PHYSICAL PERFORMANCE OF GEOMEMBRANES USED IN HEAP LEACH PAD APPLICATIONS. Huma Irfan. A thesis submitted to the Department of Civil Engineering

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1 PHYSICAL PERFORMANCE OF GEOMEMBRANES USED IN HEAP LEACH PAD APPLICATIONS by Huma Irfan A thesis submitted to the Department of Civil Engineering in conformity with the requirements for the degree of Masters of Applied Science Queen s University Kingston, Ontario, Canada (January, 213) Copyright Huma Irfan, 213 i

2 Dedication This humble work is dedicated to my mother and my father. ii

3 Abstract Geomembranes (GMB) are normally used as part of the liner system in heap leach pads. There is a need to quantify tensile strains in the geomembrane that could affect short-term puncture and long-term performance of the GMB. In this thesis, short-term tensile strains arising from indentations caused from the material placed both below and above the geomembrane are quantified, and the potential for puncturing is investigated. Experiments were conducted on 1.5 mm high-density (HDPE) and liner low density polyethylene (LLDPE) geomembranes for applied pressures up to 3 kpa. The geomembrane punctured from underliner material having gravel and sand placed directly beneath the geomembrane and a peak tensile strain of 4% was induced. Increasing the sand fraction to obtain a well graded gravel and sand underliner resulted in peak tensile strains of 14% in the geomembrane and caused no puncture. When geomembrane is underlain by geosynthetic clay liner (GCL) and compacted clay liner, the tensile strains increased with increasing deformability (due to higher water content) of the underlying material. Experiments were also conducted to examine the implications of overliner material and pressure on geomembrane strains. It was found that the overliner having gravel and some sand resulted in 18% tensile strain in the geomembrane at 2 kpa and 27% for 3 kpa. A gravelly sand overliner with some silt induced tensile strains of 9% and 12% at 2 kpa and 3 kpa respectively. None of the overliners was able to limit stains in the geomembrane to less the maximum recommended geomembrane strain proposed in the literature. A 15-mm-thick silty sand layer placed above coarsest overliner examined reduced the geomembrane strains to 2%, even when subjected to pressures of 3 kpa. iii

4 Co-Authorship This thesis contains materials submitted for publication that have been co-authored by H. Irfan and her supervisors, Dr. R.K. Rowe, and Dr. R.W.I. Brachman. The research reported in this thesis was initiated by consultation between H. Irfan, Dr. R. K. Rowe and Dr. R.W.I. Brachman. The laboratory tests were planned and conducted by H. Irfan. The analysis and interpretation of laboratory data was performed by H. Irfan under the direct supervision of Dr. Kerry Rowe and Dr. R.W.I. Brachman. Papers: Chapter 2 has been prepared as the paper: Effect of underliner on geomembrane strains in heap leach applications by R. Kerry Rowe, R. W. I. Brachman, H. Irfan, M.E. Smith and R. Thiel Chapter 3 has been prepared as the paper: Assessment of tensile strains in heap leach geomembranes from overliner materials by R. W. I. Brachman, R. Kerry Rowe and H. Irfan, These papers are yet to be submitted for publication. iv

5 Acknowledgements I would like to express my sincere gratitude to my supervisors, Dr. R.K. Rowe and Dr. R.W.I Brachman for their guidance and invaluable support during my research. This thesis would not have been possible without their support and cooperation. Also, I would like to appreciate the help and care I received from our Support and Technical Staff: Lloyd, Maxine, Cathy, Bill, Jaime, Brett, Corry and Stan. Their tremendous help made it possible for me to accomplish the experimental work for my research. My parents and sisters have given me their unequivocal support throughout, as always, for which my mere expression of gratitude likewise does not suffice. I would like to thank my friends Ms. S. Habib, Ms. S. Tariq and Ms. M. Ahmed Labeid whose continuous encouragement always pushed me to work hard and gave me confidence. I highly appreciate the help and friendship of my fellow graduate students Mr. P. Joshi, Ms. L. Ashe, Mr. F. Abdelaal, and Mr. M. Hosney. I am particularly grateful to Dr. Simon Gudina for his help in starting experiments for this research. I would like to register special thanks to my friend Ms. L. Abbas, her husband, and daughters for their love and care. Last but not least, sending many love to my close friends in Pakistan and fellow students at Queens University for their valuable support and encouragement. v

6 Table of Contents Dedication... ii Abstract... iii Co-Authorship... iv Acknowledgements... v Table of Contents... vi List of Tables... x List of Figures... xi Chapter 1 Introduction Description of Problem Current state of Practice Research Objectives and Methodology Scope of Thesis Chapter 2: Geomembrane (GMB) punctures and strains from the underliner material Chapter 3: Geomembrane (GMB) behavior for different overliner materials at large overburden pressures Format of Thesis References... 8 Chapter 2 Geomembrane (GMB) punctures and strains from the underliner material Introduction Characteristics of Heap leach Pads Underliner material vi

7 2.2.2 Geomembrane Overliner material Experimental Method Test Apparatus Underliners considered Test Procedure Results Response of Geomembrane for underliners UL1 and UL2 (Tests 1, 1A, 2) Effect of maximum particle size and grading curve with 15% fines (UL2, UL3 and UL4) Geomembrane performance for underliners UL5 and UL6 with 15% fines Geomembrane performance with a GCL over sand (UL5) underliner Geomembrane performance with a compacted clay liner HDPE versus LLDPE (Tests 2 and 2A) Summary and conclusion References Chapter 3 Geomembrane behaviour for different overliner materials at large overburden pressures Introduction Experimental details Apparatus Procedure and Materials Results Typical results for Test series 4 (OL1, 3 kpa) Variability in replicate tests conducted Discussion vii

8 3.4.1 Effect of maximum and minimum grain size (OL1 and OL2) Effect of grain size distribution (OL2 and OL3) Effect of grain size Reducing strains with a soil Protection layer Practical applications Summary and conclusions References 65 Chapter 4 Conclusions and Recommendations Effect of underliner Effect of overliner and pressure Applicability and Limitations, and Future Work References Appendix A Heap Leach Projects Details Appendix B Procedure of Scanning Indentation Appendix C Pressure versus Time plots Appendix D Tests conducted at high temperature Appendix E Test number scheming and configuration Appendix F Properties of underliner used in Chapter Appendix G Calculated strains for all tests Appendix H viii

9 Plots of Indentation geometries and strains ix

10 List of Tables Chapter 2 Table 2.1 Summary of the 19 experiments conducted at 22oC and an applied pressure of 2 kpa. All experiments except Test 2A were for a 1.5 mm HDPE geomembrane. Test 2A was for a 1.5 mm LLDPE geomembrane Table 2.2 Properties of underliners considered Table 2.3 Grain size properties of underliner materials... 4 Table 2.4 GCL thickness before and after each test... 4 Table 2.5 Index stress-strain properties (measured in the machine direction) of the 1.5-mm-thick HDPE and LLDPE geomembranes studied (Tested following ASTM D6693 unless otherwise noted)... 4 Chapter 3 Table 3.1 Summary of the 25 experiments conducted at 22 C. All experiments were for 1.5 mm HDPE geomembrane Table 3.2 Properties of material used as an overliner Table 3.3 Index stress-strain properties (measured in the machine direction) of the 1.5-mm-thick HDPE geomembrane tested following ASTM D x

11 List of Figures Chapter 1 Figure 1.1: Typical configuration of heap leach pad liner system... 1 Chapter 2 Figure 2.1: Histogram of heap leach ore height with estimated vertical stresses for 72 cases where data were available. Unit weight of ore is assumed as 2 kn/m Figure 2.2: Grain size distribution of underliners (UL1-UL6) and overliner examined (OL) in this study and bounds of underliner in projects reported by Lupo and Morrison (27) Figure 2.3: Cross section through a typical test cell used in present study and test setup for Test #1; all dimensions in mm. Underliner and overliner material shown schematically only and not drawn to scale. All dimensions are in mm Figure 2.4: Photograph of (a) underliner material UL1 as used in Test 1, (b) underliner material UL2 as used in Test 2, 2A, (c) underliner material UL3 as used in Test 3, 3A, (d) underliner material UL4 as used in Test 4, (e) underliner material UL5 as used in Test Figure 2.5: Photographs of bottom of geomembrane after test with puncture locations shown by arrows for: (a) Test 1 (UL1), (b) Test 2 (UL2) HDPE geomembrane, and (c) Test 2A (UL2) LLDPE geomembrane Figure 2.6: Strains calculated for scanned indentation for: (a) Test 1 (UL1), (b) Test1A (UL1 modified and geotextile protection over geomembrane) (c) Test 2 (UL2, HDPE), and (d) Test 2A (UL2, LLDPE). Indentations were from the top (overliner) unless a B indicates it was from the bottom (underliner) Figure 2.7: The indentation from a gravel particle in the underliner giving the maximum strain for Test 1A: (a) Deformed shape, and (b) calculated strain. Geometry, h is the height of the indentation measured from the deepest or highest (in this case highest) point of the indentation. Tensile strain plotted as positive. Note that there is tension through the entire geomembrane thickness on sides of the indentation Figure 2.8: Strains calculated for key indentations in (a) Tests 3, 3A for underliner UL3, and (b) Test 4 with underliner UL4. Indentations were from the top (overliner) unless a B indicates it was from the bottom (underliner) Figure 2.9: Strains calculated for key indentations for (a) Tests 5, 5A, 5B with underliner UL5, and (b) Test 6 with underliner UL6. All indentations were from the top (overliner) Figure 2.1: Strains calculated for key indentations for (a) Tests 7, 7A with a prehydrated GCL, and (b) Tests 8 and 8A with GCLs hydrated from silty sand subgrade under 2 kpa stress. All indentations were from the top (overliner) Figure 2.11: Strains calculated for key indentations for (a) Tests 9, 9A with a clay liner compacted at standard Proctor optimum water content, and (b) Tests 1 and 1A with a clay xi

12 liner compacted at water content of standard Proctor optimum plus 4%. All indentations were from the top (overliner) Chapter 3 Figure 3.1: Cross section through a typical test cell used in experiments; all dimensions in mm. Underliner and overliner material shown schematically only and not drawn to scale. All dimensions are in mm Figure 3.2: Grain size distribution of overliners (OL1 OL3) and underliner examined (UL) in this study Figure 3.3: Particle Shapes of the material used as OL Figure 3.4: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4 (OL1, 3kPa) Figure 3.5: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4A (OL1, 3kPa) Figure 3.6: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4B (OL1, 3 kpa) Figure 3.7: Largest calculated strains from three replicate tests of Test series 4 (OL1, 3 kpa) Figure 3.8: Largest strains calculated from each test series showing the effect of pressure and overliner Figure 3.9: Photograph of lead sheet (27 mm x 27 mm) for tests conducted at 3 kpa, (a) after using OL1 in Test 4 (b) after using OL 2 in Test 6. The marked circles are showing the major indentations scanned in the lead sheet. All the indentations are going into the plane of paper Figure 3.1: Photograph of lead sheet (27 mm x 27 mm) showing no discernable indentations after using silty Sand protection layer in Test Figure 3.11: Influence of protection layer on the maximum strain in geomembrane at 3 kpa xii

13 Chapter 1 Introduction 1.1 Description of Problem A geomembrane (GMB) is a crucial component of the liner system in heap leach pads. The primary purpose of the GMB is to minimize the loss of minerals dissolved in the pregnant liquor and, as a side benefit, minimize environment damage from these metal rich solutions that may have a very high or low ph and may contain other toxic chemicals used in the leaching process (such as cyanide). The GMB may be exposed to extremely harsh conditions introduced by the coarse material present on the two sides of the GMB, and immense pressure on the GMB that arises from the heights of the ore above the liner. A GMB used as an interlift liner in heap leach pad for the early recovery of process solution (Breitenbach 25) is exposed to coarse material on both sides. A native soil is used as underliner to minimize construction costs, and native processed granular material is preferred as overliner to maintain permeability under high loads (Lupo 25). However, coarse material and high pressure may result in larger contact forces on the GMB. Thus there is a need to quantify the deflections and strains in GMB arising from overliner and underliner induced indentations in the GMB. To act as an effective barrier, physical damage must be limited to allowable levels. Appropriate selection of the material to be used as an overliner and underliner is necessary, to 1

14 prevent GMB puncture from the overlying drainage material and limit the long term GMB tensile strains that lead to stress cracking and development of holes in the GMB over time. Alternatively suitable protection layer can be used above (or below) the GMB to minimize the risk of puncture or excessive strains. The physical conditions experienced by the GMB are exacerbated by: a) using coarse gravel in the underliner material, and b) using coarse material in the overlying drainage layer, and c) applying high pressures (up to 3MPa) to the GMB. The configuration of a typical liner system is shown in Figure Current state of Practice In heap leaching process the heap is placed on a gently sloping "pad" or within a natural valley to allow gravity drainage at the base of the facility and to facilitate the collection of PLS. The typical slope of the base liner at downhill toe is around 1 to 3 %, and no berm support at the toe is observed most commonly (Breitenbach 25). GMB used in heap leach pads application is exposed to extreme base pressure and moisture conditions (due to the continuous dripping of chemical solutions) not present in most containment applications. Current heap leaching practices are pushing the limits of geomembrane performance parameters beyond conventional working stress conditions (Thiel and Smith, 24). A GMB liner is exposed to harsh conditions by the overlying and underlying coarse material. In landfill applications, a geotextile is used as the protection layer on the top of GMB, to provide some protection from the coarse overlying drainage material and the method developed by Koerner et al. (1996) is used to select the mass of the geotextile required to minimize the risk of puncture depending for a given overlying gravel and overburden stress. 2

15 Geotextile protection layers are not normally used in heap leach pad applications for several reasons including, in particular, economics. Heap leach pad may extend over areas of 15 to 2 hectares and hence using even a low unit cost geotextile would result in very high total costs. Another issue associated with the use of a geotextile protection layer is the effect of the geotextile/geomembrane interface on leach pad stability. Occasionally a protection layer of silty sands or sand may be used on the top of the GMB as part of the overliner. When testing is done to assess liner puncture performance of leach pad liners it typically involves high-load static puncture tests where actual underliner and overliner materials are placed below and above a GMB sample and is subjected to normal pressures up to 2 kpa (Thiel and Smith, 24). After testing even though the GMB was visually inspected to determine the severity of indentations (Thiel and Smith, 24), the experience gained from their work demonstrates the need for a testing at even higher pressures to simulate liner performance in heap leach applications. Lupo and Morrison, (27) presents a GMB selection guide from experience based on the actual performance of lined facilities and from liner load tests. The choice of which geomembrane type and thickness to use in a particular design is made based on factors including expected loading conditions, the angularity of the underliner and overliner materials, estimated foundation settlement, as well as the geomembrane material properties. This guideline is meant only as a starting point for designers, and is not meant to be used in any final design. Thus the method leaves the final decision regarding design/ choice of materials to the discretion of the designer. 3

