Evaluation of failure and deformation in alluvial clay deposits during earthquake

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Although there exists relatively clear guide line on the liquefaction behavior of sands, a similar knowledge base for clays under cyclic loading needs to be developed, particularly for in-situ conditions. For the purpose of this research program thin walled clay samples have been taken from Hiroshima, Yokohama, Kumamoto, Ooita and Saga. The properties of the clays depend not only on characteristics such as their plasticity but also their sedimentary origins, fabric and stress path. A series of cyclic triaxial tests has been performed at varying plasticity indeces, overconsolidation ratios and confining pressures on undisturbed samples. These have heen compared with a further series of tests on remolded clays. As a result we have developed an empirical equation relating cyclic shear strength to the plasticity index, overburden pressure, consolidation yielding stress and overconsolidation ratio.Additionally, a series of undrained cyclic triaxial compression tests has been performed on a high plasticity marine clay. Testing was performed using various combinations of initial static and subsequent cyclic shear stress on isotropically and anisotropically consolidated specimens. The cyclic shear strength was considered first, and then, the accumulated shear strain was investigated related to the effective stress ratio at the peak cyclic stress. Based on the experimental results, a semi-empirical model is proposed for evaluating the development of pore pressure and residual shear strain during cyclic loading. The model successfully explains the behavior of clay subjected to various magnitudes of initial static and subsequent cyclic shear stresses.