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[Observatoire Gravitaire Géoazur] Geomechanical characteristics
Geomechanical characteristics

Research Studies by S. El Bedoui, Y. Guglielmi and T. Lebourg, Geoazur, UNS, Nice, France ; J.L. Pérez, LRPC, CETE Méditerranée, Nice, France.

Regarding its dimensions (1.100 meters long and 750 meters high) and the volume that threatens to collapse (around 60 million cubic meters), the gravitational movement is the most important landslide in Europe. The main risk stands in the collapse of millions cubic meters of rocks that might block the valley where the Tinée River flows and thus create an artificial dam. This one should cause first the overflow of the Saint Etienne de Tinée village, situated upstream. Furthermore, if the artificial dam came to break under the effect induced by the weight of the retained water, this should provoke a destructive wave and damage the Tinée River valley, then the Var River valley, till the immediate surroundings of Nice, where the Var River oulets into the Mediterranean Sea. Numerous monitoring and prevention actions (detour of the road, diversion tunnel for the Tinée River) have been taken, especially at the time of the main crisis, at the end of the 1980s. The whole slide is currently divided into several parts which areas and kinematics are variable, and that are summarized by the illustrated image (photo taken in October 2007).

From 1960 to 1990, the slope was significantly deformed due to the rock slide activity characterized by a 130 m high scarp in the middle slope. This rock slide activity has been continuously monitored with electronic distance meters and GPS (Geoazur Laboratory) for several years. Currently, the most active part of the rock slide shows a movement of about 0.40 m yr-1, slower than the rate of several m yr-1 recorded during the late 1980s. The moving mass has been investigated with structural, mechanical and hydrogeological studies (J.P. Follacci, 1987 ; J.P. Ivaldi et al, 1991 ; INTERRREG1, 1996 ; F. Cappa et al, 2004 ; Y. Guglielmi et al, 2005), and electrical tomography (T. Lebourg et al, 2005 ; H. Jomard et al, 2007). These studies provided a better understanding of the rock slide, including the geometry of the sliding surface (estimated to be 100 m deep) and links between metoric forcings and slope displacements. Y. Guglielmi et al, 2005 attributed the rock slide to a critical toppling. However, the slope evolution before the rapid deformation has been less well documented.

Combining a chronological study with geomorphological mapping, S. El Bedoui et al, 2009 have shown that slope evolution from a large-scale deformation (DGSD) to a rapid failure can be regarded as the evolution of creep that leads to faster and more localized displacements. They have identified three phases of slope evolution: I) a slow large-scale deformation (4 mm yr-1) with opening of trenches, over a long time period (10 ka to 5.6 ka BP); II) a localized deformation with faster displacement (13 to 30 mm yr-1) in a shorter period (3.6 ka BP); and III) a rapid failure at the slope foot in a short time period (50 years), with a very high displacement rate (> 80 mm yr-1). The result presented by the authors agrees with several previous studies, which indicates that instrumental surveys (e.g. GPS) of creeping of slopes affected by DGSD may enable the estimation of the time of a future rapid failure.