CHAPTER 8.11 Waste Piles and Dumps
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CHAPTER 8.11 Waste Piles and Dumps Document Transcript

  • 1. CHAPTER 8.11 Waste Piles and Dumps Marc Orman, Rich Peevers, and Kristin SampleThe terms mine waste piles and dumps refer to piles of waste continues to be problematic in regions where regulations androck or leached ore that carry little or no economic value at the environmental laws are not strict and enforcement is lax.time they are placed. As commodity values rise and process A significant contributor to mine waste dumps is heapmethods gain efficiency, waste piles and dumps may be reclas- leaching, which is a relatively new form of mining wheresified as ore and gain value. Also, the waste material may be low-grade ore is piled over large surface areas and irrigatedvaluable at some future time as an aggregate source, for use in with solutions. The resulting pregnant solution is then pro-riprap, drain material, or other process method that recovers cessed to recover the desired commodity. After the leachingthe commodity at lower grades or has lower acceptable rates is completed, the leached ore becomes a waste product. On aof return. Heaps are ore piles that are amenable to a leaching permanent pad, the ore material is stacked and leached in liftsprocess, both with and without the use of liners, and share until the pile reaches the final design height. Modern leach padphysical characteristics with piles and dumps. facilities can be hundreds of meters high and cover thousands of square meters in area. Alternatively, on a dynamic pad, aHISTORIC PERSPECTIVE single thin lift (5 to 10 m [16.4 to 32.8 ft]) of ore is stacked andEver since humans began to extract materials of value from leached on the pad at any one time, after which the leached orerocks, waste or other lesser-valued material has been left behind is removed and stored in a waste dump. In the past, environ-after the extraction process. For early miners, waste dumping mental issues were not a main consideration for heap leaching.was simply a matter of pushing the waste out of the way, either However, in response to environmental regulation, facilitiesdown a slope or to any other available area. Frequently, these have evolved, and many now utilize geosynthetic liners incor-waste materials ended up in drainage basins, rivers, and lakes porating a leak-detection provision.where they caused environmental harm. In 1884 in California The earliest full-size leaching projects in the United States(United States), hydraulic mining was essentially outlawed were for copper in the form of dump leaching with natural con-by the Sawyer decision because the mine waste in the rivers tainment. Subsequently, with the introduction of cyanide forhad led to flooding after streams and rivers became choked leaching gold and silver, soil liners came in vogue in the latewith solids. This law, handed down by the Ninth Circuit Court 1970s to the mid-1980s (Breitenbach and Smith 2007b). Sincein the case of Woodruff v. North Bloomfield Gravel Mining then, use of geosynthetic clay liner, high-density polyethyleneCompany, became one of the first environmental decisions in (HDPE), low-density polyethylene (LDPE), linear low-densitythe United States (U.S. Circuit Court 1884). polyethylene (LLDPE), and polyvinyl chloride (PVC), as well Over time, other regulations and laws have emerged, and as asphalt impregnated geotextiles and a few others, havestandard practices have evolved to minimize the environmen- become the standard liner materials for heap leaching.tal damage and potential hazards associated with the disposalof mining waste. Enlightened mining companies now deal TYPES OF WASTE PILES, DUMPS, AND HEAPSwith their waste products in a responsible manner, especially This section provides a description of waste piles, wastewhen negative impacts on the public may result because of dumps, and heap leach pads (both lined and unlined). Althoughimproper disposal. Nevertheless, improper disposal and han- these types of facilities are similar, the liner aspect introducesdling of mine waste continue to pose environmental hazards an additional potential for failure along the liner as part ofacross the world. While the hazards associated with mine design. On the other hand, for unlined facilities it is importantwaste disposal have decreased in most developed countries, it to consider the materials’ geochemistry (both the ore as well Marc Orman, Senior Geotechnical Engineer, Ausenco Vector, Grass Valley, California, USA Rich Peevers, Senior Engineer, Ausenco Vector, Grass Valley, California, USA Kristin Sample, Staff Engineer, Ausenco Vector, Fort Collins, Colorado, USA 667
  • 2. 668 SME Mining Engineering Handbook where the facility is going to take a long time to fill, it may Valley-fill Ridge 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 val- ley is not completely filled up-gradient. The structure is usu- ally designed with a rock drain at the bottom of the valley Cross-valley Heaped 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. Cross-valley A sidehill structure lies along the side of a slope but does Diked Pond Impoundment 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 Incised Pond Sidehill 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 cross- valley or sidehill configurations, this type of structure is typi- cally not used to impound fine-grained material or water. In Sidehill some cases, one or both sides of the ridge may require bench- Combination Impoundment ing 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 embank- ment or dump itself. These two parts may or may not be iso- lated from one another by liners. If fine wastes are impoundedSource: Zahl et al. 1992. by coarser waste, the structure is considered a dike. If theFigure 8.11-1 Mine dump configurations embankment is homogeneous and coarse, the embankment is termed a heap, such as a heap leach pad.as the resulting pregnant solutions) and the site’s hydrology toensure that natural water resources are adequately protected. Leach Dumps or Heaps Leach heaps consist of low-grade ores spread or stacked onConfigurations large platforms where the pile is irrigated with leaching solu-Collectively, waste pile, dump, stockpile, or a leach heap can tion to leach out the recoverable product of value. Althoughbe referred to as waste structures. As such, their layout gener- heap leaching has been used mostly for precious metal andally falls into the following categories, depending on the type copper ores in the past, it is now also being used for otherof waste, the purpose of the waste structure, and the physical products, such as uranium and nickel. In recent times, evenconstraints at the site. Each of the configurations is shown in municipal wastes have been leached using similar methods toFigure 8.11-1 and discussed in further detail in the following accelerate the decomposition of waste and add capacity to theparagraphs. facility. Heaps are normally placed on impermeable liners of A valley-fill waste structure, as the name indicates, fills a natural and synthetic materials (discussed in more detail latervalley. Many of the lined valley-fill leach pads require some type in this chapter).of stability berm at their toes. Construction of a lined valley- Dumps usually refer to material piles created by endfill leach pad would begin at the toe berm and progress up the dumping. Run-of-mine ore is sometimes simply dumpedvalley. Construction of a waste dump (not a leach pad) usually instead of being stacked on a leach pad and leached for eco-begins at the upstream end of the valley, and dumping proceeds nomic recovery of the contained commodity (a process knownalong the downstream face (as shown in Figure 8.11-1). For a as dump leaching). The same procedure is often used forheap leach facility, stacking should begin at the toe and proceed secondary recovery from leached ores. Dumps are generallyup the valley to avoid slope-stability problems. placed on natural soil or rock subgrade surfaces that have been The top surface is usually sloped to prevent water pond- demonstrated to have some degree of natural solution contain-ing. Stormwater run-on can be controlled by constructing ment and are normally located on sloping ground or in a valleydiversion channels up-gradient of the facility. In steep terrain, to promote drainage to the toe.
