It is necessary to fully understand the settlement of high-speed railway subgrade induced by train loading to ensure the operation safety of high-speed trains. A 1:7 reduced-scale model test was designed to investigate the settlement of subgrade under two loading methods: continuous and intermittent cyclic loading. The testing results show that an increase in load amplitude enhances the load transmission effect to the bottom of the subgrade. After 105 cycles of continuous loading, the cumulative settlement of the subgrade at depth of 0, 20, and 40 cm directly below the loading range is 3.247, 1.05, and 0.09 mm, respectively, showing significant decreases with depth. A significant rebound can be observed when the applied load is removed during the intermittent loading process, which is quite different from the results under condition of continuous loading. Thus, the intermittent effect of train load on the cumulative deformation of the subgrade cannot be ignored. In addition, to better predict the cumulative settlement of the subgrade, a prediction method based on the state evolution model was proposed and used to quantitatively analyze the testing observations. Based on the state evolution model, the predicted cumulative strains at depths of 0, 20, and 40 cm were 1.218%, 0.457%, and 0.047%, respectively, which are in good agreement with the experimental results of 1.099%, 0.48%, and 0.045%, indicating that the theoretical model can accurately predict the cumulative strain of the subgrade caused by train load. Additionally, the parameters of the state evolution model can be updated in a timely manner by applying the updated monitoring data to enhance the prediction accuracy. The current work provides an alternative method for predicting the long-term cumulative settlement of subgrade induced by the train loading, and also a basis for the optimization of high-speed railway subgrade design.

Subgrades may be subjected to intermittent cyclic loads such as traffic loads. Under these loading conditions, excess pore water pressure can accumulate in clayey soils during cyclic loading period and dissipate during resting time. The deformation behaviour of clayey soil after reconsolidation process may be different from that under consecutive cyclic loading. A series of undrained cyclic triaxial tests, including reconsolidation process between cyclic loading stages, were performed on kaolin clay. The axial strain accumulation, excess pore water pressure accumulation, deviatoric stress-strain loop and resilience modulus under different cyclic stress ratios, initial confining pressures and degrees of reconsolidation were discussed and presented. Test results show that the reconsolidation process has significant effects on the deformation characteristics of clayey soil. The coupling effects of change of void ratio and effective mean stress result in a non-monotonic relationship between normalised total axial strain and degree of reconsolidation. In addition, an increase in the degree of reconsolidation leads to an increase in the normalised excess pore water pressure increment during 2nd cyclic loading stage, regardless of cyclic stress ratio and initial confining pressure. Furthermore, the steady resilience modulus at the end of each cyclic loading stage depends on the effective cyclic stress ratio and initial confining pressure, irrespective of reconsolidation process.

The dynamic behaviors of subgrade soil are usually investigated by use of continuous loading mode in most studies; however, the dynamic loading induced by traffic loading is composed of cyclic loading and intermittent periods. Moreover, the existence of cyclic deviator stress, cyclic confining pressure, and shear stress has been already observed in the stress field induced by traffic loading. Recognizing this, intermittent cyclic loading was applied to saturated soft clay for this study. The impacts of cyclic deviator stress, cyclic confining pressure, and drained condition during intermittent periods on the deformation behaviors of soft soil were analyzed. The variations in strain increment were similar in all cases: as the number of loading stages increased, the strain increment decreased, and the difference in strain increment was more significant in the first loading stage although it could be ignored in subsequent loading stages. Furthermore, the strain increment increased with increasing cyclic stress ratio (CSR) and decreased with increasing cyclic confining pressure. Moreover, the dissipation of excess pore-water pressure induced during the cyclic loading period resulted in the increase of accumulated axial strain under intermittent partially drained conditions, while the recovery of specimen deformation during intermittent period led to the decreasing of accumulated axial strain under undrained conditions. In addition, an empirical formula of accumulated axial strain under intermittent cyclic loading was established, and the calculated results were consistent with the measured data.

Both cyclic loading on soil and intermittent periods come into play with the passing of trains. However, the existence of both cyclic deviator stress and cyclic confining pressure has already been observed in the stress field. In this paper, two types of cyclic triaxial tests, i.e., continuous cyclic loading and intermittent cyclic loading, were conducted to study the deformation behaviors of soft clay. The effects of cyclic confining pressure, duration of intermittent period (i.e., intermittent time), and loading cycles were analyzed. Compared to continuous cyclic loading, the intermittent period has a substantial influence on the deformation behavior of soft clay: the strain produced by intermittent cyclic loading is larger than that produced under undrained conditions, but less than that generated under partially drained conditions. The increment of accumulated axial strain corresponding to each loading stage under various factors was compared: an increase in intermittent time and loading cycles leads to greater degradation of the increment of accumulated axial strain, while the greater cyclic confining pressure corresponds to lower the increment of accumulated axial strain. The differences in the increment of accumulated axial strain for various loading cycles and cyclic confining pressures are significant in the cyclic loading period of the first loading stage, and can be ignored in subsequent cyclic loading periods. Besides, an empirical formula is provided to calculate the total accumulated axial strain caused by intermittent cyclic loading, and the predicted results accord well with the measured data.