Failure process and tensile stiffness of shield tunnel with reinforced concrete inner lining under internal water pressure

Abstract

Shield method have been commonly used in water conveyance tunnels. To improve the bearing capacity of a shield tunnel under internal water pressure, a reinforced concrete lining is constructed inside the segmental linings to form a composite lining structure. However, cracks typically occur in reinforced concrete inner linings. In this study, a prototype test was performed to investigate the failure process of a composite lining. Based on the test observations, the progressive failure process of the composite lining experienced four stages. In the elastic stage, the stress and deformation of the composite lining increased linearly with the internal pressure, and no cracks appeared. In the cracking stage, cracks first occurred in the inner lining near the joints and were subsequently uniformly distributed along the circumferential direction. In the joint damage stage, the cracking of the inner lining caused more internal pressure to transmit to the segmental lining. As the internal pressure increased, compressive damage occurred in the handholes of the segmental joints. Finally, in the failure stage, the damage to the joints deteriorated and more internal pressure was borne by the rebar of the inner lining. Shear failure typically occurred at the welding point of the inner-lining rebar, followed by joint fracture near the welding point. Finally, the composite lining lost its ability. The test results showed that cracking of reinforced concrete inner lining remarkably decreased the tensile stiffness of composite lining. Consequently, the joint opening and convergence of the tunnel developed at a higher rate, posing a threat to the operational safety of the tunnel. The tensile stiffness is a key factor related to the deformation and bearing capacity of a shield tunnel. Therefore, an analytical solution was established to calculate the tensile stiffness of the composite lining after cracking. The analytical solution matched the test results well. In addition, the effects of the thickness and reinforcement ratio of the inner lining were analysed. The results suggested that increasing the reinforcement was more beneficial in improving the tensile stiffness of the composite lining.