Modelling of landslides in a scree slope induced by groundwater and rainfall

Predicting the trigger of a slope failure of a steep Alpine scree slope in south-west Switzerland is challenging. The groundwater (GW) flow from snow-melting and rainfall infiltration during summer changes the susceptibility to surficial failure, which also depends on the slope angle, bedrock geometry, stratigraphy and the shear strength of the soil. Surficial failure mechanisms are investigated using prototype ground models that integrate input from field monitoring, geological observations and soil properties and account for relevant factors and constraints for physical and numerical modelling. Shallow scree deposits overlying various bedrock configurations (parallel to the slope, with and without a step) were tested under two hydrological regimes: GW flow, and GW combined with additional intense rainfall. Numerical modelling was used to study the parameter combinations that would lead to failure, and worst-case scenarios were defined in terms of the bedrock geometry and hydraulic perturbations. These results were verified using advanced physical modelling techniques in a geotechnical drum centrifuge. Physical modelling results indicated that, for a given GW condition, slope stability decreases (a) as the depth of the soil cover over the bedrock decreases and (b) the higher the bedrock step. Furthermore, a bedrock step impacts the volume and the location of the triggered failure. Rainfall exacerbates the situation.