Laboratory test and empirical model for shear modulus degradation of soft marine clays

- Shear modulus degradation is investigated by cyclic triaxial tests.
- Degradation model considering loading conditions and physical property is proposed.
- Only one set of parameter is required for prediction under various loading conditions.

The change in the shear modulus of clays under repeat environmental loads is an important factor in the design of offshore and onshore foundations. A degradation model for the shear modulus of soft clays that considers the effects of the number of cycles, the cyclic stress ratio, the loading frequency and the plasticity index is proposed. A series of undrained cyclic triaxial tests on undisturbed Shanghai marine clays were conducted. Experimental results reveal that the shear modulus degradation index has a logarithmic relationship with the number of cycles, decreases linearly with the cyclic stress ratio, and increases linearly with the loading frequency when plotted in log-log coordinates. Further study using existing data from other research shows that the degradation index decreases with plasticity index, IP. Based on the test results, a new degradation model is proposed which considers these factors explicitly. The main feature of the new model is that only one set of parameters for each type of clay is required for the prediction of shear modulus degradation. The applicability of the present model is verified by five soft clays with different plasticity indices and loading conditions.