  • 3. Waste Piles and Dumps 669Stockpiles To provide background information on flows and waterThe term stockpile refers to any pile of material that is placed quality, groundwater and surface water samples should be col-for future use. This can include material with either proven or lected before construction begins. These measurements andpotential value, material for structural fill, or other materials samples should be collected throughout the year so that sea-obtained from borrow pits or removed from stripping projects. sonal fluctuations can also be monitored and effectively evalu-Waste rock or processed material to be used as backfill can ated. All drainages and aquifers in the vicinity of the projectalso be categorized as a stockpile. These materials, which are should be tested to ensure that water quality for the entirestored for processing or future use, appear much the same as project area is well understood before the project begins.waste rock except they are normally isolated from waste mate- Initial testing of the water samples should include majorrials so they may be recovered at some later time as economi- cations and anions, metals, nitrates, dissolved and suspendedcally as possible and without being contaminated with waste. solids, salts, and organic compounds, as well as other con-Stockpiled material, such as the ore itself, may be chemically stituents that may emerge as relevant during the process andunstable, and the stockpile may require liners, caps, and/or involve potential changes to water chemistry. Samples shouldstormwater diversion structures to prevent water infiltrating be collected from dedicated monitoring wells and surface sam-the pile and causing water contamination. pling locations, both up- and down-gradient of the project site. A water quality monitoring plan should be prepared toPlacer Waste and Tailings Deposits document sample locations, sampling frequencies, and proto-During placer mining for gold or aggregates, the practice of col for collecting the samples. At a minimum, the plan shouldwashing sand and gravel to recover minerals can produce tail- contain the following items:ings with particle sizes ranging from coarse to fine (<75 µm) • Identification of the surface and groundwater sourcesand wash water, which should be treated. The coarse waste • Monitoring objectivesfraction can be disposed of using one of the methods previ- • Description of water quality parametersously described; however, the fines portion is similar to the • Sampling point descriptions and a map of their locationstailings from a milling operation. Considerations for these • Analytical procedurestypes of wastes include the placement and storage of the • Data quality control objectivestailings and treatment of the wash water to meet discharge • Data management and quality control detailsrequirements. • Sampling equipment to be used With physical constraints of space limitation and the ris- • Sample preparation and handling proceduresing cost of conventional impoundment methods for tailings • Chain of custody and data sheets to be usedstorage, the use of process items such as thickeners and filter • Reporting requirementspresses to put tailings in piles or mounds has become morecommon. With the removal of additional moisture, alternative Land Disturbancedisposal methods such as thickened tailings, paste backfill, Wherever mine waste is placed, the natural environment istreated paste backfill, and dry stacking become viable options, changed, and the process is therefore classified as land distur-which can add capacity to the facility. bance. The initial disturbance creates the potential for sedi- mentation of natural waterways caused by erosion and waterIMPACTS OF WASTE DUMPS quality degradation, which are among the major potentialWaste dumps and heaps have several (actual and potential) impacts of waste dump construction. Although waste dumpsimpacts on the environment, which must be considered as part can be designed to minimize the impacts of land disturbanceof their permitting and design. These impacts include distur- and blend in with natural surroundings as part of reclamationbance of the land, water quality issues, slope stability, and in some locations, these disturbances have been perceived byvisual effects. In the past, waste dump disasters have led to some as highly destructive to the environment. Specifically, inthe contamination of surface and groundwater, as well as mas- California, all metallic mines are now required to use wastesive slides, which have buried communities. rock to backfill all open pits as part of the state’s mine rec- Planning waste disposal facilities requires evaluating the lamation requirements. The U.S. Office of Surface Miningregulatory constraints, identifying an appropriate site, design- requires restoration to approximate original contours for sur-ing the structural and environmental integrity of the facility, face coal mining. These requirements can add considerabledeveloping an operating and maintenance plan, and develop- cost to final closure.ing a reclamation plan for future land use (Center and Zlaten Since most waste structures are not compacted, the vol-1982, and Ritcey 1989, as cited in Zahl et al. 1992). From the ume of a pile or a dump can be much greater than the volumedesign point of view, the specific issues to be considered are of the pit, which adds further to the issues of how to hide, or atthe contamination potential of the waste, slope stability, the least reduce, their impact.condition of the waste structure’s formation under normal andseismic loading, and ways to control water (both internal and Visual Impactsexternal) to the dump. Visual impacts from mine waste dumps and leach pads can be a major concern for mines located in the vicinity of populatedWater Quality areas or where the facilities will be visible from roads andWater quality impact issues associated with waste, unlined highways. Visual impact and viewshed studies are now per-dumps, or poorly constructed heap leach facilities can be a formed routinely in many areas of the world as part of initialmajor environmental concern. Waste rock should be thor- mine permitting.oughly tested at the design stage for acid-generating and In areas where the color of the rock blends with the natu-metals-leaching potential to ensure that water resources are ral color of the terrain, visual impacts will be less than in areasadequately protected.
