Geophysical investigation of the large séchilienne gravitational movement, The Alps (France)

Major gravitational movements are common in the metamorphic formations of mountain ranges and show various types of failure, such toppling, sagging and translational or rotational sliding. The different failure processes are mainly governed by the characteristics of the discontinuities (foliation, schistosity, faults and fractures) affecting the mass (Antoine et al., 1990). In the Alps, most of the gravitational movements have probably been initiated or reactivated after the retreat of glaciers some 10,000 to 15,000 years ago, and have evolved at very different rates, depending on the initial geological and topographic characteristics, as well as on the other factors contributing to lower the stability (presence of water, earthquake ground motions, climatic cycles). The instability process progresses through periods of stabilisation and reactivation and leads to slope failure after decades or centuries. The Séchilienne movement, which is located in the French Alps near Grenoble, is affecting the right southfacing bank of the Romanche river (figure 1) The slope is homogeneously made of micaschists with subvertical foliation at right angle with the valley (except in the upper part of the slope where it is folded with an axis inclined of 30° in the north direction) and is affected by 3 sets of subvertical fractures. The main family runs ENE-WSW and delimits vertical slices in the rock mass. It is clearly distinguished by several hundred meters long depressions in the morphology associated with scarps of several meters high (Vengeon, 1998). Some of these depressions are 20 m wide, attesting the long duration of the gravity-induced processes. The slope angle is about 40° in the lower part of the hill (elevation between 330 m and 950 m) and decreases to 20° between 950 m to 1100 m (Mont Sec). Near the crest, a 20 m high scarp which is followed on a distance of several hundreds meters reveals an upper collapse. The non-freshness of the scarp and the absence of glacial erosion sign and deposits show that this movement is relatively old and occurred after the last ice age. The part of the slope which exhibits signs of current instability is located in the middle of the hill, at an elevation between 700 m and 850 m, and involves a rock volume estimated to about 3 million cubic meters (Giraud et al., 1990). This area was extensively instrumented since 1988 (Evrard et al., 1990) and the measured displacements are globally oriented in a SSE direction, perpendicular to the strike of the main fractures, and dip downhill between 10° and 20°. The movement rate varies from a few cm/year to a few dm/year. Besides geological surface observations and displacement measurements, a 240 m long gallery was excavated in 1993-94 at the elevation of 710 m. It showed a succession of blocks delimited by highly fractured and sheared zones parallel to the main fracture set. No sign suggesting the presence of a sliding surface has been observed (Vengeon, 1998). Numerical modelling with the discrete element method was able to retrieve the main field observations and suggest that the movement at Séchilienne is controlled by the main discontinuities cutting the mass into blocks and includes toppling and local sliding, evolving through progressive damaging to a potential large sliding of unknown characteristics (Vengeon, 1988). The data available at Séchilienne have led some authors to propose that the hill could be affected by a massive movement, delineated to the East by the active zone and to the North by the Mount Sec scarp. No western limit is clearly visible in the topography and the thickness of this moving mass is unknown. Consequently, the volume estimations for a rock avalanche scenario are highly variable and poorly constrained, ranging from 3 to 20 hm3 (Giraud et al., 1990 ; Antoine et al., 1994) with a global mass movement between 50 to 100 h m3. The aim of this study is to try to get information at depth over the movement area by using geophysical techniques and to confront the results with existing geological data.