Research on the swelling mechanism of high-speed railway subgrade and the induced railway heave of ballastless tracks

The ballastless track has been a widely-used track form in high-speed railway (HSR), however, its application is seriously hampered by the railway heave, which is mainly caused by subgrade swelling. In order to identify the mechanism of HSR subgrade swelling and the rules of swelling-induced railway heave response, long-term longitudinal-continuous deformation monitoring and layered deformation monitoring over a heaving-affected site have been carried out. Furthermore, a coarse-grained soil oedometer was applied to explore the correlations between swelling pressure and swelling ratio of the subgrade filler and mudstone foundation to the on-site water content. With the aid of FLAC(3D)-PFC3D coupled numerical simulation, the influence of different swelling ratios for subgrade filler and mudstone foundation on the railway heave response of a double-block ballastless track under was analyzed. According to the field monitoring results, swelling deformation occurred in both the filler layer and subgrade layer of the heave section, and this combined action within the two layers resulted in railway heave displacement. Laboratory tests indicated that the swelling ratios of the subgrade filler and mudstone foundation under in-situ water content reached 0.205% and 3.325%, respectively. The swelling pressure and swelling ratio in the subgrade filler and mudstone foundation both followed a logistic function relationship with the water content. Numerical results showed that the railway heave amount followed Expassoc and Nelder distribution relationships with the swelling ratios of the subgrade filler and mudstone foundation, respectively. The research results could provide some reference value for the study on the swelling potential of expansive soils and the prediction of railway deformation in railway engineering.