Degradation of Marine Clays under Cyclic Loading

Abstract

A study of the effect of the overconsolidation ratio (OCR) on the cyclic shear modulus degradation of clay is presented. The research is based on a series of strain‐controlled, constant‐volume, cyclic, simple‐shear tests conducted on samples of an offshore Venezuelan clay consolidated to OCR=1, 2, and 4. The modulus degradation with number of cycles is successfully modeled for different OCR's using an expression applied previously to normally consolidated clays. The rate of cyclic modulus degradation decreases with increased OCR, and the dynamic stress‐strain backbone curves can be modeled by an extension of the SHANSEP method originally suggested for normalizing static stress‐strain curves. A chart is also developed for normally consolidated marine clays from Venezuela, California, and Alaska, which indicates that the modulus degradation rate parameter t, corresponding to a given cyclic strain, is similar for similar clays, with t decreasing as the plasticity of the clay increases. Also, a comparison between values of t obtained from simple shear and triaxial tests on one of the clays shows that these different test types can be compared at an equal value of the increment of deviatoric strain trajectory for one cycle. This suggests a way to apply the modulus degradation model presented in this study to the analysis of 3‐D cyclic loading situations using the incremental theory of plasticity.