By Chris Athanassopoulos and Mark Smith
Traditionally, leach pad lining systems have consisted of a single geomembrane liner placed directly over a prepared subgrade of locally available soil or, occasionally, compacted clay.
Heap fills are most commonly constructed by placing a layer of highly-permeable drainage stone (overliner) over the geomembrane, usually with perforated drainage pipe incorporated in the overliner layer. Ore is then placed on the leach pad in 3m- to 15m-thick lifts, often reaching final heights of 100m to 160m. The ore is generally crushed to a maximum particle size of 5mm to 50mm, but occasionally is crushed to a larger maximum size (e.g., 100mm to 150mm) or even leached without crushing (run-of-mine).
The ore is irrigated with a chemical solvent that dissolves the target metals from the ore. The nature of the leaching solution depends on the targeted metal.
Low pH sulfuric acid solutions are generally used to leach copper, uranium, and nickel. High pH sodium cyanide solutions are used to leach gold and silver, and high pH carbonate solutions are occasionally used to leach uranium. The metal-laden pregnant leach solution (PLS) passes down through the ore pile and is captured in a drainage system. Metals are extracted from the leach solution through various down-stream processes and the solution is then recycled back onto the leach pile.
When under load, geomembranes are vulnerable to damage from large stones both in the soil subgrade and in the overlying drainage layer (or, in some cases, when crushed ore is placed directly on the geomembrane). Although intact geomembranes are virtually impermeable, installed geomembranes will have a small number of holes due to imperfect seams or damage during construction, overliner placement, and subsequent filling operations. These holes serve as open pathways for leakage into the soil below.
Smith and Welkner (1995) estimated liner leakage rates ranging from 5 to 10,000 L/ha/day, depending on the type of liner and level of construction quality assurance (CQA). Thiel and Smith (2003) reported liner leakage rates up to 2,000 L/ha/day for one impounding valley leach facility with heads ranging from 15m to 35m, and total leakage greater than 500,000 L/day have been measured at another facility.
Often overlooked factors include over-stressing that can occur next to pipes (up to 130% of the nominal load) and potential for increased geomembrane deformation due to high temperatures in bio-leach facilities, up to 60 C (Smith, 2012).
Composite liner systems
To reduce leakage through defects, a low-permeability layer can be used beneath the geomembrane to form a “composite” liner system. The low-permeability material beneath the geomembrane is typically either a compacted low-permeability soil (clay or silt) liner or a bentonite clay-based geosynthetic clay liner (GCL).
These systems have been used for base liners for leach pads as well as covers and base liners at municipal solid waste landfills in the U.S. for more than 20 years. Compacted soil liners are typically constructed at wet of optimum moisture content to achieve a maximum hydraulic conductivity of either 10-6 or 10-7 cm/sec, depending on local regulatory requirements. While preparing soil liners wet of optimum moisture content is beneficial for hydraulic performance, it can be detrimental for slope stability, most famously demonstrated at the Kettleman Hills landfill in California (Mitchell et al., 1990).
Based on liner leakage measurements collected by the U.S. Environmental Protection Agency (EPA) at 287 landfill cells spanning 91 sites (Bonaparte et al., 2002), GCL-based composite liner systems have been shown to allow less leakage than clay-based composite liner systems. Due to improved hydraulic performance, limited availability of fine-grained soils at many mine sites, difficulty in compacting clay on steeper slopes, and speed of installation, GCLs are seeing increased use in leach pad liner systems.
Because conditions under an ore heap can be extreme—harsh chemical environments, high temperatures, enormous normal stresses aggravated by shearing or horizontal loads from sloping ground and stacking equipment, and coarse-grained angular soils—the use of geosynthetic materials in these applications requires special design considerations.
This article will discuss designing with GCLs in leach pad liners with respect to puncture protection for the geomembrane, chemical compatibility, and shear strength for slope stability. The information presented will include a review of the existing literature, the authors’ project experience, and the latest research intended to improve GCL performance in these aggressive conditions.
GCLs as puncture protection
The use of well-graded, angular stone as the overliner layer directly above the geomembrane presents obvious puncture challenges. The landfill industry has addressed this by adding nonwoven geotextiles for puncture protection, but this is often proscribed on leach pads for stability reasons. Additionally, it is common to encounter subgrade soils at mine sites containing appreciable quantities of gravel, leaving geomembranes vulnerable to puncture damage from below. In these settings, geomembrane damage could occur either with increasing normal stress (as ore is placed) or during shear displacement (due to dynamic construction loads, seismic forces, slope movement, or settlement).
Narejo et al. (2007) demonstrated that GCLs can effectively cushion geomembranes from subgrade puncture challenges, providing protection that was comparable to a 16 oz/yd2 (540 g/m2) nonwoven geotextile. Compacted soil liners, unless devoid of stones, cannot be expected to offer the same protection; in fact, under the high normal loads seen at many leach pads, any stones in the soil subgrade present increased puncture risks. (Puncture risks from such stones are increased when the soil is compacted wet of optimum, leading to greater consolidation of the fine particles around the stones.)
