Abstract
Imperfection-sensitive bifurcation of a rectangular (12 cm × 36 cm) Cam-Clay specimen emerging from a state of homogeneous in-plane compression with undrained boundaries is simulated by soil-water coupled finite deformation analysis, in which the vertical displacement at a constant rate is imposed on the top of the specimen. The analysis is carried out using a non-linear finite element method through an up-dated Lagrangean scheme, in which the bifurcation points corresponding to the first few bifurcation modes are found to be well approximated by the limit points of external load by introducing cosine-curve geometrical imperfection on both sides of the specimen. This is particularly true when a fast rate of testing is considered. Even with undrained boundaries, when the specimen includes initial imperfection, the imperfection causes the migration of pore water from the beginning. The effect of migration on the shear deformation, therefore, should be highly dependent on the rate of vertical displacement. When a slow rate of displacement (9.12 × 10-3%/min) is applied, it is found, in this study, that the mode-switching occurs due to migration and a "peak" in the load-displacement curve occurs which naturally yields a relatively low undrained shear strength. One of the possible reasons for this rate dependency of undrained shear strength of saturated clay is ascribed to the effect of pore water migration on the imperfection-sensitive bifurcation behaviour of saturated clay.
