Design Algorithm for the Puncture Resistance of PVC Geomembranes for Heap Leach Pads

Transcription

1 As published in the Proceedings of the Second Middle East Geosynthetics Conference, Dubai, UAE, November, 2009 Design Algorithm for the Puncture Resistance of PVC Geomembranes for Heap Leach Pads Michel Marcotte, GENIVAR SEC., Montréal, Qc., Canada, Robert Denis, SOLMAX International, Varennes, Qc., Canada, Éric Blond, Sageos, CCT Group, St-Hyacinthe, Qc., Canada, ABSTRACT Puncture resistance of geomembranes often determines the required material specification of both subgrade and cover layers of heap leach pad facilities. Tested under ASTM D5514, 1 mm thick PVC geomembranes have typically yielded hydrostatic failure pressures much higher than for 1, 5 mm thick HDPE geomembranes. For instance, varying packed angular gravel layers of 20mm to 100mm particle size showed a minimum hydrostatic failure pressure of 400 kpa for 1mm thick PVC geomembranes, increasing as particle diameter is reduced. These results, consistent with recent values presented by others, however demonstrate that abrasion is probably the governing factor when it comes to puncture resistance of PVC geomembranes. In this respect, it is showed that truncated cones tests results for PVC geomembranes against angular material do not correlate as well as they do for HDPE. Specific hydrostatic pressure testing with actual granular materials is therefore recommended. Testing and analysis of puncture-protective geotextiles have also demonstrated that the use of a 270 gr/m 2 non-woven needle-punched geotextile increases the minimum failure pressure of 1mm thick PVC Geomembranes to 800 kpa, e.g. ten times the allowable pressure value calculated for a 1,5 mm thick HDPE geomembranes protected by a 550 gr/m 2 geotextile. In addition, test results indicate that whereas HDPE geomembranes require direct contact with only finer granular materials to prevent puncture, the use of much coarser (e.g. up to 4 times as coarse) and hence usually less expensive granular material may be used in the case of PVC geomembranes without jeopardizing their integrity. 1. INTRODUCTION PVC geomembranes have been used extensively as hydraulic barriers for heap leach pads since the early 80 s on account of their numerous inherent engineering benefits such as their low installed costs, high puncture resistance and friction angles (Breitenbach, 2001). But increasingly demanding geomembrane mechanical properties have come of age as heap leach pads are being designed with increasing heights. According to Lupo (2005), heap leach pads need to be specifically designed for their subgrade properties, type of geomembrane, overburden and pregnant solution collection system, such as costing of the overall solutions need be considered when comparing different geosynthetic options. As an example, placement of a pre-selected subgrade material free of protruding rocks in order to prevent geomembrane puncture will obviously increase the final costs of a project as opposed to using a more natural in-situ material by selecting a less puncture-prone geomembrane type material. It then becomes advantageous to choose a geomembrane material which will fully deform elastically around protrusions such as PVC geomembranes as opposed to a material which will rely on its rigidity and puncture resistance to span over protrusions without long term creep drawbacks. To effect, this research compares the behaviour of both PVC and HDPE type geomembranes under hydrostatic load when in contact with sizeable aggregates while searching for the best and most economical engineering puncture-proof synthetic barrier solutions. All materials used for this project meet or exceed PGI 1104 (PVC geomembranes) or GRI-GM-13 (HDPE geomembranes) industry standards. 2. DESCRIPTION TEST PROTOCOL Although initial additional ASTM D5514 testing of PVC geomembranes (Denis et al. 2009) have clearly confirmed Stark et al. (2008) conclusions pertaining PVC geomembranes requiring less puncture protection than their HDPE counterparts under similar hydrostatic loads on account of their high elasticity and perfect intimate contact with protrusions (see Figure 1), the study also denoted that truncated cones did not properly simulate the true behaviour of rocks in contact with PVC type geomembranes. For example whereas 1mm

2 thick PVC geomembranes failure pressure exceeded the testing equipment limits (1050 kpa) using maximum size truncated cones (110 mm), they were only able to withstand 300 kpa of hydrostatic pressure prior to puncture while in contact with typical heap leach pad crushed heap stones of mm dia. (see Figure 2). Figure 1. PVC geomembrane and truncated cones. Figure 2. PVC geomembrane and aggregates. This basically indicates that once PVC geomembranes have fully conformed in intimate contact with the underlying truncated cones, they are liable to withstand without puncture very high hydrostatic loading thereafter, the reason being that truncated cones are relatively blunt and do not offer any real sharp edges. The natural crushed stone aggregates on the other hand are jagged and razor-sharp quickly piercing the PVC geomembrane from their abrasive motion when combining material elasticity with the increasing hydrostatic pressure. The standard ASTM D5514 test was further modified by using 7 kpa/minute pressure increments instead of the customary 7 kpa/30 minutes, as the accelerated protocol had not shown any major impact on the failure pressures as observed and suggested by Stark et al. (2008). The cross-section arrangement using 1 mm thick PVC geomembranes and crushed stones was also tested with the addition of a 270 g/m 2 non-woven needle-punched staple fiber geotextile with the following test results. All in all, 30 tests were performed. 3. TEST RESULTS Figure 3 illustrates failure pressures as a function protrusion exposed heights (exposed heights being defined as half of the protrusion s mean diameter whenever packed layer of stones is used). This approach was preferred for it is on the conservative side of the true exposed height; any other hypothesis would have moved the curves on the right hand side. Figure 3. Failure pressures of 1 mm thick PVC geomembranes in contact with crushed stone.

