Delayed response of excavations in structured clays

In this paper, the behaviour of an idealised excavation carried out in a clay sensitive to destructuration is studied through a series of finite-element analyses, employing an advanced bounding surface model developed for structured clays. Several cases are examined, in order to investigate the influence of factors including the velocity of destructuration, the damage produced by the wall construction to the soil-wall contact, the width-to-height ratio of the excavation. The case of a soil deposit insensitive to microstructural damage is also studied for comparison. The results of the numerical analyses show that the progressive dissipation of the excess pore water pressures generated during the excavation stage can damage the clay microstructure severely enough to trigger an instability phenomenon. If the clay structure deteriorates rapidly, the instability is concurrent with the dissipation of the excess pore water pressures. However, for a clay less sensitive to microstructural damage, the instability can occur towards the end of the consolidation process, or even be preceded by a deceptively stable time interval, during which small redistributions of the pore water pressure can trigger an important destructuration and the collapse of the excavation. In a final part of the paper the results of the numerical analyses are used to provide indications about the most appropriate quantities to monitor in order to provide an effective early warning of the instability phenomenon.