  • 4. 670 SME Mining Engineering Handbookwith sharp color contrasts. In flat areas, hills develop, and, in limit equilibrium analysis using one of the several prevalentmountainous terrain, ridge tops appear and grow, and drain- approaches is considered adequate to evaluate slope stabilityages are filled. By maintaining slope angles that are similar of waste dumps.to natural slopes, visual impacts may be reduced, and manycompanies are now designing dump surfaces to simulate the Failure Modesoriginal topography. However, contrasts in colors from the The basic failure modes of waste dumps must be considerednatural vegetation to rock and topsoil can take several years to during the stability evaluation and design. Detailed descrip-blend together as the revegetated slopes take hold following tions of identifiable waste dump failure modes and appro-reclamation. priate analyses are described by many in the literature (e.g., A visual impact study may include the following compo- BCMDC 1991; Caldwell and Moss 1985). Each of the mainnents, as described by the Federal Highway Administration failure modes are shown in Figure 8.11-2.(FHA 1981): Surface or edge slumping. The most common failure mode is edge slumping (crest slumping), where a thin wedge • Description of the project setting and the major viewsheds of material translates down the slope, parallel to the dump face. • Photographic study of the project from the major views This shallow failure typically originates near the crest of the • Description and analyses of the existing visual resources dump because of oversteepening. Cohesive or low-permeability and responses from people in the area waste materials allow the development of oversteepened slopes. • Renderings of the project alternatives’ views End dumping the waste in thick lifts or pushing material over • Assessment of the visual impacts of the project the dump crest also leads to a higher risk of over-steepening and alternatives edge slumping. Edge-slumping failures often occur after heavy • Possible methods to mitigate the adverse visual impacts precipitation, which leads to increased pore pressures in theAs part of the visual impact study, maps are usually produced low-permeability waste. In coarse rock-fill dumps, oversteep-and show the areas from which the project would be visible ening of the crest may develop due to initial interlocking of theaccording to different design options. The design options typi- blocks (BCMDC 1991).cally include several different ultimate elevations and possible Plane failure similar to edge slumping may occur deeperconfigurations of the waste dump or heap. within the waste dump materials. In this case, sliding occurs along a single plane of weakness within the dump, which mayDESIGN OF WASTE DUMPS have been created because of a zone of poor quality waste orThis section provides an overview of waste dump design. from dumping waste on top of snow or ice. The plane of weak-Further details regarding the design of waste dumps may be ness parallels the dump slope or daylights at the dump face.obtained from the following recommended SME-AIME pub- Shallow flow slides. Flow slides are shallow slumpinglications and from several other references cited throughout failures of saturated or partially saturated waste. Typically trig-this chapter: McCarter 1985a, 1990; and Hustrulid et al. 2000. gered by rain or snowmelt, they result in material flowing down Proper planning and design require a thorough under- the slopes due to shear failure or collapse of the soil structure.standing of the material properties of the waste rock or ore, Rotational circular failures. Rotational circular failureliner interface strengths in the case of a lined facility, and (mass failure along a curved failure surface) may occur withinfoundation conditions. In the case of a dump or heap leach, the waste as a result of excessive dump height, additionalgroundwater and seepage properties of the ore must also be loading induced during an earthquake, weak or fine-grainedunderstood in order to properly design these types of facili- waste materials, reduction in toe support, and/or high pore-ties. Studies would include a field investigation consisting of water pressures. Rotational failure surfaces may also extendmapping of soils and rock; drilling boreholes; monitoring well into the foundation if the soil is weak or high pore pressuresinstallation; excavating a test pit; sampling waste rock, ore, develop, such as within a deep fine-grained soil deposit. Creepand foundation materials; laboratory testing; and analyses. failure is also a type of rotational failure, with widespread rotational shearing characterized by bulging at the dump toeSlope Stability (BCMDC 1991).Slope instability and failure are major issues for all types of Base failure (spreading). Base failure may occur if amine waste dumps and heap leach operations. The risks and thin, weak base layer is placed over the foundation, especiallyenvironmental impacts of waste dump instability are a major if the foundation is inclined. If a slope wedge of the wasteconcern for both mine operators and regulators. A slope fail- dump translates laterally along a shear surface, the founda-ure in a waste structure could cause injuries and disruption of tion soils may spread and be squeezed ahead of the advancingoperations because of equipment burial or closure of an access dump toe. This phenomenon, known as foundation spreading,or haul road. Slope failure in a heap leach pile can lead to a liner may result in progressive failure of the overall dump (Vandrefailure and the potential release of pregnant solution, which 1980; BCMDC 1991).may result in contamination of groundwater resources, as well Block translation. Block translation (planar sliding)as a loss of revenue. In either case, there are clean-up and reme- may result from any of the inducing factors mentioned fordiation costs. Proper preplanning and design are imperative to rotational failure and is favored by steep foundation slopesavoid these types of costs. and a thin, weak soil cover or lined surface. The bulk of the Numerous factors affect waste dump or pile stability, dump slides as a rigid block along a plane of weakness. Thisincluding site topography, dump geometry, rate of stacking weak plane may be within the foundation soil, along the inter-and lift thickness, geotechnical properties, method of con- face between the dump and the foundation, or along a linerstruction, equipment loads, phreatic surface, and seismic interface.forces—all of which must be considered in the evaluation of Liquefaction. If the soil foundation or the waste dumpthe waste structure’s stability over its design life. Generally, itself is composed of liquefiable materials, and high pore-water
  • 5. Waste Piles and Dumps 671 Mine Mine Waste Waste Saturated/Partially Saturated Material Surface or Edge Slumping Shallow Flow Slides Mine Waste Mine Waste Rotational Circular Block Translation Mine Mine Waste Waste Weak Plan e Base Failure (Spreading) LiquefactionFigure 8.11-2 Failure modespressures exits, then liquefaction may pose a significant sta- During the investigation stage of design, the topographicbility risk. If liquefaction occurs in the foundation, the entire information gathered should include the entire drainage areadump may be translated or there may be progressive failure that may affect the dump, as well as identifying those areas(BCMDC 1991). that would be affected should a dump failure actually occur. Should a failure occur, the inclination of the dump foundationFactors Affecting Slope Stability will be an important factor in the dump stability as well as run-To properly design a mine waste dump for stability, the fol- out distance. Experience shows that foundation slopes steeperlowing details should be considered: than 25° typically result in lower factors of safety for slope stability. On the other hand, topographical features providing • Site topography and location lateral support or toe buttressing will improve the stability of • Dump geometry, rate of stacking, and lift thickness the waste dump. • Geotechnical properties of the waste, liner system (if Dump geometry and stacking method. The geometry applicable), and foundation of the waste dump depends largely on the dumping method, as • Methods of construction and equipment loading well as the topography of the site. The two common construc- • Seepage, phreatic surface level within the dump, and the tion methods for waste dumps include end dumping and stack- solution collection system ing material in lifts or layers. If the material is end-dumped • Seismic forces and liquefaction potential from the crest of the waste dump, the material will flow