Abstract
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.
Plain Language Summary
Much of Eastern Canada, Sweden, Norway, and parts of the Northeastern United States were flooded by the sea at the end of the last ice age, resulting in the deposition of thick clay deposits that are prone to landslides. Since landslides represent one the largest natural hazards to human life and property in these areas, understanding the mechanisms that trigger landslides is a matter of public security. Previous research examining slope stability in these deposits has assumed that small cracks (i.e., fractures) only exist in the upper 1–5 m of the clay. Evidence presented here shows that fractures can extend to depths of greater than 15 m, and are present in many hillslopes. These fractures enable water to rapidly flow through a material that is otherwise nearly impermeable, ultimately influencing the processes that control landslide initiation. This work uses statistical analyses of groundwater data from a network of wells across an 800‐km transect of the province of Quebec to show that groundwater flow systems in massive clay hillslopes are much more complicated than originally thought, and that methods used for conducting current slope stability analyses need to be updated to properly assess landslide risks in these deposits.
Key Points
Fractures were identified beneath the upper 1–5 m‐thick weathered zone of the post‐glacial marine clays at multiple locations in Quebec
Fractures allow large variations in hydraulic head at >10 m depth to occur almost concurrently with variations observed near the surface
Fractures control groundwater flow in low‐permeability media, but their hydraulic effects are not considered in slope stability analyses