Compared Micromorphology of Alpine Debris Flow Deposits and Parent Materials.
Pascal Boivin, Institute of Research for Development, IRD, L.G.I.T. - Maison des Geosciences -, Grenoble Cedex 9, 38041, France, Carole Theler, Swiss Federal Institute of Technology, Av. d'Echallens, Lausanne, 1015, Switzerland, and John Menzies, Brock Univ, Earth Sciences Dept, St. Catharines, ON L2S 3A1, Canada.
Debris flow deposits and parent materials were sampled in Val de Bagnes, Switzerland. By means of thin sections, major dissimilarities in material composition and permeability as well as in terms of microstructures have been observed. While the triggering material lacks plastic deformation, flow structures, rotations and lineations in the material of the deposits give evidence for the rapid movements taking place in debris flows as well as for fluid transport. A better understanding of the triggering process as well as of the history of the material can be provided. The materials of the triggering zone are highly permeable and show a edge to edge fabric of low stability, contrary to the ones of the deposits. As the triggering process is related to the arrangement of the material, the high permeability allows the infiltration of the water leading to saturation and eventually to the build up of positive pore pressure and to the release of a debris flow. The examination of the materials cannot establish the importance of soil plasma in the triggering process, since a matrix made of colloidal components was hardly observed in the uphill materials. It was, therfore, not possible to discuss the role of these components on soil stability although the chemical analyses revealed a higher content in organic mater and swelling clays of the uphill materials. Additional changes in the soil constituents of the deposits indicate the crushing of the trigger material, the smoothening of the more resistant clasts as well as the incorporation of sediments in the channel and from the banks. Together with previous results, these observations show that the deposit materials are very different in structure and constituents from the parent materials.