downThe size and complexity of the project, as well as the con- the slope and rest at or near the angle of repose, with the largersequences of dump failure, will typically control the extent particles rolling down to the toe of the dump (Couzens 1985).of the investigation performed to obtain this information. The angle of repose for mine waste rock typically falls withinThe investigation should be thorough enough to identify all the 35°-to-40° range, leading to steep side slopes. The factoradverse conditions and to provide reasonable certainty that of safety for the slope of an end-dumped waste pile is close tothe parameters used in the design are appropriate (Vandre 1.0. The slopes are generally not flattened or compacted until1980). closure of the waste dump. Site topography. Based on economics, dump-site loca- In comparison, layered or stacked dumps allow for ations are typically selected to minimize the distance between higher factor of safety to be maintained, because they are con-the waste source and the disposal area. The waste may be dis- structed in a more controlled manner from the bottom up. Theposed of in an area completely outside of the pit, or in-pit layers can be placed and compacted to increase the densitydumping may be preferred. and strength of the material. However, except for the heap
  • 6. 672 SME Mining Engineering Handbookleach piles, layered waste dumps are not always feasible, as strength envelope does not necessarily remain linear, and thisthey require relatively flat topography (Vandre 1980). nonlinearity of the strength envelope must be considered in Waste dumps constructed from end dumping are more the stability analysis.likely to have a loose, collapsible particle structure within The dominance of cobble- and boulder-sized rock frag-the dump than those constructed from the layered method. ments in typical waste rock imparts a dilatant behavior underCollapse will result in localized arching, which leads to low effective normal stresses and significant crushing of con-reduced normal pressures and shear strengths (Vandre 1980). tact points at high stresses, as demonstrated in the case of rock The exterior slopes of heap leach pads and waste dumps fill (Barton and Kjaernsli 1981). The friction angle of the rockare typically constructed as steep as practical during mining fill is strongly stress dependent and will be significantly loweroperations to maximize the tonnage contained in the dump. for material at the base of the dump (due to higher normalSlope-stability analyses are used to determine the maximum loads) than for material near the toe of the dump (under lowallowable overall slope angle, including benches, for main- loads). Barton and Kjaernsli (1981) estimated that the effec-taining stable slope conditions to the planned ultimate dump tive friction angle of rock fill increases by between 4° andheight (Breitenbach 2004). 8° for every 10-fold decrease in effective normal stress. The Smith and Giroud (2000) examined the effect of ore shear strength of rock fill is also influenced by the rock-fillplacement direction on the stability of a geomembrane-lined dry density, void ratio, unconfined compressive strength, uni-heap leach pad and concluded that stacking ore in the down- formity coefficient, maximum grain size, fines content, andgradient direction results in a less stable structure than stack- particle shape.ing in the up-gradient direction typically would. Laboratory testing of the mine waste is often too lim- Geotechnical properties—mine waste. The geotech- ited to accurately represent the potential material variabilitynical properties of mine waste materials vary significantly of a large volume of waste under various loading conditions.between projects and even between different phases of the Therefore, the shear strength of the mine waste for designsame project. The density, saturation, and shear-strength and analysis purposes must often be estimated based on vari-parameters of the materials forming the dump slope affect the ous inputs, including current laboratory test results, previousfailure mode and the calculated factor of safety (FS) against experience, the behavior of similar materials, and publishedsliding. Other useful information for design includes the par- literature (Vandre 1980; K.P. Sinha, personal communication).ticle size distribution, specific gravity, permeability, compres- Another aspect to consider during design is the effect ofsion index, soils classification, and degradation behavior of weathering on geotechnical properties. Waste materials thatthe waste materials. These parameters are generally based on were assumed to be durable may weather or be altered inlaboratory tests. However, field practices and construction some other way, which decreases slope stability. For exam-procedures are often not completely simulated in the labora- ple, weathering of feldspar-rich rock may result in formationtory for various reasons (e.g., equipment limits, time and bud- of clay, decreasing the effective friction angle and inhibitingget restraints), and therefore engineering judgment is required rapid drainage.in selecting properties for stability analyses. Verification test- Geotechnical properties—foundation. The founda-ing is often required during construction to ensure that the tion is a critical factor in the overall stability of the wasteparameters used during the design were reasonable, accurate, dump. The dump-site investigation should identify the generaland appropriate. geology of the site and any adverse geologic and soil condi- Waste rock is coarse material typically classified as cob- tions. The soil cover and rock weathering depths should bebles, rocks, or boulders with some fines. As previously stated, determined and the materials should be classified for design.the angle of repose for mine waste rock typically ranges from Particular attention should be paid to the presence of shallow35° to 40° and is based on factors such as particle size and groundwater, discharge areas, landslides, creeping slopes,shape, fall height, specific gravity, and amount of water pres- organic soils, clays, and dip slope bedrock structures (Vandreent. The density of waste rock materials typically ranges 1980). The subsurface exploration may include sampling,between 1.6 and 2.2 t/m3 (100–137 lb/ft3), depending on in-situ testing, and borehole geophysics, and should cater towhether the material is loose or compacted (Williams 2000). obtaining the critical parameters for design.In heap leach pads, for example, the ore is purposely stacked After soil and rock samples have been obtained during thein a loose state to maintain a high permeability, as required by investigation, laboratory testing should be performed to identifythe leaching process. As subsequent lifts are placed, the den- the pertinent geotechnical properties of the materials. The classi-sity of the lower lifts increases as they are compacted by mate- fication, strength, permeability, and consolidation properties ofrial placed on top, and therefore the shear strength of the lower the foundation materials, and how these properties are affectedlifts typically increase (Smith and Giroud 2000). Stacking or by time or saturation, should be determined. The shear strengthdumping mine waste in thick lifts results in significant vari- and thickness of the foundation soil is an important parameterability of the in-place density within each of these lifts. for slope stability and the dump failure mode. Permeability of Understanding the shear-strength behavior of the waste the foundation material will affect the generation of pore watermaterial is important for evaluating the slope stability of the pressures in the foundation, affecting the dump stability andwaste dump. Waste density and gradation variability, along with limiting the permissible dumping rate. Foundations consistingdifferences in normal and confining stresses (e.g., inside the pile of low-plasticity silts and clay soils have been blamed for form-versus at the toe or on the slope face), result in heterogeneous ing shear failure surfaces of several large (>10 Mt [11 millionshear strength throughout the pile. Generally, a linear-strength st]) dump failures (Zavodni et al. 1981). Consolidation param-envelope with a single friction-angle value over the entire eters are used for calculating expected settlement of the foun-range of stresses may be assumed for the stability analysis. dation; excessive settlement could have serious implications inHowever, dump heights achieved these days result in a much terms of the liner and collection system in case of heap leachwider range of normal stresses in the pile, over which the piles and dump failure in general.
