Étude numérique de la réduction du transport advectif de contaminants entre les fosses remblayées et l'environnement par l'utilisation de chemins d'écoulements préférentiels

Mining industry had a key role to play to support the supply of raw materials needed for the development of new technologies, the economic development of emerging countries and the energy transition. However, mine operations generate a large quantity of solid wastes which raise several geotechnical and geochemical challenges and especially regarding their disposal and management. Projected climate change may also modify the environmental conditions surface storage facilities were designed for, therefore adding further uncertainty to disposal site design and increasing the risks for instabilities. In-pit mine wastes disposal therefore appears as a promising alternative management approach which benefits from many advantages, such as the permanent isolation of mine wastes below ground (thus significantly reducing the geotechnical instability risks), or the improvement of the social acceptance of such projects by the public and regulators. However, geochemical interactions between the wastes disposed in-pit and the surrounding rock are expected and groundwater flow could contribute to disperse contaminants contained in the backfilled wastes in the environment. One of the common approaches to control the interactions between in-pit disposed wastes and the environment consists in using preferential flow paths in or around the backfilled pit to divert groundwater streamlines from the wastes, thus preventing contamination. Such preferential flow paths can be created using codisposal of different wastes with a significant hydraulic conductivity contrast. Previous studies have, however, only considered preferential flow paths placed to formed a pervious surround, despite the optimality of such a geometry was not proven. An alternate way to create preferential flow paths could be to use the damaged zone along the pit walls created by mining operations. Such an approach was yet never assessed. The objective of this research was therefore to, first, develop new numerical tools to the study the performance of preferential flow paths in and around backfilled pits, and then to propose recommendations to optimize the deviation of groundwater flow from in pit disposed wastes, using either natural or constructed preferential paths. A three-dimensional numerical approach, supported and validated by an analytical approach, was used in this study. This numerical approach required the development of new tools to facilitate data input and output to numerical models. The computer-aided design software Salomé was thus interfaced and coupled with the subsurface flow and transport code PFLO- TRAN and the Vorpalite mesher. This new interface especially aimed to ease and improve control of the creation of numerical models and was actively used during the whole research. The code PFLTORAN was also modified to simulate the groundwater flow in the damaged zone and to output new variables to enable adjoint problem solving. With these changes, PFLOTRAN became one of the first hydrogeological software to provide the sensitivities required to solve high dimensional optimization problems. Then, the accuracy of the simulation approach and of the PFLOTRAN code improvements were verified against an experimental model taken from the literature and an analytical solution. Solute transport in a laboratory-sized physical model of a backfilled pit with a pervious surround was first modeled using the numerical approach developed in the present project and was compared to experimental measurements. Comparison showed the numerical approach was able to reproduce the hydrogeological behaviour observed in the physical model. A decoupling of solute transport between the pit and the surrounding rock was also observed, thus suggesting it would be possible to focus on the processes at the pit scale, without requiring a detailed representation of the fractured rock in practice. The capacity of PFLOTRAN to accurately simulate preferential flow paths in the damaged zone was also verified by com- paring numerical simulation results with a two-dimensional analytical solution. This second validation demonstrated that PFLOTRAN was able to simulate accurately the water flow deviations in the damaged zone. The deviation of groundwater in the damaged zone around the pit was then assessed using a stochastic approach based on 124 000 numerical simulations. This approach allowed to determine general trends, and to derive new indicators to evaluate its performance related to groundwater flow in backfilled wastes and advective solute transport reduction. Results showed that the damaged zone could act as a natural pervious surround provided it was sufficiently fractured and thick and the pit was backfilled with low permeability wastes. In this case, the damaged zone was able to reduce waterflow in backfilled wastes by more than 90%. A new approach to evaluate water deviation in the damage zone was also proposed. A semi-analytic model was developped based on two parameters that could be easily determined in the field, and was ±20% accurate. Results also permitted to derive more precise engineering criteria than those available in the literature to ensure that the damaged zone would act as a natural pervious surround. Finally, this study highlighted the potential of the blast damaged zone for reducing groundwater flow contaminants transport from the backfilled pit to the environment. The last step of this study consisted in optimizing the geometry of preferential flow paths made of inert waste rocks placed in a pit backfilled with potentially contaminating tailings. A topology optimization approach (applied for the first time to a hydrogeological problem) was used to minimize groundwater flow entering the disposed wastes and the advective solute transport from the backfilled pit to the environment. A Python proto-library was created to prototype and develop the objective function and operational constraints, and improve the optimization process. Overall, results confirmed the good performance of the current practice and proved that the pervious surround geometry was suboptimal for most of the pit geometries considered. Topological optimization also allowed to propose an improved design when the regional flow was oriented along the minor axis of an elliptical pit, in which case unconventional geometries composed of waste rocks conduits through the tailings allowed to decrease the groundwater flow in backfilled tailings by approximately 40% compared to a non-optimized pervious surround alone. The results obtained in this study therefore provided new knowledge related to the characterization of preferential paths in the damaged zone around the pits, the optimization of pervious surrounds geometry, and more generally to the reduction of groundwater flow in backfilled wastes contaminants transport to the environment. This work also paves the way for future developments, in particular the field-scale validation of the damaged zone role as a containment structure, the improvement of the optimization approach by considering other operational constraints, or its use to other mining environment problems. Finally, the results of this research project contributed to reducing the uncertainties related to mine wastes disposal in open-pits, and to improving the long term physical and geochemical stability of the disposed wastes.