chapter 8.11 waste piles and dumps

667

CHAPTER 8.11

Waste Piles and DumpsMarc Orman, Rich Peevers, and Kristin Sample

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 reclas-sified 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.

HISTORIC PERSPECTIVEEver since humans began to extract materials of value from rocks, waste or other lesser-valued material has been left behind after the extraction process. For early miners, waste dumping was simply a matter of pushing the waste out of the way, either down a slope or to any other available area. Frequently, these waste materials ended up in drainage basins, rivers, and lakes where they caused environmental harm. In 1884 in California (United States), hydraulic mining was essentially outlawed by the Sawyer decision because the mine waste in the rivers had led to flooding after streams and rivers became choked with solids. This law, handed down by the Ninth Circuit Court in the case of Woodruff v. North Bloomfield Gravel Mining Company, became one of the first environmental decisions in the United States (U.S. Circuit Court 1884).

Over time, other regulations and laws have emerged, and standard practices have evolved to minimize the environmen-tal damage and potential hazards associated with the disposal of mining waste. Enlightened mining companies now deal with their waste products in a responsible manner, especially when negative impacts on the public may result because of improper disposal. Nevertheless, improper disposal and han-dling of mine waste continue to pose environmental hazards across the world. While the hazards associated with mine waste disposal have decreased in most developed countries, it

continues to be problematic in regions where regulations and environmental laws are not strict and enforcement is lax.

A significant contributor to mine waste dumps is heap leaching, which is a relatively new form of mining where low-grade ore is piled over large surface areas and irrigated with solutions. The resulting pregnant solution is then pro-cessed to recover the desired commodity. After the leaching is completed, the leached ore becomes a waste product. On a permanent pad, the ore material is stacked and leached in lifts until the pile reaches the final design height. Modern leach pad facilities can be hundreds of meters high and cover thousands of square meters in area. Alternatively, on a dynamic pad, a single thin lift (5 to 10 m [16.4 to 32.8 ft]) of ore is stacked and leached on the pad at any one time, after which the leached ore is removed and stored in a waste dump. In the past, environ-mental issues were not a main consideration for heap leaching. However, in response to environmental regulation, facilities have evolved, and many now utilize geosynthetic liners incor-porating a leak-detection provision.

The earliest full-size leaching projects in the United States were for copper in the form of dump leaching with natural con-tainment. Subsequently, with the introduction of cyanide for leaching gold and silver, soil liners came in vogue in the late 1970s to the mid-1980s (Breitenbach and Smith 2007b). Since then, use of geosynthetic clay liner, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and polyvinyl chloride (PVC), as well as asphalt impregnated geotextiles and a few others, have become the standard liner materials for heap leaching.

TYPES OF WASTE PILES, DUMPS, AND HEAPSThis 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

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

668 SME Mining Engineering Handbook

as the resulting pregnant solutions) and the site’s hydrology to ensure that natural water resources are adequately protected.

ConfigurationsCollectively, waste pile, dump, stockpile, or a leach heap can be referred to as waste structures. As such, their layout gener-ally 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 valley-fill 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 pond-ing. 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 val-ley is not completely filled up-gradient. The structure is usu-ally 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 cross-valley or sidehill configurations, this type of structure is typi-cally not used to impound fine-grained material or water. In some cases, one or both sides of the ridge may require bench-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 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 HeapsLeach heaps consist of low-grade ores spread or stacked on large platforms where the pile is irrigated with leaching solu-tion 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 eco-nomic recovery of the contained commodity (a process known as dump leaching). The same procedure is often used for secondary recovery from leached ores. Dumps are generally placed on natural soil or rock subgrade surfaces that have been demonstrated to have some degree of natural solution contain-ment and are normally located on sloping ground or in a valley to promote drainage to the toe.

