Seismic Stability of Submarine Slides

The geological profile of the submerged slopes of the continental shelf typically consists of normally to lightly overconsolidated clays with depths ranging from a few meters to a few hundred meters. In evaluating existing methods or developing new methods to assess the seismic stability of these slopes, the questions of amount of permanent displacements and time and strain rate effects should be of primary importance. Seismic slope stability is often investigated with pseudo-static methods, which reduce the dynamic loading to an inertial force applied as a static load. These analyses give a fail/safe type of answer, without addressing the overall response of the soil. On the other hand, seismic site response analysis focuses on performance, predicting stresses and strains due to shaking, but does not include the effects of the sloping ground on soil behavior. This points to the need for a comprehensive site response methodology based on an accurate description of the stress-strain-strength relationships for clayey materials. The stress state of soil elements in a slope can be replicated in the laboratory in the direct simple shear apparatus. Results of testing on San Francisco Bay Mud illustrate the importance of consolidation stress history on the behavior of clays. During a cyclic undrained test, the presence of an initial shear stress causes the strains to accumulate in its direction (i.e. the direction of the slope) through a mechanism that differs from that of a test with no initial shear stress (i.e. flat ground). Seismic site response analysis is usually one-dimensional, therefore only one, crucial direction of loading is selected. However, testing shows that the interaction of the stresses in two directions is important and should be considered for accurate predictions of the displacements due to shaking. The SIMPLE DSS model was developed specifically to describe the response of lightly overconsolidated soft clays in simple shear conditions under two-dimensional loading, as illustrated by predictions of tests on San Francisco Bay Mud. The model combines the complexity of constitutive laws that incorporate anisotropic hardening and bounding surface principles with the simplicity of the selection of the model parameters through step-by-step procedure, with the help of charts.