16 In literature more emphasis is given on the puncture potential of GMB than the strains induced. Indeed, until now strains have not been evaluated in any study related to heap leach applications. Rather the deformations are analyzed visually and ranked as severe, less severe etc. (Thiel and Smith, 24; Lupo and Morrison, 27). The most severe case of short term tensile strains is those leading to puncture of the GMB. However the paucity of data on the performance of GMB for different underliner and the GMB tensile strains arising from a coarse underliner complicates the design of GMBs for heap leach application. A rational design guideline for underliner and overliner selection is needed. As a first step to developing a rational design method for GMB underliner and overliner selection there is a need to quantify the physical demands on the GMB imposed by the choice of underliner and overliner for different geomembranes and applied stress. This will require tests that simulate the performance conditions of GMB liners and enables the quantification of the response of the GMB for the given test condition. Techniques have been developed for landfill applications that use a soft lead sheet placed beneath the GMB to preserve the permanent deformations in the GMB under loading conditions. The deformation can then be quantified and the strains are calculated using the method of Tognon et al. (2). Heap leach pads are constructed using the natural topography of the site. While economics may play a role, the trend of increasing height of ore may also arise simply because the topography and ground conditions of some sites do not allow thinner heaps (Thiel and Smith, 24). Currently ore heights are reaching the depth of 24 m resulting in pressure of more than 4 MPa on the GMB (Lupo 21). Lupo and Morrison (27) conducted a test with high pressure 4

17 on the GMB placed over a 15 cm thick layer of minus 25mm diameter gravel and covered with a 15 cm thick layer of minus 38mm diameter gravel and their results showed that a 2.5 mm LLDPE punctured at a pressure of 4.3MPa after 24 hours. In another study Lupo (25) examined a 1.5 mm LLDPE GMB after loading to.9 MPa and found no puncture, even though the liner was highly deformed. There is a need to evaluate the compatibility of different materials within liner system, but also define the operating limits of the liner. In order to ensure good long-term performance in landfill applications, the strains induced in the GMB should be less than the allowable tensile strain. Seeger and Müller (23) recommended a maximum allowable local GMB strain of 3%. Peggs et al. (25) proposed a maximum allowable strain of 6% for smooth GMBs with a stress crack resistance (SCR) of less than 15 hrs and 8% for GMB with SCR of larger than 15 hours. Heap leach pads may not need to provide the long-term ( hundreds of years) protection required of landfill liners, however prior to this study nothing was known regarding the strains induced in GMB liners used in heap leach applications with typical overliner and underliners for typical applied loads even for, short-term physical tests in the laboratory. 1.3 Research Objectives and Methodology This thesis examines the deformations and strains that occur in GMB commonly used as the part of the heap leach pads liner system, the main emphasis being placed on 1.5-mm-thick HDPE GMB overlying different underliner materials and a variety of overliner material at high pressure. The specific objectives of this research are to: Examine the circumstances under which GMB s have been and are being used in heap leach pad projects located in different parts of the world, in terms of projects status, 5

18 underliner material type and properties, GMB type and thickness, overliner material type and properties, and height of excavated ores based on information gained from collaborators (Smith and Thiel, pers. Comm.). Study the effect of different underliner material on the deformations and strains in the GMB, and to quantify the location, size and cause of punctures in the GMB when directly overlain by coarse underliner material and subjected to applied pressure of 2 kpa. Examine the effect of the presence of a compacted clay liner and geosynthetic clay liner (GCL) below the GMB, on the deformations and strains in the GMB, when directly underlying a coarse overliner material. Investigate the effect of temperature, high pressure, and overliner material on the deformations and strains of the GMB, as well as to determine the impact of a protection layer on the GMB strains due to very large (3 kpa) overburden pressure. These objectives were achieved by conducting a series of laboratory experiments designed to measure and quantify deformations and strains in the GMB. The experiments were conducted in a test apparatus previously developed to simulate harsh conditions expected at the base of landfills (Gudina 22). 1.4 Scope of Thesis This thesis represents the first examination of the effects of underliners and overliners on the strains developed in GMB for a range of conditions as described below. 6

19 1.4.1 Chapter 2: Geomembrane (GMB) punctures and strains from the underliner material The circumstances under which GMB s have been and are being used in heap leach pad projects located in different parts of the world are examined with additional materials given in Appendix A. A series of short-term laboratory tests were conducted to investigate the impact of different underliner material on a 1.5-mm-thick GMB. The effects of six different granular underliners ranging from coarse material to finer soils, as well as CCLs and GCLs, were examined with one specific coarse overliner material. The size and nature of punctures in the GMB (where present) were observed, the deformations in the GMB were measured, and the tensile strains in the GMB were calculated based on the measured deformations as described in Chapter 2. Additional test details are given in Appendices Chapter 3: Geomembrane (GMB) behavior for different overliner materials at large overburden pressures Having obtained results from different underliner material beneath the GMB, a reference underliner of silty sand was used below the GMB and the effect of different overliners was examined in Chapter 3. Experiments were conducted to simulate a variety of conditions including applied pressure, type of overliner material, temperature and protection layer. The first series of tests were conducted using a coarse overliner with applied pressures ranging between 5 and 3 kpa to examine the effect of pressure on the deformations and tensile strains in the GMB. Experiments were then conducted at applied pressures of 2 and 3 kpa to examine the deformations and strains using less coarse overliner material. The experiments were conducted at applied pressures of 3 kpa with 15-mm-thick silty sand protection layer. The 7

20 results of experiments conducted at 5 o C at 1 hours are presented in Appendix D to demonstrate the effect of temperature on the deformations and strains in GMBs. 1.5 Format of Thesis This thesis has been prepared in accordance with the regulations for a Manuscript Form thesis as stipulated by the School of Graduate Studies at Queen s University. Each manuscript is presented with the literature review, experimental procedures, results and conclusions pertinent to each contribution, but without an abstract. References, tables and figures are presented at the end of each chapter. The thesis consists of two original manuscripts in Chapters 2 and 3. The manuscripts will be submitted for publication. Additional information is included in the appendices. Units of measurement corresponding to the S.I. system (Le Système International d Unités) are used consistently throughout the thesis. 1.6 References Breitenbach, A. J. (25). Heap Leach Pad Design and Construction Practices in the 21st Century, Vector Colorado LLC, 9 p. Gudina, S. 22. Design and Development of an Experimental Apparatus to Simulate the Physical Behaviour of Geomembranes under Large Earth Pressures. M. Eng. Report, The University of Alberta, Alberta, Canada. Lupo, J. F. (21). "Liner System Design for Heap Leach Pads." Geotextiles and Geomembranes, 28(2), Lupo, J. F., and Morrison, K. F. (27). "Geosynthetic Design and Construction Approaches in the Mining Industry." Geotextiles and Geomembranes, 25(2),

21 Lupo, J. F. (25). Heap Leach Facility Liner Design. Golder Associates Inc., Lakewood, Colorado, USA, 25 p. Peggs, I.D., Schmucker, B., and Carey, P. 25. Assessment of maximum allowable strains in polyethylene and polypropylene geomembranes. In: Geo-Frontiers 25 (CD-ROM). American Society of Civil Engineers, Reston, VA. Seeger, S. and Müller, W. 23. Theoretical approach to designing protection: selecting a geomembrane strain criterion. In: Dixon, N., Smith, D.M., Greenwood, J.H., Jones, D.R.V. (Eds.), Geosynthetics: Protecting the Environment. Thomas Telford, London, pp Thiel, R., and Smith, M. E. (24). State of the Practice Review of Heap Leach Pad Design Issues. Geotextiles and Geomembranes, 22(6), Tognon, A.R., Rowe, R.K. and Moore, I.D. 2. Geomembrane Strains Observed in Large- Scale Testing of Protection Layers. Journal of Geotechnical and Geoenvironmental Engineering, 126(12):

22 Vertical pressure Overliner material Geomembrane Underliner material Figure 1. Heap Leach Pad Configuaration Figure 1.1: Typical configuration of heap leach pad liner system 1

23 Chapter 2 Geomembrane (GMB) punctures and strains from the underliner material 2.1 Introduction Heap leaching has gained wide acceptance as a relatively low cost method for the recovery of metals (Smith 24). The mined ore is crushed and placed in 5-1 m thick lifts over a geomembrane (GMB) lined pad (Breitenbach 25). A chemical solution, with the characteristics appropriate to leaching the mineral to be extracted, is applied at a controlled rate to the ore, most commonly via a drip irrigation system. As the solution percolates through the ore it dissolves the metal of interest, producing a solution referred to as the pregnant liquor or pregnant leach solution (PLS). This solution is collected at the base of the heap leach pad and directed to a recovery plant for metal recovery (Fourie et al. 21). The GMB liner serves to minimize the loss of the PLS (and hence valuable minerals as well as the process reagents) but also minimizes environmental impact due to the escape of PLS. GMBs provide an excellent barrier to the PLS except where there are holes (Rowe 212). Thus it is desirable to minimize the number of holes throughout the period when the PLS will be captured for mineral recovery and potentially for a longer period during which the escape of fluids leached from the ore could have a negative impact on the environment. Heap leach pads represent a challenging environment for any liner. The challenges include high stresses with ore heights reaching 24 m, and stresses of up to 4 MPa on the liner, having been reported (Lupo 21). Additional factors that could affect liner performance include the presence of a coarse overliner, gravel in the underliner, very high or low ph leach solution (Abdelaal et al 212), hydraulic heads of up to 6 m, and potentially high temperatures 11

24 (e.g., Thiel and Smith 24). Lupo and Morrison (27) developed general guidelines for GMB selection based on the applied load, characteristics of the foundation, overliner materials, and liner bedding materials. However, specific testing should be conducted to assess GMB liner performance for the given site conditions. The cylinder test method (as described in Environmental Agency 26, Brachman et al. 2, Lupo and Morrison 27, Shercliff 1998, and Thiel and Smith 24) is one technique used to assess the potential for GMB puncture for a given underliner and overliner. In these highload static puncture tests, the proposed underliner and overliner materials are placed below and above the GMB of interest and subjected to applied pressures up to 2 kpa (Thiel and Smith, 24). These tests focus on puncture due to vertical load. They do not represent lateral or horizontal loading that may be induced due to stacking equipment, angle of repose face angles of the first ore lift, or the relatively steep liner grades present on some pads. Because of the horizontal loading (and strain) one may also examine the condition of the liner sample coming out of a large direct shear test as representing a limiting condition for horizontal loading. Both the cylinder and the direct shear tests provide information about potential short-term puncture at the temperature at which the test is performed. While it is certainly necessary to avoid shortterm puncture, the absence of puncture does not mean that holes will not develop with time in areas where there are high tensile strains (Seeger and Müller 23, Peggs et al. 25). Yet there is a paucity of archival literature dealing with strains in GMBs used in heap leach applications. The objectives of this paper are to: (a) identify common features of heap leach pads, (b) examine the effect of the underliner on puncture and short-term tensile strains induced in 1.5 mm thick HDPE GMB, and (c) examine the relative performance of 1.5 mm thick HDPE and LLDPE GMBs under similar conditions. 12

25 2.2 Characteristics of Heap leach Pads The unit weight of the ore in a heap leach pad depends on a number of factors with typical moist values reported to range from 17.3 kn/m 3 (11 pcf) to 2.4 kn/m3 (13 pcf) with the maximum unit weight occurring during leaching (Breitenbach and Thiel 25). The present study included a review of 92 heap leach projects from 15 countries (Argentina, Brazil, Chile, Colombia, Ghana, Indonesia, Mexico, Namibia, Niger, Peru, Philippines, Poland, Turkey, USA, and Uzbekistan) to identify common features (Appendix A). Data was available regarding the height of ore at 73 of the projects examined (Figure 2.1). Approximately 51% of cases had ore heights of 5 m or less (i.e., typically less than about 1 MPa of vertical pressure), 9% were 1 m or less ( 2 MPa), but 1% were 15 m of higher ( 2.6 MPa) with a maximum height of 238 m ( 4.8 MPa). Based on this information, the experiments conducted in this study were for a pressure of 2 kpa (i.e., covering 92% of the cases) Underliner material Lupo and Morrison (27) indicated that, where possible, a native soil is used as the underliner to minimize construction costs. They indicated that typically requirements for the underliner include a non-gap graded particle distribution, a maximum particle size of 38 mm, greater than 15% fines (i.e. <.75mm), a plasticity index greater than 15%, and a saturated hydraulic conductivity of less than 1 x 1-8 m/s. Lupo and Morrison (27) presented a grain size envelope of underliner materials from several mining projects as defined by the curves UL2 and UL6 in Figure 2.2. Of the 92 cases reviewed as part of the current study (Appendix A), the underliner was described as clay in 48% of cases (although clay should probably be interpreted as soil with significant fines and these fines may not actually include significant clay in some cases), native soil in 9%, a GCL in 5%, tailings in 4%, silt/sand in 3%, and was not given in 3% of cases. 13

26 2.2.2 Geomembrane The literature indicates that the most common GMB used for a leach pad liner is 1.5 mm polyethylene (either HDPE and LLDPE) but that thicker PE is used occasionally for deeper heaps and.75 to 1. mm PVC has been occasionally used (Theil and Smith 24, Lupo 28). Data on the liner was available for 88 of the 92 cases examined in this study. This data indicated that HDPE was used in 75% of cases (presumably because of its good chemical resistance), LLDPE in 22% of cases, and PVC in only 3% of cases. Although LLDPE only represented 22% of cases examined in total, there appeared to be a trend of increasing popularity of LLDPE in the more recent cases and for heap pads in the design phases LLDPE was being considered in about 5% of cases. The thickness of GMB used was 1mm in only 5% of cases, with PVC being used for heaps less than 2 m and HDPE for heaps less than 5 m. A thickness of 1.5 mm was used in 46% of cases (4% HDPE, 6% LLDPE) with a maximum heap height of 12 m for HDPE and 9 m for LLDPE. A thickness of 2 mm was used in 45% of cases (31 % HDPE, 14 % LLDPE) with the HDPE being used of heap heights up to 238 m and LLDPE up to 16 m. The 2.5 mm GMB was only used in four cases (4%); 2 involved HDPE and heaps of 14 and 18 m in height while LLDPE was used in two cases with heaps of 16 m. Thus the review of the cases examined in this study indicates that 1.5 mm HDPE is the most commonly used GMB and 2 mm HDPE the next most common (they represent 71% of cases) Overliner material The overliner above the GMB serves a number or purposes including contribution to the drainage the PLS to a collection point. The collection of the PLS is enhanced by the use of geopipes. Typical diameters for geopipes include 1, 15, and 18 mm (Thiel and Smith 24) 14

