Abstract: The stability of tunnel is gradually undermined by the effects of unsaturated seepage and soil expansion under the condition of continuous rainfall infiltration. Hence, it is important to investigate their effects on the deformation of surrounding rock mass and stress in the supporting system. This study simulated the processes of unsaturated seepage and soil expansion with the rainfall infiltration by using the seepage module and thermal module in the finite difference software FLAC3D, respectively. FISH language was also adopted to investigate the impacts of changes in matrix suction, soil softening and soil expansion during the unsaturated seepage process. A shallow expansive soil tunnel was simulated as a case study. Then the effects of rainfall duration, permeability coefficient and expansion coefficient on the deformation of surrounding rock mass and stress in the supporting system were analyzed. The results showed that during the rainfall infiltration process, the horizontal stress of surrounding rock mass increased remarkably, whereas the vertical stress changed within a small range. Moreover, the supporting system transformed from initial vertical extrusion deformation to horizontal extrusion deformation. It was found that the permeability coefficient and expansion coefficient exerted great actions on the stress in the supporting system. When the permeability coefficient and expansion coefficient reached a particular value, the bending moment of the supporting system substantially increased, which greatly reduced the tunnel stability and safety.

Key words: rainfall infiltration, unsaturated seepage, expansive soil, tunnel, numerical analysis