The solid-to-liquid transition in the Trieves clay: the lessons from rheometric and seismic tests

Abstract : The Trièves clay (Western French Alps) is a thick Quaternary formation affected by numerous slow earthslides (slide velocity of a few cm/year to a few dm/year). Under extreme meteorological conditions (heavy rain falls, quick melting of the snow cover), these slides can turn into devastating earthflows. In order to study this solid-to-liquid transition, identification, rheometric and seismic laboratory studies were per-formed on clay samples collected at the border between the Avignonet and Harmalière landslides. Identifica-tion tests showed the wide range of liquid limits characterizing the Trièves clay. Rheometric creep tests were performed on two samples with different gravimetric water content. The clay material behaves as a thixotrop-ic yield stress fluid and is characterized by a marked viscosity bifurcation. This rheological behavior could be a key point explaining the solid-to-liquid behavior of this material. The seismic study aims at characterizing the clay in different moisture conditions and to test our capacity to measure Rayleigh wave velocity (VR) var-iations in clay from the liquid limit to a drier state. VR values ranging from 22m/s to 120m/s were successfully measured for a gravimetric water content decreasing from 40% to 26%, respectively. These results show that VR (and indirectly the shear-wave velocity) is very sensitive to a change in the clay physical state and could be used as a monitoring parameter.