27 and they are generally spaced 2 meters a part (Fourie et al. 21). This drainage layer is also comprised of gravel or coarse-grained sand with a gradation as needed to achieve the design hydraulic conductivity and shear strength under the maximum ore load while providing adequate puncture protection. In addition, the overliner needs to be selected such that it prevents damage to the underlying GMB. Ideally, a single layer will meet both objectives. However, in some cases the properties of the drainage layer may be such that it would damage the GMB and in these cases the overliner layer above the GMB may include a protection layer to separate the drainage layer from the GMB. When used, the protection layer may be a sand and gravel mixture, silty soil, or clayey soil (Lupo and Morrison, 27). However, geotextile protection layers are not commonly used in heap leach pads because of issues related to the high costs associated with construction of this layer over the large pad areas (15 to 2 ha) and concerns about stability given the very high fills, angle of repose lift slopes, relatively steep overall slopes, elevated phreatic surfaces, relatively high levels of saturation above the phreatic, and very high seismicity (design PGAs of.35 to.4 in Chile and Peru, and as high as.49 in Turkey). Typically, the drainage layer will have a minimum desired saturated hydraulic conductivity of 1 x 1-4 m/s with well-graded rounded gravel or coarse sand being preferred (Lupo 25), although values of 5x1-5 m/s are common in practice. The properties of the overliner materials were only reported for 15 of the cases examined in this study. The overliner had a maximum size of 12 mm (.5 ) in 2%, 19 mm (.75 ) in 53%, 25mm (1 ) in 13%, and 38mm (1.5 ) in 13% of these cases. The overliner thickness was reported for 52 cases examined in this study. Most commonly (29% of cases) it was 3 mm thick. However thickness varied substantially. The overliner thickness was 4 mm in 4% of cases, 5 6 mm in 27% of cases, 7-1 mm in 1% of cases, and 2 25 mm in 23% of cases. These larger 15

28 thicknesses are for dynamic or on/off pads where the stacking and off-loading equipment works directly over the overliner and the loading is repeated up to several times a year for possible 2 or more years. In these cases 1, mm tends to be the minimum for on-off pads, and 2, mm is relatively common when large mine haul trucks are used to haul the ore onto the pad, and then large rubber-tired loaders (such as Cat 992) are used to stack the ore. 2.3 Experimental Method Test Apparatus The experiments forming the present study were conducted in a cylindrical steel pressure vessel with an inside diameter of 59 mm and height of 5 mm (Figure 2.3). A vertical pressure of up to 31 kpa can be applied by fluid pressure acting on a flexible rubber bladder. The horizontal pressures developed correspond to essentially zero lateral strain due to the limit on the outward deflection provided by the very stiff steel cell (Brachman and Gudina 22; Krushelnitky and Brachman 29). Friction along the cell walls was minimized using two layers of.1-mm-thick polyethylene (PE) sheet with high-temperature bearing grease between the PE sheets. One PE layer was attached to the wall of the test apparatus while the other, moved with the overliner material. The friction treatment was protected by a series of 45-mm-wide HDPE sheets arranged in rings with a vertical spacing of 5 mm between the rings. This friction treatment has been shown to reduce the boundary friction to less than 5 (Tognon et al. 1999). With this treatment, in excess of 95% of the vertical stress has been shown to be transferred to the GMB (Brachman and Gudina 22). 16

29 2.3.2 Underliners considered Table 2.1 summarizes all the experiments discussed in this paper. Six underliners denoted UL1- UL6 (Figure 2.2) were examined with the choice being guided by the information on underliners gained from 62 projects considered in this study (discussed earlier) as well as the grain size envelope of underliner materials compiled from several mining projects by Lupo and Morrison (27). In each case, particles coarser than.6 mm were sub-angular and angular. The grading curves of the underliners are described below. The as-placed and final water content and dry densities of the underliners are given in Table 2.2. Table 2.3 gives the values of D 2, D 4, D 6, D 8 and D 1 and a parameter defined as the slope index, sx-y (which gives the relative slope of the grading curve in Figure 2.2 between two particle sizes x and y (e.g., between D 1 and D 8 s 1-8 = 1/(log 1 D 1 log 1 D 8 ) for each underliner material). Underliner UL1, used in Tests 1 and 1A, had a maximum particle size of 38 mm and negligible fines (Figures 2.2 and 2.4a). Except for the particles less than D 15 (which were smaller than for the overliner) this underliner was essentially the same as the overliner. It was selected to assess whether puncturing of the GMB would occur under these conditions. In Test 1A, additional silty-sand was used to fill in the voids between gravel particles in the top layer in contact with the GMB. Tests 2 and 2A used an underliner with grading curve UL2 (Figures 2.2 and 2.4b) which was selected to be the coarser bound of the cases examined by Lupo and Morrison (27). This material had a larger maximum particle size (8 mm) than UL1 (38 mm) but had 15% fines. Underliner UL3 (Tests 3 and 3A; Figures 2.2 and 2.4c) had the same 8 mm maximum particle size and the same 15% fines as UL2, but the distribution of particles for UL3 was very well graded as compared to UL2 which exhibits a sharp change in the slope of the grading curve at 17

30 about D4 (Table 2.3 and Figure 2.2). Thus a comparison of results from Test 2 with those from Test 3 will allow an assessment of the effect of the sand and gravel grain size distribution between the same two limits. Underliner UL4 (Test 4; Figures 2.2 and 2.4d) was well graded like UL3 and had the same 15% fines, but had a smaller maximum particle size of 1 mm. Thus a comparison of the results from Tests 3 and 3A with Test 4 allows an assessment of the effect of the maximum particle size (8 mm versus 1 mm) for a well graded material with similar fines. Underliner UL5 (Tests 5, 5A and 5B; Figures 2.2 and 2.4e) was a silty sand with a maximum particle size of 2. mm and 25 % fines. Underliner UL6 (Test 6; Figures 2.2 and 2.4f) was well graded with a maximum particle size of.9 mm and 7 % fines. It corresponds to the finer bound of the cases examined by Lupo and Morrison (27). Test series 7 and 8 involved a composite liner with the GMB over a geosynthetic clay liner (GCL) which was underlain by silty sand (UL5) foundation layer. The silty sand was compacted using standard Proctor energy to a maximum dry density of 175 kg/m 3 at standard Proctor optimum moisture content of 11.4%. The GCL (Bentofix NSL manufactured by TAG Environmental Inc., Barrie Ontario, Canada) had a minimum average roll value (MARV) bentonite mass per unit area (MA) of 366 g/m 2 and was needle-punched with a woven carrier geotextile (MARV MA = 15 g/m 2 ). The needle-punched fibers were thermally fused to the carrier geotextile. The GCL was installed with the nonwoven cover geotextile in contact with the GMB (i.e., with the woven carrier geotextile on the silt sand). 18

31 Test series 7 and 8 differed in terms of the manner in which the GCL was hydrated. For Test series 7, the GCL was hydrated for 7 days under a confining stress of 2 kpa. This resulted in an initial water content of 86±1%. For Test series 8, the GCL was placed on the silty sand foundation prepared as it was for Test series 7 (i.e., at optimum moisture content of 11.4%) but the moisture content of the underlying soil was increased to 2% (field capacity) by adding water to the subgrade before placing the GCL. The off-the-roll gravimetric water content of GCL was 7%. The GCL was hydrated from the foundation layer during the test in the cell at a pressure of 2 kpa. The thicknesses of GCL before and after each test (obtained using the measurement technique developed by Dickinson and Brachman 26) are reported in Table 2.4. Test series 9 and 1 involved a composite liner with the GMB over a compacted clay liner. The clay was Halton Till and had a liquid limit of 26%, plastic limit of 16% and 32% clay size ( 2 μm). For standard Proctor compaction, the clay had a maximum dry density of 19 kg/m3 at optimum water content, of 12%. For Test series 9, the clay was compacted at standard Proctor optimum water content of 12% (Tests 9 and 9A). For Test series 1 (Tests 1 and 1A) it was compacted at 16% (i.e., standard Proctor optimum plus 4%) with sufficient energy to achieve the same compacted dry density as in Test series 9 (i.e., the as-placed dry density was 19 kg/m 3 for both Test series 9 and 1) Test Procedure In each experiment, the underliner material (or foundation soil for the experiments with a GCL) was compacted in three 5-mm-thick lifts with a modified compaction procedure used to achieve the same compaction energy as used in the standard Proctor test but allowing for the larger size of the test apparatus compared to the standard Proctor mould. After compaction of the underliner (and placing of the GCL when a GCL was used), a 27-mm-square,.4-mm-thick, soft lead 19

32 sheet was placed in the centre of the cell to permanently preserve the deformation of the GMB after removal of the load. Except for Test 1A, no protection layer was placed above or below the GMB. In Test 1A, a nonwoven needle-punched geotextile with mass per unit area of 54 g/m 2 was placed directly above the GMB as a protection layer. A smooth 1.5-mm-thick, 57-mm-diameter GMB was then placed on top of the underliner material and lead sheet. Except for Test 2A, the GMB used was HDPE. In Test 2A, 1.5-mm-thick LLDPE was used to allow a comparison with the results from Test 2 with a 1.5- mm-thick HDPE GMB. Both GMBs were manufactured by Solmax International Inc., Varennes, Quebec. The measured tensile stress-strain properties of the GMBs are given in Table 2.5. After placement of the GMB, the 3-mm-thick overliner material was placed on the GMB at a water content of 1.2%, without compaction, achieving a bulk density of approximately 158 kg/m3. The grain size distribution of overliner material (OL) used (Figure 2.2) was based on the upper bound of the envelope given by Lupo and Morrison (27). The overliner had D 85 = 3 mm, D 6 = 19 mm, D 3 = 1 mm, D 1 = 2.5 mm, a uniformity coefficient of 7.6, coefficient of gradation of 2.1, and an estimated hydraulic conductivity of 6x1-2 m/s. A separator geotextile was placed above the overliner and then a 5 to 7-mm-thick layer of fine to medium sand was placed on the geotextile to protect the bladder from potential puncture by the overliner gravel. The sand was leveled, another geotextile placed over the sand, and a rubber bladder installed above the geotextile. The bladder was secured into place between the flanges of the test apparatus. The separator geotextiles and sand layer were assumed to have no effect on the GMB deformations and were simply to avoid rupture of the pressure bladder. 2

33 At the start each test, water pressure was applied to the bladder in increments of 2 kpa every 1 minutes (to allow the system to respond to the pressure increment) until the target pressure of 2 kpa was reached. The 2 kpa pressure was applied for 1 hours for Test series 1-6, 9 and 1, and 168 hours for Test series 7 and 8. The experiments were conducted at a temperature of 22 C. After the completion of a test, the test cell was depressurized and the materials removed to allow examination of the GMB. Each GMB sample was examined visually for scratches, signs of yielding, punctures, or notable indentations. Punctures were identified visually with the help of a back-light in a dark room. A mold of the lead sheet was cast to preserve the deformed shape of the lead sheet. The mold was made using a low-shrinkage plaster of Paris paste that was prepared according to the procedure described by Gudina (27). The major indentations were identified on the lead sheet and the surface was scanned using a Laser scanner to quantify indentations and hence allow the evaluation of the strains using the Tognon et al. (2) method. 2.4 Results All experiments were conducted with the same overliner and at 2MPa so that the differences in results are due to the other factors varied (underliner, GMB, or protection layer) rather than the overliner. The effects of these variables will be discussed in the subsections below. Unless otherwise noted the results are for 1.5-mm HDPE GMB and no protection layer Response of Geomembrane for underliners UL1 and UL2 (Tests 1, 1A, 2) Underliner UL1 was the coarsest examined. In this case (Test 1) there were nine pin-hole sized punctures (Figure 2.5a). All were from the bottom of the GMB and hence are attributed to the 21

34 underliner. Six out of nine punctures were on the sides of the indentations; the other three were at the tip. Eight of the nine punctures were outside the 27 mm square lead sheet (placed in the centre of cell) and hence the deformations and strain are not known at most of the puncture locations. Brachman et al. (211) described these punctures as ductile tears. At the one location where a puncture was over the lead sheet, the indentation was 5.4 mm deep, 25 mm wide and the maximum strain was 33%. The lead sheet was not torn at this location. The eight largest indentations in the lead sheet were scanned. All large indentations were from the bottom and resulted from gravel in the underliner. The maximum indentation depth was 8.3 mm while the average depth of the eight largest indentations was 5.8 mm; thus at the location of the puncture, the depth of the indentations was less than the average of the eight most prominent indentations in the lead sheet. The maximum strain was 4% and for the eight indentations scanned (Figure 2.6a) the average strain was 28%. Toward the centre of the lead sheet there were two indentations with strains (4% and 36%) larger than that at the puncture (33%) which coincided with the lead sheet and was located near the edge of the lead sheet. It is not known whether the fact that most punctures were outside the lead sheet is coincidence or observer effect (i.e., the use of the lead sheet to measure strains reduced the probability of a puncture where the lead sheet was present, possibly because it provided some protection to the GMB). However, it is known with certainty that there were: (a) many punctures, and (b) excessive strains at locations where there were no short-term punctures. Thus, this underliner was too aggressive for the GMB and hence unsuitable for these conditions. An experiment was conducted using the same underliner (UL1) and overliner but where silty sand was sprinkled on the top of underliner to fill in the upper voids and a 54 g/m 2 needlepunched nonwoven protection geotextile was placed on the top of the GMB. In this case (Test 22

35 1A) there were no punctures and the sand layer appears to have been effective to this extent. Since none of the punctures in Test 1 were from the overliner, the presence of the geotextile above the GMB is not considered to have affected puncturing. However despite the thin sand layer below and the geotextile protection layer above the GMB, there were still significant indentations and strains. The seven most prominent indentations (with strains 6%) were scanned. Of these, three were from the bottom and four from the top. The maximum indentation was 6 mm deep and the average depth was 4.3 mm. Thus, the indentations were a little less severe in this case than in Test 1; however the maximum strain was still large. The profile of the indentation giving the largest tensile strain (38%) is shown in Figure 2.7a and the strains calculated from this profile using the Tognon et al. (2) method is shown in Figure 2.7b. The other six indentations scanned had lower strains (Figure 2.6b) of 18% or less and were smaller than at any of the eight indentations scanned for Test 1 (minimum of 2%, Figure 2.6a). This suggests that the surface treatment of the underliner did have a beneficial effect. However with three of the four largest strains (18%, 17% and 14%; Figure 2.6b) being induced by indention from the overliner it is apparent that the 54 g/m 2 geotextile protection layer was not sufficient to prevent significant strains due to the overliner. Test 2 examined UL2 (Figure 2.2) which corresponded to the coarser bound of the cases examined by Lupo and Morrison (27). For this case, there were five punctures (Figure 2.5b) with one of them being above the lead sheet (near the edge). The strain of 34% for the puncture above the lead sheet was essentially identical to the 33% for Test 1 with UL1. All significant indentions in the lead sheet/gmb were from the underliner below. For the eight indentations considered worthy of scanning, the maximum depth was 8. mm. The strains at these indentations (Figure 2.6c) ranged from a maximum of 38% (very close to the 4% for Test 1 and 23

