Abstract: The subgrade bed layer is the core part of railway subgrade composed of coarse-grained soil (CGS), which is generally about 2.5-3.0 m thick. It is this layer that is directly subjected to long-term effect of repeated traffic loading. The deformation of the subgrade bed layer under cyclic dynamic loading is one of the key factors to evaluate the operating performance of subgrade. To study the stress-strain characteristics of CGS under cyclic loading, a series of large-scale dynamic triaxial tests is carried out at different confining pressures (simulating different depths under the effect of lateral pressure) and dynamic stress amplitudes (simulating different axle loads), to simulate dynamic loading of train and actual situation of CGS filling by one-way cyclic stress-controlled loading. The results show that, under the cyclic loading, the change for soil stiffness is closely related to the vibration number and confining pressure. According to different dynamic stress amplitudes, the dynamic strain of saturated CGS under cyclic loading can be divided into three types according to development trends of vibration times: stable, destructive and critical. Based on the experimental results, a backbone curve model is developed including confining pressure and vibration times. Compared with the traditional backbone curve model, the proposed model can reflect the variation of soil stiffness with vibration times and the actual situation of cyclic effect of train. In addition, the model can also be used to determine the dynamic strength of subgrade soil, providing reference for evaluating dynamic stress deformation stability for subgrade bed core layer of railway and subgrade bed design based on dynamic deformation control.

Key words: coarse-grained soil, cyclic loading, large-size dynamic triaxial tests, dynamic stress-strain, hysteresis loops, backbone curve