Abstract: In order to study the cyclic strength of natural Shanghai clay under cyclic loading, especially the degradation behavior of dynamic shear modulus, after sample’s isotropic consolidation, a series of undrained stress-controlled cyclic triaxial tests are conducted on natural Shanghai clay with different cyclic stress ratios (defined as , in which is the cyclic deviator stress and is the cell pressure) and different loading frequencies. The effect of dynamic stress ratio and loading frequency are investigated. According to the test results, for layer No.4 of Shanghai clays, the soil sample damage occurred until the dynamic stress ratio is 0.2. The relationship between shear modulus and strain is not affected by dynamic stress ratio and loading frequency; and shear modulus decay falling mainly in axial strain 0?1% range. For the dynamic shear modulus degradation index , it is smaller with the smaller frequency and with the larger dynamic stress ratio under the same number of cycles. Through the analysis of the effect of dynamic stress ratio and frequency on dynamic shear modulus, a mathematical model for shear modulus degradation is presented. The tests data and the model calculation are good agreement.

Key words: Shanghai clay, dynamic triaxial test, shear modulus, cyclic stress ratio, frequency