive dump height additional loading induced during an earthquake weak or fine grained waste materials reduction in toe support and or high porewater pressures rotational failure surfaces may also extend into the foundation if the soil is weak or high pore pressures develop such as within a deep fine grained soil deposit creep failure is also a type of rotational failure with widespread rotational shearing characterized by bulging at the dump toe bcmdc 1991 base failure spreading base failure may occur if a thin weak base layer is placed over the foundation especially if the foundation is inclined if a slope wedge of the waste dump translates laterally along a shear surface the foundation soils may spread and be squeezed ahead of the advancing dump toe this phenomenon known as foundation spreading may result in progressive failure of the overall dump vandre 1980 bcmdc 1991 block translation block translation planar sliding may result from any of the inducing factors mentioned for rotational failure and is favored by steep foundation slopes and a thin weak soil cover or lined surface the bulk of the dump slides as a rigid block along a plane of weakness this weak plane may be within the foundation soil along the interface between the dump and the foundation or along a liner interface liquefaction if the soil foundation or the waste dump itself is composed of liquefiable materials and high pore water pressures exits then liquefaction may pose a significant stability risk if liquefaction occurs in the foundation the entire dump may be translated or there may be progressive failure bcmdc 1991 factors affecting slope stability to properly design a mine waste dump for stability the following details should be considered site topography and location dump geometry rate of stacking and lift thickness geotechnical properties of the waste liner system if applicable and foundation methods of construction and equipment loading seepage phreatic surface level within the dump and the solution collection system seismic forces and liquefaction potential the size and complexity of the project as well as the consequences of dump failure will typically control the extent of the investigation performed to obtain this information the investigation should be thorough enough to identify all adverse conditions and to provide reasonable certainty that the parameters used in the design are appropriate vandre 1980 site topography based on economics dump site locations are typically selected to minimize the distance between the waste source and the disposal area the waste may be disposed of in an area completely outside of the pit or in pit dumping may be preferred during the investigation stage of design the topographic information gathered should include the entire drainage area that may affect the dump as well as identifying those areas that would be affected should a dump failure

actually occur should a failure occur the inclination of the dump foundation will be an important factor in the dump stability as well as runout distance experience shows that foundation slopes steeper than 25 typically result in lower factors of safety for slope stability on the other hand topographical features providing lateral support or toe buttressing will improve the stability of the waste dump dump geometry and stacking method the geometry of the waste dump depends largely on the dumping method as well as the topography of the site the two common construction methods for waste dumps include end dumping and stacking material in lifts or layers if the material is end dumped from the crest of the waste dump the material will flow down the slope and rest at or near the angle of repose with the larger particles rolling down to the toe of the dump couzens 1985 the angle of repose for mine waste rock typically falls within the 35 to 40 range leading to steep side slopes the factor of safety for the slope of an end dumped waste pile is close to 1 0 the slopes are generally not flattened or compacted until closure of the waste dump in comparison layered or stacked dumps allow for a higher factor of safety to be maintained because they are constructed in a more controlled manner from the bottom up the layers can be placed and compacted to increase the density and strength of the material however except for the heap leach piles layered waste dumps are not always feasible as they require relatively flat topography vandre 1980 waste dumps constructed from end dumping are more likely to have a loose collapsible particle structure within the dump than those constructed from the layered method collapse will result in localized arching which leads to reduced normal pressures and shear strengths vandre 1980 the exterior slopes of heap leach pads and waste dumps are typically constructed as steep as practical during mining operations to maximize the tonnage contained in the dump slope stability analyses are used to determine the maximum allowable overall slope angle including benches for maintaining stable slope conditions to the planned ultimate dump height breitenbach 2004 smith and giroud 2000 examined the effect of ore placement direction on the stability of a geomembrane lined heap leach pad and concluded that stacking ore in the downgradient direction results in a less stable structure than stacking in the up gradient direction typically would geotechnical properties mine waste the geotechnical properties of mine waste materials vary significantly between projects and even between different phases of the same project the density saturation and shear strength parameters of the materials forming the dump slope affect the failure mode and the calculated factor of safety fs against sliding other useful information for design includes the particle size distribution specific gravity permeability com

pression index soils classification and degradation behavior of the waste materials these parameters are generally based on laboratory tests however field practices and construction procedures are often not completely simulated in the laboratory for various reasons e g equipment limits time and budget restraints and therefore engineering judgment is required in selecting properties for stability analyses verification testing is often required during construction to ensure that the parameters used during the design were reasonable accurate and appropriate waste rock is coarse material typically classified as cobbles rocks or boulders with some fines as previously stated the angle of repose for mine waste rock typically ranges from 35 to 40 and is based on factors such as particle size and shape fall height specific gravity and amount of water present the density of waste rock materials typically ranges between 1 6 and 2 2 t m3 100 137 lb ft3 depending on whether the material is loose or compacted williams 2000 in heap leach pads for example the ore is purposely stacked in a loose state to maintain a high permeability as required by the leaching process as subsequent lifts are placed the density of the lower lifts increases as they are compacted by material placed on top and therefore the shear strength of the lower lifts typically increase smith and giroud 2000 stacking or dumping mine waste in thick lifts results in significant variability of the in place density within each of these lifts understanding the shear strength behavior of the waste material is important for evaluating the slope stability of the waste dump waste density and gradation variability along with differences in normal and confining stresses e g inside the pile versus at the toe or on the slope face result in heterogeneous shear strength throughout the pile generally a linear strength envelope with a single friction angle value over the entire range of stresses may be assumed for the stability analysis however dump heights achieved these days result in a much wider range of normal stresses in the pile over which the strength envelope does not necessarily remain linear and this nonlinearity of the strength envelope must be considered in the stability analysis the dominance of cobble and boulder sized rock fragments in typical waste rock imparts a dilatant behavior under low effective normal stresses and significant crushing of contact points at high stresses as demonstrated in the case of rock fill barton and kjaernsli 1981 the friction angle of the rock fill is strongly stress dependent and will be significantly lower for material at the base of the dump due to higher normal loads than for material near the toe of the dump under low loads barton and kjaernsli 1981 estimated that the effective friction angle of rock fill increases by between 4 and 8 for every 10 fold decrease in effective normal stress the sh

ear strength of rock fill is also influenced by the rock fill dry density void ratio unconfined compressive strength uniformity coefficient maximum grain size fines content and particle shape laboratory testing of the mine waste is often too limited to accurately represent the potential material variability of a large volume of waste under various loading conditions therefore the shear strength of the mine waste for design and analysis purposes must often be estimated based on various inputs including current laboratory test results previous experience the behavior of similar materials and published literature vandre 1980 k p sinha personal communication another aspect to consider during design is the effect of weathering on geotechnical properties waste materials that were assumed to be durable may weather or be altered in some other way which decreases slope stability for example weathering of feldspar rich rock may result in formation of clay decreasing the effective friction angle and inhibiting rapid drainage geotechnical properties foundation the foundation is a critical factor in the overall stability of the waste dump the dump site investigation should identify the general geology of the site and any adverse geologic and soil conditions the soil cover and rock weathering depths should be determined and the materials should be classified for design particular attention should be paid to the presence of shallow groundwater discharge areas landslides creeping slopes organic soils clays and dip slope bedrock structures vandre 1980 the subsurface exploration may include sampling in situ testing and borehole geophysics and should cater to obtaining the critical parameters for design after soil and rock samples have been obtained during the investigation laboratory testing should be performed to identify the pertinent geotechnical properties of the materials the classification strength permeability and consolidation properties of the foundation materials and how these properties are affected by time or saturation should be determined the shear strength and thickness of the foundation soil is an important parameter for slope stability and the dump failure mode permeability of the foundation material will affect the generation of pore water pressures in the foundation affecting the dump stability and limiting the permissible dumping rate foundations consisting of low plasticity silts and clay soils have been blamed for forming shear failure surfaces of several large 10 mt 11 million st dump failures zavodni et al 1981 consolidation parameters are used for calculating expected settlement of the foundation excessive settlement could have serious implications in terms of the liner and collection system in case of heap leach piles and dump failure in general geotechnical properties geosynthetics within the last 20 years gold silver and more recently copper leach pads have been constr