  • 7. Waste Piles and Dumps 673 residual strengths because of minor strains caused by installa- tion and initial loading. Residual strength conditions may also Rock or Waste Fill 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 Drain Cover Fill soil was more than 30. Groundwater and phreatic surface. The effects of water on the stability of mine waste dumps can be difficult Geomembrane to evaluate, and measures should be taken to prevent excess Prepared water from entering the dump. In order to accurately assess Subgrade 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.Figure 8.11-3 Example of heap leach pad liner system 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. Geotechnical properties—geosynthetics. Within the last Because of the leaching process, leach pads present a combi-20 years, gold, silver, and, more recently, copper leach pads nation of extreme base pressures and high moisture conditionshave been constructed with geomembrane-lined foundations not present in other lined facilities, such as landfills (Thiel and(Breitenbach 2004). Typically, LLDPE or HDPE is used as the Smith 2004). In addition, leach pads are sometimes located inbase liner. The decision is based on the elongation, strength, and highly seismic areas, raising concerns about liquefaction dueother requirements of the application, as well as economic rea- to sudden pore-pressure buildup.sons. PVC liners have been provided in specific cases, mainly An increase in the foundation water table may signifi-for economic considerations. The liner interfaces with the over- cantly decrease the FS for a deep failure through the founda-liner (the drainage material), the subgrade, or the ore material tion material, while perched water within the dump may leaditself (in case of interlift liners) create planes of weakness in to surface failures. Flow parallel to the surface of the slopethe leach pile. An example of a geomembrane-liner system may also decrease the FS significantly.for a heap leach pad is shown in Figure 8.11-3. Slides in lined Seismic forces. In seismically active regions, the slopefacilities usually occur by wedge failure along the geomem- stability of the waste structure is also evaluated for seismicbrane interface with geotextile or low-permeability subgrade loading conditions. The seismic loading, although dynamic(Breitenbach 2004), this being the weakest link in the chain. and cyclic in nature, is generally treated as a superimposedThus, the soil–liner interface strength parameters may become equivalent set of static loads, and the stability analysis forthe most critical data for evaluating heap leach stability. The this case is referred to as the pseudostatic analysis. For thesesoil–liner interface strength depends on several factors, includ- analyses, the two-dimensional mass in the limit equilibriuming normal load, rate of applied shear, soil type, density, water slope-stability model is subjected to a horizontal acceleration,content, and drainage conditions, as well as liner thickness, which represents inertia forces due to earthquake shaking andflexibility, and texture (Sample et al. 2009). is equal to an earthquake coefficient multiplied by the accel- Just as with the waste and ore material, soil–liner interface eration of gravity. The earthquake coefficient, or pseudostaticstrengths may also exhibit a nonlinear strength envelope, with coefficient, is selected based on a specified design earthquake.the friction angle generally decreasing as the normal stress Often a percentage of the maximum design acceleration inincreases. Thus, as heap leach piles are extended to greater bedrock may be used for the pseudostatic analysis. However,heights, decreases in the interface friction angle used for the selection of an appropriate pseudostatic coefficient may relystability analysis should be considered for the liner interface. heavily on engineering judgment and is often debatable. Also, To select an appropriate minimum FS against slope fail- materials within the waste dump may undergo a significanture, the designer must consider whether peak or post-peak loss of strength during earthquake shaking, which may not(residual) strengths were used for the liner interface in the be entirely understood or defined from the laboratory test-stability analysis. One method to ensure conservative design ing. Therefore, while pseudostatic analyses are a simple andfor wedge failure of a heap leach pad is to assume post-peak convenient tool, they should serve primarily as a screening(residual) strengths for the liner system. Numerous stud- method as to whether significant displacement may occuries of shear stresses for geomembrane–soil interfaces based during the design earthquake. If a low FS is calculated in theon direct shear testing have been published, and the conclu- pseudostatic analysis (e.g., <1.0), then significant displace-sions regarding peak versus post-peak strengths have been ments may occur, and displacement (deformation) analysesmixed. Post-peak strengths as low as 50% of peak strength should be performed.have been observed for geomembrane–clay interfaces (Byrne Dynamic analyses with numerical tools provide a more1994; Stark and Poeppel 1994), while other studies indi- sophisticated alternative to pseudostatic analyses. Analysescated that no strain-softening (i.e., reduction in strength with may be performed with tools such as the finite difference pro-straining) behavior occurred (Koerner et al. 1986; Masada gram FLAC, and available finite element method and bound-et al. 1994). Valera and Ulrich (2000) recommend the use of ary element method programs. Use of these tools duringpost-peak shear strength for soil–liner interfaces in stability design may depend on project budget, design requirements,analyses of heap leach pads, because the interface may reach and available resources.
  • 8. 674 SME Mining Engineering Handbook For waste dumps, the greatest stability risk posed by Reliability. For significant structures, such as wasteearthquakes is typically liquefaction of foundation materials, dumps and heap leach pads, it is critical that sources of uncer-although liquefaction may occur in susceptible waste materi- tainty in the stability analysis be acknowledged early on andals as well. If liquefaction occurs in the foundation, the entire considered in the overall design approach. As with any proj-dump may be translated or there may be progressive failure ect, economics and other physical constraints such as space(BCMDC 1991). Liquefaction due to seismic events is typi- limitation do not always allow for an overly robust design. Incally limited to 20 m (66 ft) in depth or shallower, due to the an effort to quantify uncertainty and provide a level of con-beneficial effects of confining pressure against liquefaction fidence in the safety and reliability of a design, probabilis-susceptibility (Thiel and Smith 2004). Simplified procedures tic methods have been developed and implemented in manyto evaluate liquefaction resistance in soils have been widely slope-stability software packages. Reliability methods arediscussed in the literature (e.g., Seed and Idriss 1971; Seed often used in the design of open-pit mine slopes but not as1979; Ambraseys 1988; Suzuki et al. 1995; Arango 1996; commonly in designing heap leach pads and waste dumps.Andrus and Stokoe 1997; Olsen 1997; Youd and Noble 1997; When selecting appropriate values for the input parameters ofRobertson and Wride 1998; Youd and Idriss 2001). The paper the stability analysis, the level of uncertainty in the data andby Youd and Idriss (2001) is a summary of commonly used the assumptions that are made must be clearly identified andprocedures and provides recommendations for design. considered in the design. Simplified deformation analyses. Analyses may alsoGeneral Design Considerations be performed to evaluate seismically induced deformations.All waste dumps have some risk of instability, whether due to an The pseudostatic analysis method can be used to calculate theinadequate design process or unforeseen variability of assumed yield acceleration of the sliding mass. This yield accelera-parameters. The issue of addressing uncertainty in geotechni- tion may then be used in simplified procedures for estimatingcal design has been discussed in depth by numerous authors earthquake-induced deformations, such as those provided by(Duncan 2000; Christian 2004; Whitman 1984; Christian et al. Makdisi and Seed (1978) and Bray et al. (1998). Determination1993). The trade-off between the costs of a thorough geotechni- of acceptable deformation limits may depend on several fac-cal investigation versus the risks of design uncertainty has long tors, such as regulations, engineering judgment and previousbeen a challenging management decision in geotechnical proj- experience, and acceptable risk.ects. For mine sites, significant investment is typically made in In summary, slope failure may occur in waste dumpsexploration and estimating mineral resources, and the geology by a variety of failure modes, which include surface slump-of a mine site is often more thoroughly documented than other ing, shallow flow slides, rotational circular failures, basetypes of geotechnical projects. Nevertheless, the engineer- spreading, block translation, and liquefaction. In geo-ing properties of the soil and rocks relevant to slope stability membrane-lined heap leach pads, slides typically occur byreceive less emphasis. Baecher and Christian (2003) observed wedge failure along the critical interface of the liner system.that the areas of geotechnical concern, such as slopes and waste Engineering judgment and experience must be used whendisposal facilities, are usually associated with mine costs rather selecting the appropriate analysis method for these potentialthan revenue, and, therefore, significantly less money is devoted failure modes, as well as when selecting input parametersto their site characterization and laboratory testing. for the dump materials and foundation. The reliability of One may ignore the uncertainties involved in a design, take the stability analysis results depends on whether the designa conservative approach, rely on observational methods (Peck assumptions are representative of the actual waste dump1969), or attempt to quantify the uncertainty. Geotechnical proj- conditions.ects, in general, may include a combination of these methods. Factor of safety. The most common way to take the con- Settlementservative design approach is to require a minimum calculated Waste rock settlement occurs because of particle reorienta-FS for slope failure. The methods used to calculate the FS are tion, weathering of high clay-content materials, weakeningdescribed in detail in Chapter 8.3. The minimum FS selected for of inter-particle bonding due to water, and transport of finedesign allows for some margin of error between the assumed particles through the dump (Williams 2000). The rate of settle-conditions and those that actually exist in the field, and should ment is affected by dump height, loading rate, location withinconsider the following, as outlined by Vandre (1980): the dump, and material type (Zavodni et al. 1981). Settlement is more predictable and usually less in layered dumps than in • Consequences of instability end-dumped embankments. • Thoroughness of the geotechnical investigation During placement of the waste material, initially self- • Reliability of the design assumptions weight settlement may occur or crest settlement may happen • Ability to predict adverse conditions because of compaction or surface sloughing from oversteep- • Possible construction deviations from design ening (Zavodni et al. 1981). After waste placement, primary • Engineering judgment based on past experience settlement and creep settlement occur at a decreasing rate withThe FS is calculated for normal loading conditions, as well as time and have been shown to continue for more than 10 yearsfor seismic loading when the project is located in a seismically after dump construction (Williams 2000). The majority of theactive area. In general, a minimum FS of 1.3 (for shallow fail- settlement, however, occurs within the first months after con-ures) to 1.5 (for more significant failures) is considered accept- struction (Zavodni et al. 1981).able for long-term (static) conditions (NAVFAC 1982; Vandre As the dump materials become saturated, there is a reduc-1980). The FS required for extreme adverse conditions, such as tion in strength, and collapse settlement may occur (Williamsthe design seismic event or temporary slopes, is typically lower 2000), especially in loose, end-dumped waste piles. Thethan that required for long-term stability of final waste slopes, potential for collapse can be minimized with adequate com-and a range of 1.1 to 1.3 is generally accepted. paction (Vandre 1980).
