Deep Hydraulically‐Active Fractures in Sensitive Clay Deposits: Implications for Groundwater Flow and Slope Stability

Landslides in sensitive post‐glacial marine clays are one of the major geological hazards in Canada, Norway and Sweden. Current hydrogeological conceptual models used for slope stability analyses in these deposits consider simple groundwater flow conditions within a homogenous, isotropic, massive clay deposits, where fractures are surficial features that only exist within a 1–5 m‐thick weathered zone. This study uses cross‐correlation analysis on hydraulic head data from a large network of vibrating‐wire piezometers in clay deposits along the St. Lawrence River and in the Saguenay‐Lac St‐Jean Lowlands, in Quebec, Canada, to show that hydraulically‐active fractures are present to depths of up to 16 m at 4 (possibly 6) of the 7 locations studied. These findings suggest that current conceptual models have a high likelihood of misrepresenting local flow systems, and that further field and modeling work is needed to characterize the extent and influence of these fracture networks.