ucted with geomembrane lined foundations breitenbach 2004 typically lldpe or hdpe is used as the base liner the decision is based on the elongation strength and other requirements of the application as well as economic reasons pvc liners have been provided in specific cases mainly for economic considerations the liner interfaces with the overliner the drainage material the subgrade or the ore material itself in case of interlift liners create planes of weakness in the leach pile an example of a geomembrane liner system for a heap leach pad is shown in figure 8 11 3 slides in lined facilities usually occur by wedge failure along the geomembrane interface with geotextile or low permeability subgrade breitenbach 2004 this being the weakest link in the chain thus the soil liner interface strength parameters may become the most critical data for evaluating heap leach stability the soil liner interface strength depends on several factors including normal load rate of applied shear soil type density water content and drainage conditions as well as liner thickness flexibility and texture sample et al 2009 just as with the waste and ore material soil liner interface strengths may also exhibit a nonlinear strength envelope with the friction angle generally decreasing as the normal stress increases thus as heap leach piles are extended to greater heights decreases in the interface friction angle used for the stability analysis should be considered for the liner interface to select an appropriate minimum fs against slope failure the designer must consider whether peak or post peak residual strengths were used for the liner interface in the stability analysis one method to ensure conservative design for wedge failure of a heap leach pad is to assume post peak residual strengths for the liner system numerous studies of shear stresses for geomembrane soil interfaces based on direct shear testing have been published and the conclusions regarding peak versus post peak strengths have been mixed post peak strengths as low as 50 of peak strength have been observed for geomembrane clay interfaces byrne 1994 stark and poeppel 1994 while other studies indicated that no strain softening i e reduction in strength with straining behavior occurred koerner et al 1986 masada et al 1994 valera and ulrich 2000 recommend the use of post peak shear strength for soil liner interfaces in stability analyses of heap leach pads because the interface may reach residual strengths because of minor strains caused by installation and initial loading residual strength conditions may also be reached because of cyclic loading during an earthquake k p sinha personal communication sharma et al 1997 observed that the reduction in hdpe soil interface strength after peak stress was greater when the plasticity index of the soil was more than 30 groundwater and phreatic surface the effects of water on the stability of

mine waste dumps can be difficult to evaluate and measures should be taken to prevent excess water from entering the dump in order to accurately assess the stability of the waste dump a seepage analysis should be performed to establish flows through the dump and the height of the phreatic surface water pressure buildup within the dump will lower the fs for slope stability and the potential for increases in the phreatic surface should be considered within heap leach pads the phreatic surface is often assumed to be some height above the base liner e g 1 to 3 m 3 3 to 9 8 ft based on the design of the collection system because of the leaching process leach pads present a combination of extreme base pressures and high moisture conditions not present in other lined facilities such as landfills thiel and smith 2004 in addition leach pads are sometimes located in highly seismic areas raising concerns about liquefaction due to sudden pore pressure buildup an increase in the foundation water table may significantly decrease the fs for a deep failure through the foundation material while perched water within the dump may lead to surface failures flow parallel to the surface of the slope may also decrease the fs significantly seismic forces in seismically active regions the slope stability of the waste structure is also evaluated for seismic loading conditions the seismic loading although dynamic and cyclic in nature is generally treated as a superimposed equivalent set of static loads and the stability analysis for this case is referred to as the pseudostatic analysis for these analyses the two dimensional mass in the limit equilibrium slope stability model is subjected to a horizontal acceleration which represents inertia forces due to earthquake shaking and is equal to an earthquake coefficient multiplied by the acceleration of gravity the earthquake coefficient or pseudostatic coefficient is selected based on a specified design earthquake often a percentage of the maximum design acceleration in bedrock may be used for the pseudostatic analysis however selection of an appropriate pseudostatic coefficient may rely heavily on engineering judgment and is often debatable also materials within the waste dump may undergo a significant loss of strength during earthquake shaking which may not be entirely understood or defined from the laboratory testing therefore while pseudostatic analyses are a simple and convenient tool they should serve primarily as a screening method as to whether significant displacement may occur during the design earthquake if a low fs is calculated in the pseudostatic analysis e g 1 0 then significant displacements may occur and displacement deformation analyses should be performed dynamic analyses with numerical tools provide a more sophisticated alternative to pseudostatic analyses analyses may be performed with tools such as the finite difference program flac and available

finite element method and boundary element method programs use of these tools during design may depend on project budget design requirements and available resources for waste dumps the greatest stability risk posed by earthquakes is typically liquefaction of foundation materials although liquefaction may occur in susceptible waste materials as well if liquefaction occurs in the foundation the entire dump may be translated or there may be progressive failure bcmdc 1991 liquefaction due to seismic events is typically limited to 20 m 66 ft in depth or shallower due to the beneficial effects of confining pressure against liquefaction susceptibility thiel and smith 2004 simplified procedures to evaluate liquefaction resistance in soils have been widely discussed in the literature e g seed and idriss 1971 seed 1979 ambraseys 1988 suzuki et al 1995 arango 1996 andrus and stokoe 1997 olsen 1997 youd and noble 1997 robertson and wride 1998 youd and idriss 2001 the paper by youd and idriss 2001 is a summary of commonly used procedures and provides recommendations for design general design considerations all waste dumps have some risk of instability whether due to an inadequate design process or unforeseen variability of assumed parameters the issue of addressing uncertainty in geotechnical design has been discussed in depth by numerous authors duncan 2000 christian 2004 whitman 1984 christian et al 1993 the trade off between the costs of a thorough geotechnical investigation versus the risks of design uncertainty has long been a challenging management decision in geotechnical projects for mine sites significant investment is typically made in exploration and estimating mineral resources and the geology of a mine site is often more thoroughly documented than other types of geotechnical projects nevertheless the engineering properties of the soil and rocks relevant to slope stability receive less emphasis baecher and christian 2003 observed that the areas of geotechnical concern such as slopes and waste disposal facilities are usually associated with mine costs rather than revenue and therefore significantly less money is devoted to their site characterization and laboratory testing one may ignore the uncertainties involved in a design take a conservative approach rely on observational methods peck 1969 or attempt to quantify the uncertainty geotechnical projects in general may include a combination of these methods factor of safety the most common way to take the conservative design approach is to require a minimum calculated fs for slope failure the methods used to calculate the fs are described in detail in chapter 8 3 the minimum fs selected for design allows for some margin of error between the assumed conditions and those that actually exist in the field and should consider the following as outlined by vandre 1980 consequences of instability thoroughness of the geotechnical i

nvestigation reliability of the design assumptions ability to predict adverse conditions possible construction deviations from design engineering judgment based on past experience the fs is calculated for normal loading conditions as well as for seismic loading when the project is located in a seismically active area in general a minimum fs of 1 3 for shallow failures to 1 5 for more significant failures is considered acceptable for long term static conditions navfac 1982 vandre 1980 the fs required for extreme adverse conditions such as the design seismic event or temporary slopes is typically lower than that required for long term stability of final waste slopes and a range of 1 1 to 1 3 is generally accepted reliability for significant structures such as waste dumps and heap leach pads it is critical that sources of uncertainty in the stability analysis be acknowledged early on and considered in the overall design approach as with any project economics and other physical constraints such as space limitation do not always allow for an overly robust design in an effort to quantify uncertainty and provide a level of confidence in the safety and reliability of a design probabilistic methods have been developed and implemented in many slope stability software packages reliability methods are often used in the design of open pit mine slopes but not as commonly in designing heap leach pads and waste dumps when selecting appropriate values for the input parameters of the stability analysis the level of uncertainty in the data and the assumptions that are made must be clearly identified and considered in the design simplified deformation analyses analyses may also be performed to evaluate seismically induced deformations the pseudostatic analysis method can be used to calculate the yield acceleration of the sliding mass this yield acceleration may then be used in simplified procedures for estimating earthquake induced deformations such as those provided by makdisi and seed 1978 and bray et al 1998 determination of acceptable deformation limits may depend on several factors such as regulations engineering judgment and previous experience and acceptable risk in summary slope failure may occur in waste dumps by a variety of failure modes which include surface slumping shallow flow slides rotational circular failures base spreading block translation and liquefaction in geomembrane lined heap leach pads slides typically occur by wedge failure along the critical interface of the liner system engineering judgment and experience must be used when selecting the appropriate analysis method for these potential failure modes as well as when selecting input parameters for the dump materials and foundation the reliability of the stability analysis results depends on whether the design assumptions are representative of the actual waste dump conditions settlement waste rock settlement occurs because of par

ticle reorientation weathering of high clay content materials weakening of inter particle bonding due to water and transport of fine particles through the dump williams 2000 the rate of settlement is affected by dump height loading rate location within the dump and material type zavodni et al 1981 settlement is more predictable and usually less in layered dumps than in end dumped embankments during placement of the waste material initially selfweight settlement may occur or crest settlement may happen because of compaction or surface sloughing from oversteepening zavodni et al 1981 after waste placement primary settlement and creep settlement occur at a decreasing rate with time and have been shown to continue for more than 10 years after dump construction williams 2000 the majority of the settlement however occurs within the first months after construction zavodni et al 1981 as the dump materials become saturated there is a reduction in strength and collapse settlement may occur williams 2000 especially in loose end dumped waste piles the potential for collapse can be minimized with adequate compaction vandre 1980 under dry conditions settlements of 0 3 to 7 of the waste dump height have typically been reported naderian and williams 1996 however settlements of more than 20 of the total dump height have also been documented zavodni et al 1981 various techniques can be used to monitor deformations of waste dumps with time these methods include onsite inspections surveying photogrammetry extensometers inclinometers settlement cells and laser beacons mccarter 1985b the appropriate monitoring methods are selected based on the waste dump height material and method of construction robertson 1982 describes the development and operation of effective waste dump monitoring systems seepage and drainage the same fundamental seepage principles used in the design of earth dams and levees should be considered in the design of waste piles and tailings storage facilities cedergren 1989 understanding fluid flow through waste dumps is important for evaluating both stability and environmental risks most mine waste dumps and leach piles are usually unsaturated and accurate seepage and contaminant transport modeling requires determining unsaturated soil properties fredlund et al 2003 however unsaturated soil behavior is less understood than saturated behavior and unsaturated properties and flow modeling are not always included as part of the waste dump and heap leach design in fact most geotechnical seepage calculations are based on saturated soils the fundamentals of seepage through porous media are explained in detail in chapter 8 2 the soil properties used in unsaturated flow modeling are briefly introduced here the soil parameters used in unsaturated flow modeling are derived from nonlinear equations using laboratory test data and are generally referred to as the hydraulic conductivi