  • 9. Waste Piles and Dumps 675 Under dry conditions, settlements of 0.3% to 7% of the (1938, 1956), Rawls and Brakensiek (1989), Alyamani andwaste dump height have typically been reported (Naderian Sen (1993), and Sperry and Pierce (1995).and Williams 1996). However, settlements of more than 20% Design and construction elements can significantly affectof the total dump height have also been documented (Zavodni seepage and drainage through waste dumps. The top surfaceet al. 1981). of the waste dump should be graded to prevent surface water Various techniques can be used to monitor deforma- from flowing onto the slopes. Since the 1990s, geosynthetictions of waste dumps with time. These methods include on- raincoats have been used on heap leach pads in high-rainfallsite inspections, surveying, photogrammetry, extensometers, areas to minimize storm runoff flows into the collection pondsinclinometers, settlement cells, and laser beacons (McCarter (Breitenbach 2004; Smith 2008). These raincoats also serve1985b). The appropriate monitoring methods are selected as protection against erosion and damage to the agglomeratesbased on the waste dump height, material, and method of con- (Breitenbach and Smith 2007a).struction. Robertson (1982) describes the development and When waste rock is dumped, the coarsest fraction oftenoperation of effective waste dump monitoring systems. ends up at the bottom of the dump, creating a rock drain at the base. Depending on topographic details, such rock-fillSeepage and Drainage drain sections can be significantly large and a useful tool forThe same fundamental seepage principles used in the design controlling flow, especially in places such as valley bottomsof earth dams and levees should be considered in the design where a watercourse already passes. If the flow capacity of theof waste piles and tailings storage facilities (Cedergren 1989). rock drain is exceeded, the phreatic surface may rise, loweringUnderstanding fluid flow through waste dumps is important the stability of the waste dump. Therefore, understanding thefor evaluating both stability and environmental risks. Most hydraulic behavior of rock drains is important for waste dumpmine waste dumps and leach piles are usually unsaturated, design. Hansen et al. (2005) have provided some insight intoand accurate seepage and contaminant transport modeling this issue. Additionally, the Rock Drain Research Programrequires determining unsaturated soil properties (Fredlund et was completed in Canada to study the characteristics of rockal. 2003). However, unsaturated soil behavior is less under- drains and their environmental effects (Fitch et al. 1998).stood than saturated behavior, and unsaturated properties and In heap leach pads, a properly designed and operatingflow modeling are not always included as part of the waste solution collection and liner system is critical for retrievingdump and heap leach design. In fact, most geotechnical seep- pregnant leach solution, as well as for controlling phreaticage calculations are based on saturated soils. The fundamen- surface levels within the heap. The most versatile and pre-tals of seepage through porous media are explained in detail ferred liner system currently used for heap leach pads consistsin Chapter 8.2. The soil properties used in unsaturated flow of a low-permeability soil layer overlain by a geomembranemodeling are briefly introduced here. with a drainage layer of crushed rock (overliner) on top of it The soil parameters used in unsaturated flow modeling (Breitenbach 2000). However, in the drier and remote areasare derived from nonlinear equations using laboratory test of South America, the geomembrane with the overliner isdata and are generally referred to as the hydraulic conductiv- generally considered adequate. The geomembrane liners areity function and the water storage function. To model seepage specified by their material type, thickness, and surface rough-through an unsaturated pile, these functions are required for ness, and are designed on the basis of initial and final loadingeach material in the flow path (Fredlund et al. 2003). Various conditions and the expected strains produced in the liner. Amethods of determining unsaturated soil parameters for input properly selected overliner or drainage material and a strin-in waste dump models are described in detail in Fredlund et gent construction quality-assurance program during installa-al. (2003). Some of these methods are also summarized here. tion are crucial to performance of a liner system. The overliner The hydraulic conductivity function (HCF) represents material is specified in terms of gradation, or maximum andthe conductivity of the unsaturated material at various water minimum particle size, in order to avoid puncturing the geo-contents. The HCF can be measured in the laboratory or esti- membrane, provide adequate support to the leachate collec-mated using the methods of Brooks and Corey (1964), van tion pipes, and facilitate adequate drainage. Key concerns forGenuchten (1980), Campbell (1973), and Fredlund and Xing liner system selections are summarized in Table 8.11-1.(1994). Many software packages allow users to select one ofthese methods when entering input parameters into the seep- Erosionage model. Soil–water characteristic curves (SWCCs) rep- Erosion is a natural process that cannot be stopped, only con-resent the relationship between the water content of the soil trolled. Erosion on material stacked at the angle of reposeand the soil suction, and can be measured in the laboratory can be hazardous, because of the risk of material failure andusing a variety of devices. The SWCC is also used to deter- catastrophic movement downslope, as well as sedimenta-mine the water storage function, which relates the change in tion and contamination of downstream waters. Reclamationwater content to the change in soil suction. This relationship and closure of waste dumps or piles usually requires regrad-becomes highly nonlinear as the soil desaturates (Fredlund ing for reduction in slope and seeding of vegetation. Both ofet al. 2001). these efforts will dramatically reduce erosion. Large dumps, The saturated hydraulic conductivity represents the limit- mounds, or piles are designed to control and collect runoff anding condition for unsaturated flow and is generally measured prevent material failure. The final reclaimed landform is alsoas such in the laboratory. However, if laboratory data are not an important element in long-term erosion control.available, there are multiple methods for estimating the satu- It is much easier and less costly to avoid contaminationrated hydraulic conductivity of a material indirectly. The for- before it occurs than to clean up after the fact. Because ofmulas typically relate the hydraulic conductivity to the grain this, regulators and industry are designing facilities that, fromsize distribution of the material. Some of the available meth- their inception, reduce the potential for harmful effects to theods include those by Hazen (1892), Kozeny (1927), Carman environment.
