Évaluation des propriétés chimiques, mécaniques et hydrogéologiques de géomembranes utilisées comme matériaux de recouvrement pour restaurer des sites miniers en milieu froid et acide

Geomembranes (GM) are used as fluid barrier components of cover systems for mine site reclamation to control the generation of acid mine drainage (AMD). A dozen sites with this type of cover systems have been found in the literature. On the one hand, the depths at which GMs are located at the various sites identified in Canada are less than the potential frost depth, thus subjecting them to freeze-thaw cycles (FTCs). On the other hand, at some of these sites, GM is installed in direct contact with acid generating wastes, which would expose the GM to AMD chemical attack. Nonetheless, GMs are synthetic materials with a finite lifespan, whereas in mine site reclamation, the objective is to achieve a system that would remain efficient in the very long term. Hence the origin of this thesis to assess the durability of GMs in the context of cover systems for mining reclamation in cold and acidic environments. To do so, the project was oriented on three axes by assessing first 1) the effects of FTCs on the GM properties, then 2) the effects of the AMD chemical attacks on the GM properties, and finally 3) the real GM performance installed on three sites located in Abitibi-Témiscamingue and in Mid-North of Québec. For the assessment of the effects of FTCs on the GM properties, HDPE, and linear low-density polyethylene (LLDPE) GMs were subjected to laboratory-controlled FTCs. LLDPE GMs were also examined in this part of the thesis as they offer a practical alternative to HDPE GMs. Tensile, hydraulic and oxygen sorption/diffusion properties were used as key performance. Initially, from a mechanical point of view, HDPE GMs were more rigid and had lower break properties than LLDPE GMs. The tensile properties were evaluated after 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, and 300 FTCs. Over the FTCs, a stiffening, and an increase in break properties of the two materials were observed. LLDPE GMs remain more elastic and resistant than HDPE GMs. As for the hydrogeological properties of the two GMs, they were similar initially. The hydraulic and the oxygen sorption/diffusion properties of the two GMs were then examined after 100, 200, and 300 cycles. Over the FTC, these mass transfer properties changed. The equivalent hydraulic conductivities of the two GMs have increased by an order of magnitude and are at 10-14 m/s. While the oxygen diffusion and permeation coefficients decreased by an order of magnitude of 10-14 and 10-13 m2/s, respectively. These values indicate that the GMs retain good hydrogeological properties after 300 FTCs. In order to examine the effects of the AMD exposition on the GM properties, high-density polyethylene (HDPE) GMS were artificially degraded in the laboratory. These HDPE GMs are the most used GM in cover system. This degradation was performed with three immersion baths filled with synthetic AMD and heated to 65, 75, and 85 °C for a period up to 16 months. GM samples were thus extracted monthly from the baths to assess the change in the GM chemical and mechanical properties compared to the initial properties. Hydrogeological characterization tests (permeability and sorption/diffusion tests) were also carried out at the end of the immersion periods for each bath. At the end of the degradation period, GMs remained in the first degradation stage called the antioxidants (AOs) depletion stage. Regarding the GM mechanical properties, a gain in GM elasticity and no significant change in break properties were noticed. The GM hydraulic and oxygen sorption/diffusion properties have remained in the same order of magnitude as the initial ones. The equivalent hydraulic conductivity, oxygen diffusion and permeation coefficient remained within orders of magnitude of 10-15 m/s, 10-13 m2/s, and 10-12 m2/s, respectively. Thus, there would not yet be a negative effect on the GM properties during the AO depletion stage. This stage would have lasted 44, 21, and 18 months, respectively, before reaching complete AO depletion for the 65, 75 and 85 °C baths. However, the usual temperature in the cover systems is not as high as for degradation tests. Arrhenius modeling was thus performed to estimate the AO depletion time at lower temperatures. The AO depletion time would thus be respectively 170, 80, and 40 years at temperatures of 0, 10, and 20 °C for example. Nevertheless, these results were obtained in immersion baths where the GM is exposed to AMD from both sides when, at worst, GM would be only exposed to AMD on one side. Thus, for the three temperatures of 0, 10, and 20 °C, the AO depletion times could reach approximately 600, 275, and 130 years, respectively, in the case where a drainage layer is installed above the GM. These AO depletion times would be even higher