36 UL1) to a minimum of 9%, with five indentations having 2% strain (compared to eight for Test 1 and UL1). This while UL2 was a little less aggressive than UL1, this underliner was also too aggressive for the GMB and hence unsuitable Effect of maximum particle size and grading curve with 15% fines (UL2, UL3 and UL4) Underliners UL2 and UL3 have the same maximum particle size (8 mm) and both have 15% fines but UL3 is very well graded well graded (with very consistent slope index values of s 1-8 = 1.1, s 8-6 =1.5, s 6-4 =1.6, s 4-2 = 1.5 ) as compared to UL2 which exhibits a sharp change in slope at about D 4 (slope index values of s 1-8 = 2.3, s 8-6 = 2.2, s 6-4 = 2.2, s 4-2 =.7 ;Table 2.3 and Figure 2.2) and so a comparison of the results for Test 2 with those for Tests 3 and 3A allows an assessment of the effect of the grading of the underliners on GMB performance. The effect was significant. Whereas there were five punctures in Test 2 (UL2), there were no punctures in the duplicate Tests 3 and 3A (UL3). For Test 3 there were five indentations scanned. Peggs et al. (25) suggested a maximum allowable strain in an HDPE GMB of 6% for good long-term GMB performance. Taking this as a limit, two indentations were classified as significant (i.e., having a strain greater than 6%). These two were caused by gravel in the underliner and gave strains of 13% and 9% (Figure 2.8a). The other three indentations scanned were from the overliner but the maximum strain for these indentions was 5%. In contrast, Test 2 (UL2) there were eight indentations (all from the underliner) with strains 9% (Figure 2.6c). Test 3A was a duplicate of Test 3 and was conducted to assess variability in results and confirm that the much better performance observed for UL3 than UL2 was not an anomaly. The maximum depth of indentation was 3.2 mm for Test 3 and 5.7 mm for Test 3A, compared to 8. mm for Test 2. In neither Test 3 nor duplicate 3A was there any puncture (compared to five 24

37 punctures in Test 2). To all practical purposes, the maximum strains (13% and 14%) in the duplicate experiments over UL3 (Figure 2.8a) were identical and much less than the maximum strain of 38% in Test 2 (UL2). In Test 3, the two indentations with the largest strains were from the underliner with an average strain of 11%. For Test 3A the four indentations with the largest strains were from the underliner and had an average strain of 12.5% which was very close to the 11% for Test 3. Thus there appears to be very consistent results from these duplicate tests over UL3 which, when compared to Test 2 (UL2), suggest that the shape of the grading curve (Figure 2.2) is much more significant, in terms of potential puncture and the magnitude of the strains, than the maximum particle size. To investigate the effect of the maximum particle size, two well graded underliners with 15% fines but very different maximum particle sizes (8 mm for UL3 and 1 mm for UL4) were examined. There were no punctures for either underliner (Tests 3, 3A or Test 4). The maximum depth of indentation in the lead sheet was 3.2 mm (Test 3) and 5.7 mm (Test 3A) over UL3 and 2.6 mm for UL4, however the source of the maximum indentations was different. Over UL3, two (Test 3) and four (Test 3A) of significant indentations were from the underliner; over UL4 there were only two significant indentations (i.e., giving > 6% strain) and they were from the overliner. The maximum strain in Test 4 (UL4) of 11% (Figure 2.8b) was a little smaller than the maximum strains from the underliner for Test 3 and 3A (i.e., 13% and 14%) and a little larger than the maximum strains due to the overliner in Test 3 and 3A (i.e., 5% and 9% respectively). Considering the underliners examined above (UL2, UL3 and UL4), it may be concluded that for a well graded material the maximum particle size (at least ranging from 1 mm to 8 mm as examined here) can affect the source and to a much lesser extent the magnitude of the strains 25

38 in the GMB, but that the shape of grading curve has a much greater effect on the potential for puncture and the magnitude of the strains in the GMB than the maximum particle size Geomembrane performance for underliners UL5 and UL6 with 15% fines The underliners consider above all had gravel, and in some cases cobbles. The underliners considered in this section were silty sand (UL5) and sandy silt (UL6) with no gravel in either case (Figures 2.2). In both cases it was expected that the overliner would dominate in terms of being the source of indentations; these experiments, and those for GCLs and compacted clay discussed later, were to examine how the nature of the underliner affects the indentations and strains caused by a given overliner. Triplicate experiments were conducted over UL5 (silty sand) to assess variability. The deepest indentations were 2.6 mm, 4. mm, and 3.6 mm and the maximum strains were 18%, 15% and 18% for Tests 5, 5A and 5B respectively (Figure 2.9a). There were 4, 5 and 6 indentations with strains exceeding 6% with average strains of 14.3%, 11.% and 13.7% for Tests 5, 5A and 5B respectively. Thus while there is some variability, the results are relatively consistent. The maximum strains developed over UL5 (15-18%) exceed those obtained over UL3 (13-14%) or UL 4 (11%) suggesting that the absence of gravel in the underliner does not mean smaller strains in the GMB if the underline is well graded. Test 6 (over sandy silt UL6; the finer bound of the cases examined by Lupo and Morrison 27) gave a maximum indentation of 2.6 mm which was within the range observed for UL5. The maximum strain of 13% (Figure 2.9b) was a little smaller than the maximum strains obtained over UL5 (15-18%; Figure 2.9a). There were three indentations exceeding 6% strain 26

39 with an average strain of 1.7% for these indentations (compared to % over UL5). The strains obtained with UL6 were similar to those obtained with UL3 and slightly greater than obtained for UL4 discussed above. Of the granular underliners examined (UL1-UL6), the best performance was for the well graded gravelly sand with some silt (UL4) with all indentations being from the overliner (as they were for ULs 5 and 6 as well) but where it offered sufficient support to minimize the strains in the GMB due to the overliner. Nevertheless even in this case the maximum strain of 11% is almost double the maximum recommended by Peggs et al. (25) for ensuring good long-term performance of the GMB Geomembrane performance with a GCL over sand (UL5) underliner The first GCL case (Tests 7, 7A) involved a GCL underlain by silty sand (UL5) foundation layer where the GCL was hydrated to 86% moisture content at low stress (2 kpa) before application of the heap leach loading which resulted in final water contents of 66% and 62%. The partially prehydrated thickness just prior to the experiment (Table 3) was 8.7 (± 1.6) mm and 9.3(± 1.6) for Tests 7 and 7A respectively. The loading reduced these GCL thickness to 5.3 (± 2.4) mm and 6. (± 2.) mm. Part of this reductions (as represented by the changes in the average thickness) was due to consolation under 2 kpa average stress. However the increase in the variability of the thickness represents the effect of local indentations in the GCL due to gravel particles in the overliner. As a result, in this case the largest indentations were 5.1 mm and 8. mm deep and the maximum strains (Figure 2.1a) were 14% and 21% for duplicate Tests 7 and 7A respectively. In both cases there were six indentations corresponding to strains exceeding 6% and the average strains for these indentations were 11.% and 15% for Tests 7 and 7A 27

40 respectively. The variability in these duplicate tests was much greater than that observed for any of the cases with a granular underliner alone. In the second case (Tests 8, 8A), the GCL was hydrated from the foundation layer during the test, under a pressure of 2 kpa, to a final water content of 55%. The off-the-roll thickness just prior to the experiment (Table 3) was 8.5 (± 1.3) mm and 7.9 (± 1.1) for Tests 8 and 8A respectively. The hydration under 2 kpa load resulted in final GCL thickness to 6.5 (±.6) mm and 5.8 (±.7) mm. Thus hydration under greater stress (Test Series 8) appeared to give less variability in thickness than was obtained when the GCL had been partially prehydrated before significant load was applied (Test series 7). For Test series 8, the deepest indentations were 3.3 mm and 5.2 mm and the maximum strains were 12% and 18% for Tests 8 and 8A respectively. There were three and five indentations with GMB strains in excess of 6% with average strains for these indentions of 9.7% and 14.8% for Tests 8 and 8A respectively. Thus again there was more variability between duplicate tests than was observed for granular underliners but slightly smaller strains when the GCL was hydrated under load than when it was partially prehydrated before loading. Test series 7 and 8 suggests that when on a relative deformable (partially prehydrated) GCL, the indentations and strains are very sensitive to the arrangement of the particles in the overliner (much more so than over a firmer foundation). As was the case with the granular underliners, the strains in the GMB may be too large for good long-term performance but there were no punctures for any of the experiments involving GCLs (or ULs 3-6) for the conditions examined Geomembrane performance with a compacted clay liner The experiments on clay compacted at its optimum water content gave maximum indention depths of 5.4 mm and 6. mm and maximum strains of 25% and 18% for Tests 9 and 9A 28

41 respectively. There were 7 and 5 significant indentations (i.e., with strains exceeding 6%) with average strains of 15.9% and 14.6% for Tests 9 and 9A respectively. When the clay was compacted at a water content of optimum plus 4%, the maximum indentation depths increased to 8.5 mm and 7. mm while the maximum strains increased to 36% and 33% for Tests 1 and 1A respectively. Again there were 7 and 5 significant indentations but this time with average strains of 25.3% and 23.6% for Tests 1 and 1A respectively. These are very large strains although there were no punctures (either above or outside the lead sheet) for any compacted clay case examined. The strains in the GMB obtained when the liner as compacted 4% wet of optimum were considerably greater than those when it was compacted at optimum, indicating again, that the more deformable the foundation the greater the strains induced in the GMB from a given (coarse) overliner HDPE versus LLDPE (Tests 2 and 2A) To assess the effect of HDPE versus LLDPE, 1.5-mm-thick GMBs of HDPE and LLDPE (Table 4) were used over underliner UL2, all other things being the same. The LLDPE was from the upper end of the LLDPE range of densities (to give better chemical resistance). The results for Test 2 using HDPE were discussed above and here only comparative numbers are given with respect to Test 2A with the LLDPE. In both cases there were punctures (five in Test 2 and three in Test 2A; Figures 2.5 b and c) with one puncture above the lead sheet in each case. For the HDPE, the puncture occurred at a location with a maximum strain of 34% and in the LLDPE it corresponded to an indentation with a maximum strain of 17% (at a location relatively close to the middle of the lead sheet; Figure 2.5c). As might be expected from two tests with a similar underliner and overliner, the indentation geometries observed for HDPE and LLDPE were 29

42 similar. The maximum depth of the indentations were 8. mm and 7.6 mm for Test 2 and 2A respectively and in both cases all major indentations were from gravel in the underliner. The maximum strains were 38% and 31% (Figure 2.6 c and d) although for Test 2A with the LLDPE there were only two strains greater than 2% (compared to five for Test 2) but, as noted above, there was a puncture at a strain of 17% with the LLDPE. The difference in strains between the two tests is attributed to the variability associate with using coarse gravel in two tests where the strains will be highly dependent on the location and orientation of individual gravel particles rather than the effect of the choice of GMB. Thus, at least for this LLDPE, there was no apparent improvement in performance for the LLDPE versus the HDPE over the same underliner (UL2). 2.5 Summary and conclusion A review of 92 heap leach projects from 15 countries indicated that: In approximately 51% of cases, the ore heights were 5 m or less (i.e., 1 MPa of applied pressure) although these were either older cases or dynamic heaps. In 9% of cases the heaps were 1 m or less ( 2 MPa), but in 1% of cases the ore height exceeded 1 m with a maximum of 238 m ( 4.8 MPa). The underliner was contained considerable fines (possibly clay) in 48%, native soil in 9%, a GCL in 5%, tailings in 4%, and silt/sand in 3% of cases, but was not given in 3% of cases. HDPE was used in 75% of cases, LLDPE in 22% of cases, and PVC in only 3% of cases; although LLDPE was being considered in about 5% of cases currently in the design phase. 3

43 1.5-mm-thick GMB was the most common, being used in 46% of cases (4% HDPE, 6% LLDPE). 2-mm-thick GMB was used in 45% of cases (31 % HDPE, 14 % LLDPE). 2.5-mm-thick GMB was only used in 5% of cases. The overliner had a nominal size of 12 mm in 2%, 19 mm in 53%, 25mm in 13%, and 38mm in 13% of cases where it was reported. Experiments were conducted in a cylindrical steel pressure vessel with an inside diameter of 59 mm and height of 5 mm. All experiments were conducted with the same (gravel, some sand) overliner at 22 o C and 2MPa. Attention was focused of the effect of different underliners on puncturing and GMB strains in a 1.5 mm HDPE GMB, although the effects of a protection layer and a 1.5 mm LLDPE GMB were also examined. For the conditions examined, the following conclusions were reached: When the GMB was over an underliner of gravel with some sand or a gravel and sand, there was a significant number or punctures and strains of 38-4% in the GMB. Thus, these underliners were not suitable for the conditions examined. When the underliner was a well graded sand and gravel with some silt, there were no punctures and the maximum GMB strain from the underliner was 13% and 14% in the duplicate experiments. This underliner had the same 8 mm maximum particle size and 15% fines as a gravel and sand underliner which exhibited a sharp change in the grading curve at about D 4 for which there were five punctures and a maximum strain of 38%, demonstrating the critical role of the underliner grading curve. 31

