Mechanisms and characteristics for deterioration of shield tunnels under surface surcharge

The metro shield tunnel in soft soil area is investigated and analyzed. It is shown that overlarge transverse deformations of the shield tunnel are induced due to the surface surcharge, which is an allowable magnitude according to the current computational theories, and it brings about problems of breakage and leakage of longitudinal segment joints. Through the scaled model tests, numerical simulations and theoretical analyses, the research results show that the additional vertical earth pressure on shield tunnel induced by the uniform surface surcharge is not the uniform pressure, and the vertical earth pressure on the top of shield tunnel is larger than the uniform surface surcharge. The additional earth pressure around the shield tunnel caused by surcharge is more negative for preventing tunnel deformation as the tunnel passes through softer soil. The longitudinal segment joints are the weakest points on the segment ring in the process of transverse deformation. Finally, the design concept of rigid linings for shield tunnels in soft soil areas is proposed, and some suggestions for enhancing the strength and stiffness of longitudinal segment joints are given.