1,7 Million Square Meters PVC Heap Leach Pad Case History

Transcription

1 1,7 Million Square Meters PVC Heap Leach Pad Case History Robert Denis, Solmax International Inc., Varennes, Qc., Canada Michel Marcotte, Genivar, Montreal, Qc., Canada Draft paper submitted for publication GeoAfrica 2009 Abstract This is the case history of one of the largest PVC geomembrane single project installation; a 1,7 million square meter of 0,75mm PVC geomembrane heap leach pad for the world s largest privately-owned copper mine in Chile. Heap leaching is a well utilized extraction mining technology using geomembranes as hydraulic barriers designed to recuperate orecontaining pregnant solutions in addition to protecting the environment from huge volumes of heavily concentrated contaminants. In addition to its sizeable surface area, the project, completed in 2008, is particularly significant by the logistics of its complete site installation without mining operations ever being interrupted, its high-tech dielectric panel shop assembly and its innovative infrared pre-fabrication non-destructive testing technology. The project is further valued by the substantial financial savings in both materials and time for the owners as compared to other traditional design options. The paper also itemizes all other additional inherent engineering advantages offered by PVC geomembranes, followed by a proposed hydraulic loading puncture-safe design algorithm for PVC geomembranes for heap leaching facilities based on widely accepted industry methodology and additional new published research and laboratory experimentation test results. Introduction Minera Escondida is an open pit mine on one of the world s largest porphyry copper deposit system, producing copper concentrate by flotation of sulphide, and copper cathodes by leaching oxide ore. It is located 3000 meters above sea level within Chile s Atacama Desert, one of the world s driest and most sterile regions. Minera Escondida is owned by a consortium of mining interests headed by BHP Billiton (57%) and has been in operation since At an average of 230,000 tonnes/day ore throughput, it is the largest privately-owned copper mine in Chile and one of the world s largest by the same token, representing 23% of Chile s annual copper production and 8% of the world s annual copper production. Employing over 4000 labourers and contractors, Minera Escondida boasts on its own 2.5% of Chile s Annual Gross Domestic Product. In addition, the mine s proven and probable reserves exceed 2000 Mt of copper sulphide ore at 1.24% total copper, 1700 Mt of low-grade copper sulphide ore at 0.55% total copper, as well as 290 Mt of copper oxide at 0.73% acid-soluble copper (Minera Escondida, 2007). Description of the Heap Leaching Mining and Extracting Process Oxide ore is produced at Minera Escondida as a by-product of its sulphide mining operations. Run-of-mine oxide ore blended with surface stockpiles are crushed and placed onto heap leach pads, whereby pregnant leached solutions are recuperated and treated in a Solvent Extraction / Electrowinning plant producing 140,000 tonnes per year of fine copper cathodes.

