PVC Liner for Heap Leach Pad

Transcription

1 PVC Liner for Heap Leach Pad Michel Marcotte, eng., M.Sc.A., GENIVAR Eric Blond, eng., M.Sc.A., Sageos, CCT group Abstract Puncture resistance of each liner material determines the required specifications of both the under and the overliner in Heap Leach Pad facilities. Tested under ASTM D5514, 1.0mm thick PVC liner shows minimum hydrostatic failure pressures much higher than the one used for 1.5 mm HDPE. An angular packed gravel subgrade tested for dimensions between 20 and 100 mm showed a lower limit for 40-mil PVC of 50 psi (350 kpa), increasing as the particle s diameter declines. These results, consistent with recent values presented by others, show, however, that abrasion is surely an important factor to consider when puncture resistance is looked at for PVC. In that respect, it is shown that truncated cone test results for PVC against angular material do not correlate as well as they do for HDPE. Specific testing is therefore recommended. The use of a 270 gr/m 2 non-woven needle-punched geotextile increases the lower limit failure pressure of the liner to 125 psi (875 kpa), ten times the allowable value calculated for 60-mil HDPE with a 550 gr/m 2 geotextile. In summary, the observed behaviour and low variability of results reduce the global costs for both under and overliner by quadrupling the acceptable maximum particle diameter for operating pressure under 350 kpa. Finally, thickness and formulation quality are shown as important parameters to take into account when selecting such a material. 1

2 1 INTRODUCTION First used in the mining industry as a dampproofing material, PVC geomembrane has a sizeable role in terms of documentation and references. Breitenbach (----) notes that it was first used in Utah in the 70s for evaporation tanks and, starting in 1979, for gold and silver leaching preparation facilities in Montana and Southern California. By the end of the 90s, geomembrane was used around the world. There are many reasons for this situation, but knowledge of geomembrane behaviour, competitive costs for manufacturing and installation on site of pre-assembled panels, as well as geomembrane s attractive puncture and abrasion resistance are among the most compelling reasons for its selection as dampproofing material. Breitenbach (----). Those in the field are also aware of the quality of manufacturers and installers. The increased height and size of works are creating more and more constraints for the dampproofing system. Lupo (----) thus stresses that designing a proper dampproofing system for leaching treatment facilities can no longer go by the "cookbook" but must be tailored specifically to each project. Each of the following elements is important in designing a dampproofing system. Foundation materials Underliner Dampproofing membrane Overliner All solution collection and air injection systems Aside from questions on foundation materials and potential differential settlement, the following three issues deal with the interaction between the underliner, geomembrane and overliner. As a result, the following article in fact deals with the cost of these three elements combined. In fact, the advantage of anticipating at the design phase all of the costs associated with selecting one dampproofing system over another is that it helps more accurately compare the overall economic value, beyond the cost of the membrane alone. Protective geotextile, specific granular material and preparation of the receiving surfaces must be added to the cost of the membrane itself in order to properly assess the economic consequences of selecting one membrane rather than another. One of the determining factors in the price of a dampproofing system is its resilience (survivability) to load systems; its resilience often depends on properties specific to the dampproofing membrane, unless materials that are able to offset such a shortcoming are added. Insofar as maintaining a seal under all circumstances and over time is the primary quality demanded of a membrane, it is evident that its ability to effortlessly contour the surfaces it is in contact with is a key component of its resilience. Deformability is, as a result, more important than any capacity to resist deformations to which it is subjected by the contact surface during construction or, as applicable, over a longer term, especially as the constraints involved can exceed 3000 kpa in new structures (Lupo, Thiel and Smith, ). In practice, when locally imposed deformation exceeds its ability to contour the contact surface, it will fail and thus perforate. In some cases, excessive accumulated deformation that affects the membrane s long-term hydraulic behaviour in a given location is akin to a shear. This deformation is called a puncture.