SidehillImpoundment

Sidehill

Cross-valleyImpoundment

Ridge

Heaped

Diked Pond

Incised Pond

Combination

Cross-valley

Valley-fill

Source: Zahl et al. 1992.Figure 8.11-1 Mine dump configurations

Waste Piles and Dumps 669

StockpilesThe 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 mate-rials so they may be recovered at some later time as economi-cally 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 DepositsDuring placer mining for gold or aggregates, the practice of washing sand and gravel to recover minerals can produce tail-ings with particle sizes ranging from coarse to fine (<75 µm) and wash water, which should be treated. The coarse waste fraction can be disposed of using one of the methods previ-ously 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 ris-ing 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 DUMPSWaste 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 distur-bance 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 mas-sive slides, which have buried communities.

Planning waste disposal facilities requires evaluating the regulatory constraints, identifying an appropriate site, design-ing the structural and environmental integrity of the facility, developing an operating and maintenance plan, and develop-ing 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 QualityWater quality impact issues associated with waste, unlined dumps, or poorly constructed heap leach facilities can be a major environmental concern. Waste rock should be thor-oughly 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 col-lected before construction begins. These measurements and samples should be collected throughout the year so that sea-sonal fluctuations can also be monitored and effectively evalu-ated. 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 con-stituents 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 sam-pling 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 proto-col 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 DisturbanceWherever mine waste is placed, the natural environment is changed, and the process is therefore classified as land distur-bance. The initial disturbance creates the potential for sedi-mentation 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 rec-lamation requirements. The U.S. Office of Surface Mining requires restoration to approximate original contours for sur-face coal mining. These requirements can add considerable cost to final closure.

Since most waste structures are not compacted, the vol-ume 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 ImpactsVisual 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 per-formed routinely in many areas of the world as part of initial mine permitting.

In areas where the color of the rock blends with the natu-ral 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 drain-ages 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 compo-nents, 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 typi-cally include several different ultimate elevations and possible configurations of the waste dump or heap.

DESIGN OF WASTE DUMPSThis 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 pub-lications and from several other references cited throughout this chapter: McCarter 1985a, 1990; and Hustrulid et al. 2000.

Proper planning and design require a thorough under-standing 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 facili-ties. 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 StabilitySlope 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 fail-ure 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 reme-diation 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 con-struction, 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 ModesThe basic failure modes of waste dumps must be considered during the stability evaluation and design. Detailed descrip-tions of identifiable waste dump failure modes and appro-priate 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, oversteep-ening 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 weak-ness 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 trig-gered 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 pore-water 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 founda-tion 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 inter-face 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 sta-bility risk. If liquefaction occurs in the foundation, the entire dump may be translated or there may be progressive failure (BCMDC 1991).

Factors Affecting Slope StabilityTo properly design a mine waste dump for stability, the fol-lowing 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 con-sequences 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 loca-tions are typically selected to minimize the distance between the waste source and the disposal area. The waste may be dis-posed 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 run-out 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 construc-tion methods for waste dumps include end dumping and stack-ing 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 con-structed 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

MineWaste

MineWaste

Saturated/PartiallySaturated Material

MineWaste

MineWaste

MineWaste

MineWaste

Block Translation

Shallow Flow Slides

Rotational Circular

Surface or Edge Slumping

Weak Plane

LiquefactionBase Failure (Spreading)

Figure 8.11-2 Failure modes


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 main-taining 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 down-gradient direction results in a less stable structure than stack-ing in the up-gradient direction typically would.

Geotechnical properties—mine waste. The geotech-nical 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 par-ticle size distribution, specific gravity, permeability, compres-sion 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 labora-tory for various reasons (e.g., equipment limits, time and bud-get restraints), and therefore engineering judgment is required in selecting properties for stability analyses. Verification test-ing 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 cob-bles, 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 pres-ent. 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 den-sity of the lower lifts increases as they are compacted by mate-rial 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 vari-ability 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 frag-ments in typical waste rock imparts a dilatant behavior under low effective normal stresses and significant crushing of con-tact 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 effec-tive friction angle of rock fill increases by between 4° and 8° for every 10-fold decrease in effective normal stress. The shear strength of rock fill is also influenced by the rock-fill dry density, void ratio, unconfined compressive strength, uni-formity coefficient, maximum grain size, fines content, and particle shape.

