Analysis on the effect of buried depth-initial water content of the tunnel in expansive rock

There are countless engineering disasters induced by expansive strata in the tunnel project. Nowadays, most of the research focuses on the basic characteristics of expansive rock and soil, while the influence about coupling buried depth and different initial water content of surrounding rock on the tunnel lining is still rarely involved. Based on the principle of temperature and humidity equivalence, this paper discusses the behavior of rock as well as the mechanical properties of lining under the interaction of different buried depth and initial water content by using FLAC3D numerical simulation. It was found that the initial water content show an obvious effect on the evolution about the plastic area of rock compared with the buried depth of the tunnel. For shallow tunnels with a water content of less than 10 %, the plastic zone of the vault will penetrate to the surface after water absorption and expansion. Under the coupling effect, buried depth inhibits the development of the plastic zone to a certain extent. For different initial water contents of the surrounding rock, the displacement of the tunnel adds with the burial depth. The swelling of rock will significantly reduce the safety of the primary support, and when the burial depths of the tunnel increase to 150 m, the security coefficients of some primary support structures are less than 2, which does not meet the need in standard. The safety factor of secondary support is mainly influenced by the expansion force, not the ground pressure (represented by buried depth). The findings may be valuable reference for the analysis as well as evaluation of the security as well as stability of tunnels in expansive mudstone.