Stability analysis model for a tunnel face reinforced with bolts and an umbrella arch in cohesive-frictional soils

Face bolting and umbrella arch techniques are widely adopted to increase the stability of tunnels in conventional tunnelling. Nevertheless, owing to incomplete theoretical analysis models, their design mainly depends on the judgment and experience of engineers. Therefore, a safety factor analysis model and a concise design flow for a reinforced tunnel face are established based on the limit equilibrium method and the strength reduction technique, in which the failure modes of a tunnel face and an unsupported span as well as the effects of face bolts and an umbrella arch are considered. In this method, the role of the face bolts is to improve the tunnel face stability by generating supporting pressure, which is stimulated by the cable model. The role of the umbrella arch is to improve the stability of both the tunnel face and the unsupported span by the load transfer effect, which is simulated by the Winkler beam model. To validate the accuracy of this method, the calculation results are contrasted with those obtained by other typical theoretical models and numerical software, which show good agreement. The stability of the Yenikapi-Unkapani tunnel is analyzed, and the practicability of the proposed model is determined. Finally, parametric analyses are conducted to discuss the effects of face bolts and an umbrella arch on the stability of a tunnel face and an unsupported span.