  • 10. 676 SME Mining Engineering HandbookTable 8.11-1 Key concerns for liner system selection USLE, and the Revised USLE. However, this model is forEngineering and Design Concerns Construction Concerns agricultural situations where the slopes are much flatter than those used for waste rocks dumps. Several computer codes • Liquid containment: liner integrity • Shipping to site: container rolls • Operational and closure stability: versus boxes such as SIBERIA and CEASAR are being used that utilize interface friction strength, • Installation, deployment, and digital terrain models and mathematical algorithms to pre- flexibility, nonplanar anchorage seaming dict both erosion and deposition. The various versions of the • Chemical and temperature • Ease of repair: local liner USLE calculate erosion loss only. These codes have their own compatibility expertise, equipment disadvantages as well, such as the need for rigorous calibra- • Subgrade and overliner: • Site access: storage area, tion (Hancock 2009). gradation, permeability, lift perimeter access, slopes placement, compaction, surface • Dynamic and static loading preparation conditions: cover fill, roads, Leach Pads • Long-term exposure: ultraviolet traffic, ultimate load Leach pads are designed to allow leaching solution to pass (UV), oxidation/aging, animals, • Weather and climate: UV, wind, through the stacked material, which is then collected on a liner and other biological attack rain, ice, temperature changes, system with collection pipes to be conveyed to the process • Puncture resistance: subgrade stress cracking, expansion/ facility. These systems catch and collect all meteoric water as and overliner fill type contraction • Flexibility: differential foundation • Cold weather installation and well, and the ponds must be sized to capture a design storm. A settlement, installation, puncturing cover: frozen subgrade, safety detailed water balance is usually calculated to size the ponds • Tensile, tear, and seam strength: • Grade change adaptability: and to understand the water needs of the pad and the process liner uniformity, thickness steep grade gravitational forces, facility. In a properly designed leach pile, the material stacked • Contact between composite corners, benches, pipe boots on the pad should have a high enough infiltration rate to prevent geomembrane liner and • Tie-ins for expansion facilities excessive solution flowing on the surface and on the side slopes. low-permeability clayey soil • Overall cost to construct: subgrade materials, labor, schedule Any such flow is captured in lined trenches around the pad.Source: Mark E. Smith and RRD International (Adapted from Smith 2008). ACID ROCK DRAINAGE Acid rock drainage (ARD) occurs whenever unoxidized sul- fide material is exposed to the atmosphere and water. Dumps, The erosion potential of the waste needs to be charac- piles, or stacks of material are particularly susceptible toterized, and, where possible, higher erosion-potential mate- ARD due to the permeability of the material, availability ofrial should be capped with a material that has lower erosion atmospheric oxygen, and amount of material that can comepotential. An example is the sodic waste rocks in Australia. in contact with meteoric and surface water flow. This topic is covered in detail in Chapter 16.5 and is briefly touched uponRock Dumps here.Rock dumps are generally designed with a slight grade on the Design criteria for ARD prevention for large dumps ortop deck to allow rainfall to flow to a collection system and be mounds include chemical characterization of the material,conveyed to a collection pond. A similar system is placed at acid–base accounting (ABA), compartmentalizing the dump/the toe of the dump as well. The water in the ponds is tested pile into discrete cells for material buffering control, andregularly and treated if required. The collection systems are run-on/runoff control.normally part of the larger mine-wide stormwater controlplan. A good stormwater management plan, which will pre- Chemical Characterizationvent ponding of water against the safety berm at the crest of Some large mines in Nevada (United States) use ABA and buildthe dump, helps avoid washouts of the slide slopes on active the waste rock facility to confine potentially acid-generatingrock dumps. Generally, the operational toe of a dump will be material in cells composed of acid-consuming material. Thisoffset to accommodate the ultimate toe of the reclaimed dump, creates a net acid-neutralizing environment. In order to do this,at a 2:1 (horizontal to vertical) or 3:1 slope, allowing room for good characterization of the material needs to be completed.minor ravel and washouts on the side slopes. With modern production analytical capability and mine dis- Another important erosion consideration for slopes is patch systems, material that is not ore can be characterized andtheir shape. Concave slopes can reduce erosion (McPhail and routed to a specific location on the dump. If the material bal-van Koersveld 2006). Naturally eroded features are concave ance is not net acid neutralizing based on the ABA, the dumpsin shape, and by emulating this with wider catch benches on may need to be placed on a low-permeability layer and cappedthe lower elevations of the dump or pile, eroded material from upon closure.the upper levels is slowed and deposited on the lower levels. Waste characterization can also include tests for total and The configuration of the dump design is an important soluble metals, such as the U.S. Environmental Protectionconsideration in stormwater management planning. The run- Agency’s (EPA’s) toxicity characteristic leaching procedureon controls for a valley fill are much more complicated than and the State of California’s waste extraction test. Testing forfor a ridge crest or heaped dump, as the entire design storm pH in water flowing from dumps is important, because lower-flow of the drainage needs to be conveyed around the dump pH water is more likely to contain metals that have beenor pile. In all cases it is important to keep rainfall from native leached out of the waste rock.ground separate from what falls on the dump, as the latter maybe contaminated, whereas the former should not be. Run-On and Runoff Erosion has typically been modeled in civil applications The run-on component of meteoric water is controlled basedusing the U.S. Department of Agriculture Universal Soil Loss on a stormwater management plan. Stormwater collectionEquation, of which there are several variations, including the systems need to be well thought out and based on the mineoriginal Universal Soil Loss Equation (USLE), the Modified plan, topography, and required maintenance. A mine-wide