3 2 DESCRIPTION OF TESTS In the context of the article submitted by Stark et al in 2008, the expected impact of its publication and the results obtained during an initial testing series last December, further testing was planned on standard granular materials in order to assess the PVC s behaviour under such a load and compare it to the values submitted by Narejo, Wilson-Fahmy and Koerner in 1996 (NW-F & K). As initial results had challenged the use of truncated cones in ASTM D5514 testing, which then did not seem very representative of actual behaviour, a choice was made to measure puncture behaviour using sizeable angular stone with materials encountered at leaching treatment sites. Replacing truncated cones with three categories of angular stone, i.e mm, mm and mm, thirty (30) tests were done on a 1.0-mm (40-mil) PVC sheet whose standard properties are set out in the table below. Table 2.1 Standard properties of tested PVC Properties Value Coefficient of variation Thickness 0.99 mm 1.3% Density Machine direction Breaking strength (kn/m) % Elongation at rupture % Modulus of elasticity at 100% % Transverse direction Breaking strength (kn/m) % Elongation at rupture % Modulus of elasticity at 100% % The testing protocol followed standard ASTM D , procedure B (increasing pressure in increments of 1 psi (7 kpa) until failure and recording of the failure pressure. Failure was recognized as a sufficient loss of pressure to prevent it from being increased, corresponding to membrane puncture. The approach was modified according to the Stark trials for tests involving PVC geomembrane (increasing pressure at a speed of 1 psi/min rather than 1 psi/30 min), which did not note any impact by load speed on failure pressures (Stark et al, 2008). The variability of the materials behaviour will be assessed by repeating the tests under the most critical conditions, i.e., conditions that are assumed to be associated with rock abrasiveness, not material deformability. From the very first trials, the issue of abrasiveness was raised. Aside from the truncated cones which were replaced by gravel, the equipment used is the same as used in the initial phase, with a maximum pressure capacity of 150 psi (1050 kpa). 3

4 Figure 2.1 Equipment used: Pressure chamber and technical set-up 3 PRESENTATION OF RESULTS As shown in the figure below, failure pressures measured on PVC sheets are relatively consistent with respect to the semidiameter of the largest particle. This characterization is representative of a tight arrangement of particles as opposed to an isolated particle. Figure Average results of puncture measurements 4

5 In Figure 3.1, the middle curve shows the average values for the results obtained, while the upper and lower curves show the standard deviations associated with the results. The greater variability in the results below 50 mm has to do with the number of lower values and, in particular, a result that is much higher than average. In fact, all of the results obtained were factored into the average in order not to underestimate it. The impact of this decision must be kept in mind. While the use of geotextile in heap leach pads is relatively uncommon (Lupo, ----), the impact of adding geotextile was measured for all diameters tested. The material used was 270 gr/m² nonwoven needle-punched geotextile placed between the PVC sheet and stone as puncture protection. As shown in the graph below, the results obtained are at the operating limit of the instrumentation used. However, the failure noted at 125 psi (875 kpa) for the intermediate size suggests proximity to the actual ceiling pressure. Thus, for 20 mm, the failure pressure is above 150 psi; at 50 mm, it is above 125 psi. As shown, these results have been retained for the purpose of comparison and should in all cases be specifically checked in the framework of a project. Figure 3.2 Puncturing with non-woven geotextile 5

6 4 DISCUSSION The bulk of the discussion provided regarding the results obtained is based on the approach developed by Stark et al (2008) for the results presented by Narejo, Wilson-Fahmy and Koerner in Based on certain hypotheses, the latter noted that using truncated cones was a good simulation of the heaviest load placed on HDPE by angular stone. Once of the assumptions made is that there is little friction on the membrane as it deforms on the surface of the stone (or cones). The anchoring on either side of a peak holds the membrane as if in a clamp, forcing deformation between one peak and the next. If the deformation required to contour the rock surface (or surface between the cones) is below the sheet s maximum deformation level, there is equilibrium and failure does not occur. Otherwise, the deformation on the peak or between two or three peaks exceeds the membrane s capacity, and it fails. The figure below shows the correlation between the HDPE membrane s behaviour on angular stone and on truncated cones. Figure Results presented by NW-F & K (shown in psi) With this diagram, the authors determine that there is a relationship between the maximum (final) pressure and an exposed height H. (1) P final = 450/H 2 > 50kPa By observing the linear relationship between adding a protective geotextile and the new failure pressure, the formula is changed, taking the following shape, where M is the geotextile s surface mass. P final = 450*M/H 2 6