ty function and the water storage function to model seepage through an unsaturated pile these functions are required for each material in the flow path fredlund et al 2003 various methods of determining unsaturated soil parameters for input in waste dump models are described in detail in fredlund et al 2003 some of these methods are also summarized here the hydraulic conductivity function hcf represents the conductivity of the unsaturated material at various water contents the hcf can be measured in the laboratory or estimated using the methods of brooks and corey 1964 van genuchten 1980 campbell 1973 and fredlund and xing 1994 many software packages allow users to select one of these methods when entering input parameters into the seepage model soil water characteristic curves swccs represent the relationship between the water content of the soil and the soil suction and can be measured in the laboratory using a variety of devices the swcc is also used to determine the water storage function which relates the change in water content to the change in soil suction this relationship becomes highly nonlinear as the soil desaturates fredlund et al 2001 the saturated hydraulic conductivity represents the limiting condition for unsaturated flow and is generally measured as such in the laboratory however if laboratory data are not available there are multiple methods for estimating the saturated hydraulic conductivity of a material indirectly the formulas typically relate the hydraulic conductivity to the grain size distribution of the material some of the available methods include those by hazen 1892 kozeny 1927 carman 1938 1956 rawls and brakensiek 1989 alyamani and sen 1993 and sperry and pierce 1995 design and construction elements can significantly affect seepage and drainage through waste dumps the top surface of the waste dump should be graded to prevent surface water from flowing onto the slopes since the 1990s geosynthetic raincoats have been used on heap leach pads in high rainfall areas to minimize storm runoff flows into the collection ponds breitenbach 2004 smith 2008 these raincoats also serve as protection against erosion and damage to the agglomerates breitenbach and smith 2007a when waste rock is dumped the coarsest fraction often ends up at the bottom of the dump creating a rock drain at the base depending on topographic details such rock fill drain sections can be significantly large and a useful tool for controlling flow especially in places such as valley bottoms where a watercourse already passes if the flow capacity of the rock drain is exceeded the phreatic surface may rise lowering the stability of the waste dump therefore understanding the hydraulic behavior of rock drains is important for waste dump design hansen et al 2005 have provided some insight into this issue additionally the rock drain research program was completed in canada to stud

y the characteristics of rock drains and their environmental effects fitch et al 1998 in heap leach pads a properly designed and operating solution collection and liner system is critical for retrieving pregnant leach solution as well as for controlling phreatic surface levels within the heap the most versatile and preferred liner system currently used for heap leach pads consists of a low permeability soil layer overlain by a geomembrane with a drainage layer of crushed rock overliner on top of it breitenbach 2000 however in the drier and remote areas of south america the geomembrane with the overliner is generally considered adequate the geomembrane liners are specified by their material type thickness and surface roughness and are designed on the basis of initial and final loading conditions and the expected strains produced in the liner a properly selected overliner or drainage material and a stringent construction quality assurance program during installation are crucial to performance of a liner system the overliner material is specified in terms of gradation or maximum and minimum particle size in order to avoid puncturing the geomembrane provide adequate support to the leachate collection pipes and facilitate adequate drainage key concerns for liner system selections are summarized in table 8 11 1 erosion erosion is a natural process that cannot be stopped only controlled erosion on material stacked at the angle of repose can be hazardous because of the risk of material failure and catastrophic movement downslope as well as sedimentation and contamination of downstream waters reclamation and closure of waste dumps or piles usually requires regrading for reduction in slope and seeding of vegetation both of these efforts will dramatically reduce erosion large dumps mounds or piles are designed to control and collect runoff and prevent material failure the final reclaimed landform is also an important element in long term erosion control it is much easier and less costly to avoid contamination before it occurs than to clean up after the fact because of this regulators and industry are designing facilities that from their inception reduce the potential for harmful effects to the environment rock dumps rock dumps are generally designed with a slight grade on the top deck to allow rainfall to flow to a collection system and be conveyed to a collection pond a similar system is placed at the toe of the dump as well the water in the ponds is tested regularly and treated if required the collection systems are normally part of the larger mine wide stormwater control plan a good stormwater management plan which will prevent ponding of water against the safety berm at the crest of the dump helps avoid washouts of the slide slopes on active rock dumps generally the operational toe of a dump will be offset to accommodate the ultimate toe of the reclaimed dump at a 2 1 horizontal to vertical or 3 1 slop

e allowing room for minor ravel and washouts on the side slopes another important erosion consideration for slopes is their shape concave slopes can reduce erosion mcphail and van koersveld 2006 naturally eroded features are concave in shape and by emulating this with wider catch benches on the lower elevations of the dump or pile eroded material from the upper levels is slowed and deposited on the lower levels the configuration of the dump design is an important consideration in stormwater management planning the runon controls for a valley fill are much more complicated than for a ridge crest or heaped dump as the entire design storm flow of the drainage needs to be conveyed around the dump or pile in all cases it is important to keep rainfall from native ground separate from what falls on the dump as the latter may be contaminated whereas the former should not be erosion has typically been modeled in civil applications using the u s department of agriculture universal soil loss equation of which there are several variations including the original universal soil loss equation usle the modified usle and the revised usle however this model is for agricultural situations where the slopes are much flatter than those used for waste rocks dumps several computer codes such as siberia and ceasar are being used that utilize digital terrain models and mathematical algorithms to predict both erosion and deposition the various versions of the usle calculate erosion loss only these codes have their own disadvantages as well such as the need for rigorous calibration hancock 2009 leach pads leach pads are designed to allow leaching solution to pass through the stacked material which is then collected on a liner system with collection pipes to be conveyed to the process facility these systems catch and collect all meteoric water as well and the ponds must be sized to capture a design storm a detailed water balance is usually calculated to size the ponds and to understand the water needs of the pad and the process facility in a properly designed leach pile the material stacked on the pad should have a high enough infiltration rate to prevent excessive solution flowing on the surface and on the side slopes any such flow is captured in lined trenches around the pad acid rock drainage acid rock drainage ard occurs whenever unoxidized sulfide material is exposed to the atmosphere and water dumps piles or stacks of material are particularly susceptible to ard due to the permeability of the material availability of atmospheric oxygen and amount of material that can come in contact with meteoric and surface water flow this topic is covered in detail in chapter 16 5 and is briefly touched upon here design criteria for ard prevention for large dumps or mounds include chemical characterization of the material acid base accounting aba compartmentalizing the dump pile into discrete cells for material buffering control a

nd run on runoff control chemical characterization some large mines in nevada united states use aba and build the waste rock facility to confine potentially acid generating material in cells composed of acid consuming material this creates a net acid neutralizing environment in order to do this good characterization of the material needs to be completed with modern production analytical capability and mine dispatch systems material that is not ore can be characterized and routed to a specific location on the dump if the material balance is not net acid neutralizing based on the aba the dumps may need to be placed on a low permeability layer and capped upon closure waste characterization can also include tests for total and soluble metals such as the u s environmental protection agency s epa s toxicity characteristic leaching procedure and the state of california s waste extraction test testing for ph in water flowing from dumps is important because lowerph water is more likely to contain metals that have been leached out of the waste rock run on and runoff the run on component of meteoric water is controlled based on a stormwater management plan stormwater collection systems need to be well thought out and based on the mine plan topography and required maintenance a mine wide stormwater management plan is required for mines in the united states and these plans are site specific the runoff component of meteoric water is controlled by the waste facility s design and may be included in the overall stormwater plan important considerations for the runoff plan include material classification treatment requirements and appropriate sizing of ponds and catchments in the case of a waste rock dump water that infiltrates the dump will be contained at the toe of the dump only the surface flow will be contained on the top closure and reclamation closure and reclamation of heaps and piles are necessary for environmental ecological and health and safety reasons and in some instances for economic reasons such as to recover bonds posted during the permitting or construction phase the closure process typically requires detoxification of heap leach facilities and reclamation usually means decreasing the slopes of heaps and dumps covering the area with growth media and reseeding vegetation where appropriate rock dumps that have acid drainage issues require ongoing treatment of the water flowing from these facilities detoxification of leach pads is required in the united states and typically involves lengthy periods of rinsing to reduce the cyanide or other toxic content of solutions circulating in the heap sloping the waste facility to moderate slopes of 3 1 horizontal vertical or flatter is typically required capping of dumps and leach facilities with semipermeable capping material allows for the establishment of a growth medium for planting vegetation which is the best way to prevent erosion as discussed previously the