Laboratory testing of the mine waste is often too lim-ited 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 vari-ous 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 exam-ple, weathering of feldspar-rich rock may result in formation of clay, decreasing the effective friction angle and inhibiting rapid drainage.

Geotechnical properties—foundation. The founda-tion 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 condi-tions. 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 classi-fication, 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 form-ing shear failure surfaces of several large (>10 Mt [11 million st]) dump failures (Zavodni et al. 1981). Consolidation param-eters are used for calculating expected settlement of the foun-dation; 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 constructed 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 rea-sons. PVC liners have been provided in specific cases, mainly for economic considerations. The liner interfaces with the over-liner (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 geomem-brane 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, includ-ing 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 fail-ure, 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 stud-ies of shear stresses for geomembrane–soil interfaces based on direct shear testing have been published, and the conclu-sions 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 indi-cated 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 installa-tion 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 combi-nation 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 signifi-cantly decrease the FS for a deep failure through the founda-tion 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 accel-eration 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 test-ing. 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 displace-ments 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 pro-gram FLAC, and available finite element method and bound-ary element method programs. Use of these tools during design may depend on project budget, design requirements, and available resources.

PreparedSubgrade

Geomembrane

Drain Cover Fill

Rock or Waste Fill

Figure 8.11-3 Example of heap leach pad liner system


For waste dumps, the greatest stability risk posed by earthquakes is typically liquefaction of foundation materials, although liquefaction may occur in susceptible waste materi-als 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 typi-cally 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 ConsiderationsAll 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 geotechni-cal 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 geotechni-cal investigation versus the risks of design uncertainty has long been a challenging management decision in geotechnical proj-ects. 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 engineer-ing 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 proj-ects, in general, may include a combination of these methods.

Factor of safety. The most common way to take the con-servative 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 investigation• 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 fail-ures) to 1.5 (for more significant failures) is considered accept-able 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 uncer-tainty in the stability analysis be acknowledged early on and considered in the overall design approach. As with any proj-ect, 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 con-fidence in the safety and reliability of a design, probabilis-tic 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 accelera-tion 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 fac-tors, 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 slump-ing, shallow flow slides, rotational circular failures, base spreading, block translation, and liquefaction. In geo-membrane-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.

SettlementWaste rock settlement occurs because of particle reorienta-tion, 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 settle-ment 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 self-weight settlement may occur or crest settlement may happen because of compaction or surface sloughing from oversteep-ening (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 con-struction (Zavodni et al. 1981).

As the dump materials become saturated, there is a reduc-tion 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 com-paction (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 deforma-tions of waste dumps with time. These methods include on-site 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 con-struction. Robertson (1982) describes the development and operation of effective waste dump monitoring systems.

Seepage and DrainageThe 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 under-stood 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 seep-age calculations are based on saturated soils. The fundamen-tals 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 conductiv-ity 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 esti-mated 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 seep-age model. Soil–water characteristic curves (SWCCs) rep-resent 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 deter-mine 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 limit-ing 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 satu-rated hydraulic conductivity of a material indirectly. The for-mulas typically relate the hydraulic conductivity to the grain size distribution of the material. Some of the available meth-ods 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 study 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 pre-ferred 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 rough-ness, 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 strin-gent construction quality-assurance program during installa-tion 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 geo-membrane, provide adequate support to the leachate collec-tion pipes, and facilitate adequate drainage. Key concerns for liner system selections are summarized in Table 8.11-1.

ErosionErosion is a natural process that cannot be stopped, only con-trolled. 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 sedimenta-tion and contamination of downstream waters. Reclamation and closure of waste dumps or piles usually requires regrad-ing 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.


The erosion potential of the waste needs to be charac-terized, and, where possible, higher erosion-potential mate-rial should be capped with a material that has lower erosion potential. An example is the sodic waste rocks in Australia.

Rock DumpsRock 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 pre-vent 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 slope, 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 run-on 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 pre-dict 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 calibra-tion (Hancock 2009).

