Application of geotechnical and geophysical field measurements in an active alpine environment

Abstract

A gravelly scree slope in the Meretschibach catchment, a location in the Swiss Alps in the vicinity of Agarn, canton Valais, has been observed to deform downslope at up to 0.5 m p.a. The potential instabilities at this site include surficial landslides, some of them originally thought to be triggered by an increase in pore water pressure with a subsequent loss of shear strength as a consequence of rainfall infiltration and rockfalls. A programme consisting of monitoring, laboratory testing and investigation was developed, to perform a thorough soil characterisation needed in order to produce a realistic ground model. The long-term geotechnical monitoring included in situ soil temperature, suction as well as volumetric water content measurements using dielectric permittivity and time domain reflectometry (TDR) sensors. This data was complemented by electrical resistivity tomography (ERT) to provide extensive knowledge on the depth to bedrock and to validate the volumetric water contents in specific locations. The datasets are completed by recordings from two nearby weather stations. Seasonal changes of precipitation and temperature were reflected in corresponding trends in all measurements. A comparison of volumetric water content records was obtained using capacitance and time domain reflectometry (TDR) sensors with ERT, yielding reasonable agreement. The resulting ground model, which integrates all currently available parameters, delivers the essential information and boundary conditions for predicting and validating slope instabilities in the future, using numerical and physical modelling.