3 As shown on Figure 3, the minimum failure pressure for the unprotected 1 mm thick PVC geomembrane in contact with a 20 mm dia. exposed height crushed stone is about 90 psi ( 600 kpa), increasing to well over the testing equipment s pressure limit of 150 psi (> 1000 kpa) with the addition of a relatively light 270 g/m 2 puncture-protective geotextile. On the other hand the same 1 mm thick PVC geomembrane in direct contact with 30 mm dia. and even larger stones sees its corresponding failure pressures reduced and stabilizing at about 60 psi ( 400 kpa) without the puncture protective geotextile and at 120 psi ( 800 kpa) with geotextile. 4. DISCUSSION The previous test results need now to be compared with the traditional Koerner et al. (1996) HDPE puncture analysis whereby, as opposed to PVC geomembranes, truncated cones are believed to better represent the actual mode of puncture failure for HDPE geomembranes under hydrostatic loading conditions. Whereas abrasion is the major puncturing mode for PVC geomembranes, abrasion (e.g. friction) is deemed inconsequential between HDPE geomembranes and the apex of protrusions. As a result, the rather rigid HDPE geomembranes will first span across apexes, and eventually deform in between by elongation under increasing hydrostatic pressure and/or long term creep. Failure will eventually be attained if the deformation is large enough to exceed the elastic limit of the HDPE material with corresponding thinning at the apex locales (see Figure 4). Figure 4. Thinning of HDPE geomembranes over a protrusion. Extracted from Koerner et al (1996), the figure 5 illustrates the applicability of truncated cones when modeling the puncture mode of HDPE geomembranes in contact with natural stones. Figure 5. Close relationship between truncated cones and natural angular stone (after Koerner et al. 1996).

4 Using the above graph, Koerner et al. establish a simple basic relationship between failure pressures (P ultimate ) and exposed heights (H) as follows; P ultimate = 450/H 2 > 50 kpa [1] This relationship is then further developed through experimentation with puncture-protective geotextiles and enhanced failure pressures where M is the mass per surface area; P ultimate = 450 x M/H 2 [2] This last equation with its modification factors (the subject of which is beyond the topic of this paper) constitute the main design algorithm for the puncture protection of HDPE geomembranes. More recently, Stark et al. (2008) have compared this information with new test results in an effort to supply the industry with a comparable design algorithm for PVC geomembranes, coming up with the following compounded graph containing the Koerner et al. HDPE curve with a corresponding critical cone height of 12 mm (which is the maximum permitted stone size in direct contact with a polyethylene geomembrane, a criteria still used today), and the Stark et al. PVC curve with a critical cone height of about 66 mm. But as previously mentioned, it is the author s opinion that truncated cones do not accurately represent the failure mode of PVC geomembranes as can be seen by layering in the corresponding current experimental test results onto Figure 6. As a result, differing from the Stark et al. conclusions, the current 1 mm thick PVC test results performed on actual granular material do demonstrate advantageously heightened minimum failure pressures of 400 kpa for exposed heights of up to 50 mm as compared to 1, 5 mm thick HDPE s mere 50 kpa minimum failure pressure over a maximum particle size of 12 mm. Figure 6. Compounded graph after Koerner et al., Stark et al. and new test results. Figure 7 then compares the Koerner et al. algorithm results for puncture protection of a 1, 5 mm thick HDPE géomembrane, including appropriate factor of safety with the actual 1 mm thick PVC test results of this study, yielding the following data in Figure 8. As it can be seen, a 1 mm thick PVC geomembrane even without puncture-protective geotextile sustain a higher failure pressure than a 1, 5 mm thick HDPE geomembrane

5 with a 550 g/m2 puncture protective geotextile for any effective protrusion height. Creep effect of the HDPE behaviour is in fact considered. Figure 7. Compounded graph after Koerner et al., and new test results. Figure 8. Comparative failure pressures between different cross-sections.