Leach PadsLeach 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 DRAINAGEAcid rock drainage (ARD) occurs whenever unoxidized sul-fide 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, and run-on/runoff control.

Chemical CharacterizationSome 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 dis-patch systems, material that is not ore can be characterized and routed to a specific location on the dump. If the material bal-ance 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 lower-pH water is more likely to contain metals that have been leached out of the waste rock.

Run-On and RunoffThe 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

Table 8.11-1 Key concerns for liner system selection

Engineering and Design Concerns Construction Concerns

• Liquid containment: liner integrity• Operational and closure stability:

interface friction strength, flexibility, nonplanar anchorage

• Chemical and temperature compatibility

• Subgrade and overliner: gradation, permeability, lift placement, compaction, surface preparation

• Long-term exposure: ultraviolet (UV), oxidation/aging, animals, and other biological attack

• Puncture resistance: subgrade and overliner fill type

• Flexibility: differential foundation settlement, installation, puncturing

• Tensile, tear, and seam strength: liner uniformity, thickness

• Contact between composite geomembrane liner and low-permeability clayey soil subgrade

• Shipping to site: container rolls versus boxes

• Installation, deployment, and seaming

• Ease of repair: local liner expertise, equipment

• Site access: storage area, perimeter access, slopes

• Dynamic and static loading conditions: cover fill, roads, traffic, ultimate load

• Weather and climate: UV, wind, rain, ice, temperature changes, stress cracking, expansion/contraction

• Cold weather installation and cover: frozen subgrade, safety

• Grade change adaptability: steep grade gravitational forces, corners, benches, pipe boots

• Tie-ins for expansion facilities• Overall cost to construct:

materials, labor, schedule

Source: Mark E. Smith and RRD International (Adapted from Smith 2008).


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 over-all 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 RECLAMATIONClosure 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 treat-ment 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 ROCKSome 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 tail-ings, 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 min-imized, 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 reason-able 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 aver-aged over the disposal area for at least a 1-year period. In some cases at inactive sites, the regulations allow for a radon concen-tration 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 site.

The typical cover includes, from bottom to top, the fol-lowing 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 spe-cific 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 require-ments 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

Figure 8.11-4 Typical cover for uranium mill tailings


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 cal-culator 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 pro-cessed 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.

Typically, the effective diffusion coefficient of radon in unconsolidated soil material with low moisture content is in the order of 1.0–6 m2/s (1.08–5 ft2/s). The upper limit is rep-resented by the radon diffusion coefficient in open air, which is approximately 1.1 # 10–5 m2/s (1.18–4 ft2/s). At the lower extreme, in a fully saturated soil material, the radon diffusion coefficient may be as low as 1.0–10 m2/s (1.08–9 ft2/s).

ACKNOWLEDGMENTSThe authors thank the following for their assistance in prepar-ing this chapter: Krishna Sinha, corporate technical director, who acted as technical reviewer; and Peter Holland, senior geologist, for the uranium section.

REFERENCESAlyamani, M., and Sen, Z. 1993. Determination of hydrau-

lic conductivity from complete grain-size distribution curves. Ground Water 31(4):551–555.

Ambraseys, N. 1988. Engineering seismology. Earthquake Eng. Struct. Dyn. 17:1–105.

Andrus, R., and Stokoe, K. 1997. Liquefaction Resistance Based on Shear Wave Velocity. Technical Report NCEER-97-0022. Buffalo, NY: National Center for Earthquake Engineering Research, State University of New York at Buffalo. pp. 89–128.

Arango, I. 1996. Magnitude scaling factors for soil liquefac-tion evaluations. J. Geotech. Eng. 122(11):929–936.

Baecher, G., and Christian, J. 2003. Reliability and Statistics in Geotechnical Engineering. New York: John Wiley and Sons.

Barton, N., and Kjaernsli, B. 1981. Shear strength of rockfill. J. Geotech. Eng. 107(GT7):873–891.