2 Details of the process are as follow; 1. Mined ore is first placed in designated stockpiles designed to segregate ore on the basis of their type and grade. 2. The ore is then crushed and conveyed to agglomeration areas in order to reduce segregation of coarse and fine components. 3. Concentrated sulphuric acid ( 96%) is added to the ore as a pre-treatment process. 4. The ore is then spread over geomembrane-lined leach pads overlaid by a matrix of perforated drainage pipes (typically 100 to 150mm diameter, 2m center to center) and drip irrigated with dilute sulphuric acid solution ( 2% & ph2) in order to dissolve the copper from its embedding rock matrix. 5. As the acidic solution percolates through the heap for several weeks, it slowly leaches out the copper in copper sulphate form. 6. The copper sulphate pregnant solution is then collected and sent to a solvent extracting plant. 7. Through emulsifier and filtration processes, a clean and strong (45-55g/l) copper sulphate solution is obtained from the solvent extracting plant, ready for electrowinning. 8. Electrowinning cells contain multiple horizontal stacks of stainless steel cathode plates and lead alloy anodes. 9. DC current is applied between the electrodes and through the copper sulphate electrolyte yielding to the electro-deposition of highly pure copper at the cathodes. Types of Heap Leach Pads Heap leach pads come in a variety of types depending of the nature of the natural mine settings and/or the type of operational process. Minera Escondida is of the static type which is characterized by relatively flat and horizontal expanses (shallow sloping bases) with moderately high overburdens. They are further characterized by the one time placement of their first and lower heap layer hence the static quality of their designation as in nonremovable. Dynamic heap leach pads differ from their static counterparts on account of the multiple re-use of their first and only heap layer. Contrary to static pads whereby numerous layers are piggybacked in sequence following the leaching processes of the lower layers, dynamic pads only allow the placement of a single layer which is removed and replaced following each completed leach phase. Dynamic pads thus usually require the use of hefty and highly mechanically resistant liners such as typically thick (e.g. 2mm) HDPE (High Density Polyethylene) geomembranes to prevent damage by excavating equipment while replacing heap layers. Valley Fill heap leach pads are associated with typical steep slopes of their basal constituents imitating the natural terrain geometry of valley troughs. Valley Fills are often characterized by toe dams and maximized ore/liner friction resistance against downward drag as called for from enhanced heap stability requirements. Dump leach pads constitute the fourth and last type and are closely related to static leach pads differing mainly from their super high heaps. Dump pads thus also require enhanced ore/liner friction resistance in order to prevent catastrophic failures from sliding block instability of the heap piles which is expected to settle to up to 15% on account of consolidation from ore degradation, erosion and progressive stacking. In addition, since typical heap slope angles correspond to the ore s natural angle of repose which can be fairly steep, and since the interface between ore and geomembrane is saturated, adequate friction must be supplied in order to prevent heap failure

3 from flowslides. For that matter, protective geotextiles are seldom used since they weaken the shear plane supporting the heaps. Generic Static Heap Leaching Pads Precisions Natural terrain on mine property is cut and filled to large smooth and usually rectangle surface areas with 0.5 to 1% down slopes. Top subsoil layers are typically constituted from either selected fill (rock free) or amended with a relatively high percentage fine cohesive material in order to provide a suitable geomembrane bedding free of any potentially puncturing vectors such as angular aggregates and debris. The prepared subgrade is then covered with an impermeable thermoplastic liner e.g. geomembrane, superposed by a matrix of slotted drainage pipes (typically corrugated and regularly spaced) connected to a series of collector pipes and/or open lined swales for the collection and transport of the pregnant solutions. It is crucial to note at this point that in spite of this standard state-of-practice drainage collection systems, very high standing hydrostatic leachate heads, as high as 10m in static leach pads and even 60m in valley fills for instance, are often observed and have been reported as such by Thiel and Smith (2004). As shown in Figure 1, this hydrostatic loading often constitutes the major and most important long acting constraint upon the geomembrane, representing the prime puncturing vector. Figure 1. From Wilson-Fahmy, Narejo and Koerner (1996), hydrostatic loading represented by the block arrow, stretching the geomembrane (geometric cord) beyond its yield, to permanent deformation and thinning at the protrusion s aperture, potentially leading to puncture. A global protective/drainage granular layer may be installed on top of the geomembrane/pipe system should the overlying ore s granulometry and hardness represent a potential geomembrane puncturing source, and/or should it not offer adequate permeability and transmissivity at the leachate collection level. Granular material is also usually compulsory anyhow around thermoplastic pipes in order to prevent creep ovalization and possible collapse from the overburden. The agglomerated ore is then usually delivered to the heap leach pads by either overland conveyors such as radial stackers or off-road vehicles in 4 to 8 meter lifts. Leaching solution

