Evidence of Deep Hydraulically Active Fractures in Clay Deposits (Québec, Canada) and Numerical Simulation of Their Impacts on Groundwater Flow and Slope Stability

This study applies cross-correlation analysis to hydraulic head data from a large network of vibrating-wire piezometers installed in sensitive glaciomarine clay deposits across the St. Lawrence and Saguenay-Lac St-Jean Lowlands in Quebec, Canada. The results reveal the presence of hydraulically active fractures near slopes, extending to depths of up to 16 meters. These findings challenge traditional models that assume clay deposits remain unfractured below a shallow weathered zone, commonly referred to as the crust. The presence of fractures facilitates rapid groundwater movement, leading to significant variations in hydraulic head that were previously believed to be attenuated at depth due to the clay’s low permeability. To assess the broader implications, we compared field data with the results of steady-state groundwater flow models that incorporate fracture scenarios. Two slope geometries with contrasting groundwater flow dynamics were analyzed, each under different fracture configurations. The hydrogeological modeling outcomes were then integrated into a slope stability model to examine how fractures influence stability. The results indicate that fractures can enhance hydraulic head by acting as preferential pathways for infiltration. However, they may also lower hydraulic head by accelerating water discharge through the slope face. Consequently, from a hydrogeological standpoint, fractures can stabilize or destabilize slopes depending on the prevailing groundwater flow system. Since this study focuses exclusively on the hydrogeological effects of fractures, future research should explore their coupled hydromechanical impacts.