Athanassopoulos et al. (2009) presented example calculations that demonstrated the improved PLS recovery rate afforded by adding a GCL below the geomembrane could potentially translate to hundreds of thousands of dollars per year of added revenue, which during the life of the project, would exceed the cost of the initial investment in the GCL.
Athanassopoulos et al. (2009) performed a series of short-term static puncture tests on geomembrane-only and geomembrane/GCL composite liner systems at normal stresses as high as 5,172 kPa, or a high equivalent ore depth of 270m—for a factor of safety (FS) of 1.5, this equates to 180m 1.5 for the nominal load, or 138m near the pipes were stresses are higher. The results of the high-load puncture testing showed that geomembranes alone are generally expected to experience more puncture damage (puncturing or strain deformation past yield) from the overliner than a geomembrane with an underlying GCL. Geomembrane samples subjected to stresses greater than 2,586 kPa (135m of ore) experienced more than 300 permanent deformations per m2. A geomembrane sample tested alone at the highest normal stress, 5,172 kPa, also had two punctures in a 0.09m2 area, each measuring 2mm in diameter. The GCL’s benefit, in terms of reducing biaxial strains in the geomembrane, appeared to increase with increasing normal stress.
Fox et al. (2013) performed a series of direct shear tests on geomembranes in contact with a compacted clay subgrade containing 20% gravel to evaluate potential for shear-induced puncture damage. In a leach pad setting, especially those on steeper ground, shear forces could occur as a result of heavy loading or unloading equipment, seismic forces, or settlement causing downdrag along lined slopes. Shear forces will also be present at the leading face of any heap for the first lift of ore.
Tests were performed both with and without a GCL placed between the geomembrane and the compacted clay liner at normal stresses ranging from 348 to 4,145 kPa (18m to 220m of ore) to evaluate the level of protection provided by the GCL. Results indicate that geomembranes are vulnerable to severe puncture damage due to shear displacements under high normal stress and that including a GCL between the geomembrane and the compacted clay liner can essentially eliminate such damage.
GCLs are factory-manufactured liners containing sodium bentonite clay with a hydraulic conductivity of about 5 × 10-9 cm/sec with deionized water. The amount of bentonite swelling and, therefore, the GCL hydraulic conductivity, can be influenced by the presence of divalent cations (e.g., calcium and magnesium), high ionic strength, or extreme pH solutions. GCL chemical compatibility is therefore an important consideration in heap leach pad applications because high-pH cyanide solutions are used to leach gold and silver, and low-pH sulfuric acid solutions are generally used to leach copper, uranium, and nickel laterites.
Relative to high-pH dilute cyanide leaching solutions, CETCO (2000) presented hydraulic conductivity results showing that GCLs are compatible with gold PLS, with a long-term hydraulic conductivity on the order of 10-9 cm/sec.
Relative to sulfuric acid leaching solutions, Jo et al. (2001) found that sodium bentonite exhibited approximately a 50% decrease in swell at pH values less than 3. As part of the same study, GCL permeability values on the order of 10-6 to 10-5 cm/sec were measured at pH values less than 2.
Ruhl and Daniel (1997) found that when exposed to strong acid, a GCL’s buffering capacity was not exhausted until after 15 pore volumes of flow. At the low water flow rates expected in a liner, it may take years for the first 15 pore volumes to flow through the liner. By this time, the liner will likely be covered and compressed by several hundred feet of ore so the GCL’s permeability will be greatly reduced.
Athanassopoulos et al. (2009) performed permeability tests on both an intact GCL sample and a GCL sample that had been pierced by a large stone during an earlier high-stress puncture test. The permeant solution for these tests was a copper PLS (pH = 2; EC =37 mS/cm) collected from an active copper leach heap in the southwestern U.S.
Permeability testing was performed at effective stresses ranging from 34.5 to 3,166 kPa (2 to 170 m of ore), to span the range of operational stages of a typical copper heap leach facility. The results of these permeability tests showed that at low effective stress, the GCL permeability was greater than 10-6 cm/sec.
Because effective stress was increased to simulate increasingly higher ore heights on the liner, the permeability decreased significantly, reaching values less than 10-10 cm/sec at 1,440 kPa (75m of ore). Interestingly, similar behavior was exhibited by the punctured specimen, as shown in Figure 1.
The improvement in GCL hydraulic conductivity with increasing effective stress, even in harsh chemical solutions, is similar to the findings of Daniel (2000) and Thiel and Criley (2005). This is important because post-construction defects will most likely occur at greater ore depths.
Research is under way to evaluate the use of polymer-amended bentonites and bentonite-polymer nanocomposites for improved GCL hydraulic performance in contact with low-pH mining solutions (Scalia et al., 2011). Encouraging early results with low-pH copper PLS are expected to be published in 2015.