44 Experiments conducted with two well graded underliners with 15% fines but very different maximum particle sizes of 8 mm and 1 mm gave maximum strains of 13-14% (for the sand and gravel with some silt) and 11% (gravely sand, some silt); however in the former case the maximum strain was due to an indentation from the underliner while in the second case the maximum strain was due to an indentation from the overliner. The shape of grading curve had a much greater effect on the potential for puncture and the magnitude of the strains in the GMB than the maximum particle size. For silty sand and sandy silt underliners (with no gravel in either case) the maximum GMB strains were 15-18% and 13% respectively and all the strains were due to gravel in the overliner. Of the granular underliners examined, the best performance was for the well graded gravelly sand with some silt which offered sufficient support to minimize the strains in the GMB due to the overliner while not inducing any strains directly from the underliner. Nevertheless even in this case the maximum strain of 11% is almost double the maximum recommended by Peggs et al. (25) for ensuring good long-term performance of the GMB. For a GCL partially hydrated prior before application of the heap leach loading, the maximum strains were 14% and 21% for duplicate tests. The magnitude of strains and variability was slightly reduced when the GCL was hydrated from the foundation layer under 2 kpa pressure during the test, with the maximum strains in duplicate test being 12% and 18%. In both cases there was more variability between duplicate tests than was observed for granular underliners, indicating a greater sensitivity to the arrangement of the particles in the overliner when the GCL was above the GCL than when above a granular underliner. 32

45 Duplicate tests of GMBs on clay compacted at optimum water content gave maximum strains of 25% and 18% compared to maximum strains of 36% and 33% for duplicate tests with a GMB over clay compacted at a water content 4% wet of optimum (the plastic limit). These are very large strains although there were no punctures for the compacted clay cases examined. The more deformable the foundation, the larger are the indentations and strains induced by a given overliner. Thus the short- and long-term performance of a GMB will not only depend on the grain size distribution and the size of particles in the underliner, but also on the deformability of the underliner and the interaction between the overliner particle size distribution with that deformability. This and the fact that there were GMB strains well in excess of 6% in all cases examined, suggests the need for a future study of the effects of different overliner grainsize distributions on the strains developed in GMBs. Tests conducted with a 1.5-mm-thick HDPE and 1.5-mm-thick LLDPE GMB over a gravel and sand underliner, indicated very similar behaviour with puncturing of the GMB and maximum strains exceeding 3% on both cases. Thus, at least for the conditions examined, there was no apparent improvement in performance for the LLDPE versus the HDPE. A 54 g/m2 geotextile protection layer above the GMB was not sufficient to prevent significant strains (18%, 17% and 14%) in the GMB due to the overliner. 33

46 2.6 References Abdelaal, F.B., Rowe, R.K., Smith, M. and Thiel, R. (211) OIT depletion in HDPE geomembranes used in contact with solutions having very high and low ph, 14th Pan- American conference of Soil Mechanics and Geotechnical Engineering, Toronto, October, paper #483, CD-ROM, 7p. Brachman, R.W.I., Rowe, R.K., Irfan, H., and Gudina, S High-pressure puncture testing of HDPE geomembranes, 64th Can. Geotech. Conf., Toronto, CD-ROM, 7 p. Brachman, R.W.I., and Gudina, S. 22. A new laboratory apparatus for testing geomembranes under large earth pressures, 55th Can. Geotech. Conf., Niagara Falls, ON, pp Brachman, R.W.I., Moore, I.D., Rowe, R.K., 2. The design of a laboratory facility for evaluating the structural response of small- diameter buried pipes. Canadian Geotechnical Journal 37, Breitenbach, A. J. (25). Heap Leach Pad Design and Construction Practices in the 21st Century, Vector Colorado LLC, 9 p. Breitenbach, A. J., and Thiel, R. (25). A tale of two conditions: Heap leach pad versus landfill liner strengths. North American Geosynthetics Society (NAGS)-Geosynthetic Institute (GSI) Conference. Proceedings from a conference held in Las Vegas, Nevada, USA, December, 25. Canadian Geotechnical Society 26. Canadian Foundation Manual, 4th Edition. The Canadian Geotechnical Society c/o BiTech Publisher Ltd. 34

47 Dickinson, S., and Brachman, R.W.I. 26. Deformations of a geosynthetic clay liner beneath a geomembrane wrinkle and coarse gravel, Geotextiles and Geomembranes, 24(5): Environmental Agency, 26. Methodology for cylinder testing of geomembranes in landfill sites, Environmental Agency, U.K. Fourie, A. B., Bouazza, A., Lupo, J. F., Abrão, P. (21). Improving the performance of mining infrastructure through the judicious use of geosynthetics. 9th International Conference on Geosynthetics. Proceedings from a conference held in Guarujá, Brazil, May 21, pp Gudina, S. 27. Short-term Physical Response of HDPE Geomembranes from Gravel Indentations and Wrinkles. PhD thesis, Department of Civil Engineering, Queen s University, Kingston, Ontario. Krushelnitzky, R.P., and Brachman R.W.I. 29. Measured deformations and calculated stresses of high-density polyethylene pipes under very deep burial, Canadian Geotechnical Journal, 46(6): Lupo, J. F. (21). "Liner System Design for Heap Leach Pads." Geotextiles and Geomembranes, 28(2), Lupo, J. F. (28). Liner system design for tailings impoundments and heap leach pads. Tailings and Mine Waste. Proceedings from a conference held in Vail, Colorado, USA, October p. 35

48 Lupo, J. F. (27). Design and Operation of Heap Leach Pads.Golder Associates Inc., Lakewood, Colorado, USA, 42 p. Lupo, J. F., and Morrison, K. F. (27). "Geosynthetic Design and Construction Approaches in the Mining Industry." Geotextiles and Geomembranes, 25(2), Lupo, J. F. (25). Heap Leach Facility Liner Design. Golder Associates Inc., Lakewood, Colorado, USA, 25 p. Peggs, I.D., Schmucker, B., and Carey, P. 25. Assessment of maximum allowable strains in polyethylene and polypropylene geomembranes. In: Geo-Frontiers 25 (CD-ROM). American Society of Civil Engineers, Reston, VA. Rowe, R.K (212). Short and long-term leakage through composite liners, The 7th Arthur Casagrande Lecture, Canadian Geotechnical Journal, 49(2): Seeger, S. & Muller, W. (23). Theoretical approach to designing protection: selecting a geomembrane strain criterion. In: Dixon, N., Smith, D.M., Greenwood, J.H., Jones, D.R.V. (Eds.), Geosynthetics: Protecting the Environment. Thomas Telford, London, pp Shercliff, D.A., Designing with the cylinder test. In: Proceedings of the Polluted and Marginal Land Conference, Brunel University, London. Smith, M. E. (24). Applying the "Seven Questions" to Heap Leaching. The Mining Record, (6). Thiel, R., and Smith, M. E. (24). State of the Practice Review of Heap Leach Pad Design Issues. Geotextiles and Geomembranes, 22(6),

49 Tognon, A.R., Rowe, R.K. and Moore, I.D. 2. Geomembrane Strains Observed in Large- Scale Testing of Protection Layers. Journal of Geotechnical and Geoenvironmental Engineering, 126(12):

50 Table 2.1 Summary of the 19 experiments conducted at 22oC and an applied pressure of 2 kpa. All experiments except Test 2A were for a 1.5 mm HDPE geomembrane. Test 2A was for a 1.5 mm LLDPE geomembrane Underliner Test Designation 1 UL1 1A UL1* 2 2A UL2 3 3A UL3 4 UL4 5 5A 5B UL5 Underliner Description Gravel, some sand Gravel and Sand Sand and Gravel, some silt Gravely Sand, some silt Silty Sand 6 UL6 Sandy Silt 7 GCL 7A hydrated at 2 kpa Silty Sand 8 8 A GCL hydrated at 2 kpa Silty Sand 9 9A 12% w Clay 1 1A 16% w Clay * A thin layer of silty sand was placed over the gravel Underliner description is based on classification given in Canadian Foundation Engineering Manual (24) 38

51 Table 2.2 Properties of underliners considered Test Underliner Initial Water content (ω)% Final Water content (ω)% Initial dry density ρ dry (kg/m 3 ) Final Dry density ρ dry (kg/m 3 ) 1 UL1 1A 2 2A 3 3A UL1* UL2 UL3 4 UL4 5 5A 5B UL5 6 UL6 7 7A GCL hydrated at 2 kpa 8 GCL hydrated at 8 A 2 kpa 9 9A 1 1A 12% w 16% w

52 Table 2.3 Grain size properties of underliner materials Grading Curve Slope Index 2 (mm) 4 (mm) 6 (mm) 8 (mm) 1 (mm) s 1-8 s 8-6 s 6-4 s 4-2 UL UL UL UL UL Slope Index: = indicates relative slope of the grading curve in Figure 2.2 between two particle sizes (e.g., between D 1 and D 8: s 1-8 = 1/(log 1 D 1 log 1 D 8 ) Table 2.4 GCL thickness before and after each test Test Initial thickness (mm) Final thickness (mm) Mean Minimum Maximum Mean Minimum Maximum A A Table 2.5 Index stress-strain properties (measured in the machine direction) of the 1.5-mm-thick HDPE and LLDPE geomembranes studied (Tested following ASTM D6693 unless otherwise noted) HDPE LLDPE* Property Mean Standard deviation Mean Standard deviation Yield strength (kn/m) Break strength (kn/m) Yield strain (%) Break strain (%) Crystallinity (%)(ASTM E794) * Abdelaal et al. (212) 4

53 Figure 2.1: Histogram of heap leach ore height with estimated vertical stresses for 72 cases where data were available. Unit weight of ore is assumed as 2 kn/m 3. 41

54 Percent finer by mass SILT ( mm) Lupo and Morrison,(27) Finer bound UL6 UL5 SAND (.6-2. mm) UL4 UL UL3 Lupo and Morrison, (27) Coarser bound Grain size (mm) GRAVEL ( mm) UL2 OL COB > 6mm Figure 2.2: Grain size distribution of underliners (UL1-UL6) and overliner examined (OL) in this study and bounds of underliner in projects reported by Lupo and Morrison (27). 42

55 Applied pressure P Rubber bladder Sand Seperator geotextile Overliner Geomembrane (GMB) Lead sheet Underliner 59 Figure 2. GLLS Set up for TEST No.1 (All Dimensions in mm) Figure 2.3: Cross section through a typical test cell used in present study and test setup for Test #1; all dimensions in mm. Underliner and overliner material shown schematically only and not drawn to scale. All dimensions are in mm. 43

56 (a) (b) (c) (d) (e) (f) Figure 2.4: Photograph of (a) underliner material UL1 as used in Test 1, (b) underliner material UL2 as used in Test 2, 2A, (c) underliner material UL3 as used in Test 3, 3A, (d) underliner material UL4 as used in Test 4, (e) underliner material UL5 as used in Test 6. 44

57 (a)test 1 (b)test 2 (c)test 2A Figure 2.5: Photographs of bottom of geomembrane after test with puncture locations shown by arrows for: (a) Test 1 (UL1), (b) Test 2 (UL2) HDPE geomembrane, and (c) Test 2A (UL2) LLDPE geomembrane. 45

58 (a) Test 1 B B B B B B B B B B B B B B B B B B B B B B B B Figure 2.6: Strains calculated for scanned indentation for: (a) Test 1 (UL1), (b) Test1A (UL1 modified and geotextile protection over geomembrane) (c) Test 2 (UL2, HDPE), and (d) Test 2A (UL2, LLDPE). Indentations were from the top (overliner) unless a B indicates it was from the bottom (underliner). 46

59 Tension Compression Figure 2.7: The indentation from a gravel particle in the underliner giving the maximum strain for Test 1A: (a) Deformed shape, and (b) calculated strain. Geometry, h is the height of the indentation measured from the deepest or highest (in this case highest) point of the indentation. Tensile strain plotted as positive. Note that there is tension through the entire geomembrane thickness on sides of the indentation. 47

60 (a) Test 3 and 3A B B B B B B Figure 2.8: Strains calculated for key indentations in (a) Tests 3, 3A for underliner UL3, and (b) Test 4 with underliner UL4. Indentations were from the top (overliner) unless a B indicates it was from the bottom (underliner). 48

61 Figure 2.9: Strains calculated for key indentations for (a) Tests 5, 5A, 5B with underliner UL5, and (b) Test 6 with underliner UL6. All indentations were from the top (overliner). 49

62 Figure 2.1: Strains calculated for key indentations for (a) Tests 7, 7A with a prehydrated GCL, and (b) Tests 8 and 8A with GCLs hydrated from silty sand subgrade under 2 kpa stress. All indentations were from the top (overliner). 5

63 Figure 2.11: Strains calculated for key indentations for (a) Tests 9, 9A with a clay liner compacted at standard Proctor optimum water content, and (b) Tests 1 and 1A with a clay liner compacted at water content of standard Proctor optimum plus 4%. All indentations were from the top (overliner). 51

64 Chapter 3 Geomembrane behaviour for different overliner materials at large overburden pressures 3.1 Introduction Geomembranes (GMB) are used in heap-leach mining applications to line the bottom of heap leach pads (Thiel and Smith 24). Heap-leaching involves passing a chemical solution through a pile of crushed ore to extract the desired mineral (Lupo 21). In these applications, it is important to minimize leakage through the base to limit environmental impact from the chemical solution and also increase mineral and solution reagent recovery. Leakage through the GMB can be reduced by limiting the number of holes that develop in the GMB (Rowe et al. 24). Holes can occur during placement of overlying materials or from local indentations from overlying or underlying coarse particles when subject to the weight of material above the GMB. Current practice to minimize the number of holes is to conduct performance tests with proposed soil materials, often conducted for 24 to 1 h to assess whether the GMB punctures or not (e.g., Lupo and Morrison 27, and Thiel and Smith 24). The focus of these short-term tests is almost exclusively on whether the GMB punctures or not. In addition to whether there is puncture in a short term test, there may also be a need to limit the tensile strains that develop in the GMB to ensure adequate long term performance (Rowe et al. 24). While much is known on the GMB strains that may develop in municipal solid waste landfill applications (e.g., Tognon et al. 2, Brachman and Gudina 28), there is a paucity of data on the tensile strains that may develop for heap-leach GMBs. 52

65 Tensile strains developed in heap-leach GMBs from the material placed beneath the GMB (termed as the underliner) have been reported in Chapter 2. These results indicated that the grain size, grain size distribution and compressibility of the material below the GMB affect the resulting GMB strains. The objective of this chapter is to quantify the short term tensile strains induced in the GMB from heap-leach materials placed on the top of GMB, termed as the overliner. Physical experiments conducted in a.59 m diameter pressure vessel at applied vertical pressures up to 3 kpa for 1 hours at a temperature of 22ᵒC are reported for three different overliner materials. Results are reported for tests with a soil protection layer on the top of the GMB to examine its effectiveness at reducing GMB strains. 3.2 Experimental details Apparatus A cylindrical pressure vessel made of steel with an inside diameter of 59 mm and a height of 5 mm and is capable of applying vertical pressures of up to 31 kpa (Figure 3.1) was used to examine the response of a GMB to different overliner materials. Vertical pressure was applied by using fluid pressure acting on a flexible rubber bladder. Horizontal pressures develop corresponding to conditions of zero lateral strain by limiting the outward deflection of the test apparatus. To reduce friction on the vertical boundaries of the test apparatus, a friction treatment comprising two layers of.1-mm-thick polyethylene sheets and high-temperature bearing grease between the PE sheets was used. One sheet was attached to the wall of the test apparatus while the other, was able to move downward with the overliner material. The friction treatment was protected by 6 HDPE strips that were cut to be 45 mm wide and placed in front of the friction treatment in rings, with a vertical spacing of 5 mm between the rings to minimize binding. With 53

