Stiffness degradation of soft clay under cyclic loading considering intermittency effect

During subway operation period, the soft soil foundation is subjected to periodic vibration and intermittent loading, and changes of mechanical properties of soft soil during intermittency will affect its stiffness degradation, which has been ignored in most related previous researches. Hence, considering different dynamic stress ratios and unit intermittent and vibration durations, triaxial tests of undrained continuous and intermittent cyclic loading were designed, where samples were consolidated by real K0, and the long-term stiffness softening rule of undisturbed soft clay under train discontinuous load was studied. Then by comparing the test results of continuous and intermittent loading, it was found that intermittent had a weakening effect on stiffness degradation, which can reduce the residual softening index under large cycle number and shorten the cycle number during rapid softening period. Finally, based on the test results, the influence mechanism of intermittent was analyzed. It was found that soft soil was affected by the excitation during initial loading, and the dynamic adjustment of the soil structure was retained to the intermittency period, which led to continuous pore water pressure generation, stiffness degradation of soft soil, and increase of the average softening rate. After a certain number of cycles, because of the structure adjustment of the soft soil under intermittent and vibration loading in the earlier stage, the soil structure entered the steady state in advance, partial pore water pressure was dissipated, and stiffness loss was restored, which led to the decrease of softening rate in the next vibration. The restored stiffness during the intermittencies was accumulated as the vibration times increased, and the final residual softening index was significantly increased. Under the same conditions, with longer intermittent duration, the initial softening rate was greater, and the attenuation of the softening rate was more significant with the increase of the vibration times, so the weakening of the overall softening degree was more obvious, and the vibration times required to enter the softening stationary phase was fewer.