Shear strength and
Since many leach pads involve a combination of steep slopes and high normal stress, shear strength and heap stability is a critical design consideration. Historically, laboratory direct shear devices were limited to loads less than 690 kPa, representing approximately 36m of ore, which falls far short of the maximum loads expected in most leach pads. Recently, universities and commercial laboratories have learned to perform shear testing at higher normal stresses. The following is a summary of some of these tests involving GCLs.
Athanassopoulos et al. (2009) tested textured geomembranes against needle-punched reinforced GCLs, at normal stresses up to 2,758 kPa (146m of ore). Results, summarized in Table 1, showed peak secant angles ranging from 30° at 517 kPa (27m of ore) to 20° at 2,758 kPa (146m of ore), and corresponding large displacement secant angles decreased from 14° to 7°.
Thielmann et al. (2013) evaluated geomembrane/GCL interface strengths for ultra-high normal stresses up to 4,144 kPa (220m of ore). Tests were conducted with the geomembrane/GCL liner materials placed between a lower layer of sand and an upper layer of coarse gravel, to replicate common leach pad field conditions. The peak friction angles (Table 1) ranged from 22° at 348 kPa (18m of ore) to 15° at 4,144 kPa (220m of ore), and corresponding large displacement secant angles decreased from 13° to 5°.
Table 1 is useful in showing how interface friction angles change with increasing normal stress. For slope stability analyses, the most important area of the heap is toward the toe (inside the “stability zone”). If a slope stability analysis is performed with a non-linear failure envelope, the sections of the liner under the deepest part of the ore would have the lowest peak and large displacement friction angles; however, in the critical “stability zone,” toward the toe, the normal loads are lower and therefore the friction angles, and thus resistance to sliding, will be higher.
Breitenbach and Swan (1999) and Parra et al. (2010) observed that liner components placed in contact with coarse soils are expected to see an increase in strength, because of local out-of-plane deformation, or “dimpling,” of the liner components under the gravel particles. These results suggest that the common practice of performing direct shear tests using rigid backing plates is conservative (perhaps overly conservative) with respect to shear strength of composite liners that are overlain by coarse soils.
At extremely high normal stress, the interface strength between a textured geomembrane and a needle-punched, reinforced GCL can exceed the strength of the needle-punched reinforcement and the critical interface could occur internally within the GCL. The critical normal stress associated with this failure mode transition depends on the specific materials (e.g., GCL peel strength, geomembrane texturing, and asperity height) and testing conditions (e.g., hydration/consolidation, displacement rate) and, as such, can vary widely.
Recent laboratory studies noted that this failure mode transition occurred at normal stresses ranging from 692 kPa (Fox and Ross, 2011), to 2,072 kPa (Thielmann et al., 2013), to 2,758 kPa (Athanassopoulos et al., 2009). Triplett and Fox (2001) observed no GCL internal failures for geomembrane/GCL interface tests conducted at normal stresses as high as 486 kPa. McCartney et al. (2009) observed no GCL internal failures in a database of 534 geomembrane/GCL interface tests performed at normal stresses as high as 965 kPa. The variability of normal stress at failure mode transition further highlights the need for project-specific shear tests using representative site materials and project-specific conditions.
Observations of internal shear failure of needle-punched reinforced GCLs have thus far been limited to the laboratory, because there are no known cases of internal shear failure of needle-punch reinforced GCLs in the field (Fox and Ross 2011; Koerner 2012). Nonetheless, the potential for both interface and internal failure should be considered for designs that subject hydrated GCLs to high normal stress levels. (Note that unless the GCL is installed on a subgrade prepared at optimum, or wet of optimum moisture content, the assumption of a fully hydrated GCL across the entire failure plane is a conservative one.)
of leach pads with
Breitenbach and Athanassopoulos (2013) presented a series of slope stability calculations for a hypothetical ore heap study section containing a leach pad liner with a very low strength bentonite clay to geomembrane liner interface (friction angle = 7°). The calculations evaluated the benefit of incorporating non-planar features, such as stability berms, shear keys, or backwall benches, along critical portions of the liner system.
For one example study section (Figure 2), the use of stability berms (or “speed bumps”) along the toe of the structure improved the FS against sliding from a clearly undesirable FS = 0.89 to a favorable FS = 1.4. These findings have important design implications for leach pad liner systems with low interface strengths.
The incorporation of non-planar features in liner systems containing GCLs can improve overall stability and maintain an adequate FS, even in worst-case loss of GCL internal strength due to either rupture, pullout, or degradation of the needle-punched reinforcing fibers, or reduction in geomembrane/GCL interface strength due to extrusion of hydrated bentonite through the woven geotextile component of the GCL. Note that for many leach pads the inclusion of an undulating subgrade can also reduce grading costs.
GCLs are increasingly used in leach pads because of improved containment, the limited availability of fine-grained soils at many mine sites, speed of installation, and reduced risk of cost overruns and construction delays. Because conditions under an ore heap can be extreme—harsh chemical environments, high temperatures, enormous normal stresses, and coarse-grained angular soils—the use of geosynthetic materials and GCLs in these applications requires special design considerations, including: puncture protection, chemical compatibility, and shear strength and slope stability improvement methods.
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