final landform should be concave if possible with shallower slopes at the base of the facility in addition many agencies are requiring certain randomness to the final landform avoiding stretches of linear slopes and ridges many operations do concurrent reclamation where they slope and plant segments of their facilities to reduce the overall operating footprint and possibly to recover a portion of their bond an important factor to consider is the longevity of the closure system many waste facilities are looking at closure periods in hundreds of years and waste facilities containing radon or other radioactive material are looking at thousands of years of containment natural material will last longer than synthetic materials and this must be considered in the design of a facility handling radioactive material radioactive waste rock some waste rock can be radioactive and may require special design considerations uranium mill tailings have received a lot of attention because of their radioactive properties and as a result are designed for long term disposal phosphate mining and processing produce phosphogypsum tailings which may also contain trace levels of radioactive material fipr 2010 phosphogypsum tailings have been used as fertilizers and for other uses however the epa has banned the use of phosphogypsum with an average radium 226 concentration of 10 pci g picocuries gram for agricultural application fipr 2010 as a result of phosphate mining currently 0 909 billion t 1 billion st of phosphogypsum waste materials are stacked in the state of florida and about 27 3 million new metric tons 30 million short tons are generated each year uranium tailings contain low levels of radioactive radium 226 ra 226 has a half life of 1 620 years and decays into the odorless and colorless gas radon 222 which has a half life of 3 8 days inhalation of ra 226 is known to lead to lung cancer because of the radioactive properties of uranium tailings the standard practice is to design the impoundments for long term disposal typically 1 000 years to avoid erosion over this type of time frame slopes of the piles need to be minimized and natural forms of containment should be utilized in the united states the design of uranium tailings impoundments and covers falls under regulations in the uranium mill tailings radiation control act of 1978 these regulations require that a cover be designed to produce reasonable assurance that the radon 222 release rate does not exceed 20 pci m2 s for a period of 1 000 years to the extent reasonably achievable and in any case for at least 200 years when averaged over the disposal area for at least a 1 year period in some cases at inactive sites the regulations allow for a radon concentration of 0 5 pci l above the background concentration the regulations also state that the tailings should be disposed of in a manner that no active maintenance is required to preserve the conditions of the si

te the typical cover includes from bottom to top the following layers thicknesses are variable 0 61 m 2 ft radon infiltration barrier consisting of clay 0 15 m 0 5 ft thick capillary break layer consisting of coarse sand fine gravel 1 07 m 3 5 ft thick water storage soil layer consisting of fine grained soil 0 15 m 0 5 ft thick surface erosion protection layer soil rock mixture consisting of 80 soil 20 riprap boulders vegetated surface for water balance control the actual thickness of the radon infiltration barrier in a specific case would be based on calculations of radon flux at the surface of the compacted soil layer an example design is shown in figure 8 11 4 the soil type would be selected from available borrow sources that can satisfy performance requirements for permeability and radon attenuation the compaction requirements would be determined with tests and calculations of saturated hydraulic conductivity and radon attenuation a uranium mill tailings cover calculator is available on line at www wise uranium org ctch html this calculator determines the radon fluxes and concentrations in multilayer uranium mill tailings and cover systems and optimizes the cover thickness to satisfy a given flux constraint the calculator is a clone of the raecom radiation attenuation effectiveness and cover optimization with moisture effects code rogers and nielson 1984 the input data include radium 226 activity concentration if the value is unknown it can be estimated from the grade of ore processed in the uranium mill radon 222 emanation fraction the fraction of the total amount of rn 222 produced by radium decay that escapes from the soil particles and gets into the pores of the soil radon 222 effective diffusion coefficient porosity moisture content and minus 200 sieve fraction this chapter introduces the engineer to electrical power use in mining and concentrator plants drawing upon mining equipment designs of the last decade today s mine power systems are complex and subject to numerous technological and environmental constraints it is no longer possible to treat the subject with the indifference shown in the past power distribution equipment is partly stationary and partly mobile and subject to extreme levels of dust moisture and vibration mining and concentrator machinery create electrical loading that is often cyclic and variable designing and maintaining such an electrical distribution system is challenging and demanding and requires specialist knowledge of both mining and electrical engineering effective mine management requires that those who are responsible for production and safety be familiar with the mine s electrical distribution system typical mining and concentrator plant power requirements today are 120 mw for a 100 000 t d metric tons per day plant in the utility substation power is transformed from transmission distribution voltage to commonly 22 to 3

3 kv and then carried to major load areas to supply large industrial consumers directly or to power distribution substations that step the voltage down it is the responsibility of the mining company to select the voltage best suited to its needs the choice depends primarily on the amount of power purchased it is not safe to assume that a power company can serve a large mine complex from existing primary distribution lines or even from the transmission system the problem stems from the fluctuating nature of mine loads for example large excavators in surface mines can require high peak power for a short time followed by regenerative peak power all cycling within a span of 45 seconds fluctuating loads can create voltage and frequency variations beyond the limit set for other utility customers similarly most large draglines and shovels require 4 16 to 30 kv to start up several terms are used to describe the operation of a power system these terms are applicable not only in system design operation and maintenance but also in utility company billing the sum of the electrical ratings for all equipment in an electrical operation gives the total connected load expressed in kilowatts kw kiloamperes ka kilovolt amperes kva or amperes a many loads operate intermittently especially for mining equipment with varying load conditions accordingly power demand is frequently less than the connected load this fact is important in the design of a mine power system the system should be designed for what the connected load actually uses rather than the total connected load obviously these considerations have great impact on power system investment ieee has standard definitions for load combinations and their ratios because of the importance of assessing equipment power demands the most important definitions are the following ieee 141 1986 demand electrical load for an entire complex or a single piece of equipment averaged over a specified time interval the time or demand interval is generally 15 minutes 30 minutes or 1 0 hour demand is generally expressed in kilowatts kilovolt amperes or amperes peak load maximum load consumed or produced by one piece or a group of equipment in a stated time period it can be the maximum instantaneous load the maximum average load or loosely the maximum connected load over the time period maximum demand largest demand that occurs during a specified time period demand factor ratio of the maximum demand to the total connected load diversity factor ratio of the sum of the individual maximum demands for each system part or subdivision to the complete system maximum demand load factor ratio of the average load to the peak load both occurring in the same designated time period this can be implied to be equal to the ratio of the actual power consumed to the total connected load in the same time period for example consider a power cable supplying several mining se

ctions in a mine or concentrator plant the sum of the connected loads on the cable multiplied by the demand factor for these loads yields the maximum demand that the cable must carry when applied to current this is the maximum amperage good demand factors for mine power systems are in the range 0 7 0 8 depending on the number of operating sections the lower value is used when there are fewer producing units the demand factor can be extended to include estimates of average load for instance the sum of the average loads on a cable multiplied by the demand factor provides the average load on the cable a prime application here is for approximating the current that a conductor is expected to carry the demand factor and the diversity factor can be applied to many other electrical applications such as estimating transformer capacities protective circuitry continuous ratings and the load that a utility company must supply the load factor can be used to estimate the actual loads required by equipment here the total connected load multiplied by the load factor yields an approximation of the actual power consumed the average load factor in underground mining tends to be low due mainly to the cyclic nature of machine operation but also to the use of high horsepower motors for performing specific functions sometimes for only a small fraction of the possible running time the peak load is normally one basis that utility companies use to determine power bills most often one month is the specified time period demand meters are often installed at the utility company s metering point design criteria the goal of the engineer is to provide an efficient reliable electrical system at maximum safety and for the lowest possible cost the types of information made available to the engineer include the following expected size of the mine anticipated potential expansion types of equipment to be used haulage methods to be used available power from the utility company amount of capital assigned for the electrical system designing a safe reliable electrical power distribution network for an industrial plant requires detailed planning to avoid installation commissioning and operational problems and limitations of future plant expansions basic questions regarding mine operation scenarios need to be answered and careful attention must be paid to the following location of equipment altitude and earthquake zone for installations at 1 000 m above sea level design must be suitable for operation at higher altitudes in earthquakeprone areas design must be as specified in the international building code icc 2009 which replaced the older uniform building code safety of electrical technicians and other personnel reliability of operation simplicity maintainability and selective system operation voltage regulation to handle fluctuations in voltage resulting from changes in electrical load high medium low voltage levels de

