HYDROCHEMISTRY, A MARKER OF LONG-TERM EVOLUTION OF UNSTABLE FRACTURED SLOPES, TOWARDS LARGE MOVING ROCK MASSES Insight from a multi-scale approach in the high Tinée valley (Mercantour, France) and in the Rosone slope (Gran Paradiso, Italy)
In fractured media, the factors that make evolve a stable fractured media towards a landslide highlight the water effect as an aggravating and a triggering factor of slope instabilities. Long term water effect on stability is studied from spring water chemistry, which gives data about the weathering state in the slope (aggravating factor of the instability) and about infiltration event in the massif (triggering factor). Thus, hydrochemistry enables to propose a conceptual model of flow in unstable slope. A multi scale approach, in two alpine valleys, of relations between the flows and the instability structures highlights the draining role of these structures. Slopes comparison enables to discuss the long term evolution of the effect between water seepages and slope destabilization. Slow movements (toppling, sagging and creep) decompress the upper part of the slope, increase the infiltrated water volumes and create perched aquifers in the decompressed areas. With the destabilization, the water / deformation coupling evolves, from high water pressure effect in low storative fractures to a water high charging effect in the high storative zone, in the upper part of the slope, and can initiate a rupture. Hydrogeochemical equilibrium, observed in stable slope, is modified by large landslide crisis. Thus, sulfate dissolution evolves with active deformation of the slope. This observation enables to demonstrate the possibility to use hydrochemistry to monitor unstable slopes that can record slow deformation in the slope.