66 this friction treatment, the boundary friction has been shown to be reduced to less than 5 (Tognon et al. 1999). For the size of the test apparatus and the applied friction treatment, at least 95% of the pressure applied at the top is calculated to reach the elevation of the GMB (Brachman and Gudina 22) Procedure and Materials Table 3.1 summarizes all the tests discussed in this paper. All tests were conducted with conditions simulating a heap leach pad liner with a GMB overlying an underliner soil material and backfilled with a graded soil material termed as the overliner. The components of the test setup from the bottom to the top are discussed in this section. The grain size curves of material used are given in Figure 3.2. The grain size envelope of underliner materials from several mining projects compiled by Lupo and Morrison (27), and the data collected from several heap leach projects compiled in Table A.1 was used as guidance to select the material placed beneath the GMB. The silty sand underliner used in all tests is towards the finer bound of the envelope reported by Lupo and Morrison (27). The silty sand was compacted in the cell to its Standard Proctor maximum dry density (175kg/m 3 ) at its Standard Proctor optimum moisture content (11.4%) in three 5-mm-thick lifts. After compaction and leveling of the underliner, a 27 mm by 27 mm,.4mm thick, soft lead sheet was placed in the center of the cell to permanently preserve the deformation of the GMB. The lead sheet was placed on the top of underliner. A smooth GMB of 1.5mm thickness with a 57-mm diameter was then placed on top of the underliner material and lead sheet. The GMB used in the all the tests was 1.5 mm thick HDPE manufactured by Solmax International Inc., Varennes, Quebec (with an internal Queen s 54

67 reference name of Solmax Ab). Measured tensile stress strain properties from dog bone shaped specimens of the tested GMBGMB are summarized in Table 3.3. After placement of the GMB, the 3-mm-thick overliner material was placed on the GMB. The grain size envelope of overliner materials from several mining projects compiled by Lupo and Morrison (27) was used as guidance to select the material placed above the GMB in these tests. The grain size curves of the overliners (OL) used in the tests is shown in Figure 3.2. Three different types of overliner material were used and were placed in cell without compaction (a scoop full of gravel particles was gently dropped from a constant height of approximately 5 mm), achieving a bulk density of approximately 155 kg/m 3. OL1 was the coarsest material tested and the gradation for this material matched to the coarser bound of the envelope by Lupo and Morrison (27). OL1 is well graded gravel with D 1 = 5 mm, D 85 = 3 mm, D 6 = 18 mm, D 3 = 8.8 mm, D 1 = 2 mm, a uniformity coefficient (C u ) of 9., coefficient of curvature (C c ) of 2.2. OL2 had similar grain size shape as OL1 but had a smaller maximum and minimum grain size, and it matched the finer bound of the Lupo and Morrison (27) envelope. OL2 had D 1 = 25 mm, D 85 = 17 mm, D 6 = 8 mm, D 3 = 1.9 mm, D 1 =.18 mm, C u = 44, and C c = 2.5. OL3 had the same maximum a grain size and a similar amount of fines as OL2 but had much more sand with D 1 = 25 mm, D 85 = 1 mm, D 6 = 2 mm, D 3 =.23 mm, D 1 =.5 mm, Cu = 4, and C c =.5. The coarse particles were crushed from granite that produced sub-angular and angular particles. Figure 3.3 shows the particle shape of different gravel sizes. A 15 mm thick layer of silty sand protection layer was placed on the top of GMB in only Test series 9. The same soil as that used for the underliner was used for the protection layer. 55

68 A separator geotextile and a 5 to 7mm-thick layer of fine to medium sand were placed on top of overliner. This sand was used to protect the bladder from potential puncture by the coarse gravel. The sand was leveled and a rubber bladder was then installed above the sand with another layer of separator geotextile between the fine sand and the bladder. The bladder was secured into place between the flanges of the test apparatus to provide a mechanical seal. To start each test, water pressure was applied to the bladder in increments of 2 kpa every 1 minutes (to allow the system to respond to the pressure increment) until the required pressure of 2 or 3 kpa was reached. Breitenbach and Thiel (25) have reported typical unit weights of crushed ore in heap leach pads to be between 17.3 to 2.4 kn/m 3. Assuming a unit weight of 19 kn/m 3 and considering 95% of the applied pressure to reach the level of GMB, applied pressures of 2 and 3 kpa corresponds to ore height of approximately 1 and 15 m. The largest pressure in each test was applied for 1 hours. The experiments were conducted at a temperature of 21 ± 2 C. After the completion of the test, the test cell was depressurized and the materials above the GMB were carefully exhumed to allow examination of the GMB. The GMB was visually examined for any scratches, signs of yielding, punctures, or notable indentations. A mold of the lead sheet was then cast to preserve the deformed shape of the lead sheet. The major indentations were identified on the lead sheet and their deformed surfaces were measured using a laser scanner (Appendix B). 56

69 3.3 Results Typical results for Test series 4 (OL1, 3 kpa) The GMB did not puncture in any of the tests in this study and hence the focus of this chapter is on the tensile strains that developed in the GMB. Strains in the GMB were calculated from the measured deformed shape using the method developed by Tognon et al. (2). To illustrate how strains were calculated a typical deformed shape of an indentation producing the maximum tensile strain in Test 4 is plotted in Figure 3.4 (a). In this and all other similar plots, h is the vertical distance above the deepest point of the indentation. The components of membrane and bending strain of the GMB calculated using Tognon et al. (2) are given in Figure 3.4 (b). Tensile strains are taken as positive. Membrane strains (i.e. in-plane stretching) were calculated to increase from zero at the deepest point to a maximum roughly half way up the indentation, while the absolute magnitude of the bending strains (i.e. strains from local changes in curvature) were calculated to be largest at the deepest point and near the top of the indentation. When combining the two components, the resulting strains calculated for the top and bottom GMB surfaces are shown in Figure 3.4 (c). The largest over all tensile strain in all tests generally occurred on the bottom surface and along the side slope of the indentation, located roughly half way up. For example in Test 4 a peak strain of 27% occurred on the bottom surface and 1.5 mm away from the deepest point of the indentation and the maximum all tensile strain was 18% located 3.5 mm away from the deepest point. Three replicate tests were conducted for Test series 4, denoted as Tests 4, 4A and 4B. The deformed shapes leading to maximum strains in each of 4A and 4B are presented in Figure 3.5 and Figure 3.6. For Test 4A the maximum strain of 18% was calculated at the side of the indentation and 3.5 mm away from the deepest point. The shape of the indentation producing the 57

70 maximum strain in Test 4 and Test 4A are similar, but the size of two indentations varies. The indentation in Test 4 is 2. mm deep, and 12 mm wide, and for Test 4A the indentation is almost the same depth, but slightly wider (Figure 3.5a). The indentation producing the maximum strain of 27% in Test 4B is 8. mm deep and 45 mm wide (Figure 3.6a), which is almost six times deeper and four times wider than the indentation in Test 4 but the resulting maximum strain values for both indentations, are the same. For Test 4B the maximum strain also occurred on the side of the indentation and 8. mm away from the deepest point. The key point observed from these measurements is that the steepness of the side slope of the indentation, and its width to depth ratio is the factor governing the resulting strain. In all indentations, the maximum strain was located at the steeper side of the indentation because as the indentation is deeper and narrower the elongation on the side of the slope tends to be larger, thus resulting in higher tensile strains Variability in replicate tests conducted Strains were calculated for at least five of the most prominent indentations in the lead sheet for each test. The plot of the indentation geometry and strains for the indentation producing largest strain from each test are presented in Appendix D, and the largest three strains from each test are reported in Table 1. Triplicate tests were conducted for five of the nine test series. For Test series 6, 7, 8 the maximum strain obtained from each test in a series was one of the three largest strains values calculated for the series. For example for Test series 6, Tests 6, 6A, and 6B gave the maximum strains of 14%, 13%, and 15% respectively and these were the three largest values for the whole test series and the reproducibility of the maximum strain was generally good in replicate tests. In 58

71 contrast, Figure 3.7 shows that for Test series 4 the four largest strains (27%, 27%, 26%, and 22%) were from Tests 4 and 4B while the fifth largest (18%) was the maximum from Test 4A. For Test series 3, the four larges strains (18%, 17% 18% and 17%) were for Tests 3 and 3B with the fifth largest strain (15%) being the maximum strain for Test 3A. Based on the evaluation of all the data, it was considered that the largest three values of strains calculated for a test series were the best representation of strain for a series instead of considering the maximum value from each test. For the remainder of test series conducted in duplicate, the maximum strain for each test was among the largest two strain values from that series. 3.4 Discussion Effect of maximum and minimum grain size (OL1 and OL2) The influence of pressure on GMB strain using OL1, which corresponds to the coarser bound of the cases reported by Lupo and Morrison (27), is shown in Figure 3.8. The two or three largest values of strains from each test series are plotted in Figure 3.8. For the duplicate tests conducted at 5 kpa, a maximum tensile strain of 6% was calculated, which increased to 8% at 1 kpa. Triplicate tests were conducted at pressure of 2 kpa and 3 kpa, and the maximum strains were 18% and 27% respectively. There is an overall trend of maximum strain increasing linearly proportional to the applied pressure as shown in Figure 3.8. Results with OL1 and OL2 are compared in Figure 3.8. Overliners OL1 and OL2 were both well graded with a similar shape to the grain size distribution curve but different D 1 (5 mm for OL1 and 25 mm for OL2), D 1 (2 mm for OL1 and.18 mm for OL2), and OL1 had no grains finer than.6 mm while OL2 had around 18% grains finer than.6 mm. Thus, a 59

72 comparison of the results for OL2 with those for OL1 allows an assessment of the effect of the maximum and minimum grain size on GMB performance. The effect of using OL2 instead of OL1 was significant for pressures of both 2 kpa and 3 kpa. The indentations in the GMB for OL1 at 3 kpa were more pronounced and deeper than for OL2. Based on the indentations captured in the lead sheet (Figure 3.9), the maximum strain at 3 kpa for OL1 was 27% compared to 14% for OL2. For a pressure of 2 kpa, the maximum strain decreased from 18% with OL1 to 12% with OL2. Thus having a smaller maximum particle size and more sand to fit in around the larger particles reduced local indentations in the GMB from overliner particles and reduced GMB strains by a factor of nearly two at 3 kpa Effect of grain size distribution (OL2 and OL3) Overliners OL2 and OL3 had the same maximum particle size (25 mm) and a similar amount of fines (1-15%), but OL3 had 7% of the material finer than 4.75 mm compared to 45% for OL2 (Figure 3.2). This difference in the shape of the grain size distribution curve resulted in smaller GMB strains for the OL with more sand size material (Figure 3.8). At 2 kpa, the maximum GMB strain was 12% for OL2 as compared to 9% for OL3. A similar effect was observed at 3 kpa with the maximum strain being 15% for OL2 and 12% for OL Effect of grain size The performance observed for OL3 was better than OL1 and OL2. To confirm that this was not an anomaly, triplicates tests were conducted. The maximum strains (8%, 9% and 9%) in the triplicate tests for OL3 (Figure 3.8) were much less than three largest strain of 27%, 26% and 27% using OL1 at 3 kpa. Thus there appears to be very consistent results from the triplicate tests. Comparing the maximum strains for OL3 with those for OL1, suggests that the maximum 6

73 and minimum grain size is significant in terms of magnitude of the strains in addition to the shape of the grain size curve (Figure 3.2) as discussed in the previous section Reducing strains with a soil Protection layer In municipal solid waste landfill applications, thick sand protections layers have been shown to be very effective at limiting local indentations and hence strains in GMBs from overlying coarse, poorly graded gravel (e.g., Tognon et al. 2, Brachman and Gudina 28). A 15 mm thick silty sand protection layer was therefore placed beneath OL1 and on top of GMB in Test series 15 to see how effective it would be at limiting strains with heap-leach overliner materials. No discernable indentations were observed in the lead sheet after the test (Figure 3.1). The resulting strains were no greater than 2% for duplicate tests conducted at 3 kpa. The calculated strain here was not from overlying gravel particles but from minor (around.2mm -.23mm) differential settlement of the underliner. A comparison of the maximum strains observed for tests conducted at 3 kpa is shown in Figure Practical applications It is of interest at this point to examine how the calculated strains compare with long-term tensile strain limits that have been proposed in the literature. For example, Peggs et al. (25) have suggested a maximum allowable strain for HDPE GMBs of 6-8% (depending on its stress-crack resistance; 6% being relevant to the GMB used in these experiments), while Seeger and Müller (23) have proposed a more stringent limit of 3% for municipal solid waste landfill GMBs. A 6% strain limit is reached at an applied vertical pressure of only 5 kpa with coarse overliner OL1 directly on top of the GMB, which corresponds to heap-leach pad depths less than 61

74 25 m. Taking even the most optimistic proposed limit of 8% strain, the results here suggest that pressures no greater than 1 kpa can be applied with OL1 directly on top of the GMB. At higher pressures, the finer overliners OL2 and OL3 were effective at reducing GMB strains, but even with these materials, linear interpolation of test results to lower pressures suggests a 6% strain limit is exceeded at vertical pressures greater than approximately 1 kpa and 14 kpa for OL2 and OL3 (heap-leach pad depths of around 5 and 7 m, respectively). Thus while a finer overliner may help reduce tensile strains, for the high vertical pressures expected in deep heap-leach pads, the silty sand protection layer tested would be able to reduce GMB strains to below even the more stringent 3% value. In such a case, it is likely that the tensile strains will be governed by more global strains induced by differential settlements of the base and along side-slopes as discussed by Lupo (21) that should be assessed and mitigated with good geotechnical design. It is acknowledged that there are challenges in placing, and retaining after rainfall, silty sand as an overliner on the GMB for anything but relatively flat slopes that would need to be carefully considered. The concept of limiting local tensile strains for heap leach GMBs may appear to be a dramatic departure from the current prevailing heap leach approach of solely preventing GMB puncture in relatively short-term (e.g., 24-1 h) performance tests. Clearly, if it punctures in a short-term test, then those holes will be present in the long-term; however, if the GMB does not experience short-term puncture because of its significant ability to withstand short-term ductile elongation that does not necessarily mean that it will not rupture in the long-term from environmental stress-crack if tensile strains are excessive. Hence, there is a need for discussion amongst stakeholders of these sorts of heap leach applications as to the potential implications of these large tensile strains. Conceivably, an argument could be made that stress relaxation will 62