signated hv mv lv and breaker technology transformer sizes and impedances protection monitoring and control philosophy load flow and network harmonic studies harmonic distortion levels at the point of common coupling pcc electromagnetic compatibility emc issues and power factors system efficiency grounding and lightening and potential for expansion of these safety reliability and simplicity are closely related depend on good preventive maintenance and are of vital concern in the cramped inhospitable environment of a mine operation especially in underground mines routine and dedicated maintenance should be performed only by electrically trained personal and training for these tasks must be provided importance must be paid to the mean time between failures mtbf the failure rate increases as the number of installed components in a system increases similarly the mean time to repair mttr must also be carefully considered safety is enhanced with reliability measures such as adequate interrupting capacity current limiting capability and selective system operation adequate interrupting capacity and current limiting capability ensure protection during a disturbance current limiting when applied to grounding is perhaps the most significant personnel safety feature of mine electrical systems selective system operation is a design concept that minimizes the effect of system disturbances although initial cost is important it should never be the determining factor in the design of a mine s power system high cost equipment can easily offset first costs by reducing operating costs when designs maximize safety and reliability using the data available the task of the mine electrical engineer is to select one combination of power equipment provide power or circuit diagrams estimate equipment operating and maintenance costs set system specifications and receive and assess proposals from suppliers the engineer must possess a firm knowledge of mine power systems and operating plans no two mines are exactly alike so there can be no standard mine electrical system the engineer must resort to fundamental concepts an awareness of what has worked in the past and a clear understanding of technological operational environmental and safety constraints power supply and distribution equipment solutions the increase in power plant size in the last decade has been accompanied by an evolution in mine power systems toward higher complexity key aspects of concern must now include the following overhead power transmission voltage level hv substation hv mv and lv technologies overhead power transmission power requirements for a 100 000 t d plant can typically reach 120 mw as a consequence of almost yearly increases in grinding equipment size involving for example huge mine hoists transport belts and pumps in addition concentrator plants are usually built in remote areas where power supply and ne

twork conditions are not always adequate the trend in load behavior characteristics over the last few years from network linear loads to nonlinear loads requires sufficient network shortcircuit power capability to operate the electrical equipment correctly adjustable speed drives constitute 80 of the total installed power in today s modern mine operations resulting in 70 90 mw nonlinear loads from 120 mw of installed power the short circuit power at the pcc defines whether a plant is normally operable or special measures must be taken the best engineering practices specify the following if the short circuit power at the pcc in megavoltamperes is a factor of 6 times the total nonlinear installed power in megawatts no special measures need be taken if the factor is 6 consideration must be given to implementing special filter and compensation units voltage compensators or in the worst case rotating synchronous condensers the minimum short circuit power at the pcc must be 400 mva for a 100 000 t d plant the maximum shortcircuit level must be kept within the breaker protection capability there are in principle four remedial approaches approach 1 replace single lines with two parallel lines for plants in remote areas overhead power line lengths from the next power station can easily be up to 400 km the transportability of electrical energy is reduced significantly in such cases because of the inductive resistance of the line a solution is to run not just single lines but two parallel lines figure 9 1 1a for example a single overhead line whose short circuit power is 700 mva at the beginning of the line furnishes at 300 km only 40 of its original value 280 mva two parallel lines furnish 60 of their original value 420 mva approach 2 increase voltage in the lines an alternative method of boosting short circuit power in overhead lines is to increase the line transmission voltage figure 9 1 1b for example a 230 kv overhead line whose short circuit power is 700 mva at the beginning of the line furnishes at 300 km only 40 of its original value 280 mva a 400 kv line furnishes 70 of its original value 490 mva approach 3 use direct current another solution for distances of 600 km is to use dc transmission figure 9 1 2 dc transmission by overhead line or by cable at high voltage 750 1 000 kv is an alternative at these distances relevant factors are the voltage drop from the beginning to the end of the line and the ohm resistance there is no inductive line resistance a significant advantage over ac transmission is the nonattenuation of short circuit power along the line approach 4 produce supporting power directly at the plant power production directly at the plant is technically advantageous first there is no attenuation full power is available at the plant second network short circuit power on the pcc is improved this approach makes sense for plants located in remote areas fo

r example an additional on site power generator of 50 mva increases network short circuit power to 350 mva and solves many problems that are characteristic of weak networks however the advantages of this solution are largely outweighed by the disadvantages including the need to transport fuel to remote areas and the logistical and technical problems at high altitudes voltage level the network connection voltage is generally determined by the power utility and depends on the required power of the new facility capacity of the local network location of utility substations and utility network voltages typical loads that utility authorities allow for connection to their system are listed in table 9 1 1 utility planning authorities dictate their requirements and rules in most cases when permitting connection to their electrical grid typical considerations include the following ownership of the substation total harmonic distortion limits set by the authorities to typically 5 power factor requirements set by the authorities to typically 0 9 maximum power drawn short circuit contribution from plant to grid equipment and installation standards that must be fulfilled protection and metering requirements load shedding systems hv substation after the coupling voltage has been determined with the utility the hv substation can be considered factors to consider include the following current requirements lighting and voltage insulation levels short circuit levels environment site conditions potential pollution earthquake and atmospheric conditions common engineering practice in the last two decades is to indicate the most used configuration in terms of load and short circuit levels for hv switchgear and associated equipment as shown in table 9 1 2 the substation should be located as close as practical to the load without compromising safety and access to the site and plant during construction and future expansions the location should be chosen so as to minimize contamination from dust sources such as stockpiles and from airborne chemicals to minimize power outages redundant incoming power supplies should be considered use of underground cables and surge arrestors rather than overhead line construction should be considered when designing the substation this solution minimizes exposure to lighting strikes and provides a good safety margin for inadvertent accidental contact with live hv connections by heavy machinery or personnel depending on the configuration from the overhead line i e whether there is one line or two lines from different sources often multiple hv mv transformers 50 to 80 mva each are installed on the mv side the transformers also feed three or four distribution systems that are connected by selectable tie breakers this configuration covers all plant operational conditions including failure of one of the transformers because of the operational flexibility with this configuratio

n it is essential that a switching concept for the tie breakers be implemented the concept must respect not only the plant load but also power factors and network harmonics usually automatically operated tap changers are installed on the primary side of the transformers tap changers provide the correct voltage to the plant and mine consumers and compensate for voltage variations in the hv feeder line or voltage changes due to varying load requirements in the plant tap changers are usually switchable in 2 5 and 5 steps it is critical that the function of the tap changers be coordinated as soon as the distribution bars on the secondary side are connected i e the tie breaker is closed one tap changer must be designated master and the others slaves later if more plant loads are added it can be relatively simple to boost the transformers for more power with forced cooling a delta primary connection and wye y shaped secondary connections are preferred for standard two winding power transformers and are commonly used in mine power centers the wye secondary connection provides an easy means for resistance grounding the delta primary connection isolates the distribution circuit from the utilization circuit with respect to ground currents the delta wye connection stabilizes the secondary neutral point and minimizes production of harmonic voltages in certain situations a delta secondary connection may be specified and the primary connection may be a delta or a wye if neutral grounding is desired or required a grounding transformer is needed to derive a neutral grounding point the point of common coupling pcc is the point where the utility ends and the plant starts this point defines the following level of voltage distortion from the utility to the plant and contribution from the plant to the utility needed to design the harmonic filters the most frequently used standard is ieee 519 1992 power factor from the plant to the utility needed to design power factor correction equipment a value of 0 95 is generally required allowable voltage fluctuations needed to design equipment for the rated load and to protect it from over and undervoltages network short circuit power in megavolt amperes needed to design harmonic filters and converter transformer impedances a typical power distribution configuration for a mine and plant operating at 100 000 t d is shown in figure 9 1 3 hv and mv technologies hv and mv technologies relate primarily to the types of switchgear system used switchgear systems are of two types air insulated switchgear ais and gas insulated switchgear gis figures 9 1 4 and 9 1 5 conventional metal clad ais systems fully fitted with draw out circuit breakers cbs are in common use for primary distribution circuits either vacuum or sulfur hexafluoride sf6 circuit breakers can be used both provide similar operating and technical characteristics vacuum circuit breakers are lower in c

