1 Introduction

Natural temperature variations are thought to play a role in rock slope instabilities. Thermal and and geotechnical in situ measurements have been carried out in sites were the succession of thermomechanical effects are suspected to have led progressively to failure. Parallel laboratory tests show the damage of specimens subjected to thermal fatigue. Site and laboratory observations are briefly described below and elements of interpretation are proposed.

2 In situ observations

In situ observations accredited the role of temperature variations in the progressive destabilization of rock slopes. We refer below to 2 French sites, but Vargas et al. (2004, 2009) made similar observation in Brazil.

2.1 A gneissic rock mass

For several years, thermal and geotechnical in situ measurements have been collected at the "Rochers de Valabres" site (Southern French Alps), a gneissic rockmass, to examine surface thermomechanical phenomena and their potential contribution to rock fall triggering. In fact, in May 2000 (Fig. 1) the site experiments a 2000 m3 rock fall during a period with any particular rainfall event, seismic activity, or freeze-thaw period and we suspect it could be linked to the repetition of thermal variation (Gunzburger et al. 2005). The site was monitored with different equipment to record the temperature variation in the rock-mass and the potential induced effect. The measured data were analyzed based on analytical solutions and numerical modelling.

2.2 A limestone cliff

At "La Roque Gageac", a village located near Bergerac at the right bank of the Dordogne, a cliff composed of sandy limestone was subjected to rupture accredited to temperature fluctuations.

The site experienced 4 rock falls since 1920, which resulted in three deaths. Among the potential causes for these rock falls, temperature variations were considered as one of the possible preparatory factor and the site is monitored with temperature gauges and strain measurement and under study.

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