Approche géomécanique des causes d'envasement. Application à la rade de Lorient

The silting-in of estuaries and harbours has important consequences for management. Si/tation problems are generally only studied from a sedimentological point of view. Our aim is to bring a geotechnical dimension to these phenomena, and to find methodic approach including soil behaviour. Geotechnical investigations are a way of identifying the nature andbehaviour affine sediments. Laboratory and in-situ experiments. however, are difficult with this kind of material. Stability studies show that the security ratio, regarding the circular rupture of slopes, is sufficient. Two original approaches, of cyclic mechanical stresses acting on the sediments, are suggested. ln the first, we show an increase of piezometric levelin tidal slopes. ln the second, tidal pore pressure can induce liquefaction phenomena in the first meter of submerged mud.An original apparatus is able ta regionalise the soft mud thickness, and to predict areas of instability.Such a methodology will be useful for a knowledge of siltation in other harbours