Evidence of hydraulically active fractures in Champlain Sea clay deposits and their impact on transient groundwater flow dynamics and slope stability.

ABSTRACT: Landslides in Quebec represent the largest natural risk to human life and civil infrastructure. Because 80% of the reported landslides in the province occur in sensitive marine clay deposits, there is a practical interest in improving the understanding of the mechanisms that trigger landslides in these deposits. Slope stability modeling usually assume homogeneous clay properties and a steady-state pore pressure distribution. However, data analysis from more than 200 piezometers arranged in over 45 nests across 12 monitoring sites in Champlain Sea clay have shown that large and rapid variations in hydraulic heads can be observed at great depths. In addition, synchronism was observed between the piezometers that are located at different depths in the same nest. These behaviors are not compatible with the hydraulic properties of intact clays, which could be an indicator of the presence of hydraulically active fractures up to 15 meters of deep. Given the evidence of the presence of fractures, this research aims to explore their potential impact on transient groundwater flow dynamics and the slope stability. Using a hydrogeological model, the pore pressure distribution in a slope with discrete fractures was simulated and the results were subsequently integrated into a slope stability model. The hydrological simulation shows that depending on the direction of the groundwater flows the pore water pressures increase or decrease compared to the base case scenario without fractures. These results will allow the delineation of the slope stability response to transient groundwater flows in a slope with fractures and, consequently, a better understanding of the mechanisms that trigger landslides.