Elasto-plastic behavior of naturally deposited clay during/after sampling

Changes in soil structure and overconsolidation in highly structured natural deposited clay as a result of sampling and setting in laboratory tests, together with their effects on the subsequent mechanical behavior, are numerically simulated in terms of unconventional elasto-plastic mechanics. To describe the mechanical behavior, a Super/subloading Yield Surface Cam-clay model is employed. This model allows a description of the decay of soil structure, loss/ increase of overconsolidation and evolution of anisotropy with ongoing plastic deformation from the process of the soil sampling through the unloading with swelling up to the subsequent tests. The sampled clay subjected to Isotropic unloading and /or one-dimensional unloading with swelling down to the first stage of vertical loading level, displays a response to some degree, different from that of the soil at the original sampling site; for example, an overconsolidated sample subjected to Isotropic unloading with swelling will display rewinding behavior in a subsequent undrained triaxial compression test, and "secondary consolidation" /"delayed compression" in a one-dimensional consolidation test. However, one-dimensional unloading with swelling, which is followed by one-dimensional compression test, will give better prediction of the in-situ "consolidation yield stress" than Isotropic unloading. Artificially disturbed clay subjected to repeated undrained shearing after sampling suffers a significant loss of structure. This results in a drop in peak strength under subsequent monotonic undrained shearing tests, and in the compression index in one-dimensional compression tests, as described by Schmertmann.