Volumetric water content determination by TDR sensors and Decagons in gravelly soils

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.