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.@en

Debris flow and landslide events in an alpine environment depend on factors such as slope inclination, soil and rock mass characterization, vegetation, rainfall infiltration, ice degradation and snowmelt. If rain infiltrates into the ground, the degree of saturation will increase, with a subsequent decrease in suction, and effective stress in the soil. The shear resistance decreases accordingly, which can trigger failure in critical cases, although the loads that act in the soil may remain almost constant. A study site is located in a steep alpine slope of a lateral valley of the Rhone valley in Valais. This paper focuses on the preliminary geotechnical monitoring, which includes installation of thermistors, TDR (time domain reflectometry) sensors and Decagon (capacitance/frequency domain technology) sensors to measure the volumetric water content and recording of data to assess the effect of rainfall infiltration on the reduction of shear resistance in the soil. The process of laboratory calibration, placement of the sensors in gravelly soil in the field, and early results from the monitoring will be presented and discussed. Determination of the volumetric water content at discrete points in a slope will correlate to the degree of saturation, and can, through tensiometer measurements of the soil suction or a suitable Water Retention Curve, give valuable information to calculate the change in soil resistance.@en