Determination of minimum overburden depth for underwater shield tunnel in sands: Comparison between circular and rectangular tunnels

With the development of global urbanization, the utilization of underground space is more critical and attractive for civil purposes. Various shapes of shield tunnels have been gradually proposed to cope with different geological conditions and service purposes of underground structures. Generally, reducing the burial depth of shield tunnel is conducive to construction and cost saving. However, extremely small overburden depth cannot provide sufficient uplift resistance to maintain the stability and serviceability of the tunnel. To this end, this paper firstly reviewed the status of deriving the minimum sand overburden depth of circular shield tunnel using mechanical equilibrium (ME) method. It revealed that the estimated depth is rather conservative. Then, the uplift resistance mechanism of both circular and rectangular tunnels was deduced theoretically and verified with the model tests. The theoretical uplift resistance is consistent with the experimental values, indicating the feasibility of the proposed equations. Furthermore, the determination of the minimum soil overburden depth of rectangular shield tunnel under various working conditions was presented through integrated ME method, which can provide more reasonable estimations of suggested tunnel burial depth for practical construction. Additionally, optimizations were made for calculating the uplift resistance, and the soil thickness providing uplift resistance is suggested to be adjusted according to the testing results. The results can provide reference for the design and construction of various shapes of shield tunnels in urban underground space exploitation.