Face Stability of Tunnels in Soft Rocks

Abstract

This chapter aims to investigate stability of a tunnel face in weak rock masses at the ultimate limit state in the context of the kinematical approach of limit analysis. Several predicting models are proposed for assessing necessary supporting pressures or safety factors of a tunnel face under different conditions, considering underground water seepage, reinforcement techniques and material non-homogeneity. These deterministic models give good predictions with high computational efficiency compared to results given by numerical modelings. Soft rocks, a common geomaterial in nature, show some basic features, such as low strength, strong plasticity, and highly weathering. Engineering constructions in soft rocks are often difficult due to their mechanical characteristics. When excavating in soft rocks, the tunnel face, a high-risk region, can collapse because of the stress release and the low strength of the surrounding rock masses. For closed-face tunnels using tunneling boring machines, shield machines can provide support pressures to tunnel face. It permits to compensate earth pressures as well as underground water pressures. For openface conventional tunnels, no face support is conducted, but some auxiliary techniques, such as face reinforcement with dowels and umbrella pipe, can be employed to maintain a safe excavation. Thus, it is very interesting to investigate tunnel face stability in soft rock masses.