  • 11. Waste Piles and Dumps 677stormwater management plan is required for mines in theUnited States, and these plans are site specific. The runoff component of meteoric water is controlled bythe waste facility’s design and may be included in the over-all stormwater plan. Important considerations for the runoffplan include material classification, treatment requirements,and appropriate sizing of ponds and catchments. In the caseof a waste rock dump, water that infiltrates the dump will becontained at the toe of the dump. Only the surface flow will becontained on the top.CLOSURE AND RECLAMATIONClosure and reclamation of heaps and piles are necessary forenvironmental, ecological, and health and safety reasons, andin some instances for economic reasons such as to recoverbonds posted during the permitting or construction phase. The closure process typically requires detoxification ofheap leach facilities, and reclamation usually means decreasingthe slopes of heaps and dumps, covering the area with growth Figure 8.11-4 Typical cover for uranium mill tailingsmedia, and reseeding vegetation where appropriate. Rockdumps that have acid drainage issues require ongoing treat-ment of the water flowing from these facilities. Detoxification into the odorless and colorless gas radon-222, which has a halfof leach pads is required in the United States and typically life of 3.8 days. Inhalation of Ra-226 is known to lead to lunginvolves lengthy periods of rinsing to reduce the cyanide or cancer. Because of the radioactive properties of uranium tail-other toxic content of solutions circulating in the heap. ings, the standard practice is to design the impoundments for Sloping the waste facility to moderate slopes of 3:1 long-term disposal, typically 1,000 years. To avoid erosion(horizontal/vertical) or flatter is typically required. Capping over this type of time frame, slopes of the piles need to be min-of dumps and leach facilities with semipermeable capping imized, and natural forms of containment should be utilized.material allows for the establishment of a growth medium for In the United States, the design of uranium tailingsplanting vegetation, which is the best way to prevent erosion. impoundments and covers falls under regulations in theAs discussed previously, the final landform should be concave Uranium Mill Tailings Radiation Control Act of 1978. Theseif possible, with shallower slopes at the base of the facility. regulations require that a cover be designed to produce reason-In addition, many agencies are requiring certain randomness able assurance that the radon-222 release rate does not exceedto the final landform, avoiding stretches of linear slopes and 20 pCi/m2/s for a period of 1,000 years to the extent reasonablyridges. Many operations do concurrent reclamation, where achievable, and in any case for at least 200 years when aver-they slope and plant segments of their facilities to reduce the aged over the disposal area for at least a 1-year period. In someoverall operating footprint and possibly to recover a portion cases at inactive sites, the regulations allow for a radon concen-of their bond. tration of <0.5 pCi/L above the background concentration. The An important factor to consider is the longevity of the regulations also state that the tailings should be disposed of inclosure system. Many waste facilities are looking at closure a manner that no active maintenance is required to preserve theperiods in hundreds of years, and waste facilities containing conditions of the site.radon or other radioactive material are looking at thousands The typical cover includes, from bottom to top, the fol-of years of containment. Natural material will last longer than lowing layers (thicknesses are variable):synthetic materials, and this must be considered in the design • 0.61-m (2-ft) radon/infiltration barrier consisting of clayof a facility handling radioactive material. • 0.15-m- (0.5-ft-) thick capillary break layer consisting of coarse sand/fine gravelRADIOACTIVE WASTE ROCK • 1.07-m- (3.5-ft-) thick water storage soil layer consistingSome waste rock can be radioactive and may require special of fine-grained soildesign considerations. Uranium mill tailings have received a • 0.15-m- (0.5-ft-) thick surface erosion protection layerlot of attention because of their radioactive properties and as a (soil/rock mixture) consisting of 80% soil, 20% riprapresult are designed for long-term disposal. Phosphate mining bouldersand processing produce phosphogypsum tailings, which may • Vegetated surface for water balance controlalso contain trace levels of radioactive material (FIPR 2010). Phosphogypsum tailings have been used as fertilizers The actual thickness of the radon/infiltration barrier in a spe-and for other uses. However the EPA has banned the use of cific case would be based on calculations of radon flux at thephosphogypsum with an average radium-226 concentration surface of the compacted soil layer. An example design isof >10 pCi/g (picocuries/gram) for agricultural application shown in Figure 8.11-4. The soil type would be selected from(FIPR 2010). As a result of phosphate mining, currently 0.909 available borrow sources that can satisfy performance require-billion t (1 billion st) of phosphogypsum waste materials are ments for permeability and radon attenuation. The compactionstacked in the state of Florida, and about 27.3 million new requirements would be determined with tests and calculationsmetric tons (30 million short tons) are generated each year. of saturated hydraulic conductivity and radon attenuation. Uranium tailings contain low levels of radioactive A uranium mill tailings cover calculator is availableradium-226. Ra-226 has a half life of 1,620 years and decays on-line at www.wise-uranium.org/ctch.html. This calculator
  • 12. 678 SME Mining Engineering Handbookdetermines the radon fluxes and concentrations in multilayer Breitenbach, A. 2004. Improvement in slope stability perfor-uranium mill tailings and cover systems, and optimizes the mance of lines heap leach pads from design to operationcover thickness to satisfy a given flux constraint. The cal- and closure. GFR Eng. Solutions 22(1).culator is a clone of the RAECOM (Radiation Attenuation Breitenbach, A., and Smith, M. 2007a. Geomembrane raincoatEffectiveness and Cover Optimization with Moisture Effects) liners in the mining heap leach industry. Geosynth. IFAIcode (Rogers and Nielson 1984). The input data include 25(2):32–39. Breitenbach, A., and Smith, M. 2007b. La historia de las • Radium-226 activity concentration (if the value is Geomembranas en la Industria Minera. Lima, Peru: unknown, it can be estimated from the grade of ore pro- Mineria and Medio Ambiente. cessed in the uranium mill); Brooks, R., and Corey, A. 1964. Hydraulic Properties of • Radon-222 emanation fraction (the fraction of the total Porous Media. Hydrology Paper No. 3. Fort Collins, CO: amount of Rn-222 produced by radium decay that escapes Colorado State University. from the soil particles and gets into the pores of the soil); Byrne, R. 1994. Design issues with strain-softening inter- • Radon-222 effective diffusion coefficient; faces in landfill liners. In Proceedings of Waste Tech ’94 • Porosity; Landfill Technology, Charleston, SC. • Moisture content; and Caldwell, J., and Moss, A. 1985. Simplified stability analysis. • Minus #200 sieve fraction. In Design of Non-Impounding Mine Waste Dumps. EditedTypically, the effective diffusion coefficient of radon in by M.K. McCarter. New York: SME-AIME. pp. 49–61.unconsolidated soil material with low moisture content is in Campbell, J. 1973. Pore pressures and volume changes inthe order of 1.0–6 m2/s (1.08–5 ft2/s). The upper limit is rep- unsaturated soils. Ph.D. thesis, University of Illinois atresented by the radon diffusion coefficient in open air, which Urbana-Champaign.is approximately 1.1 # 10–5 m2/s (1.18–4 ft2/s). At the lower Carman, P.C. 1938. The determination of the specific surfaceextreme, in a fully saturated soil material, the radon diffusion of powders. J. Soc. Chem. Ind. Trans. 57:225.coefficient may be as low as 1.0–10 m2/s (1.08–9 ft2/s). Carman, P.C. 1956. Flow of Gases Through Porous Media. London: Butterworths Scientific Publications.ACKNOWLEDGMENTS Cedergren, H. 1989. Seepage, Drainage, and Flow Nets, 3rdThe authors thank the following for their assistance in prepar- ed. New York: John Wiley and Sons.ing this chapter: Krishna Sinha, corporate technical director, Christian, J. 2004. Geotechnical engineering reliability:who acted as technical reviewer; and Peter Holland, senior How well do we know what we are doing? J. Geotech.geologist, for the uranium section. Geoenviron. Eng. 130(1):985–1003. 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