1. PhD Student, Univ. Joseph Fourier, Laboratoire Sols, Solides, Structures, Domaine Univ., B.P. 53, 38041 Grenoble Cedex 9 France.
2. Professor, Univ. Joseph Fourier, Laboratoire Sols, Solides, Structures, Domaine Univ., B.P. 53, 38041 Grenoble Cedex 9 France.