ost but can expose electrical distribution systems to overvoltages from motors during the switching off cycle this problem can be eliminated by using fused vacuum contactors fvcs rather than circuit breakers fvcs should be considered for high short circuit levels or high mv motor switching cycles their use provides mv motor protection reduces required cable sizes and thus costs and prolongs device life by reducing the amount of let through energy incurred these relative performances of these three ais technologies are compared in table 9 1 3 the cable sizes required for use with various fvc and cb configurations are listed in table 9 1 4 gis systems offer several advantages over ais systems they are much more compact 20 15 m as compared with 75 75 m they are available up to 500 kv and are minimally affected by power derating factors they can easily accommodate a duplicate bus bar system they are suitable for use in site conditions that are arduous with high contamination high altitude or extreme climatic conditions they can become costeffective when consideration is given to substation real estate civil works excavations and geographical location systems are available that provide visible isolation and equipment grounding facilities for operator verification thus ensuring safe and practical isolation of the electrical plant primary distribution network voltage should be determined according to a site s distribution distances and powerflow requirements which depend in turn on site size location and conditions current requirements lighting voltage insulation levels and short circuit levels are all important synchronous condensers synchronous condensers have played a major role in voltage and reactive power control for many years figure 9 1 8 a synchronous condenser rotates or spins freely unconnected to the network its relatively small frequency controlled motor rotates increasingly faster until it reaches the synchronous speed synchronous condensers have been connected at both subtransmission and transmission voltage levels to improve stability and maintain the voltage within desired limits under varying load conditions however they are used mainly to supply a portion of the required converter reactive power and to provide system reinforcement as needed they are a proven solution for increasing short circuit power in plants and also increasing the network s parallel resonance synchronous condensers have an inherent advantage over capacitors in that they are functionally synchronized with the power system their field is controlled so as to induce them to either generate or absorb reactive power a further strong advantage is their ability to ride through small network disturbances because of their large accelerated masses the short circuit improvement at the point where a synchronous condenser connects with a 15 mva condenser is 100 mva this performance efficiency is considered good in that total fu

ll load losses are 1 of the condenser rating drives and motors almost all relevant process drives today are adjustable speed drives traditional power distribution for fixed speed drives applies now only to smaller motors adjustable speed drives usually have their own distribution philosophy sag mill drives large autogenous grinding ag and semiautogenous grinding sag mills are usually powered by adjustable speed drives either gearless mill drives gmds or geared singleor twin pinion drives sag mill drives are larger than ag mill drives in both size and power in new installations power ratings for sag mill drives can range up to 28 mw adjustable speed drives are always coupled by converter transformers that are connected to the mv distribution system the transformer is a converter transformer not a distribution transformer the converter transformer must handle current harmonics and nonstandard secondary voltages produced by the drive a general rule for sizing converter transformers is that the sum of their power ratings in megavolt amperes should be 1 9 times the power of the drive in megawatts for example the 21 mw drive shown in figure 9 1 9 requires converter transformers with a total power rating of 40 mva 3 14 mva normally three converter transformers are connected by one mv breaker to the mv bus distribution system the secondary voltage of the transformers is relatively low depending on the cycloconverter configuration in the range 1 000 1 800 v an isolation switch is mounted for safety reasons between the cycloconverter and the motor a controlled rectifier unit is used for excitation and connected by a converter transformer to the mv bus bar with a voltage of 22 kv for average size drive motors converter transformers are typically triple wound if the primary winding is in a delta configuration and one of the two secondary windings is in a star configuration with a phase shift of 30 a 12 pulse configuration is possible on the transformer figure 9 1 9 typical start up torque is up to 130 of the nominal network current start up current is very soft and is not a problem in terms of power distribution maximum power is achieved only with the rated torque at the rated speed thus adjustable speed drives are generally considered electrical network friendly in comparison with fixed speed drives which can draw start up currents of 600 of the nominal network current for 25 mw drive motors converter transformers should be quadruple wound in an 18 pulse configuration because of the physical limits of the semiconductor figure 9 1 10 and to minimize creation of harmonics cycloconverters are fuseless in the event of a shortcircuit in the motor or the cycloconverter current is limited only by the impedance of the converter transformers cycloconverters are designed to survive short circuit currents for 200 ms which is longer than an mv breaker normally takes to trip 40 80 ms normally three transf

ormers are connected by one mv breaker to the mv bus distribution system as an example the motor in cycloconverter operation is designed for 5 730 v ball mill drives for 7 mw ball mills drives are usually fixed speed and geared wound rotor induction motors with secondary starter or synchronous motors are usually used with adequate start up equipment to limit the start up current to the network in terms of power distribution drives with fixed speed motors are more challenging to use than are those with adjustable speed motors because of their high start up currents the first harmonic created is the 5th in the six pulse configuration except for interharmonics followed by the 7th 11th 13th and so on for 7 mw ball mills drives are usually adjustablespeed gmds adjustable speed geared single or twin pinion drives are also used for adjustable speed drives in the range 7 22 mw the primary converter transformer is connected in the same manner as for a sag mill directly to 13 8 22 33 kv on the mv bus the converter transformer winding vectors are similar to those for the sag configuration for small gmds converter transformers are often in a six pulse configuration figure 9 1 11 the primary transformer can be connected to the mv distribution bus to 4 16 13 8 or 22 kv maximum high pressure grinding rolls high pressure grinding rolls hpgrs are another option for grinding or milling figure 9 1 12 hpgrs were first used in the minerals industry in the 1990s and are known for their high material breaking efficiency their main application is for hardrock gold and copper ore they have twin drives in a typical power range of 2 2 mw a grinding circuit usually consists of two to four hpgr sets followed by two or three ball mills hpgrs have standard 2 300 3 300 or 4 160 v motors with reinforced bearings the drives are mainly frequency converters one drive receives power from a single triplewound 5 6 mva transformer the other drive handles highload peaks up to 200 of the nominal torque and must be able to share the load between the two rolls in a fast controlled manner this load behavior must be taken into consideration in the power distribution conveyor belts conveyor belts today are equipped depending on the application with direct on line motors and hydraulic or gearbox slip ring motors with resistor starters or variable frequency drives figure 9 1 13 larger conveyor systems are driven by several motors in the power range from hundreds of kilowatts up to 2 mw at the head and tail end depending on belt width and length and on conveyor profile overland conveyor belts can curve with uphill and downhill sections of many kilometers and are controlled mainly by variable frequency drives vfds downhill applications use mainly active front end regenerative drives where the drive electrically breaks the belt and braking energy is supplied back to the grid l chinger et al 2006 vfd supply voltages are as fol

lows for lv drives 500 or 690 v for mv drives 3 3 4 16 or 6 kv the load behavior is a constant torque load with a relatively low dynamic for startup and braking typical start up or braking torque is between 1 15 and 1 5 times nominal torque the supply power range for one conveyor can vary from several hundred kilovoltamperes to 15 mva cyclone feed pumps the drives of cyclone feed pumps have recently undergone significant performance changes a large plant typically has two to four 1 500 kw pumps pumps are usually controlled by frequency converters and operated at low speed the pump motor is a scalar controlled induction motor and often does not have a gearbox but is coupled directly to the pump to achieve low speed 600 200 rpm operation the motors are designed with 16 to 30 poles operation with a frequency converter guarantees a soft start with low network load the supply voltage of the frequency converter transformer is from 4 160 v to 22 kv advantages of this configuration are that it is adaptable to process requirements energy savings lack of energy wasting throttle valves and maintenance intensive gearbox and the high efficiency of the drive system a disadvantage is the higher investment cost for the motor mine hoists underground mining relies on hoists to transport people equipment and ore between the mining zone and the surface in many mines vertical shafts are combined with a ramp however ramps are not technically or economically viable in mines that are deep or have poor rock stability in such mines it is common to sink both a production and a service shaft the shafts can also be used for ventilation purposes the main considerations for a mine hoist are absolute operational safety reliability energy efficiency and productivity grid stability must be considered because of changing load cycles during hoist operation mine hoists are of two types friction and drum figure 9 1 14 modern hoists are powered by ac motors smaller hoists with power requirements of 1 500 kw normally use a gearbox with high speed induction motors larger hoists use a direct drive with an overhung synchronous motor speed control is achieved by a voltage source inverter vsi frequency converter modern hoists are direct drive systems that is the motor is coupled directly without the use of a gearbox to the singleor double drum hoist the power of a friction type hoist system is in the range of 5 to 10 mw the main components of a hoist system include a synchronous motor 4q frequency converter drive motoric and generatoric hydraulic brake system control system for friction rope oscillation and so on figure 9 1 15 shows the electrical design for the friction type mine hoist shown in figure 9 1 14 dragline excavators dragline excavators are one of the largest mining machines for overburden removal in strip mines they can have any of three major drive systems 1 mg sets with digital field excitation 2 conventio