75 reduce the tensile stresses associated with these indentations, and thereby, there will be no rupture by environmental stress cracking. However, despite relaxation of tensile stresses, tensile strains remain and with small long-term creep displacements of materials beneath the GMB, there may be propagation of a crack in a GMB indentation such that it can rupture and form a hole even though it was subjected to constant applied force (Sabir 21). Therefore it is suggested here that consideration be given for limiting tensions in the GMB when designing a protection layer from materials above the GMB. However, there is a paucity of data as to what magnitude to which local tensile strains in HPDE GMBs should be limited. Presumably, selection of this limit should involve considerations of what is an acceptable leakage rate and the time frames involved. 3.5 Summary and conclusions Physical experiments were conducted in a.59 m diameter pressure vessel at applied vertical pressures up to 3 kpa for 1 hours at a temperature of 22 C to quantify the tensile strains that developed in a 1.5-mm-thick high density polyethylene GMB for heap leach mining applications with one particular silty sand soil tested beneath the GMB as the underliner. While the GMB did not puncture in any of the short-term tests conducted, tensile strains that developed in the GMB were quantified. The following may be concluded for the conditions examined: 1. Influence of grain size and grain size distribution: Three different soil materials in directly above the GMB (overliners) were tested and it was found that both grain size and grain size distribution impacted the maximum tensile strain. The coarsest overliner tested was OL1 (a coarse gravel with some sand) which matched the upper bound gradation of real heap leach cases reported by Lupo and Morrison (27). The grain size distribution curve for overliner OL2 (gravel and sand) had a similar shape as OL1 but a smaller maximum particle size and more fine 63

76 sand; this case matched Lupo and Morrison s finer bound of overliner materials. In terms of grain size, the maximum strain decreased from 18% with OL1 to 12% with OL2 at an applied pressure of 2 kpa. Overliner OL3 (sand and gravel) had the same maximum particle size and similar amount of fines as OL2, but with a more well-graded distribution than OL2. Thus, in terms of grain size distribution, the maximum strain decreased from 12% with OL2 to 9% with OL3 at an applied pressure of 2 kpa. 2. Protection layer: Even with the coarse overliner OL1 and at an applied pressure of 3 kpa, a silty-sand soil protection layer was very effective at reducing local indentations in the GMB from overliner particles and was able to reduce the tensile strain in the GMB to 2%. 3. Practical implications: For a given vertical pressure, the results quantify how the GMB strains may be reduced by having either a finer overliner or by adding a soil protection layer. If the 6% limit proposed by Peggs et al. (25) is adopted as the tensile strain limit for the GMB, pressures no greater than 5 kpa can be applied with OL1 directly on top of the GMB, which corresponds to a heap-leach pad depth less than 25 m. At higher pressures of 2 and 3 kpa (heap leach pad depths of around 1 to 15 m), a finer overliner reduced the GMB strains, but the maximum values exceeded 6% by factors of at least 1.5 to 2, even for the finest overliner examined. At these high pressures, a sand protection layer would be effective at reducing local GMB tensile strains from the overliner to 2%. 64

77 3.6 References Brachman, R.W.I., and Gudina, S. 28. Gravel contacts and GMB strains for a GM/CCL composite liner. Geotextiles and Geomembranes 26 (6), pp Brachman, R.W.I, and Gudina, S. 22. A new laboratory apparatus for testing geomembranes under large earth pressures. In: Proceedings of the 55 th Canadian Geotechnical conference, Niagara Falls, ON, Canada, pp Lupo, J. F. (21). "Liner System Design for Heap Leach Pads." Geotextiles and Geomembranes, 28(2), Lupo, J. F., and Morrison, K. F. (27). "Geosynthetic Design and Construction Approaches ithe Mining Industry." Geotextiles and Geomembranes, 25(2), Peggs, I.D., Schmucker, B., and Carey, P. 25. Assessment of maximum allowable strains in polyethylene and polypropylene geomembranes. In: Geo-Frontiers 25 (CD-ROM). American Society of Civil Engineers, Reston, VA. Rowe, R.K., Quigley, R.M., Brachman, R.W.I., and Booker, J.R. 24. Barrier systems for waste disposal, 2nd ed., Spon Press, London. Seeger, S. & Muller, W. (23). Theoretical approach to designing protection: selecting a geomembrane strain criterion. In: Dixon, N., Smith, D.M., Greenwood, J.H., Jones, D.R.V. (Eds.), Geosynthetics: Protecting the Environment. Thomas Telford, London, pp Thiel, R., and Smith, M. E. (24). State of the Practice Review of Heap Leach Pad Design Issues. Geotextiles and Geomembranes, 22(6),

78 Tognon, A.R., Rowe, R.K., and Brachman, R.W.I Evaluation of side wall friction for a buried pipe testing facility. Geotextiles and Geomembranes, 17(4): Tognon, A.R., Rowe, R.K. and Moore, I.D. 2. Geomembrane Strains Observed in Large- Scale Testing of Protection Layers. Journal of Geotechnical and Geoenvironmental Engineering, 126(12):

79 Table 3.1 Summary of the 25 experiments conducted at 22 C. All experiments were for 1.5 mm HDPE geomembrane. Test No. Overliner Pressure Largest Tensile (kpa) Strains (%) 1 OL1 5 6,5,2 1A OL1 3,4,5 2 OL1 1 7,5,3 2A OL1 8,4,2 3 OL1 2 18,17,14 3A OL1 15,12,11 3B OL1 18,17,14 4 OL1 3 27,26,14 4A OL1 18,15,13 4B OL1 27,22,16 5 OL2 2 11,1,9 5A OL2 12,9,9 6 OL2 3 14,9,8 6A OL2 13,8,8 6B OL2 15,1,8 7 OL3 2 8,6,5 7A OL3 9,6,4 7B OL3 9,8,6 8 OL3 3 11,7,6 8A OL3 12,1,6 8B OL3 12,9,6 9 OL1 + PL 3 2, 1 9A OL1 + PL 2, 2 67

80 Table 3.2 Properties of material used as an overliner D 1 (mm) D 3 (mm) D 6 (mm) D 85 (mm) C u C c OL OL OL Table 3.3 Index stress-strain properties (measured in the machine direction) of the 1.5-mm-thick HDPE geomembrane tested following ASTM D6693. Property Mean Standard deviation Yield strength (kn/m) 27 1 Break yield strength (kn/m) 46 5 Yield elongation strain (%) 24 2 Break elongation strain (%)

81 Applied pressure P Rubber bladder Sand Seperator geotextile Overliner liner GM Lead sheet Underliner liner 59 Figure 2. GLLS Set up for TEST No.1 (All Dimensions in mm) Figure 3.1: Cross section through a typical test cell used in experiments; all dimensions in mm. Underliner and overliner material shown schematically only and not drawn to scale. All dimensions are in mm. 69

82 Percent finer by mass OL 1 OL 2 OL 3 UL Grain size (mm) Figure 3.2: Grain size distribution of overliners (OL1 OL3) and underliner examined (UL) in this study. 7

83 (a)ol 1 D 8 D 5 (b)ol 2 D8 D 5 (c)ol 3 D 8 D 5 25 mm Figure 3.3: Particle Shapes of the material used as OL. 71

84 (a) (b) Tension Compression (c) Tension Compression Figure 3.4: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4 (OL1, 3kPa). 72

85 (a) (b) Tension Compression Tension Compression Figure 3.5: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4A (OL1, 3kPa). 73

86 (a) (b) Tension Compression (c) Tension Compression Figure 3.6: (a) Measured deformed shape of geomembrane indentation, (b) calculated membrane and bending components of strain, and (c) calculated strains for the top and bottom surfaces of the geomembrane for the indentation with the maximum strain in Test 4B (OL1, 3 kpa). 74

87 Figure 3.7: Largest calculated strains from three replicate tests of Test series 4 (OL1, 3 kpa). 75

88 Effect of Pressure and OL 3 Max Tensile Strain (%) OL 1 OL 2 OL Pressure (kpa) Figure 3.8: Largest strains calculated from each test series showing the effect of pressure and overliner 76

89 Figure 3.9: Photograph of lead sheet (27 mm x 27 mm) for tests conducted at 3 kpa, (a) after using OL1 in Test 4 (b) after using OL 2 in Test 6. The marked circles are showing the major indentations scanned in the lead sheet. All the indentations are going into the plane of paper. 77

90 Figure 3.1: Photograph of lead sheet (27 mm x 27 mm) showing no discernable indentations after using silty Sand protection layer in Test. 78

91 3 3 Max Strains (%) OL1 OL2 OL3 PL 2 1 Max Strains (%) 5 OL1 OL2 OL3 PL Overliner Figure 3.11: Influence of protection layer on the maximum strain in geomembrane at 3 kpa. 79

92 Chapter 4 Conclusions and Recommendations A laboratory study of the short-term physical response of a 1.5mm Geomembrane (GMB) when subjected to conditions prevailing at the base of heap leach pad liner has been reported. Local indentation strains and punctures of the GMB caused by overlying and underlying materials were quantified for stresses up to 3 kpa. This chapter summarizes the principal findings of the study; detailed conclusions are provided at the end of each chapter. Suggestions for possible future work are provided. 4.1 Effect of underliner GMB liners were tested with a variety of underliner material beneath the GMB. The results of tests conducted at applied pressure of 2 kpa with well graded gravel having some sand overliner with maximum grain size (D 1 ) of 4 mm and D 85 = 3 mm, D 6 = 19 mm, D 3 = 1 mm, D 1 = 2.5 mm, and coarser to finer underliner, clay and GCL directly underlying a 1.5-mmthick GMB were presented in Chapter 2. These results provide the first ever evaluation of strains developed in a GMB overlying a gravel having some sand, well graded underliner material (D 1 =4 mm, D 85 = 3 mm, D 6 = 19 mm, D 1 = 1 mm). For all underliner types, major indentations and, in some cases, punctures were identified and the deformations and strains of the GMB were quantified. The gravel and sand underliner material resulted in deeper indentations in the GMB, and consequently, larger strains than the sand and gravel, gravelly Sand, silty Sand, sandy Silt and CCL and GCL underliners. For the gravel and sand underliner, a peak tensile strain of 4% was calculated and the.59 m dia GMB was punctured at nine locations. A sand and gravel underliner material having the greater maximum particle size but with more sand particles (D 1 8

93 = 8mm, D 85 = 2 mm, D 6 = 2.1 mm, D 1 =.45 mm) gave a peak tensile strain of 14% and no punctures in the GMB. The maximum strains induced in the GMB with a silty sand and sandy silt underliner were in the range of 13-18%. When a dry GCL was placed beneath the GMB peak tensile strains of 18% were observed, while for a hydrated GCL the peak tensile strain was 21%. GMBs on CCLs with two moisture contents were examined. For the CCL with a moisture content of 12% (standard Proctor optimum) and no protection above the GMB, the peak strain was 25%, while for the CCL at a moisture content of 16% (4% wet standard Proctor optimum) the peak tensile strain was 36%. The tested configurations should not be used without protection, because the strains calculated for the GMB for all tests exceeded the allowable strain limits proposed by Peggs (25), and Seeger and Müller (23). 4.2 Effect of overliner and pressure The influence of different overliners above the GMB and their impact on GMB with increasing pressure was investigated in Chapter 3 for a silty sand underliner. Three types of overliner materials were examined for pressures ranging between 5 kpa to 3 kpa that were maintained for 1 hours. For applied pressures of 5 and 1 kpa, the coarsest overliner OL1 (D 1 = 5 mm, D 85 = 3 mm, D 6 = 18 mm, D 3 = 8.8 mm, D 1 = 2 mm, resulted in maximum strains of 5% and 8% respectively. Increasing the pressure on the GMB resulted in a substantial increase in GMB strain maximum strains of 18 % and 27% being observed for pressures of 2 and 3 kpa respectively. The strain induced in the GMB for OL2 material having maximum particle size of 25 mm and greater portion of sand (D 85 = 17 mm, D 6 = 8 mm, D 3 = 1.9 mm, D 1 =.18 mm) were less than for OL1. Specifically, the maximum strain for OL2 was 12% and 15% at 2 and 3 kpa respectively. Increasing the sand fraction to obtain OL 3 (D 1 = 25 mm, D 85 = 1 mm, D 6 = 2.1mm, D 15 =.75) further reduced the maximum strain in the GMB to 81

94 9% and 12% for 2 kpa and for 3 kpa respectively. None of these three overliners was able to limit the GMB strains to less than the maximum allowable values of 6-8% suggested by Peggs et al. (25) or 3% suggested by Seeger and Müller (23) for vertical pressures of 1 kpa and higher. However, a 15-mm-thick silty sand protection layer did provide an effective protection and reduced the strain in the GMB to less than 2% even at applied pressure of 3 kpa. Hence, the silty sand protection layer was effective for even very large pressures. 4.3 Applicability and Limitations, and Future Work The results reported in this thesis examined the physical response of the GMB when tested under short-term physical loading and apply only to the specific conditions tested. The results (deformations, strains and punctures) reported here are expected to underestimate those resulting for longer periods of loading. Further work is required to fully understand and predict the longterm response of high-density polyethylene GMBs used in heap leach pads. In particular, the effect of the harsh environmental conditions at the base of heap leach pads (e.g., high moisture contents, presence of chemicals, high and low ph, and elevated temperature), should be simulated. The current study focused on 1.5 mm HDPE GMB with only one test for one 1.5 mm thick LLDPE. There is a need to quantify the response for other types of LLDPE and thicknesses of GMBs under the same test conditions. Future study should also focus on the time-dependent response of the components examined including underliner materials, GMBs, overliner materials, and protection layers. 82

95 4.4 References Peggs, I.D., Schmucker, B., and Carey, P. 25. Assessment of maximum allowable strains in polyethylene and polypropylene geomembranes. In Proc. Geo-Frontiers 25 (CD-Rom), ASCE, Reston, Va. Seeger, S., and Müller, W. 23. Theoretical approach to designing protection: Selecting a geomembrane strain criterion. In Proc. 1st IGS UK Chapter, Geosynthetics: Protecting the Environment, N. Dixon, D.M. Smith, J.R. Greenwood and D.R.V. Jones, Eds., Thomas Telford, Nottingham, UK,

