Study On Dynamic Behavior Of Saturated Soft Clay Under Unidirectional And Bidirectional Cyclic Loading

Soft clays are widely distributed in the prosperous coastal areas in China. A great deal of structures will be constructed with the economical development and urbanization. Each Severe earthquake brings great damages and losses to people in China, which lies in a high-frequency earthquake region. Therefore, study on dynamic properties of saturated soft clay during earthquakes is of great theoretical and practical importance in seismic design.Previous studies on the dynamic behavior of clays have only considered the seismic loading conditions corresponding to transverse earthquake motions. Many unidirectional cyclic loading tests are performed to simulate seismic loading. In fact, the longitudinal earthquake motions have great effects on the dynamic behaviors of soft clay during strong earthquakes. In such a case, bidirectional cyclic loading is more suitable for simulating seismic loading. Studies concerned this problem have rarely been done, for laboratory testing equipment is not generally available for performing tests which model bidirectional loading conditions.Based on previous researches, a series of undrained monotonic and cyclic triaxial tests were performed on typical Hangzhou soft clay by GDS bidirectional dynamic triaxial system. The effects of initial shear stress, cyclic stress ratio, loading frequency, radial cyclic stress ratio and phasic difference on dynamic behaviors of saturated soft clay were investigated. Several conclusions useful to seismic design in engineering practice are obtained.The coupling effect of initial shear stress and load rate is investigated by monotonic triaxial tests. It is observed the increase of load rate makes up the loss of strength for the application of initial shear stress. Nonlinear stiffness degradation behavior is also observed during the tests. However, the original Duncan-chang model can only describe linear stiffness degradation. The Duncan-chang model is modified by incorporating the monotonic stiffness degradation model.The effects of cyclic stress ratio, initial shear stress, loading cycle and loading frequency on dynamic strain, pore water pressure, stress path, critical cyclic stress ratio and cyclic degradation are investigated by unidirectional cyclic loading tests. The effect of the factors above on turning dynamic strain is analyzed. Taking the turning dynamic strain as the failure criterion, the dynamic strength of soft clay subjected to unidirectional cyclic loading is obtained. Based on the tests, an empirical cyclic degradation model for unidirectional cyclic loading is built up to model the relationship between degradation index and cyclic number. Another empirical model is also proposed by normalizing the secant shear modulus in each unload-reload loop to modeling the stiffness degradation within a cycle during unidirectional cyclic loading.The effects of cyclic stress ratio, radial cyclic stress ratio, initial shear stress and phasic difference on dynamic strain, pore water pressure, stress path, critical cyclic stress ratio and cyclic degradation are investigated by bidirectional cyclic loading tests. The dynamic strength of soft clay subjected to bidirectional cyclic loading is obtained. The threshold radial cyclic stress ratio is also obtained for Hangzhou soft clay. Based on the tests, an empirical cyclic degradation model for bidirectional cyclic loading is built up to model the relationship between degradation index and cyclic number. Another empirical model is also proposed by normalizing the secant shear modulus in each unload-reload loop to modeling the stiffness degradation within a cycle during bidirectional cyclic loading.An indirectly analysis method is proposed to obtain accumulative plastic strain by modeling the relationship between degradation index and accumulative plastic strain and incorporating degradation index model. Then the accumulative plastic strain models for unidirectional and bidirectional cyclic loading can be easily obtained just by introducing a comprehensive impact parameter.The initial backbone curve is modeling by modified Duncan-chang model. The stiffness degradation model within a cycle is applied as the amplifying coefficient of Masing rules. In such a case, Iwan model is modified by incorporating the cyclic degradation model and accumulative plastic strain model. The dynamic stress-strain relationships for unidirectional and bidirectional cyclic loading are simulated by the modified models. A comparison between experimental and predicted stress-strain curves shows a good agreement.