6 Figure 9. Comparative fail/pass criteria between different cross-sections. Actually, even in contact with 75 mm effective protrusion height, a 1 mm thick PVC geomembrane with a 270 g/m 2 puncture-protective geotextile would fare well (safety factor of 2) under a 300 kpa hydrostatic pressure, and definitely better than a 1, 5 mm thick HDPE geomembrane with a 550 g/m2 puncture-protective geotextile which would fail anyway as illustrated in Figure 9. This means that natural materials in contact with a PVC containing cross-section may contain sizeable angular stones which can drastically reduce the overall costs of a project. Considering the cost of surface preparation added to the one of material required to cover the liner, the cost of the liner itself may appear less important than the proper means to insure its integrity. Marcotte et al (2009) showed for more that m2, the selection of thicker geomembrane combined with appropriate Construction Quality Assurance programs including leak detection surveys ensure the performance and the durability of confinement works. It is believed that permanent monitoring of the field work by an independent third party during construction is the only efficient approach to flaws and unpredicted problems that always occur during the construction stage. The liner thickness reduction should only then be considered when compensate with the added security of a specifically stringent CQA program that would include a leak detection survey. Thiel et al (2005) have shown that for most heap leach gold and copper sites, the cost benefit from performing a geoelectric leak detection initially could, in general, lead to savings of USD net present value per hectare of liner area or more throughout the life of the site. Moreover, protective cushions intended to protect a geomembrane from loads and punctures caused by long term material interaction should not be selected arbitrary upon realisation of pseudo field tests or past habits. Appropriate lab testing would do better Blond et al (2003).

7 5. CONCLUSIONS PVC geomembranes offer great advantages as hydraulic barriers in heap leach pad design, of which puncture resistance is prime. As opposed to more rigid polymers such as HDPE which may be stressed beyond their useful material limits in contact with protrusions and under high hydrostatic loading, highly deformable PVC geomembranes conform intimately without permanent deformation constituting a material of choice akin to all other engineered building products which are able to absorb and release high stresses or retain them without creep. PVC geomembranes also offer alluring benefit/cost ratio solutions from both competitive installed prices and related savings such as less rigid natural material requirements. On one hand, cone testing does not reflect the behaviour of PVC géomembrane in direct contact with angular material. Deformability, shear and abrasion conduct the puncture behaviour of PVC while creep does for HDPE. On the other hand, as PVC geomembranes behaviour is the result of a composite compounding, lab testing, plant and on-site quality control are also an important part of long term overall performance. Finally, designing liners for containment nowadays is more a question of choice between typical properties and specifications than calculations. PVC in this respect offers more than engineers usually think for it is a well known material, supported by a lot of case histories, with strong industrial support behind. REFERENCES ASTM D5514 Standard Test Method for Large Scale Hydrostatic Puncture Testing of Geosynthetics, American Society for Testing and Materials, West Conshohocken, Pennsylvania, USA. Blond E., Bouthot, M., Vermeersch, O., and Mlynarek, J. (2003), Selection of Protective Cushions for Geomembranes Puncture Protection, Proceedings of the 56th annual conference of the Canadian Geotechnical Society, Winnipeg, Canada. Breitenbach, A.J. (2001), Long-term performance of PVC Geomembrane in the Mining Industry, GFR Engineering Solution. Magazine Denis R. and Marcotte M.(2009), 1,7 Million Square Meters PVC Heap Leach Pad Case History, GeoAfrica 2009 Proceedings, Cape Town, South Africa. Koerner, R.M. and Wilson-Fahmy, R.F., Narejo D., (1996), Puncture protection of geomembranes, Part I, II and III, Examples, Geosynthetics International, 3, nº 5. Lupo, J.F., (2005), Heap Leach facility liner design. Proceedings of the North American Geosynthetics Society (NAGS)/GRI19 Conference, Las Vegas, Nev., December 2005 Marcotte M., Rollin A.L. and Charpentier C., (2009) The importance of liner thickness and CQA implementation in landfills, Geosynthetics 2009, Salt Lake City, USA, February 2009 Rollin A.L., Marcotte M., Chaput L., Caquel F. (2002), Lessons Learned from Geo-electrical Leaks Surveys, Proceedings Geosynthetics 2002, Nice pp Solmers Internal Files Stark, T.D., Boerman, T.R., and Connor, C.J. (2008), Puncture resistance of PVC geomembranes using truncated cone test, Geosynthetics International, 15, nº 6. Thiel R., and Smith, M.E., (2004), State of practice review of heap leach pad design issues, Journal of geotextiles and Geomembranes, Vol.22, no. 6, pp Thiel, R., Beck A. and Smith, M.E. (2005). The Value of Geolectric Leak Detection Services for the Mining Industry, Geo-Frontiers 2005 Proceeding, January 24 26, 2005, Geotechnical special publications and GRI-18, Austin, Texas.