Research Studies by F. Cappa, Geoazur, UNS, Nice, France.

Friday 4 December 2009 by Thomas Lebourg

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In a rocky massif, the hydrogeochemical measurements of the water/rock interactions enable to (1) locate aquifer zones where water flows, and (2) assess transit and renewal time of water. Besides, the measurements allow to (3) establish a first estimation of water pressures at the massif scale.

F. Cappa et al, 2004 have developed an original approach based upon measurements related to the geochemistry and discharge of spring waters and to surface displacements on the La Clapière landslide site (Southern Alps) in order to study couplings between hydraulic behaviours and slope mechanics.

This new approach has enabled to point out a clear correlation between the variations of water chemistry and displacement velocities of the slope. What is highlighted is that the acceleration periods of the La Clapière landslide are synchronous with the periods of increasing discharge of springs and the decrease of sulfate and nitrate concentrations in water. The results reveal that hydromechanical effects inside the aquifers of the slope play an important part in the global dynamics of the slope. The study shows that the monitoring of geochemistry of spring waters coupled to a fine monitoring of surface displacements is an efficient method for the in-situ characterization of transfers in groundwaters and of their mechanical impact on the dynamics of an unstable slope. Moreover the study shows that the data can be directly used in thermo-hydro-mechanical numerical models in order to analyze (1) the evolution of the properties of the media (permeability, porosity, resistance,...) when shapes change, and (2) the influence of fluids on the processes leading to instability.

 Comparison of the variations of sulfate concentrations in water with the displacement velocities of the La Clapière landslide over the 12/98 to 09/99 period. The increase of the displacement velocity is conconmittant with a high decrease in sulfate rates. These variations follow a heavy rainfall period over the slope. The figure shows that water chemistry is a reliable indicator of slope dynamics.

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