Dynamic Properties of Soft Clay for Wide Strain Range

A systematic experimental study has been conducted for dynamic properties of natural soft clay employing an improved cyclic triaxial apparatus. Undisturbed samples with various, void ratios and plasticity indices picked up from a alluvial clayey layer have been tested to reveal the following important facts: (1) The rate of the time-dependent increase of the shear modulus of clay can be approximated by a simple experimental equation in which only plasticity index is used. (2) The damping ratio of clay remarkably decreases with time implying that in situ clay will exhibit smaller damping ratio than that measured in the laboratory. (3) The strain-dependent variation of the shear modulus ratio is very sensitive to the plasticity index of the cohesive soil. The higher the plasticity, the lower will be the rate of the strain-dependent decrease of the modulus. (4) Based on the experimental results on clays with different consolidation histories, the current practice of combining the in situ small strain modulus with the laboratory modulus ratio vs. strain curve to estimate the in situ large strain modulus to be used in dynamic response analyses of grounds may be justified for engineering purposes.