Analysing the kinematic response to groundwater fluctuations of a slow moving landslide in varved clays, using two alternatives of the equation of motion

Abstract

The relation between groundwater fluctuations and displacement velocities are analysed for a slow moving deep-seated landslide. The landslide, which developed in varved clays, is situated near Monestier-duPercy, on the «Trièves Plateau» in the French pre-Alps. The landslide, which started in the fifties, moved over a long time period with varying (also zero) velocities. Over longer periods, a temporally attenuating trend in the amount of displacement could be observed. In recent years during the nineties, inclinometer measurements showed a mean displacement around 30 mm yr–1. The inclinometer profiles show a shear band of less than one meter with a more or less rigid plug on top. Two models based on the equation of motion were tested on these displacement profiles in combination with piezometric observations from an open stand pipe nearby: The AC-model uses the complete equation of motion including the local and convective acceleration terms. The NA-model ignores these terms making it a sort of steady state model. It appeared that for slow moving landslides, differences in calculated velocities and displacements between the two models can be neglected for gradual rising and falling groundwater levels. The landslide monitoring did not show a simple linear relation between groundwater level fluctuation and displacement velocity. Back calculation of the viscosity for the various time intervals within a selected test period between 23/11/1992-24/08/1995 shows an exponential relation ship between excess shear stress and shear strain rate. It can be explained by a rate dependency of an apparent viscosity, caused by generation of a rate dependent excess pore pressure or by a rate dependent change in thickness of the shear band.