7 By adding certain modification factors, discussion of which is beyond the scope of this article, they developed an approach that is still used today. Using this data and adding new data based on ASTM D5514 truncated cone testing, in 2008, Stark et al noted results that were fairly different from the results presented by Narejo, Wilson-Fahmy and Koerner (1996) on HDPE. The simplified figure below presents for comparison two families of curves, each associated with a typical behaviour. Figure 4.2 Results of Stark et al 2008 The first family covers the behaviour of rigid materials such as HDPE; the second, PVC family shows results that are similar in form but lagging with respect to exposed height. Based on the results, Stark et al (2008) determine a critical cone height (CCH) below which failure pressures would be much higher than the pressures noted during testing and beyond the capacity of the instrumentation used. Clearly, as a result, the pressures would be much higher than the pressures obtained by Narejo, Wilson-Fahmy and Koerner (1996) for HDPE. In practice, the CCH for PVC would be in the order of 66 mm, whereas it is only 6 mm for HDPE, justifying the maximum recommended diameter of 12 mm, beyond which appropriate protection would be required (Narejo, Wilson-Fahmy and Koerner, 1996). Using the value comparison, Stark et al (2008) change the NW-F & K form by adding the modification factor MF CCH, multiplying by 10 the value of the maximum pressure obtained according to exposed 7

8 height without geotextile. By excluding the effect of geotextile from the formula used, the approach recognizes that geotextile will have a different behaviour with PVC than observed with the HDPE. 4.1 COMPARISON OF RESULTS OBTAINED Adding the results from this study stresses the issue of determining an exposed height or effective protrusion height that would be representative of the actual system of loads on the ground. From a practical perspective, if the failure pressure makes it easy to fix the values, it is harder to determine the exposed height or protrusion height. The value s variability in the load area can affect the results obtained from one assembly to another. The variability will also be greater than the variability observed in cone testing. As stressed earlier, it was determined that, for this study, the exposed height would be set at half the maximum diameter of stones used, as in the NF-W & K study. The value represents a tight stone configuration in contrast with isolated stone, whose impact on the modification factors doubles the weight per unit area of geotextile required. Lastly, it was agreed that the freshly crushed stone used was similar to the angular stone used in the Wilson-Fahmy, Narejo and Koerner study (1996). This change meant that the curves for rounded and subrounded gravel could be eliminated from Figure 4.2, yielding the following graph: Figure Results of NW-F & K comparable with testing in these trials A look at the graph stresses the correlation between the behaviour of HDPE vs. angular stone and vs. truncated cones (ASTM 5514). The addition of the results obtained in these trials is as follows: 8

9 Figure Results for PVC compared with the results of NW-F & K Given the number of tests done for each type of base material and the variance between the behaviour of the PVC and that used for HDPE in the NW-F & K calculation formulae, it is clear that the PVC has a substantial advantage in terms of the maximum contact pressure for a thickness of 1.0 mm compared with 1.5 mm for the HDPE. Similarly, by comparing the results obtained with the results presented by Stark et al in 2008, the difference between the behaviour with cones and with angular stones is clearly shown in the figure below. Figure 4.5 Comparison of test results with the results of Stark et al 2008 The comparison shows that the behaviour deduced from the cone behaviour is not the same as the behaviour obtained from direct contact with angular stone. The shift to the right refutes the idea of a diameter of stone below which no protection is required. 9

10 If diameter does not explain everything, abrasion must do the rest; a look at the samples suggests this answer. 4.2 ABRASION AND SURFACE EFFECT In figure 4.5, a comparison of the results from this testing series and the results presented by Stark et al (2008) suggests questions regarding the failure mode of PVC membrane under a puncture load. Particle diameter may not have as critical an importance as was thought compared with rock abrasiveness. This observation is spotlighted by the shift in curve height toward smaller diameters. In practice, the PVC s CCH will be smaller than the CCH determined by truncated cone testing. Lastly, as rock orientation is random, exposure or non-exposure to a ridge that is abrasive to the membrane is also random, explaining the variability of results when rocks are in direct contact (without geotextile) with the membrane. However, when a geotextile is installed between the membrane and the rock, this compensates for the rock s abrasiveness, substantially increasing the failure pressure and probably reducing result variability. 4.3 DIAMETER OF GRAVEL (UNDERLINER AND OVERLINER) As shown previously, the puncture behaviour of PVC thus depends not only on the abrasive characteristics of the rock the PVC is in contact with, but also on the dimension of the particles, the balance between these two parameters being the purview of specific testing recommended during the design phase. 4.4 MEMBRANE THICKNESS As stressed by NW-F & K and confirmed by Stark et al 2008, a membrane s puncture resistance is directly proportional to its thickness. The following formula allows us to correct the values obtained for a given thickness to assess the values expected for a different thickness. Considering a thickness of Ep 1 and a failure pressure of P 2, the expected value P 2 for thickness Ep 2 would be: (1) P 2 = Ep 2 X P 2 / Ep 2 In practice, therefore, the selection of membrane thickness is also a method for reducing perforation risk. Marcotte et al (2007) acknowledge that "increasing thickness of polymeric liner is one serious way of reducing leaks occurrence and insure better imperviousness. An increase from 1 mm to 2 mm thick geomembrane can reduce by 6 fold the number of leaks. As a last resort, the approach with respect to the selection and application of protective materials will be guided by the project's economic analysis. Here, selecting the appropriate factor of security is important and must be stressed. 10

