The traffic loading is a typical cyclic loading with variable confining pressure and always lasts long, and is believed to have a significant effect on the subgrade soil, especially for the subgrade filled with soft clay. However, the mechanics have yet to be fully understood. Given that the duration of traffic loading lasts long enough, the partially drained conditions should be considered for the soft clay under the long-term cyclic loading, rather than the undrained conditions adopted commonly by most previous researches. In this study, 28 cyclic tests were conducted on the remolded saturated soft clay, utilizing both constant confining pressure and variable confining pressure under partially drained and undrained conditions. The effect of cyclic confining pressure and different drainage conditions is analyzed in relation to the evolution of pore pressure and deformation behaviors. Incorporating both the cyclic confining pressure and cyclic stress ratio, a concise pre-diction model of permanent strain is proposed and validated by the experimental results.

The railway foundation is subjected to periodic loads and time intervals during operation. Previous tests on permanent deformation of soft soil by applying the continuous cyclic loading under undrained or drained conditions cannot fully reflect the actual situation. To evaluate the cyclic triaxial behaviors of saturated soil under more realistic loading conditions, a series of bidirectional cyclic triaxial tests were designed. These tests considered the influence of drainage conditions and variable confining pressure throughout the intermittent and cyclic loading phases. The experimental results demonstrate that the progression of cumulative axial strain and excess pore water pressure under intermittent cyclic loading exhibits notable distinctions compared to previous tests conducted under continuous cyclic loading. Besides, the cumulative axial strain and excess pore water pressure of test samples under variable confining pressure conditions are affected by the cyclic stress ratio (CSR) and inclination of cyclic stress path (eta ampl). When CSR and eta ampl increase simultaneously, the cumulative axial strain of the soil tends to increase more significantly. It means that the increase in cyclic confining pressure leads to an increase in the generation of pore water pressure in the sample. This, in turn, reduces the effective stress and shear strength of the soil, resulting in a greater cumulative axial strain, regardless of the CSR. The present study highlights the importance of considering drainage conditions, variable confining pressure, and intermittent time when assessing the cyclic triaxial characteristics of soil subjected to cyclic train loads. The findings of this research emphasize importance of considering these factors when accurately evaluating the behavior of the soil under such loading conditions.