96 Appendix A Heap Leach Projects Details A.1. Introduction Table A.1 is listing features of the 92 heap leach pad projects in different parts of the world. The table has detail information about different components of the heap leach pad, such as the metal extracted, type of leach pad, status of the project, vertical depth of ore, overliner material and thickness, underliner material characteristics, and geomembrane type and thickness. Table A.1. Heap leach pad projects details Metal Type Location Status Max Vert. Depth of Ore (m) OL Thickness (mm) Au NI Valley Argentina Op Au NI Valley Peru Op 18 5 Au NI Valley Peru Op 16 Au Static Peru Op Cu Static Chile Op UL Material & Thickness Clay, 3mm Clay, 3mm or GCl Clay, 3mm or GCL Au I Valley Peru Op 14 4 Clay, 3 Cu Leveled Valley Peru Op 13 6 Clay, 3mm Leach Pad Liner (**) 2. & 2.5mm HDPE 2.mm HDPE 2. & 2.5mm LLDPE 2.mm HDPE 2.mm HDPE 2. & 2.5mm SST HDPE 2.mm HDPE Comments Double GM in impoundme nt; OL only used in gentle sloping areas Cerro Verde Pad 1 Fases 1 & 2 84

97 Au Nevada, USA Op Cu Static Chile Op Au Au Cu I Valley Static Leveled Valley Colorado, USA Nevada, USA Op 125 Op 122 Peru Op 12 Cu Static Chile Op 1 4 (19mm gravel in impoundm ent area) Au NI Valley Peru Op 1 1, silty SAND with some gravel, with a GCL on top of the compacted soil 1-7 m/s 3mm Tailings Clay, 3mm U Static Niger Op 1 Silt/sand Au Static Peru Op 1 7 Au Static Peru Op 1 3 Ag Ag Au NI Valley NI Valley Static Nevada, USA Nevada, USA Nevada, USA Clay, 3mm Clay, 3mm Op 9 Clay Op m/s Clay Op m/s Cu Static Chile Op 85 None Au NI Valley Peru Op 84 3 Au NI Valley Peru Op 8 5 Au NI Valley Peru Op 8 1, Ag NI Valley Peru Op 8 3 Clay, 3mm Clay, 3mm Clay, 3mm 2.mm HDPE 1.5mm HDPE 2.mm HDPE 1.5mmHD PE 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE 2.mm HDPE 2.mm HDPE 2.mm HDPE Double 2.mm HDPE 2.mm HDPE 1.5mm HDPE 1.5mm LLDPE 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE OL: 1% passing 38 mm, 75% passing 19 mm, and 5-9% passing 1 mm. k minimum is 1x1-5 m/s. OL -25mm Unintention ally impounding to approx 25m above GMB OL -25mm OL -12mm OL -38mm La Virgin Santa Rosa Pad 11 85

98 Au Static Peru Op 8 3 Cu Static Chile Op 75 3 Clay, 3mm Au Mexico Op 7 None Au NI Valley Peru Op 64 5 Clay, 3mm Cu Ni Valley Peru Op 6 Tailings, 3mm Au Static Chile Op 6 3 Au I Valley Mexico Op 6 Cu NI Valley Peru Op 6 Au NI Valley Chile Op 55 Au Static Ghana Op? 5 Clay; GCL on steep slopes Clay, 3mm Clay, 3mm Au Static Mexico Op 5 Clay/silt Au Static Peru Op 4 5 Clay, 3mm or GCl Cu NI Valley Argentina Op 36 Au Static California, USA Op 25 Clay, 3 Cu Dynamic Chile Op 1 2, Cu Dynamic Conversion Chile Op 7 Cu Dynamic Chile Op 7 2, Cu Dynamic Chile Op 7 2, Sand, 15mm 2.mm HDPE 1.5mm HDPE 1.5mm LLDPE 1.5mm HDPE 1.5mm HDPE 2.mm LLDPE 1.5mm HDPE 1.5mm HDPE 1.5mm LLDPE 1.5mm LLDPE 1.5mm HDPE 2.mm HDPE 1.mm PVC 1.5mm HDPE 1.5mm HDPE OL- ore OL -1mm crushed ore OL Crushed ore Two layer OL: bottom 4mm - 19mm; top 1,6mm - 1mm ore and waste rock Old static heaps are being converted to dynamic Two layer OL: bottom 5mm - 19mm; top 1,5mm - 1mm ore Two layer OL: bottom 5mm - 19mm; top 86

99 Cu Dynamic Chile Op 7 2, None Cu Dynamic Chile Op 7 2, Cu Dynamic Chile Op 7 2, None Cu Dynamic Chile Op 6 2, Cu Dynamic Chile Op 6 2, Cu Dynamic Peru Op 4 Ore Cu Dynamic Peru Op 4 2, Au NI Valley Peru C 15 5 Au NI Valley Peru C 9 5 Cu Leveled Valley Peru C 13 6 Cu Static heap Peru D 1 5 Ni Static Turkey D 5 Cu Dynamic Chile D 7 2,5 Au I Valley Alaska, USA D Tailings, 3mm Clay, 3mm Clay, 3mm Clay, 3mm Clay, 3mm 1-7 m/s, 3mm 1.5mm HDPE 1.52.mm HDPE 1.52.mm HDPE 1.5mm HDPE 1.52.mm HDPE 2.mm HDPE 2.mm HDPE 2.mm HDPE 2.mm HDPE 2.mm HDPE 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE 2.mm HDPE 1,5mm - 1mm ore Two layer OL: bottom 5mm - 19mm; top 1,5mm - 1mm ore Two layer OL: bottom 5mm - 19mm; top 1,5mm - 1mm ore Two layer OL: bottom is fine gravel, upper is coarse ore Cerro Verde Pad 4B Double GM in impoundme nt: 2.mm HDPE top & bottom. Ore will not be crushed ( run of 87

100 Cu Leveled Valley Peru D 15 6 Au NI Valley Mexico FS 1 Ag NI Valley Peru FS 1 Cu Static Brazil FS 78 4 Clay, 3mm or GCL Clay, 3mm or GCL Clay, 3mm Clay, 3mm Ni Static Brazil FS 7 Clay Ni Static Philippines FS 5 U Dynamic Namibia FS 6 2,5 Ni Dynamic Colombia FS 4 2, Au NI Valley Colombia FS Au Static Colombia FS Cu Static Peru PFS 16 Cu Static Brazil PFS 1 Ni Static Poland PFS 5 35 Ni Dynamic Indonesia PFS 6 Ni Cu Dynamic I Valley South America Arizona, USA PFS 4 to 6 Shut down mm Clay, 15mm Tailings, 3mm Clay, 3mm Clay, 3mm or GCL Clay, 3mm or GCl Clay, 3mm Clay, 3mm Clay, 3mm Clay, 3mm GCL 2.mm HDPE 2.mm LLDPE 2.mm LLDPE 2.mm HDPE or LLDPE 2.mm LLDPE 1.5mm HDPE 2.mm LLDPE 2.mm HDPE 2.mm HDPE 2.mm HDPE 2.mm & 2.5mm LLDPE 1.5mm LLDPE 2.mm LLDPE 2.mm LLDPE 2.mm LLDPE 2.mm DST LLDPE mine ). OL only used in gentle sloping areas OL - ore crushed to - 1mm Two layer OL: bottom 5mm - 19mm; top 2, leached crushed ore (ripios) Natural clay foundation soils Poland Nickel Operations ceased in 211 due to 88

101 Au Static Ghana Closed 5 Au Static Ghana Closed 5 Ag Ag Au Au Au Au Static Static Static Static Static Static Nevada, USA Nevada, USA Nevada, USA S. Dakota, USA Nevada, USA Nevada, USA 5 5 Closed 4 3mm, -19mm Clay, 3mm Clay, 3mm Clay, 3mm Closed 3 Clay Closed 25 3 Clay, 3mm Closed 25 3 None 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE 1.5mm HDPE Double: 2.mm HDPE over compacted clay sealed with sprayon bitumen 1.mm HDPE Compacted clay, 3mm 1.5mm HDPE ore permeability problems Ag Static Nevada, 3mm, Clay, 4 USA -19mm 3mm Ag Static Mexico 4 Double GM Au Static Nevada, 1.5mm Relief Closed 25 3 USA HDPE Canyon Ag Static Compacted Nevada, Closed 2 None clay/silt, USA 3mm Au Au Ag Static Static Static California, USA Nevada, USA Idaho, USA Closed 2 3 Clay, 3mm Closed 2 None Closed 2 3 None Double GMB: 1.5mm HDPE over 1.mm HDPE 1.mm PVC Compacted clay, 3mm Closed due to large, deep-seated slope & foundation failure putting tailings 89

102 Ag Static Uzbekistan 8 Cu Dynamic Chile Closed 7 Ag Dynamic Nevada, USA 7 3mm protective under 25mm drainage Clay, 3mm 1.5mm HDPE Asphalt Concrete Asphaltic concrete impoundme nt at risk Au Dynamic S. Dakota, USA Closed 7 5 Clay, 6mm Double: 2.mm LLDPE over Asphaltic concrete top liner over 1.5mm HDPE bottom Original pad did not include top LLDPE but leakage rates in LCRS exceeded permit allowances. Unless otherwise noted, GMBs are generally smooth both sides except that textured bottom side is commonly used in the outer stability zone and not necessarily reflected in this table; where multiple thicknesses are reported; where two GMB thicknesses are noted the, the thicker liner is used where the ore is deepest. O = operations FS = feasibility study PFS = pre-feasibility study C = Construction D = detailed design I = impounding NI = non-impounding ROM = run of mine, uncrushed ore Clay = nominal 1-8 m/s unless otherwise noted 9

103 A.2. Summary of Heap Leach Project Table The heap leach pad projects are divided in two phases. The term Built Phase is used for projects in operational, construction and closed phase. The Design Phase has the projects in the design, feasibility or prefeasibility stage. Important trend observed in the table are summarized in forms of tables and figures. Table A.2a Relationship between GMB type and Ore height Build Phase GMB Type HDPE Thickness (mm) Ore Height (m) Total No. of Cases > < LLDPE PVC 1 2 TableA.2b Relationship between GMB type and Ore height Design Phase GMB Type HDPE LLDPE Thickness (mm) Ore Heights (m) Total No. of Cases > <

104 Table A.3. Relationship between Metal, Ore height and Underliner Project Status Build Design Metal Ore height (m) Underliner Au Clay or GCL Cu Tailings, Sand, Clay U 1 Silt/sand Au 152, 238 Clay Cu 15 Clay or GCL U 6 Tailings Ni 5, 6,4,6 Clay 92

105 Figure A.1: Plot of project status versus number of cases Figure A.2: Plot of type of underliner material versus number of cases 93

106 Figure A.3. Plot of particle sizes used in overliner material versus number of cases Figure A.4: Plot of overliner thickness versus number of cases 94

107 Figure A.5: Plot of geomembrane thickness for the number of cases in built phase Figure A.6: Plot of geomembrane thickness for the number of cases in design phase 95

108 Figure A.7. Plot showing comparison for the type of geomembrane used in current projects and past projects 96

109 Appendix B Procedure of Scanning Indentation The elastic component of the geomembrane deformation is recoverable after removal of the load that s why a soft lead sheet having thickness of.4mm is used to preserve deformations in the geomembrane. (BAM 1995; Gallagher et al. 1999; Zanzinger, 1999; Tognon et al. 2). A mold of the lead sheet is prepared to provide base support and to preserve deformed shape of lead sheet. Plaster of Paris (POP) powder is mixed with water in.7:1 ratio. The mix is easy to prepare and sets within approximately 3 minutes. The grout has low shrinkage and dimensional stability as the change is side lengths of the mold in negligible (Gudina, 27). A high resolution laser scanner (with a resolution of.1 mm) is used to measure the geomembrane indentations. Five to seven most prominent indentations in the lead sheet are scanned for every test. Every indentation is scanned for two axes with the same point of origin. It was made sure that at least one of the axes lies along the steepest side of indentation. A typical setup of the laser scanner and lead with the most prominent indentations marked is shown in Figure B.1. A photograph of the puncture in lead sheet observed in Test 2A is shown in Figure B.2. 97

110 Laser Scanner Indentation Figure B.1. A photograph showing lead sheet after test and a laser scanner for measuring the indentations. 98

111 (a) Puncture location (b) Puncture 3 mm Figure B.2. (a) Photograph of the lead sheet having indentation after Test 2A, (b) picture of the puncture caused in the lead sheet from the material placed beneath the geomembrane. 99

112 References Bundesanstalt für Materialforschung und prüfung [Federal Institute for Material Research and Testing] (BAM) Anforderungen an die Schutzschicht für die Dichtungsbahnen in der Kombinationsdichtung, Zulassungsrichtlinie fur Schitzschichten [Requirements for Protection Layers for Landfill Geomembrane Liners in Composite Liners], Berlin (in German) Gallagher, E.M., Darbyshire, W. and Warwick, R.G Performance testing of landfill geoprotectors: background, critique, development, and current UK practice. Geosynthetics International 6 (4): Gudina, S. 27. Short-term Physical Response of HDPE Geomembranes from Gravel Indentations and Wrinkles. PhD thesis, Department of Civil Engineering, Queen s University, Kingston, Ontario. Tognon, A.R., Rowe, R.K. and Moore, I.D. 2. Geomembrane Strains Observed in Large- Scale Testing of Protection Layers. Journal of Geotechnical and Geoenvironmnetal Engineering, 126(12): Zanzinger, H Efficiency of geosynthetic protection layers for geomembrane liners: performance in a large-scale model test. Geosynthetics International, 6 (4):

113 Appendix C Pressure versus Time plots To start each experiment, water pressure was applied to the bladder in increments of 1 kpa, every 1 minutes for 1 kpa and 2 kpa every 1 minutes until the required pressure of 2 kpa or 3 kpa was reached. Figure C.1 shows the pressure increment scheme for the tests conducted. A water booster pump is used to enhance the pressure on the test system, and pressure transducer is used to record the pressure on the cell apparatus. The data is monitored after every 24 hours to see the actual pressure on the test cell. The pressure reading is plotted against time and given in Figure C.2 (a) for Test 3 and (b) for Test 1. The plot is shows the reading of pressure after 5 minutes. The data of first 24 hour after the pressure is held constant is plotted. The pressure fluctuates between 2±15 and 3±15 kpa. 11

114 Pressure held constant for 1 hours Pressure held constant for 1 hours Pressure held constant for 1 hours Figure C.1. Pressure increment scheme for tests conducted 12

115 (a) Test 3 Reading Interval 5 min (b) Test 4 Reading Interval 5 min Figure C.2. Plot of Pressure versus Time (a) Test 3, (b) Test 1. 13