Abstract
Abstract. Slow-moving landslides show complex mechanical and fluid interactions. They show among others non linear intrinsic viscosity of the shear zone, undrained loading effects and the generation of excess pore water pressure. The parameterization of hydrological and geomechanical factors by field and laboratory tests to describe the movement pattern of these landslides is difficult. It is a challenge to simulate accurately the de- and acceleration of these landslides and particularly, to forecast catastrophic surges. In this paper the relation between groundwater fluctuation and landslide velocity for two deep-seated landslides of the Trièves Plateau (the Monestier-du-Percy landslide and the Saint-Guillaume landslide) is analysed. Inclinometer measurements, showing the displacement in depth after 1–2 months periods, showed on both landslides shear band deformation within 1 m. At the Monestier-du-Percy landslide, depending on the position, the shear band depths vary between 25.0 m and 10.0 m. At the Saint-Guillaume landslide, the inclinometers detected several slip surfaces inside the clays, at respectively 37.0 m, 34.5 m, and 14.0 m depth. Two simple geomechanical models are developed to describe these displacements in depth in relation to measured groundwater fluctuations. Calibration of the models using the friction angle delivered no constant values for different measuring periods. It appeared that calibrated (apparent) friction values increase with increasing groundwater levels. The paper discusses the possibility of the generation of negative excess pore water pressures as a feed back mechanism, which may explain the complex displacement pattern of these landslides developed in varved clays.
Highlights
In the French Alps, hundreds of landslides have occurred in the varved clays of the Trieves Plateau, around 50 km south of Grenoble (Fig. 1a)
The monitoring system has been complemented since 2003 by a geophysical investigation, yielding more spatially detailed information on the geometry and depth of the slip surface and the bedrock (Tatard, 2006). These results have suggested that the slip surfaces develop first at the clay/bedrock interface where the bedrock is close to the surface and within the clay layer
We may expect that a large part of the slip surfaces of the Monestier-du-Percy and Saint-Guillaume landslides are nearly parallel to the lamination of the clays, and that due to the movement the friction angles are in a residual state
Summary
In the French Alps, hundreds of landslides have occurred in the varved clays of the Trieves Plateau, around 50 km south of Grenoble (Fig. 1a). These clays are glacio-lacustrine sediments deposited in glacially dammed lakes during the Wurm maximum episode, and are characterised by an alternation of silt laminae of 1–20 mm in thickness (Giraud et al, 1991; van Asch et al, 1996). Slope stability calculations indicate that the groundwater levels of the deep-seated slides have to rise nearby the topographic surface in order to trigger or accelerate the movements (Vuillermet, 1992). Stresses the importance of negative excess pore water pressures on their kinematics
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