Pseudo-three-dimensional experimental study on tunnel face stability in weak and fractured rock mass

Tunnelling in weak and fractured rock mass will inevitably induce excessive stress or deformation of surrounding rock, which results in tunnel face collapse accidents. In order to understand the failure modes and progressive failure characteristics, the weak and fractured rock mass of grade Ⅳ according to Chinese codes is taken as the reference prototype. This rock mass is firstly equivalent for isotropic continual stratum, and then modeled by similar materials. Subsequently, model tests are carried out to simulate tunnel excavation under different loading modes. The test results show that, the tunnel face collapses in dome-style in the slow loading mode, while in the fast loading mode, it collapses in chimney shape and reaches ground surface. In the initial stage after excavation, the lateral displacement at the upside of tunnel face is relatively large; with the increase of overlying loads, the lateral displacement at the downside of tunnel face grows drastically and forms the final failure face from bottom to top. After tunnel excavation, the position of the maximal horizontal stress increment locats at the upside of tunnel face near the line center, but the location of maximal vertical stress increment lies in the top of tunnel face profile.