SOIL-WATER COUPLED BEHAVIOR OF HEAVILY OVERCONSOLIDATED CLAY NEAR/AT CRITICAL STATE

The time dependent elasto-plastic response of a heavily overconsolidated soil has been illustrated with simple yet typical examples that a heavily overconsolidated clay under loading exhibits either hardening or softening due to the migration of pore water. In the examples studied triaxial tests are numerically simulated, where the test is considered not as a test of a soil element but as a test of the soil mass with well defined boundary conditions. In order to describe the behavior of heavily overconsolidated soils, the subloading surface concept first developed by Hashiguchi and Ueno (1977) is newly applied to the original Cam-clay model. From this study, the following conclusions are obtained : (1) The subloading surface Cam-clay model describes the typical shear behavior of heavily overconsolidated soils such as the hardening procedure that occurs above the critical state line with a considerable volume expansion. (2) The hardening and softening of heavily overconsolidated soils due to the migration of pore water have non-local characteristics because of Darcy's law. Based on this gradient nature with positive permeability, plastic instability can be well simulated by the usual finite element computation. (3) As the results of coupling with Darcy's law, the total shear behavior of the triaxial soil specimens exhibits an apparent "time dependent" nature particularly for the "stress-dilatancy" characteristic. This can be observed equally under both undrained and drained conditions for triaxial experiments.