Insight into Subgrade Long-Term Uplift Characteristic of Swelling Red-Bed Controlled by Temperature and Cyclic Load：From Laboratory Experiments

Abstract Subgrade long-term uplift of high-speed railway threatens operational safety, which is usually caused by rock swelling in red-bed rock regions. In this study, we present results on red-bed mudstone and siltstone to trace the properties of rock swelling under different heat treatment temperatures and to evaluate the swelling of siltstone after cyclic load simulating train operation. The results show that drying temperature before the swelling experiment affects the initial water content and microfractures of rock samples. Initial water content affects their swelling properties, and the swelling rate is positively correlated with the heat treatment temperature (mudstone, R2 = 0.49; siltstone, R2 = 0.73). In addition, cyclic load alters the microstructure of the rocks, increasing to further release of swelling. Compared to the normal load, the shape of the particles on the failure surface is closer to ellipsoids under cyclic loading. Swelling rates were positively related to critical stress ratios (CSRs) (R2 > 0.89) and negatively related to circumferential pressure (R2 > 0.54); no significant correlation with loading frequency was observed. Based on the above experimental results, the mechanism for the subgrade uplift of the Neijiangbei station is discussed. This study provides not only an experimental basis for the swelling mechanism of red-bed rocks but also insight for the mechanism of subgrade uplift at the red-bed rock regions.