BCMDC (British Columbia Mine Dump Committee). 1991. Investigation and Design of Mine Dumps–Interim Guidelines. Report prepared for B.C. Ministry of Energy, Mines and Petroleum Resources by Piteau Associates Engineering. Victoria, BC: BCMDC.

Bray, J., Rathje, M., Augello, A., and Merry, S. 1998. Simplified seismic design procedure for geosynthetic-lined, solid-waste landfills. Geosynth. Int. 5(1-2):203–235.

Breitenbach, A. 2000. Heap Leach Pad Design and Construction Practices in the 21st Century. Littleton, CO: SME.

Breitenbach, A. 2004. Improvement in slope stability perfor-mance of lines heap leach pads from design to operation and closure. GFR Eng. Solutions 22(1).

Breitenbach, A., and Smith, M. 2007a. Geomembrane raincoat liners in the mining heap leach industry. Geosynth. IFAI 25(2):32–39.

Breitenbach, A., and Smith, M. 2007b. La historia de las Geomembranas en la Industria Minera. Lima, Peru: Mineria and Medio Ambiente.

Brooks, R., and Corey, A. 1964. Hydraulic Properties of Porous Media. Hydrology Paper No. 3. Fort Collins, CO: Colorado State University.

Byrne, R. 1994. Design issues with strain-softening inter-faces in landfill liners. In Proceedings of Waste Tech ’94 Landfill Technology, Charleston, SC.

Caldwell, J., and Moss, A. 1985. Simplified stability analysis. In Design of Non-Impounding Mine Waste Dumps. Edited by M.K. McCarter. New York: SME-AIME. pp. 49–61.

Campbell, J. 1973. Pore pressures and volume changes in unsaturated soils. Ph.D. thesis, University of Illinois at Urbana-Champaign.

Carman, P.C. 1938. The determination of the specific surface of powders. J. Soc. Chem. Ind. Trans. 57:225.

Carman, P.C. 1956. Flow of Gases Through Porous Media. London: Butterworths Scientific Publications.

Cedergren, H. 1989. Seepage, Drainage, and Flow Nets, 3rd ed. New York: John Wiley and Sons.

Christian, J. 2004. Geotechnical engineering reliability: How well do we know what we are doing? J. Geotech. Geoenviron. Eng. 130(1):985–1003.

Christian, J., Ladd, C., and Baecher, G. 1993. Reliability applied to slope stability analysis. J. Geotech. Eng. 120(12):2180–2207.

Couzens, T. 1985. Planning models: Operating and environ-mental impacts. In Design of Non-Impounding Mine Waste Dumps. Edited by M.K. McCarter. New York: SME-AIME.

Duncan, M. 2000. Factors of safety and reliability in geo-technical engineering. J. Geotech. Geoenviron. Eng. 126(4):307–316.

FHA (Federal Highway Administration). 1981. Visual Impact Assessment for Highway Projects. Washington, DC: FHA.

FIPR (Florida Institute of Phosphate Research). 2010. Phosphate primer. www1.fipr.state.fl.us/phosphateprimer. Accessed January 2010.

Fitch, M., Thompson, M., Kavanagh, M., and Kovach, W. 1998. The Environmental Effects of Rock Drains: Results from the Drain Rock Research Program. British Columbia Mine Reclamation Symposium 1998.

Fredlund, D., and Xing, A. 1994. Equations for the soil–water characteristic curve. Can. Geotech. J. 31:521–532.

Fredlund, D., Wilson, G., and Barbour, S. 2001. Unsaturated soil mechanics and property assessment. In Geotechnical and Geoenvironmental Engineering Handbook. Edited by R. Rowe. New York: Springer.

Fredlund, M., Fredlund, D., Houston, S., and Houston, W. 2003. Assessment of unsaturated soil properties for seepage modeling through tailings and mine wastes. In Proceedings of Tailings and Mine Waste ’03. Lisse, Netherlands: Balkema. pp. 149–157.


