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P. Stehly, P. Gouédard, L. Roux, and C. De-barros, Sens-Schönfelder for helpful discussions. We are grateful to A. Peltier and Meteo France for providing us with respectively extensometer and meteorological data. We thank F. Renard, I. Manighetti, and G. Poupinet for constructive comments concerning the manuscript. This work was supported by ANR (France) under contracts