Evaluation and prevention of soil liquefaction at Fort-de-France (Martinique)
Résumé
Soil liquefaction describes a phenomenon whereby a loose soil loses strength and stiffness, due to an increase of pore water pressure, in response to an earthquake shaking. At Fort-de-France some studies have revealed several area where soil liquefaction risk is important. These areas are already urbanized or will be by future program of urbanization. The Prevention Plan of Natural Risks (Territorial Plan of prevention) of the city does not contain prescriptions or recommendations for constructions in liquefaction area, while this phenomenon must be integrated into any program of development of Territory. Then a R&D agreement has been establish between the city of Fort-de-France and BRGM (the French Geological Survey) in order to: - increase knowledge of soil liquefaction susceptibility, - refine liquefaction hazard cartography, - establish adequate geotechnical in situ tests for soil liquefaction potential assessment in Martinique's context, - establish recommendations for prevention and construction in liquefiable areas. The study leans on a database with 1678 boreholes. This database stores all information described in the borehole log sheets such as lithological description and geotechnical information relative to liquefaction characterization. This information essentially come from various archives and from a specific survey includes to this study. In order to evaluate thickness of liquefiable soil in each point of the study area, a multi-layer 3D geological model has been developed using geostatistical methods. For the susceptibility analysis, the method used was based on geological information and results of identification laboratory tests (in accordance with European rules Eurocode8). For the intensity analysis, procedures developed by Robertson & Wride (1998) and Iwasaki (1984) were adopted. Knowledge of liquefiable thickness and of liquefaction susceptibility and intensity allows the cartography of liquefaction hazard area.