Heavy haul rail transport certainly demands a more robust and stable railway foundation as the subgrade soils are prone to instability at higher axle loads. The current paper presents a series of cyclic triaxial tests to investigate the response of different subgrade soils with varying cyclic stress ratio (CSR). Theoretical models based on the Modified Cam Clay (MCC) model are extended to predict the experimental results. Two different hypotheses are proposed and discussed in this paper, namely, Model A: yield surface changes its size, and Model B: yield surface changes its shape, during cyclic loading. The results indicate that when CSR exceeds a critical threshold (CSR_c), the excess pore water pressure (EPWP) and axial strain (ε_ac) increase rapidly as the number of cycles (N) reaches a certain critical level (N_c,i). This causes soil instability and leads to specimen failure. At the same level of CSR, the faster the cyclic load is applied (i.e., at higher frequencies), the greater is the number of cycles required to trigger soil instability. The proposed models predict the inception of soil instability under cyclic rail loading with reasonable accuracy for Australian heavy haul track conditions.

重载铁路运输当然需要更坚固、更稳定的铁路基础，因为路基土壤在更高的轴载下容易不稳定。目前的论文提出了一系列循环三轴试验，以研究具有不同循环应力比 (CSR) 的不同路基土的响应。基于改良凸轮粘土 (MCC) 模型的理论模型被扩展以预测实验结果。本文提出并讨论了两种不同的假设，即模型 A：屈服面改变其尺寸，模型 B：屈服面在循环加载期间改变其形状。结果表明，当 CSR 超过临界阈值 (CSR _c ) 时，超孔隙水压力 (EPWP) 和轴向应变 ( ε _ac) 随着循环次数 ( N ) 达到某个临界水平 ( N _c,i )而迅速增加。这会导致土壤不稳定并导致试样失效。在相同的 CSR 水平下，施加循环载荷的速度越快（即频率越高），触发土壤不稳定所需的循环次数就越多。所提出的模型以合理的精度预测澳大利亚重载轨道条件下循环铁路载荷下土壤不稳定性的开始。