Evaluation of Instability and Residual Deformation of Soft Cohesive Soils under Earthquake-induced Cyclic Lording

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Based on the results from laboratory investigation using cyclic and direct-simple shear tests on clays and their consideration into modeling for numerical analysis of behavior of embankments on soft ground, the following is pointed out :l) Degradation of stiffness and strength of cohesive soils triggered by undrained cyclic loading in triaxial conditions can be uniquely correlated to the magnitude of shear strain observed during cyclic loading. This is common with normally consolidated and overconsolidated specimens.2) Numerical analysis using a computer program for dynamic response analysis called SADAP incorporating the above-mentioned finding was carried out to obtain the design chart for predicting the earthquake-induced settlements as a parameter of the maximum acceleration and the overconsolidation ratio (OCR). A practical availability of the design chart was proved by comparing with the settlements observed in the results of centrifuge tests on the river dyke founded on soft clay.3) The results from numerical analysis were also given in the from of the design chart to forecast whether the dyke or the ground leads to instability during earthquakes.4) Through the results not only from the numerical analysis but also from the centrifuge tests, the effectiveness of preloading or surcharging was recognized as a countermeasure for increasing the stability of river dykes founded on soft clay.