nal ac drives lv or mv variable speed with squirrel cage ac motors and gears 3 gearless ac drives with gearless ring motors for hoist and drag drums most draglines built between 1960 and 1990 use ward leonard mg sets and dc motors draglines built today use conventional or gearless ac motors the new systems are almost maintenance free more efficient overall and contain better drive controls supply power usually varies from 4 mva for small draglines up to 25 mva for large draglines higher voltage and short circuit capacity are desired for better performance standard dragline supply is typically 22 kv drive system voltage is typically 6 kv for the synchronous motors of an mg set and 1 400 or 690 v for ac drives draglines with mg set drive systems usually have two to four sets some larger draglines even have six sets two extra sets for the walking mechanism older systems are commonly upgraded with programmable logic controllers plcs and digitally controlled field excitation modules conventional ac drives have been successfully installed on smaller and even a few larger draglines a typical application is a multiple drive dragline figure 9 1 16 where one supply unit feeds several motor inverters over a common dc bus bar the system shown in the figure has two hoist motors two swing motors and two drag motors typically two to four such sets are installed on a dragline the common dc bus allows energy sharing among the motors some motors run in motor mode some in generating mode usually a regenerative insulated gate bipolar transistor igbt based supply unit is used to feed excessive braking energy back into the grid these units usually have better power factors 1 and low total harmonic distortion levels for weak grids they protect the drive against voltage dips or even short blackouts modern units have an adjustable leading or lagging power factor and can be used for reactive power compensation the duty cycle of dragline motion is complex and demands high drive dynamics fast controls and high overload ability on the part of the motors gearless drive systems are currently state of the art for draglines gearless technology has emerged from mill and mine hoist applications where gearless ring motors integrated into a drum have proven to be reliable and efficient the ac ring motors integrated into hoist and drag drums are powered by mv variable speed drives primary crushers main crushing motors can be across the line starting softstarting or variable speed depending on crusher size type and application for hard rock crushing typically one motor is mv across the line starting for applications where speed regulation is necessary vfds are the best choice because they can maintain constant speed seizer rotation regardless of the type or size of material fed into them such drives adjust torque on the shaft to maintain speed and a continuous quality of crushed material the crusher can also be used fo

r massflow control depending on the type of crusher and the motor s type and speed range crusher sizes are in the lower megawatt range shovels today most shovels have static dc drives with digital field control since the 1980s ac shovels have been used increasingly small electric rope shovels with mg sets are still available but are becoming obsolete most shovel suppliers now promote ac technology bucket wheel and bucket chain excavators bucket wheel and bucket chain excavators are used in continuous mining applications where trucks are not used figures 9 1 17 through 9 1 19 show three of many possible configurations overburden and ore are transported by conveyor belts spreaders are used to spread the overburden on the reclamation side of the pit continuous mining is efficient for thick homogeneous seams of ore generally lignite excavators are complex systems a bucket wheel excavator typically has three or four conveyor belts a propel system with 3 to 12 crawlers multiple winches for hoisting a bucket wheel and swing drives the supply power of an excavator can reach 16 mva the total drive power depends on the size of the excavator and can be 10 mva the supply voltage is usually 4 16 13 8 22 or 30 kv vsds are used for propel swing hoist belt bucket wheel or bucket chain drives excavators are custom built according to the geological situation the drives and power distribution systems for spreaders and reclaimers are similar heel excavator in that it drives on rails and the propel mechanism usually has about 60 motors the bucket chain is driven by two large gearless ring motors supplied by a cycloconverter whose supply power is 2 mva and motor speed is 13 rpm the voltage for the bucket chain drive is 1 400 v stackers and reclaimers move on a well defined path and handle stockpiles of defined size and so can usually be completely automated for unstaffed operation automation is accomplished by means of vfds plc systems and global positioning systems or laser positioning systems for collision protection stockpile scanning and ore handling when combined with automated ore quality detection systems stackers and reclaimers can significantly increase the overall efficiency of a material handling plant other design considerations since the 1990s plants are often installed at high altitudes in the range 3 000 4 600 m installation at high altitude requires special attention in dimensioning and design in addition many such places are in active earthquake zones and tend to have high snow loads low temperatures high wind speeds and other harsh environmental conditions all such factors must be considered in the design of a power distribution system design for high altitude four principal aspects must be considered when dimensioning equipment for high altitude cooling insulation heat radiation and utilization cooling at increasing altitude the density of air decreases and therefore also does its

cooling capability this situation can be remedied by two measures 1 increase or adjust airflow to obtain the desired cooling effect or 2 install larger equipment with lower output power insulation at increasing altitude the density of air decreases and therefore also does its insulation capability factors to use in correcting voltage ratings for reduced insulation capability due to altitude are listed in table 9 1 7 for example at 4 000 m a unit of equipment rated at 690 v can be used at only 500 v 690 v 0 73 for mv distribution a unit of equipment rated at 33 kv can be used only on a 22 kv bus heat radiation at increasing altitude copper bars tend to heat up progressively more cables and copper bars from the power distribution must be dimensioned accordingly and this effect must be considered in the design of power distribution utilization how standard motors can be used depends on altitude for ambient temperatures in the range 30 40 c the maximum design temperature at sea level is reduced by the following amounts insulation class b motor 0 8 c for each 100 m above 1 000 m insulation class f motor 1 0 c for each 100 m above 1 000 m design for earthquakes shock waves radiate from fault fracture zones and arrive at the earth s surface as complex multifrequency vibratory ground motion with both horizontal and vertical components the response of buildings and mechanical constructions to earthquake ground motion depends on their strength of construction ductility and dynamic properties the basis for design and construction in areas of intense seismic activity is defined in the standards of the international building code icc 2009 the code standardizes magnitudes of seismic activity and catalogs areas of known activity into defined earthquake zones for example most mines in chile and peru are situated in ibc zone 4 distribution and process equipment such as drives converter power distribution buildings and switchhouses are required to withstand the expected magnitude of excitation in a defined earthquake zone equipment configuration is also relevant in general taller and heavier constructions are larger in mass and have lower natural resonance frequencies that can be close to typical earthquake frequencies design for power self generation it is generally less expensive and more reliable to purchase electric power from a utility company than to operate one s own generating plant however a mine may be located in a remote area far from a utility transmission or distribution system making self generated electric power the only feasible alternative generating plants for mines are typically powered by diesel engines or by coal oil or gas fired boilers or even water power some mines purchase electric power but also use diesel powered generators for standby electric power if the primary source of power is lost a generator can be started quickly and used to supply standby power to critical equip

ment such as ventilation fans and personnel or elevator hoists and sensitive process parts special attention must be paid to overvoltage and also undervoltage conditions that might result from power self generation network voltage is generally less stable for selfgenerated power than for interconnected networks singleload units can easily use up to 20 of the total installed power sag mills can use 20 30 mw ball mills can use 12 18 mw mine hoists can use 10 mw and draglines can also have high power demands in concentrator and mining areas not using one or two mills can reduce system load by up to 50 a generator does not adapt to new load conditions nearly as quickly or stably as does the utility company power can take up to 10 seconds possibly more to stabilize during which time the system voltage and frequency fluctuate with potentially dangerous consequences various scenarios must be studied in detail and the consequences of the scenarios must be considered in system design installed equipment must be capable of coping with those conditions the protection and destruction level of the equipment must be known and designed for accordingly cables cables carry electricity from the substation where power is taken from the utility company lines to the point of utilization by a mining machine pump conveyor belt or other equipment unit many variations in mine distribution are possible and several types of cables can be put to similar use the cable type recommended depends on the application some cables remain stationary for years others are moved frequently cables that are connected to mining machines are called portable cables u s federal regulations use the term trailing cables for the specific type of portable cable that is used in mines msha 1981 trailing cables are flameresistant and flexible in underground mines cables that feed the power center or are attached to the high voltage side must be moved when the power center is moved which generally is not often similarly in surface mines cables that feed from switchhouses or unit substations to mobile equipment are moved only occasionally and are not connected directly to a machine stationary cables can be feeder or portable cables cable moving is a recurring task both underground and aboveground trailing cables can be placed on reels or spools to facilitate moving reeled cables are often used on shuttle cars as are mobile cable reels on surface excavators cable selection is based on a number of parameters including current carrying ability voltage rating and configuration the basic components of a cable are its conductor insulation and jacket there may also be fillers binding shielding and armor the conductor is surrounded by insulation and covered by a jacket for optimal flexibility cable conductors are composed of many wires combined into strands and a number of strands combined to form the conductor conductors are either copper or alumi

num the latter is cheaper and lighter but lower in conductivity the cross sectional area of a conductor is important for mechanical strength and is closely related to currentcarrying capacity insulation is required to withstand stress from heat voltage and physical abuse it must be specially designed not only to protect mine personnel from electric shock but also to separate power and grounding circuits effectively excessive heat is particularly destructive to insulating compounds the main sources of heat are ambient temperature and power loss in cable conductor resistance the maximum normal continuous current that a conductor can carry safely is directly related to cable heating and the term ampacity is often used to describe this current level the ampacity rating is usually based on the maximum rise in conductor temperature with the temperature limit chosen on the basis of the specified life expectancy of the cable insulation the temperature class always given in degrees celsius describes the maximum allowable sustained conductor temperature at an ambient temperature of 40 c cable used in a confined space can overheat with continuous current at the cable s ampacity rating this is especially true for cable wound on a reel either for storage or for mobility the cable s ampacity must be derated in these cases ampacity and derating factor tables are available in regulations from various governing bodies the most common insulating compounds for cable are neoprene styrene butadiene rubber sbr ethylene propylene rubber epr and cross linked polyethylene xlp sbr is used in 600 v trailing cables it has good elasticity and flexibility a 75 c temperature rating and good resistance to damage by crushing from runovers and rockfalls epr has replaced sbr in many trailing cables because for the same insulation thickness it has a higher voltage rating 2 000 v and higher temperature rating 90 c xlp is also rated at 90 c and is used in hv mine feeder and portable strip mining cables however it is stiff and therefore not recommended for reeling applications typical cable voltage ratings are listed in table 9 1 8 the main purpose of the cable jacket is to protect the inner components and hold the assembly in the designed configuration common jacket materials are neoprene nitrile butadiene rubber plus polyvinyl chloride nbr pvc and chlorosulfonated polyethylene cspe also called hypalon armored cables are used in some borehole applications the heavy metallic jacket affords extra protection to the conductors and insulation flat cables are commonly used on mining machinery such as shuttle cars that have cable reeling devices the flat shape allows increased length on a cable reel and is less susceptible to run over damage round cables are typical on all other mining equipment low voltage ac mining machines commonly use unshielded type g or type g gc cables current and voltage regulation are the major concerns

