1 Introduction

The stability of fractured rock slopes is strongly related to the geometrical and mechanical characteristics of natural discontinuities, which concentrate stresses at their tips giving rise to progressive failure of rock bridges. Fracture mechanics makes it possible to take this phenomenon into account assuming discontinuities as cracks and studying their triggering and propagation inside rock bridges. As an example, a Linear Elastic Fracture Mechanics approach to the back analysis of a rockfall triggering phenomenon is presented in this paper, carried out through the code GEOF (Scavia 1995; Castelli 1998), based on the BEM technique of the Displacement Discontinuity Method (DDM, Crouch and Starfield 1983).

2 Description of the rockfall and definition of the geometrical model

The simulated instability occurred in 2004 from an approximately vertical slope made of Urgonian limestone and located in the Vercors Sub-alpine Chain (South-western France).

After the failure a detailed geological and structural survey has been carried out by Frayssines (2005), leading to a complete description of the detaching niche (scar) and the surrounding slope. The main part of the scar is limited by (Figs. 1a and 1b): a mean surface (A) roughly parallel to the front, a bedding joint at the top (B), a lateral sub-vertical surface on the western side (C). The existence of fresh rock zones was observed and measured during the survey in the lower part of surface (A). This zones (52 m2) correspond to the 5% of the total surface and don't show any sign of alteration, in contrast with the rest of the scar, which is coated with a calcite crust.

A detailed digital model shows that the cliff surface presented a prominent overhang before the failure. The failure mechanism can then be a topple, due to tensile fracturing of intact rock zones. The unstable volume is characterized by a prismatic shape with average height (H) around 20 m, width (w) around 2.5 m and length (L) around 50 m, as shown in Fig. 1c.

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