Multiscale responses of gap-graded soil under the combined effect of train vibration and seepage erosion

Abstract

Leakage-induced soil loss in the metro tunnel alters soil properties, threatening the safety of both tunnel structure and surrounding environment. The soil under train operation is subjected to the cyclic loads transmitted by the train track, which are different from constant or monotonically changing boundaries. The cyclic effect of train vibrations results in significant differences in the response of the seepage erosion around the tunnel. To investigate the combined effect of train vibration and seepage erosion on the mechanical responses of soils, the coupled computational fluid dynamics and discrete element method (CFD-DEM) was employed to study the behaviors of gap-graded soil during seepage erosion under different vibration frequencies (i.e., 10 Hz, 30 Hz, 50 Hz) and amplitudes (i.e., 0.1 mm, 0.2 mm, 0.4 mm). The results show that as the vibration frequency and amplitude increase, the effect of vibration on seepage erosion becomes more severe. The average increase of the loss of non-eroded fine particles all exceeded 25 %, and the average fine particle loss ratio reaches 84 % across all vibration conditions. The mechanism of the combined effect of train vibration and seepage erosion is revealed from the macroscopic and microscopic perspective, including fine particle distribution, particle and flow field velocity, coordination numbers, contact force, particle trajectory, and coarse particles pore structure. It is found that vibration affects the seepage erosion process mainly by disrupting coarse–fine particle contacts, whose loss of strong contacts in coarse–fine contact (C–F) exceeds 65 %. The higher the vibration frequency and amplitude, the faster the adjustment of the coarse particle skeleton structure, which is approximately 3 s faster than in no vibration case. Vibrations can affect the mechanical properties of the soil during seepage erosion by reducing the coordination number of particles and decreasing the proportion of strong contacts in coarse–fine particle contacts. This study helps to explore the relationship between train vibration and seepage erosion.