Hancock, G. 2009. Understanding soil erosion and land-scape evolution using computer based predictive models. Vignettes, Science Education Resource Center. http://serc .carleton.edu/vignettes/collection/35461.html. Accessed November 2009.

Hansen, D., Zhao, W., and Han, S. 2005. Hydraulic perfor-mance and stability of coarse rockfill deposits. Water Manage. 158(4):163–175.

Hazen, A. 1892. Some Physical Properties of Sands and Gravels. Annual Report. Lawrence, MA: Massachusetts State Board of Health. pp. 539–560.

Hustrulid, W.A., McCarter, M.K., and van Zyl, D. JA., eds. 2000. Slope stability in surface mining. In Stability of Waste Rock Embankments. Littleton, CO: SME.

Koerner, R., Martin, J., and Koerner, G. 1986. Shear strength parameters between geomembranes and cohesive soils. Int. J. Geotext. Geomembr. 4(1):21–30.

Kozeny, J. 1927. Veber Kapillare Leitung des Wassers im Boden Wien. Akad. Wiss. 136,271.

Makdisi, F., and Seed, H. 1978. Simplified procedure for esti-mating dam and embankment earthquake-induced defor-mations. J. Geotech. Eng. Div. 104(GT7):849–867.

Masada, T., Mitchell, G., Sargand, S., and Shashikumar, B. 1994. Modified direct shear study of clay liner/geo-membrane interfaces exposed to landfill leachate. Int. J. Geotext. Geomembr. 13(3):161–179.

McCarter, M. 1985a. Monitoring stability of waste rock dumps. In Design of Non-Impounding Mine Waste Dumps. Edited by M.K. McCarter. New York: SME-AIME.

McCarter, M.K., ed. 1985b. Design of Non-Impounding Mine Waste Dumps. New York: SME-AIME.

McCarter, M.K. 1990. Design and operating considerations for mine waste embankments. In Surface Mining, 2nd ed. Edited by B.A. Kennedy. Littleton, CO: SME. pp. 890–899.

McPhail, G., and van Koersveld, A. 2006. Optimizing the ero-sional performance of store and release covers through appropriate landform design. In Mine Closure 2006, Proceedings of the First International Seminar on Mine Closure, Perth, Australia, September. www.metago.com/documents/Closure_Rehabilitation_Minimising_the _Legacy.pdf. Accessed November 2009.

Naderian, A., and Williams, D. 1996. Simulation of open-cut coal mine backfill behavior. In Proceedings of the National Symposium on the Use of Recycled Materials in Engineering Construction, May 1996, Sydney, Australia. Barton, ACT, Australia: Institution of Engineers.

NAVFAC (Naval Facilities Engineering Command). 1982. Foundations and Earth Structures, Design Manual 7.2. Alexandria, VA: U.S. Department of the Navy.

Olsen, R. 1997. Cyclic liquefaction based on the cone pen-etration test. In Proceedings of the NCEER Workshop on Evaluation of Liquefaction Resistance of Soils. Buffalo, NY: National Center for Earthquake Engineering Research. pp. 225–276.

Peck, R. 1969. Advantages and limitations of the observa-tional method in applied soil mechanics. Geotechnique 19(June):171–187.

Rawls, W., and Brakensiek, D. 1989. Estimation of soil water retention and hydraulic properties. In Unsaturated Flow in Hydrologic Modeling Theory and Practice. Edited by H. Morel-Seytoux. Boston: Kluwer Academic Publishers.

Robertson, A. 1982. Deformation and Monitoring of Waste Dump Slopes. Vancouver, BC: Steffen, Robertson and Kirsten.

Robertson, P., and Wride, C. 1998. Evaluating cyclic lique-faction potential using the cone penetration test. Can. Geotech. J. 35(3):442–459.

Rogers, V.C., and Nielson, K.K. 1984. Radon Attenuation Handbook for Uranium Mill Tailings Cover Design. NUREG/CR-3533. Washington, DC: U.S. Nuclear Regulatory Commission.