4 irrigation is then supplied and achieved at rates between 4 to 8 l/hr/m2. This generic crosssection certainly represents the most preferred heap leach pad liner system. So-called single composite soil/geomembrane liner systems, ensure heap stability at the bottom interface from adequate ore/liner friction resistance against lateral motion, and minimize both capital and operational costs from its simple design and ease of construction aspects. And as shown in Figure 2, Minera Escondida s heap leach pad cross-section is indeed noteworthy from the absence of a global protective/drainage layer, as the crushed ore was deposited directly onto the liner. All drainage pipes were however covered locally with small mounds of selected ore material for pipe collapse prevention. But by far the most significant difference from other similar projects is the exclusive use of a 0,75mm thick flexible PVC geomembrane (polyvinyl chloride) in lieu of the customary HDPE (high density polyethylene) liner version. Slotted Pipes 2m Support Aggregate 0,75mm PVC Underliner Subgrade Figure 2. Minera Escondida cross-section. Flexible PVC Flexible PVC is a thermoplastic material which has been extensively used as a geomembrane constituent as far back as the mid 1950 s. It is actually one of the earliest surviving member of the geomembrane developmental phylum having been utilized as recently again for some of the world s largest geomembrane projects, see Bérubé, Diebel, Rollin and Stark (2006). PVC geomembranes offer some very sought after specific engineering properties. For instance, whereas polyethylene geomembranes are essentially composed of both amorphous and crystalline molecular groupings leading to well-known typical mechanical and behavioural properties such as relatively limited elasticity (13% elongation at yield), PVC possesses a 100% amorphous structure yielding maximum elasticity until break (> 350% elongation). Furthermore, since PVC is constituted by an important percentage of additives such as plasticizers, stabilizers and ageing retardants, it may be specifically formulated to meet different service conditions such as resistance to increased UV exposure, specific contaminants and very low temperature environments. Manufacturing-wise, PVC may either be transformed into geomembrane sheets by extrusion or by calendering processes which is usually the method of choice for thinner films (e.g. < 1mm). Since PVC geomembranes are characteristically produced in narrower standard rolls ( 2m wide) than other types of thermoplastic liners such as polyethylene geomembranes, shop prefabrication of large panels is usually called for in order to reduce the extent of future on-site welding. Prefabrication also

5 enables the assembly of custom-sized panels in a fully controlled environment which may either be accordion folded or rolled upon completion according to the preferred method of on site deployment. This greatly advantageous feature is enabled by the intrinsic high elasticity of the material as compared to the relative stiffness of polyethylene geomembranes which prevents any folding at the risk of permanently damaging the liner. In a nutshell and as a general rule, highly elastic yet sufficiently resilient materials usually constitute premium choices for building materials as evidenced by the engineering realm at large as they allow high stress controlled cyclic absorption and release without permanent deformation or damage. Elasticity is prime material property from airplane wings design to suspended bridges and steel structure buildings to mention but a few examples. Eleven Generic Caveats between PVC and HDPE geomembranes 1) Puncture Resistance Although PVC geomembranes typically display a lower puncture resistance than HDPE geomembranes for similar thicknesses and under similar testing protocols, PVC geomembranes require much greater displacements prior to failure on account of their superior elasticity. As illustrated in Figure 3, even thinner PVC geomembranes compare advantageously to HDPE geomembranes when displacement (e.g. mechanical work and/or energy) is focused upon, which will turn out to be as will be seen later, one of the key deciding factors for Minera Escondida final design. Areas under the curves indicate Mechanical Work required to puncture the liners 1,5mm HDPE Load 0,75mm PVC Displacement Figure 3. Schematic stress-strain curve of both 1,5mm HDPE and 0,75mm PVC geomembranes after Peggs (1992).

6 Figure 4 further depicts PVC s high elasticity/displacement and intimate contact with the truncated cones of the ASTM D 5514 Standard Test Method for Large Scale Hydrostatic Puncture Testing of Geosynthetics. Figure 4. PVC geomembrane perfectly conforming without any puncture to the truncated cones of the ASTM D 5514 Hydrostatic Puncture Test; photograph taken immediately following the removal of the pressured vessel, prior to the material s natural rebound to its original state. 2) Chemical Resistance Although HDPE is generally better chemically resistant to a wider range of contaminants than PVC for similar thicknesses and under similar testing protocols, PVC is still adequately resistant to both typical copper heap leaching sulphuric acid solutions as well as gold heap leaching sodium cyanide solutions when laboratory tested (CGT, 2007). Care should nevertheless be exercised when dealing with very highly concentrated solutions for both types of polymers. In all cases, engineeringwise it becomes irrelevant whether one of products ages more or faster than the other if both will still continue to adequately perform their impervious function under the anticipated specific operational conditions during the whole life expectancy of the project. 3) Ageing From Loss of Plasticizers PVC geomembranes have long been criticized for their loss of plasticizers over long term usage especially when UV exposed. Although this fact cannot be denied, it needs to be put in proper perspective. Plasticizers are chemical compounds that need to be formulated into PVC geomembranes in order to confer their superior elasticity. But plasticizers have been known to slowly migrate outwards. This phenomenon has actually been drastically reduced with the advent of higher molecular weight plasticizers as required by modern PVC geomembrane industry-standard specification such as the PGI-1104 which requires molecular weights higher than 400 (PGI, 2004). Nevertheless, as shown on Figure 5, it has been demonstrated that plasticizer loss is asymptotic and levels off at around 50% albeit in a neutral ph yet exposed environment.

