In previous train operations, traffic loads were typically considered continuous, disregarding the intermittent effects of successive trains on subgrade loess. To investigate the cumulative plastic strain behavior and critical dynamic stress of subgrade loess under intermittent train loads, a series of dynamic triaxial tests were conducted considering factors such as cyclic stress ratio, confining pressure, and frequency. The deformation characteristics of subgrade soil under different stress levels were analyzed, and the dynamic behavior of specimens was categorized based on the development trends of strain rate and cumulative plastic strain. Then the critical dynamic stress levels for plastic shakedown and plastic creep states were determined. The results indicate that intermittent effects suppress the development of cumulative plastic strain and excess pore water pressure in the soil. The more cycles of the unloading-drainage stage the soil undergoes, the stronger its resistance to failure. Under intermittent loads, cumulative plastic strain increases with higher cyclic stress ratios and frequencies. When the cyclic stress ratio is constant, the increase in confining pressure enhances soil stiffness, but this increase is insufficient to counteract the strain induced by greater dynamic stress amplitude, resulting in increased cumulative strain. Combining cumulative plastic strain and plastic strain rate, a classification standard for the deformation behavior of subgrade loess under intermittent loading conditions was established, and the critical dynamic stress was identified. The critical dynamic stress increases with higher confining pressure but decreases with frequency. Accordingly, empirical formulas for critical dynamic stress concerning confining pressure and frequency were proposed. These findings are crucial for understanding the mechanism of intermittent train load effects and analyzing subgrade settlement.

In this paper, a comprehensive series of dynamic triaxial tests were conducted to delve into the influence of temperature and moisture content on the behavior of frozen silty clay. Upon scrutinizing the experimental outcomes under prolonged reciprocal cyclic loading, insights were gained into how varying temperatures and moisture contents impact the cumulative permanent strain (CPS) and critical dynamic stress (CDS) of frozen clay. The results show that the variation curves of CPS with the number of cyclic loadings show significant changes at different temperatures and moisture contents. Additionally, based on the assessment of vertical CPS recorded at the 100th and 1000th loading iterations, criteria for assessing the plastic stability and plastic creep threshold of frozen silty clay were devised. Consequently, an analysis was conducted to delineate the correlation between the variation in vertical cumulative strains and the dynamic stresses applied within the frozen clay, resulting in the formulation of a series of correlation curves. The relationship between the changes in CDS affected by different temperatures and water contents were analyzed. The CDS under the plastic stability and plastic creep limits showed a slowly increasing trend with decreasing temperatures and a slowly decreasing trend with increasing water contents.

The cumulative deformation and fatigue failure of roadbeds induced by dynamic loads are fundamental considerations in road traffic design. To gain a more comprehensive understanding of the impact of drainage conditions and loading cycles on the performance of roadbeds composed of granite residual soil in southern China under various loading modes, this study conducted high-cycle dynamic triaxial tests using a DDS-70 dynamic triaxial apparatus. Through analysis of sample deformations, pore pressure development, and changes in critical cyclic stress ratio under different simulated waveforms, it was observed that the simulated waveform significantly influences the dynamic characteristics of the soil, with the half-sine wave proving effective in simulating the complex dynamic stress caused by traffic vehicles. Meanwhile, the study revealed uncertainties in the development of cumulative deformation under undrained conditions, thus indicating a need for dynamic tests to be conducted under drained conditions to more accurately replicate the effects of traffic loads. Additionally, the deformation of samples at 1000 cycles can serve as a crucial reference for estimating final deformation, which is essential for determining sample types and obtaining key parameters of foundation soil. This approach can help reduce testing workload and save time and costs.

The shakedown state of the subgrade is crucial for the sustainable design and long-term stability evaluation of pavement structures. In order to characterize the plastic deformation and shakedown behavior of subgrade soil in seasonal frozen regions, cyclic triaxial tests were conducted on the thawed subgrade soil after seven cycles of freeze-thaw. The influences of the numbers of cycle loading, the amplitude of cyclic deviator stress, and the confining stress were considered variables. The evolution features of accumulative plastic strain, accumulative plastic strain rate, and critical dynamic stress were experimentally analyzed. Based on the shakedown theory, the ensuing discoveries were that the accumulative plastic strain response-behavior of thawed subgrade soil was typically divided into plastic shakedown, plastic creep, and incremental collapse under the long-term cyclic loading. Furthermore, the shakedown standard for thawed subgrade soil was also proposed based on the evolution of the accumulative plastic strain rate. The critical dynamic stresses can be obtained by the proposal formula to determine the different plastic deformation ranges.