in sizing cable power conductors for an application the effective continuous current through a cable power conductor must be less than the cable ampacity with correct derating factors applied the voltage drop across the distribution and utilization systems must be such that voltages at load are within allowable tolerances for trailing cables to machines current is often the determining factor because these cables are almost always relatively short for feeder cables that serve many loads however lengths are often so great that voltage drop becomes a principal concern even though cable size may be adequate in terms of ampacity and voltage drop other factors can affect decisions about conductor size including tensileload weight and available short circuit current of these weight can be particularly important because cable should not be too heavy for miners to handle the maximum conductor size is usually considered to be 4 0 awg american wire gauge for three conductor cables several methods can be used to determine cable current including full load current effective current demand and application of a load factor regardless of the method used typical current requirements for mining machinery change continuously over time the extremely wide variability of mining conditions makes it difficult to define current levels for any part of a given duty cycle with precision thus the use of full load current is recommended the primary concern for voltage conditions is that satisfactory voltage must be at the machine terminals for proper startup and operation the allowable voltage tolerance on machine motors is usually 10 of the rated voltage maintaining adequate voltage is one of the more difficult problems in mining and is often the main constraint on mine expansion for a thorough voltage study of a mine and its concentrator plant all impedances and all loads in the power system must be known a circuit diagram must then be prepared and calculations performed to determine whether voltage levels at the machines are satisfactory analyses must be performed not only for normal load conditions but also for start up of critical motors if calculated voltages are below those tolerated system impedance must be reduced the most convenient way to do so is to increase cable conductor sizes cable couplers are complex plugs and sockets used throughout a mine distribution system to connect mobile machinery to trailing cables to connect cables with one another and to connect cables to power centers switchhouses and substations their complexity is directly related to the mine environment in which they are used they must resist damage be sturdy enough to withstand repeated use prevent electrical hazards be watertight and dustproof and withstand heat and cold some models are rated explosion proof the coupling mechanism must be easy to use yet secure hv couplers sized at 15 kv 500 a are used to connect switchhouses and min

e power centers to join hv cables and for hv machines they accommodate three power conductors one or more grounding conductors and one or more groundcheck conductors lv and mv couplers sized at 225 400 600 800 and 1 200 a are used primarily to connect mobile equipment to power centers and junction boxes and to connect 600 1 000 v cables they are sturdy in construction but less complex than are hv couplers power distribution arrangements power distribution arrangements can involve surface overhead line distributions back distributions surface mine distributions and underground mine distributions surface overhead line distribution arrangements surface transmission and distribution of electric power is most commonly achieved by means of overhead conductors the conductors use air space for insulation over most of their length their elevation protects them from contact with personnel and equipment overhead conductors are arranged in various configurations to reduce line to line contact due to wind ice loading or sudden loss of ice load and may include different combinations of power neutral and static conductors aluminum conductors with steel reinforcement are commonly used because of their strength and relatively low price but special applications may call for other materials such as copper the types of overhead line installation used for mining are similar to those used in utility distribution systems pole lines are typically used to supply equipment in surface mining and to feed surface facilities related to mining the lines are normally installed on single wooden poles that can carry up to six conductors including three for power one for grounding one ground check pilot and one static pole lines can be relatively permanent installations such as those feeding plants shops and other surface facilities and long term pit baselines or ring mains temporary poles mounted in portable bases such as concrete filled tires are commonly used in surface mining operations to carry power into the pit basic distribution arrangements basic distribution systems for industrial applications are radial secondary selective primary selective primary loop or secondary spot configurations ieee 141 1986 radial configurations are the most popular in mining although other configurations are used where special circumstances call for greater system reliability surface mines have of course greater flexibility than do underground mines and use a wider range of configurations secondary spot configurations which are the most popular for large facilities in other industries are uncommon but can be suitable for preparation and milling plants a radial distribution system in its simplest form consists of a single power source and substation supplying all equipment radial systems are relatively inexpensive to install because equipment is not duplicated they can be expanded easily by extending the primary feeders a secondary

selective distribution system consists of a pair of secondary substations connected through a normally open tie breaker it is both flexible and reliable if a primary feeder or substation fails the bad circuit can be removed from service and the tie breaker can be closed automatically or manually maintenance and repair of either primary circuit is possible without incurring a power outage by shedding nonessential loads for the period of reduced capacity operation if substation requirements are greater than 5 000 kva economics often justify this double ended arrangement surface mine distribution arrangements mine power systems are of three types transmission or subtransmission distribution and utilization distribution and utilization systems can vary greatly but in some mines both functions can be handled on the same system the location of the mine substation is usually an economic compromise between the cost of running transmission lines and power losses in primary distribution from the main substation power is distributed to the various load centers in the operation incoming utility transmission should be extended to as close to the load as practical subtransmission circuits primary switchyards and main substations are almost always located in areas unaffected by the mining operation surface mine power distribution in its simplest radial form consists of a substation distribution and a power center feeding the mining equipment figures 9 1 20 and 9 1 21 this arrangement is common in small surface operations where the distribution voltage is commonly 15 000 or 4 160 v but can be as low as 2 300 v in older equipment although most strip mines use radial distribution secondary selective distribution is also used figure 9 1 22 theory of compressed air compressed air is used extensively in mining and is often referred to as the fourth utility behind water electricity and diesel in essence compressed air is stored energy and when controlled it can be used for production purposes energy from compressed air is used to drive pneumatic equipment such as air motors actuators instrumentation and pneumatic tools it can also be used to cool and clean components or parts and to blow off waste material a particularly valuable application in mines industry also uses compressed air as an active part of processes that require a supply of clean dry air for example in cleaning the finished product the effects of compression are a decrease in gas volume increases in gas pressure and temperature and an increase in the concentration of contaminants cost and productivity comparisons compressed air is a form of energy that provides beneficial features such as being safer to use in particular situations allowing the use of lightweight power tools being more economical to use and convey being cleaner than some other forms of energy and being easier to work with although electricity per kilowatt hour may cost less th

an compressed air it is safer and more convenient to work with compressed air particularly underground compressed air is widely used underground because of the safe nature of pneumatic motors compared to electrically driven tools especially in areas where there may be flammable gases and sparks that would pose a risk a primary use underground tends to be in driving pneumatic tools such as drills large quantities of air are also used in removing dust the air could be supplied by piping networks or in the case of mobile equipment by onboard compressors the energy savings from the use of compressed air systems and air compressors could be 20 or more particularly if variable speed compressors are used to tailor consumption to current demand applications and air driven equipment compressed air is used widely in mines both in powering equipment and in providing air under pressure for operations such as clearing debris it is used both in the handheld tools of semimechanized tunneling and with larger equipment for fully mechanized work uses include hammer drills where the air both powers the hammer and clears away the broken rock the air may be supplied either by portable compressors or through lines fed from large static compressors at a central location usually on the surface in the case of an underground mine compressed air can be used at almost every step of a mine s production process for instance a copper mine could use it in the extraction screening crushing milling mechanical sorting flotation filtering and drying operations typical surface applications for air in mining include service air to operate tools bagging operations dust removal and mineral filtration systems to remove buildups underground uses include handheld drills operation of pneumatic tools and water pumps and power machinery the amount of flow required is related to the application medium pressure in the range of 350 to 1 400 kpa 50 to 200 psi will cover 90 of the applications which vary from pneumatic conveying to pneumatic actuators and tools to auxiliary ventilation fans the air can be used to operate machinery or pneumatic tools such as the wrenches and tools used in cutting and grinding other applications include cleaning processing areas also benefit from dry air and the pressure required is typically in the range of 480 to 690 kpa 70 to 100 psi mineral filtration and separation operations on the other hand require higher pressures typically 1 380 to 1 590 kpa 200 to 230 psi a term that is often used is free air delivery or fad which is the volume of air exiting the compressor expressed in terms of the inlet pressure and temperature conditions using the ideal gas law free air is defined as air at normal atmospheric conditions o neil 1939 the difference between the fad and the actual intake volume represents losses within the compressor thus when comparing models it is important to be aware that some manufacturers