7 70 60 Plasticizer Loss (%) Loss of Plasticizers in Exposed 0,25mm PVC Geomembrane Years in Service Figure 5. PVC Plasticizer Loss vs. Years in Service after Morrison and Starbuck (1984). Effects of Plasticizer Loss on Mechanical Properties of PVC Tensile Strength and Modulus (kpa) Tensile Strength Elongation (%) Modulus Elongation Plasticizer Loss (%) Figure 6. Effects of Plasticizer Loss on Mechanical Properties after Morrison and Starbuck (1984).

8 What is telling is the order of magnitude of the loss as opposed to its absolute value. When this 50% loss of plasticizers is plotted as a function of the material s mechanical properties as shown in Figure 6, the material displays predictable stiffening as both modulus and tensile strength are increased as plasticizers are lost; which by itself is not a detrimental effect since the material has gained strength at the expense of elasticity which still hovers at around 250%. So once again, although HDPE is not afflicted with loss of plasticizers and is generally better UV resistant than PVC, engineering-wise it becomes irrelevant whether one of products ages more or faster than the other if both will still continue to adequately perform their impervious function as anticipated for the life expectancy of the project under operational conditions. It is also important to remember that PVC geomembrane UV resistance may always be further improved by the increased addition of UV protective agents if need be. In any case, at the end of the day, when heap leach pads are considered, liner will always be buried and then protected from UV effect. 4) Low Temperature Brittleness Although HDPE fares better than PVC at low temperature resistance for similar thickness and under similar testing protocols, most heap leach applications will be required to perform at temperatures well above standard PVC s low temperature brittleness. Once again, engineering-wise it becomes irrelevant whether one product is better resistant at lower temperatures than the other if both will still continue to perform their impervious function under the specific anticipated operational conditions during the whole life expectancy of the project. And as for increased UV resistance, additional low temperature resistance may always further be improved by the increased addition of low temperature agents if need be. 5) Elasticity As previously mentioned, PVC geomembranes truly offers superior elasticity properties. In addition to their elevated elongation at break (>350%), and as opposed to HDPE geomembranes, PVC geomembranes do not display any yield point whatsoever (although some limited hysteresis may linger as previously mentioned). In other words, save from some limited residual hysteresis (approx. 10% from observations) and for all intents and purposes, PVC will regain its original shape and properties when released from tensile strain that will have stretched it just prior to its break point. This simple property is actually tremendously much more appealing engineering-wise. In a nutshell, PVC geomembranes are always elastic up to their break point as shown in Figure 7, which is not the case for HDPE as it will deform permanently if stretched beyond its yield point, translating into a technical failure. As a matter of fact and as shown in Figure 8, save from material relaxation considerations, it is a well known practice for engineers to stay away from this critical point as designs will usually not permit any loading beyond 25 or 30% of yield (this practice is even usually universally applicable to all materials, let alone geomembranes).

9 Typical PVC Stress-Strain Curve Break δ Elastic Deformation 350% % Figure 7. Schematic typical PVC Geomembrane Stress-Strain Curve. Typical HDPE Stress-Strain Curve TECHNICAL FAILURE! Break Point ~100 % of Yield δ Yield ~ 30% of Yield 12% Plastic Deformation % 700 % Figure 8. Schematic typical HDPE geomembrane Stress-Strain Curve.