11 4.5 IMPORTANCE OF CREEP However, recent studies by Koerner et al (2008) suggest that even the relatively conservative parameters proposed by NW-F & K 1996 do not correctly factor in the importance of HDPE creep in potential failure, or reaching an unacceptable level of constraints for the membrane (yield). 4.6 PVC FORMULATION Lastly, the nature of the components that make up a PVC sheet suggest that an appropriate formulation must be selected for a project with specific conditions. Many additives and products can improve the behaviour of a PVC sheet so as to guarantee appropriate behaviour based on the loads involved. 4.7 QUALITY INSURANCE AND LEAK DETECTION Most of the leaks found on site by leak detection methods refer to construction activities and more specifically the covering of the liner (Peggs et al. ----). Application of a construction quality assurance program as well as leak detection survey must then be included in the cost of liner systems and could then lead to the reduction of the normal leak rate (Blond et al., 2003) 4.8 APPLICATION Considering, for the purpose of the discussion, that the hydrostatic condition is the worst with respect to puncture loads on the membrane, it is possible to compare the results obtained during this work and the data from NW-F & K 1996 in this context. In a geostatic condition, the pressure, although greater, is only applied at the point of contact, reducing general deformation by that extent. Moreover, some local deformations favour an arching effect, which will also reduce other local conditions. Figure 4.5 Diagram of a hydrostatic condition 11

12 Using the data from Table 4b in the NW-F & K article for angular stone and adding the value for the limit hydrostatic pressure estimated for 40-mil PVC with non-woven needle-punched 270 gr/m² geotextile, we see that, for an exposed height of 25 mm (i.e. 50 mm at its maximum size), the PVC with geotextile performs better than anticipated by NW-F & K for a 1.5-mm HDPE with a 550 gr/m² geotextile. The table and graph below summarize the results for angular stone (highly abrasive). Table 5.1: Comparison of results with no factor of security for variability Effective height (max. gravel dimension) According to NW-F & K (Table 4b) All factors of security applied HDPE and 270 gr/m² HDPE and 550 gr/m² Trial series Phase II No factors of security 40-mil PVC 40-mil PVC and 270 gr/m² 6 mm (12 mm) 147 psi 260 psi > 150 psi * > 210 psi * 12 mm (25 mm) 26 psi 60 psi > 120 psi * > 180 psi * 25 mm (50 mm) psi 70 psi 130 psi 38 mm (76 mm) psi 120 psi Note: The values with an asterisk are projected and must be confirmed. The values shown in Table 5.1 put the benefits of PVC into perspective for the applications recommended by NW-F & K. Although presented without a factor of security, which depends on the designer's choice and current practices, the difference between the final results and practice spotlights the manoeuvring room associated with the required margin of safety. Referring to Table 4b in NW-F & K 1996 (page 665), it is clear that adding a non-woven needle-punched geotextile as puncture protection at least doubles the result for the final resistance level, which exceeds 150 psi (1,000 kpa). The graph below uses the values obtained from the tests to highlight the substantial practical difference. Figure 5.5: Application of results with no factor of security The comparison has some substantial consequences. In fact, selecting a 40-mil PVC membrane with a 270 gr/m² non-woven needle-punched geotextile compares favourably with a 60-mil HDPE to which a 550 gr/m² geotextile must be added to achieve the same result. 12