Sample, K., Camus, C., and Sinha, K. 2009. A holistic assess-ment of slope stability analysis in mining applications. In Proceedings of the 2009 SME Annual Meeting and Exhibit. Littleton, CO: SME.

Seed, H. 1979. Soil liquefaction and cyclic mobility evalu-ation for level ground during earthquakes. J. Geotech. Eng. 105(2):210–255.

Seed, H., and Idriss, I. 1971. Simplified procedure for eval-uating soil liquefaction potential. J. Geotech. Eng. 97(9):1249–1273.

Sharma, H., Hullings, D., and Greguras, F. 1997. Interface strength tests and applications to landfill design. In Proceedings of Geosynthetics ’97. Long Beach, CA: Industrial Fabrics Association International. pp. 913–926.

Smith, M. 2008. Emerging issues in heap leaching technology. In Eurogeo4: 4th European Geosynthetics Conference. Easley, SC: International Geosynthetics Society.

Smith, M., and Giroud, J. 2000. Influence of the direction of ore placement on the stability of ore heaps on geomembrane-lined pads. In Slope Stability in Surface Mining. Edited by W.A. Hustrulid, M.K. McCarter, and D.J.A. van Zyl. Littleton, CO: SME. pp. 435–438.

Sperry, J., and Pierce, J. 1995. A model for estimating the hydraulic conductivity of granular material based on grain shape, grain size, and porosity. Ground Water 33(6):892–898.

Stark, T., and Poeppel, A. 1994. Landfill liner interface strengths from torsional-ring-shear-test. J. Geotech. Eng. 120(3):597–614.

Suzuki, Y., Tokimatsu, K., Koyamada, K., Taya, Y., and Kubota, Y. 1995. Field correlation of soil liquefaction based on CPT data. In Proceedings of the International Symposium on Cone Penetration Testing, Vol. 2. pp. 583–588.

Thiel, R., and Smith, M. 2004. State of the practice review of heap leach pad design issues. J. Geotext. Geomembr. 22(6):555–568.

U.S. Circuit Court for the Northern District of California. 1884. RG 21, Civil Case 2900, Woodruff v. North Bloomfield Gravel Mining Company.

Valera, J., and Ulrich, B. 2000. Geomembrane/soil interface strength relationships for heap leach facility design. In Proceedings of Tailings and Mine Waste ’00. Rotterdam: Balkema. pp. 193–201.

van Genuchten, M. 1980. A closed form equation for predict-ing the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.


Vandre, B. 1980. Tentative Engineering Guide: Stability of Non Water Impounding Mine Waste Embankments. Ogden, UT: U.S. Forest Service.

Whitman, R. 1984. Evaluating calculated risk in geotechnical engineering. J. Geotech. Eng. 110(2):145–188.

Williams, D. 2000. Assessment of embankment parameters. In Slope Stability in Surface Mining. Edited by W.A. Hustrulid, M.K. McCarter, and D.J.A. van Zyl. Littleton, CO: SME.

Youd, T., and Idriss, I. 2001. Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. J. Geotech. Geoenviron. Eng. 127(4):297–313.

Youd, T., and Noble, S. 1997. Magnitude scaling factors. In Proceedings of the NCEER Workshop on Evaluation of Liquefaction Resistance of Soils. Buffalo, NY: National Center for Earthquake Engineering Research. pp. 149–165.

Zahl, E.G., Biggs, F., Boldt, C.M.K., Connolly, R.E., Gertsch, L., Lambeth, R.H., Stewart, B.M., and Vickery, J.D. 1992. Waste disposal and contaminant control. In SME Mining Engineering Handbook. Edited by H.L. Hartman. Littleton, CO: SME.

Zavodni, Z., Trexler, B., and Pilz, J. 1981. Waste Dump Foundation Investigations and Treatment. Workshop on Design of Non-Impounding Mine Waste Dumps, SME, November.

chapter 8.11 waste piles and dumps

Business

waste dumps

waste products

dealtypes of waste piles

description of waste

ore piles

disposalof mining waste

ore material

valley crossvalley