10 6) Long Term Creep According to the Wilson-Fahmy, Narejo and Koerner algorithm for puncture protection of PVC geomembranes as proposed by Stark, Boerman and Connor (2008) results of long-term puncture tests show that a creep modification factor is not required for PVC geomembranes when used without a geotextile. This is due to the PVC geomembrane showing no long-term creep related puncture without the use of a puncture protective geotextile. As a matter fact, the geotextile may not even allow the PVC to stretch/deform enough to take advantage of the increase in puncture resistance due to deforming around the protrusion. An example of the consequence of this observation (see Figure 9) is that a 500 g/m2 non-woven geotextile would for instance be required for the protection of a 1,5mm HDPE geomembrane, whereas no cushion geotextile would be required for a 0,75 PVC geomembrane under the following conditions; Angular stone: 38mm maximum diameter packed stone layer (19mm Effective Protrusion Height) Hydrostatic loading condition Harsh leachate condition: heap leach mining solutions Overburden: 50m high of 11.8 kn/m3 material Required Safety Factor: 3 This exercise basically highlights again PVC s high elasticity benefit. Figure 9. Pressure and Geotextile Protection, from Marcotte (2008), modified from Koerner (1996); maximum pressure (height of water in meters) on a 1,5 mm thick HDPE geomembrane protected with a nonwoven needle-punched geotextile placed on a packed stone subgrade.

11 7) Stress Cracking Stress cracking is associated with highly crystallized polymer structures such as high density polyethylene. Although proper resins are formulated nowadays to prevent it, poor geomembrane processing parameters as well as poor welding procedures may induce the phenomena as either procedure will increase the crystallinity percentage of the constituting resins. Stress cracking is further enhanced by harsh environments such as those offered by reacting chemicals. PVC, on the other hand, possesses a 100% amorphous structure and hence will never be subjected to the stress cracking failure mode even when used under typical heap leach pad conditions. 8) Heap Stability at the Ore/Liner Interface PVC offers great surface friction coefficient essential to the sliding block stability of heap leach piles. This surface friction is achieved from the material s high elasticity which enables aggregates to literally settle into the geomembrane with their deep imprints as the geomembrane will partially conform around protrusions. This same holds true with the underside interface between the geomembrane and its subgrade as the geomembrane will be in very intimate contact with the natural soils. In addition, and always on account of their high elasticity and deformability, PVC geomembranes will offer great adhesion with all cohesive natural materials. 9) Ease and Rapidity of Installation PVC geomembranes greatly simplify on-site installation procedures on account of their numerous inherent advantages starting with the various and abundant methods of site assembly permitted, including both high-tech and low-tech options which can be seen as an advantage in remote and poorly serviced areas whereby installation and repairs in an event of emergency may easily be performed by unskilled labourers. Industry-approved assembly methods include both automated hot wedge and hot air fusion welding, manual hot air welding, solvent and solvent bonded welding. Prefabricated panels of up to 5000m2 will also drastically reduce the extent of field work. Field assemblies are further facilitated when using PVC geomembranes since their coefficient of thermal expansion in conjunction with their high deformability will prevent the formation of large thermally expansive waves which are a true hindrance when assembling geomembrane panels. PVC geomembranes relative flimsiness will also contribute to make field installations faster and safer by reducing wind action concerns as they do not display as much aerodynamism as other more rigid polymers. 10) Advanced Manufacturing and Construction Quality Control Programs As opposed to yesteryear, modern PVC geomembrane technology has nothing to envy from its HDPE geomembrane counterpart as both now offer the same level of confidence from both their manufacturing and construction industry approved assembly and quality control methods including fully automated wedge welding and air pressure testing. The PVC geomembrane technology now enables as deeply