if the GM is installed on a sand support layer to avoid the exposure to AMD. The AO depletion time could respectively reach 1000, 480 and 220 years at temperatures of 0, 10, and 20 °C. The design of the cover systems is thus of great importance to ensure the durability of the GM. It should be noted that what has been studied here is only the first stage of the GM degradation. There are two more stages. That means GM's service life would be even longer than the aforementioned orders of magnitude. To assess the actual performance of GMs in cover systems, two exhumation campaigns were carried out in 2019 and 2020. These campaigns thus made it possible to recover samples of smooth HDPE GM, respectively aged 20 and 13 years from two sites Mid-North and Normetal where GMs are potentially exposed to AMD and would undergo FTC, and a 10-year-old textured HDPE GM samples from the Aldermac site, which would have potentially undergone FTCs. The exhumations were carried out on two zones for each site to assess the potential influence of strain: an unsettled area where the GM would not have undergone differential deformations and a settled area where a depression was observed from the surface and where the GM would have been strained. The first characterizations carried out on these GMs were differential scanning calorimetry to measure the oxidative-induction time (OIT) to obtain overall characterization of the GMs chemical degradation stage. Infrared spectrometry was also performed to obtain the carbonyl index (CI) to assess the GM superficial degradation. CI would thus indicate the effect of the exposure to AMD. Tensile, permeability and oxygen sorption/diffusion tests were also performed on samples from each zone (unsettled and settled) of the three sites. OIT measurements indicated that GMs remained in the AO depletion stage as the OITs were well above the cut-off value of 0.5 min for a GM completely devoid of AO. CIs did not demonstrate the effects of the potential exposure to AMD. The tensile properties of the exhumed GMs remained in compliance with the standard requirements for a new GM even after years of service. The equivalent hydraulic conductivities, oxygen diffusion and permeation coefficients of GMS are in the respective orders of magnitude of 10-14 m/s, 10-13 and 10-12 m2/s. The comparison of the results obtained from the unsettled and settled zones showed that the strain would lead to GM thinning. This thinning would lead to a faster degradation of GMs but also to a slight decrease in their mass transfer properties. Comparing the results of smooth GMs with the textured one also shows that the latter would have lower durability. The work of this thesis has thus shown that at the scale of the material, GMs have good fluid barrier properties and could last up to 1000 years depending on the exposure condition of the GMs, the temperature, the configuration, and the physical stability of the cover systems for mine site reclamation in cold and acidic environments. However, the scale considered in this thesis remains small and only concerns GMs without physical defects (holes or cracks). These defects would thus constitute the limiting factors of the effectiveness of GMs and should be assessed in the future. It should also be remembered that GM is only one of the different components of the cover systems. Research needs to be done to explore the roles of each component and their interactions. Field-wide investigations will also need to be performed both during installation and during the service life of the cover systems. This thesis is thus intended to be the first steps in the assessment of cover systems including GM by providing the first information on the durability of these synthetic materials in the context of mine sites in cold and acidic environments. From a practical point of view, it is essential to avoid the installation of GMs on sites where severe deformations are expected. These deformations could induce cracks of the GMs and thus interfere with the desired sealing system. It would also be necessary to discard the options of configurations where the GM would be installed directly above the mining wastes which could accelerate its degradation, A support layer, made of sand for example, is crucial. A draining protective layer should also be installed over the GM to prevent water build-up and then to improve its durability. During the construction, strict quality assurance and quality control standards must be adopted to limit as much as possible any defect that could be generated and to repair them immediately. The cover systems should be instrumented, for example, to monitor temperature and oxygen profiles above and below the GM. GM strain should also be monitored. Then finally, an in-service quality control protocol should be adopted to regularly monitor the change of the various properties of the GMs over time, for example every 10 years.