13 In the context of heap leach pads, this situation has consequences even though the use of protective geotextile is fairly rare. In fact, the situation substantially reduces pressure on the selection of the underliner and overliner. Also, in practice, a 300-mm layer of gravel with a maximum size of 50 mm could, without shielding, be used as an under- or overliner for PVC, subject to specific pre-work testing on the stone involved, if the applied hydrostatic pressure is less than 350 kpa. In fact, the design lead must develop the approach for this application specifically for each project. The table below, presented as a go/no-go guide, shows the value of the PVC solution in the framework of heap leach pads. Based on a hydrostatic load of 200 kpa, the table compares the acceptability of the proposed solutions, showing that, with a factor of security of 2 (FS=2), PVC would accept an angular infrastructure (underliner) with a maximum stone dimension of 75 mm. Figure 5.1 Table for selecting PVC vs. HDPE 13

14 5 CONCLUSION AND RECOMMENDATIONS According to the testing performed for this study, it is clear that the 1.0-mm (40-mil) PVC membrane in direct contact with angular gravel delivers superior puncture performance in terms of its limit hydrostatic pressure than a 1.5-mm (60-mil) HDPE membrane. Comparing these results with those presented by Narejo, Wilson-Fahmy and Koerner (1996) shows substantial differences for a bed of gravel with diameters ranging from 20 to 100 mm, and confirms the principle of the results presented by Stark et al (2008) for ASTM-D5514 truncated cone testing. Where the maximum dimension of the gravel in contact with the membrane exceeds 12 mm, the PVC membrane s puncture behaviour becomes a key argument for choosing it as a technical alternative to rigid membranes which, when chosen, are often thicker and protected by adding layers with smaller maximum particle dimensions. This advantage is even more appealing due to the fact that these materials are most appropriate for subgrades that are relatively soft or compactable. Lastly, PVC s dimensional stability in comparison with HDPE will also reduce folds, which are in and of themselves a condition that can cause substantial damage to the sheet in the event of heavy pressure. Moreover, the effect of time will not reduce the quality of the behaviour over the long term. In all cases, laboratory testing is recommended to pinpoint the value of the maximum pressure to use for design based on the size and angularity of soils that are in contact with the membrane, as well as the thickness and specific properties of the membrane chosen. Finally, selecting a dampproofing membrane in the framework of ore leaching preparation work is not only based on the puncture performance of the materials used for testing. The impact of the products involved and the selected polymer s compatibility, effect of the temperature at the base of piles, formulation of the selected membrane, lifespan of the operation, availability of materials and specific conditions of a project must also be factored in when justifying the selection of one material over another. 14

15 BIBLIOGRAPHY ASTM D (2001), Standard Test Method for large hydrostatic Puncture Testing of Geosynthetics, ASTM International, West Conshohocken, PA, USA Blond E., Bouthot, M., Vermeersch, O., Mlynarek, J. (2003), Selection of Protective Cushions for Geomembranes Puncture Protection. Proceedings of the 56th annual conference of the Canadian Geotechnical Society, Winnipeg, October Breitenbach, A.J. (-----), Long-term performance of PVC Geomembrane in the Mining Industry Internal Files /SOLMERS Jacquelin, T., Bone, C., Marcotte, M. and Rollin, A.L. (2008), Recent results in geoelectrical leak location in the Chilean mining industry, Proceedings of Cancun Conference, Cancun, Mexico, March Koerner, R.M. and Wilson-Fahmy, R.F., Narejo D., (1996), Puncture protection of geomembranes, Part III, Examples, Geosynthetics International, 3, nº 5, pp Lupo, J.F., ( ), Heap Leach facility liner design 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 Narejo D., Koerner, R.M. and Wilson-Fahmy, R.F., (1996), Puncture protection of Geomembranes Part II, Experimental, Geosynthetics International, 3, nº 5, pp Narejo, D. and Corcoran G., Geomembrane protection, GSE Design manual, First Edition Stark, T.D., Boerman, T.R., and Connor, C.J. (2008), Puncture resistance of PVC géomembranes using truncated cone test, Geosynthetics International, 15, nº 6, pp 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 Wilson-Fahmy, R.F., Narejo D. and Koerner, R.M., (1996), Puncture protection of Geomembranes Part I, Examples, Geosynthetics International, 3, nº 5, pp