12 exhaustive Quality Control Programs as HDPE technology, as every step of its manufacturing, pre-assembly and site installation processes are fully quantitatively documented. Furthermore, since a high percentage level of panel fabrication is performed in-house under controlled conditions, PVC assemblies are considered most highly consistent. 11) Procurement and Installation Costs And last but not least, as it will be demonstrated subsequently, the use of PVC geomembranes may represent sizeable financial benefits over the HDPE geomembrane option. Additional Rationale behind Minera Escondida s preferred PVC Geomembrane Option Notwithstanding the previously-mentioned generic inherent benefits of using PVC geomembranes for Static-type heap leach pads, the following specific issues were instrumental in finalizing the engineering design for Minera Escondida; 1) Great elasticity over local subsidence The natural subgrade at Minera Escondida is associated with a very peculiar soil property as it contains very high percentages of acidic solution soluble salts which represent serious concerns when building acidic solution heap leach pad atop. Any leakage from the acid containing leach pad would obviously partially dissolve the subgrade rather rapidly, thereby forming small karstic voids. On account of their high elasticity PCV geomembranes offer a much better a safer option than more rigid materials as PVC will stand a better chance to conform intimately to the new subgrade geometry by filling in the voids. More rigid materials would first bridge over voids and would then risk permanent plastic deformation which is equivalent to submitting the material beyond its recommended allowable stress. Additional discussion on this phenomenon may be found in Smith (1995). 2) Great elasticity under seismic activities Chile is situated within a very active seismic zone and the site of the world s largest earthquake ever recorded (9.5 on the Richter Magnitude Scale in May 1960). It is also frequently rattled especially in the Antofagasta Region which borders the Atacama Desert were Minera Escondida is located. The use of highly elastic PVC geomembrane is especially well suited for that region. 3) Simultaneous geomembrane filed installation & 24/7 mining operations and ease of installation Another key factor in retaining the PVC option was that the field installation and work scheduling did not deter the mine off its regular mining operations.

13 90m Not to Scale Field Seams 2400 m 4 x 300m x 11m PVC Panels 4x 300m x 11m PVC Panels Stacker 20m x 15m PVC Prefab Panels Conveyor Field Seams Figure 10. Schematic representation of initial work plan. Field installation was basically performed in but a few steps as can be visualized on Figure 10. The first step consisted of deploying and welding two parallel lines, of either four or five 300m long x 11m wide prefabricated PVC geomembrane panels each approximately 90m apart. A 20m wide alley was thus left unlined to accommodate the placement and ensuing circulation of the wheeled radial stacker. A tie-in panel, 20m wide x 15m long, was subsequently deployed and welded to both parallel panel tracks followed by the partial installation of the gravity-driven slotted drain pipe matrix system on top of the assembled PVC geomembrane panels. The radial stacker was then put to work for the placement of the first heap layer over the prepared partial leach pad. Once the first tailings design level reached, the stacker was then advanced in order to allow the deployment and assembly of the next tie-in panel. The procedure was then repeated for the full 2.4 km baseline of the pad, hence completing the first full heap leach lane. A total of five adjoining lanes were added in the same manner, completing the 2400m x 540m heap leach pad. Mining operations were hence allowed to progress uninterrupted. Figure 11. Installation of drainage pipe besides operating stacker at Minera Escondida.

14 4) High Quality Factory Prefabrication All factory prefabricated panels were assembled by Solmax International Inc. at their Canadian plant through dielectric welding. Dielectric welding (a.k.a. Radio Frequency or R.F.) is accomplished by submitting overlapped adjoining PVC sheets to a high frequency alternating current (30 MHz) between two sealing electrodes. The high frequency energy produces localized heating by exciting the polarized PVC molecules causing them to melt and allowing fusion welding when pressure is simultaneously applied. Welding is thus controlled by three quantified parameters; sealing current (R.F. Power), sealing dwell time and applied pressure. In total, 440 panels of 11m x 300m as well as 850 panels of 15m x 20m were dielectrically preassembled at a production rate of 100,000m2 per week, hence requiring 17 weeks to completion. All completed panels were rolled and strapped onto wooden pallets, and shipped via 74 maritime containers (12m long). A very extensive and qualitative quality control program was performed during prefabrication. In addition to industry-standard quality control procedures such as equipment calibration protocols and destructive testing, innovative 100% web inspection was performed with a Holiday-type pinhole detector (see Figure 12), as well as 100% infrared non-destructive testing of all factory welds. Infrared non-destructive testing is a proprietary method developed by Solmax in collaboration with the Centre de Recherche Industriel du Quebec (CRIQ) a provincial governmental research center. It enables automated image analysis of the weld s heat signature, identifying defects such as weld gaps (see Figure 13a), improper overlaps (see Figure 13b) and Fishmouths (see Figure 13c), thereby ensuring weld continuity. Figure 12. Pinhole Holiday Type Detector Figure 13. a) Weld gap, b) improper overlap, c) fishmouth

