In-situ geophysical and hydro-geochemical monitoring to infer landslide dynamics (Pégairolles-de-l'Escalette landslide, France)

Abstract

The Pégairolles-de-l'Escalette landslide is located in the central part of the Languedoc Region (Southern France). It corresponds to a deep-seated landslide (>50 m of thickness) with extremely slow slip displacement (3–4 mm/year). In this area, the current landslide activity is associated with intense precipitation events (300–500 mm in a few days) and the related dissolution of Triassic evaporite layers at depth. Considering a relatively simple geological context and a landslide mainly controlled by slope hydrogeology, this site constitutes a natural observatory to study the impact of large rain events on slope kinematics.This work is based on complete and original instrumentation deployed in two nearby boreholes since 2012. The landslide is investigated down to 65 m depth by two in situ permanent observatories for geophysical (electrical resistivity and deformation, metric sensor spacing, daily and seasonal data acquisition) and hydrogeochemical monitoring (pressure, pH, temperature, electrical conductivity, fluid sampling – 4 depths, monthly data acquisition). The data recorded during the five years of investigations help us to characterize the active slip zones in the underground as well as the seasonal dynamics of the different hydrogeological units within the slope. We also observe a heavy rainfall event characterized by both electrical resistivity and geochemical changes down hole, which suggest different time responses of the system to the meteorological solicitations. These first results point out the relevance of the downhole monitoring to progress towards a better understanding of internal landslide processes in relation to climate forcing.