15 The retained material for the job was a 0,75mm flexible PVC single-sided faille-finish geomembrane meeting the PGI-1104 Specification (faille-finish is an embossing process that enhances the friction properties of a material). Both prefabricated and field seam requirements were slightly modified in order to reflect the material s faille finish limitations (the PGI-1104 being applicable to smooth materials only). Seam shear strength requirements were hence lowered from 58.4 lbs/in (10 kn/m) to 57 lbs/in (9,7 kn/m). On the other hand, seam peel strength requirements were increased from 15 lbs/in (2,6 kn/m) to 20 lbs/in (3,5 kn/m) in order to take advantage of the pre-fabricated dielectric welding process and subsequent automated field hot air welding. 5) Small thermal expansion waves Flexible PVC geomembranes typically have a much lower coefficient of thermal expansion than polyethylene geomembranes. In addition to the material s relative high flexibility and suppleness, thermal expansion wave frequency and amplitude are minimized, thus easing the heap spreading process as well as guaranteeing an unhindered drainage scheme. 6) Economic factors Substantial financial savings were achieved by using a 0,75mm single-sided faillefinish (lightly textured) PVC geomembrane in lieu of a 1,5mm single-sided textured HDPE geomembrane. The following comparative cost analysis is the actual 2007 Minera Escondida project bid offer for 1,700,000 m 2 : 0,75mm PVC 1,5mm HDPE Delivered unit costs* (US) 2.45/m2 3.40/m2 Installation unit costs (US) 0.55/m2 0.90/m2 Total unit costs (US) 3.00/m2 4.30/m2 Total costs (US) 5.1M 7.3M TOTAL SAVINGS 2.2M $US * Includes prefabrication Additional Project Field Data All structural field panel assembly seams were performed through fully automated hot air welding dual track welders, at an average speed of 3m/mn under 30C ambient temperature. All seams were non-destructively air channel tested under 70 kpa air pressure with a maximum 10% allowable 2 minute pressure loss. Destructive testing was performed at every 150 linear meters. Both calibration and destructive testing were verified using PGI 1104 seam requirements. All patches were THF (tetrahydrofuran) solvent bonded and air lanced.

16 Additional Discussion Further laboratory research was recently performed by the authors (Solmax, 2008) in an effort to enhance the reproducibility of the Fahmy-Narejo-Koerner algorithm as modified by Stark, Boerman and Connor (2008). To that effect, additional ASTM D 5514 Hydrostatic Puncture Tests were performed with the same material used for Minera Escondida with remarkable results as no puncture were observed at the testing apparatus physical limitations, i.e. 110mm high truncated 1200 kpa (see Figure 4), which basically indicates that once PVC geomembranes have fully conformed in intimate contact with the underlying protrusions, they are liable to withstand without puncture very high hydrostatic loading thereafter. These observations nevertheless need to be tempered when protrusions denote extremely sharp edges such as the ones liable to incur bodily harm when rubbed against as evidenced by a further laboratory test set (protrusions have indeed been observed with a very sharp natural mm, 60mm d 50 granular (approx. 66mm Critical Cone Height) at 300 kpa (see Figure 14) Figure 14. Very angular mm granular with sharp edges. Figure 15 is the graphical representation of the author s proposed modified algorithm for PVC puncture protection under hydrostatic loading which takes into account the aforementioned laboratory test results in addition to Stark s values (Stark et al., 2008), and from which the following formula is fitted: Whereby; p allow = 4500 M A / (H-66) 3 MF S MF PD MF A FS CR FS CBD p allow = the maximum allowable pressure (kpa) M A = mass per unit area of the protection geotextiles (g/m 2 ) H = cone height (mm) MF S = modification factor for the protrusion shape (dimensionless) MF PD = modification factor for packing density (dimensionless) MF A = modification factor for soil arching (dimensionless) FS CR = partial factor of safety for creep (dimensionless) FS CBD = partial factor of safety for chemical/biological degradation (dimensionless)

17 Figure 15. Author s proposed algorithm for PVC puncture protection under hydrostatic loading. The famous Wilson-Fahmy-Narejo-Koerner geomembrane puncture design algorithm that has been extensively used since its inception for HDPE geomembrane dimensioning can now be adapted to PVC geomembranes as well if proper adjustment is considered. More testing is required still to better relate the true behaviour of flexible liner to reliable formula. In that specific case, PVC deformability exceeds the known value of any protective geotextile and the impact of suck discrepancy is not yet fully understood. However, thick non-woven geotextile will always reduce the impact of abrasion by reducing the contact friction angle as well as the sharp edges. Conclusions Highly deformable PVC geomembranes represent a most interesting option when heap leaching on account of their numerous inherent beneficial engineering properties as well as potential substantial financial savings as exemplified by this case story. By nature of their high elasticity, PVC geomembranes offer designers with unequal material flexibility constituting a material of choice akin to all other engineered building products which are able to absorb and release high stresses without permanent deformation, or retain them without

18 creep; a definite alluring advantage over more rigid products, appreciated by engineers of all trades. Although HDPE geomembranes have long been established as an industry-standard for heap leaching, modern PVC geomembrane technology should definitely not be overlooked as it now rivals HDPE on all aspects including applicability, quantification of all manufacturing and installation processes, and design methodologies. To that effect, recent technical developments and industry-standards are now adorning PVC with the same specification rigor and design algorithm approaches that have associated with HDPE for the past decade. Governing Institutions such as the PVC Geomembrane Institute (PGI) have for instance upgraded and standardized their recommended technical properties specification to include a minimum plasticizer molecular weight in order to both dispel the erroneous generic ephemeral and fragile nature of PVC geomembranes and to standardize the industry around reliable and proven building materials. Fully automated and quantifiable fabrication and installation methods are step for step similar to the HDPE technology including dual weld air channel non-destructive continuity testing. And best of all, PVC will often lead to appreciable savings for facility owners, representing lower installed costs as well as lower subgrade preparation costs on account of the more giving nature of flexible PVC geomembranes. REFERENCES Minera Escondida, (2007), Minera Escondida Limitada Annual Report Thiel, R. and Smith, M. E., (2004), State of the Practice Review of Heap Leach Pad Design Issues, Journal of geotextiles and Geomembranes, Vol. 22, No. 6, pp Wilson-Fahmy, R.F., Narejo, D. and Koerner, R.M., (1996), Puncture Protection of Geomembranes Parts I, II & III, Geosynthetics International, Vol. 3, No. 5, pp Bérubé, D., Diebel, P., Rollin, A. and Stark, D., (2006), Massive Mining Evaporation Ponds Constructed in Chilean Desert, Geosynthetics, Vol. 25, No.1, pp Peggs, I.D., (1992), PVC Geomembranes in Municipal Landfill Liners and Covers, PVC Geomembrane Institute Document, March ASTM D 5514, Standard Test Method for Large Scale Hydrostatic Puncture Testing of Geosynthetics, ASTM International, West Conshohocken, PA, USA. CGT, (2007), Canadian General Tower, Internal Laboratory Test Results. PGI, (2004), PVC Geomembrane Institute Standard Morrison, W.R. and Starbuck, J.G., (1984), Performance of Plastic Canal Linings, REC- ERC-84-1, United States Bureau of Reclamation. Stark, T.D., Boerman, T.R. and Connor, C. J., (2008), Puncture Resistance of PVC Geomembranes Using the Truncated Cone Test, Geosynthetics International, Vol. 15, No. 6. pp

19 Marcotte, M., (2008), Poinçonnement et Déformabilité des Géomembranes, Solmax International Inc. Internal Research Report. Smith, M.E., (1995), "PVC-lined Copper Leach Pads in Chile", Geotechnical Fabrics Report, Vol. 13, No. 2, March 1995, pp Solmax, (2008), Internal Laboratory Test Results.