Minimum cover depth estimation for underwater shield tunnels

This paper presents an investigation to estimate the minimum cover depth of underwater shield tunnels. Five different conventional methods for minimum cover depth estimation were reviewed and validated based on a case history of underwater shield tunneling in the Swan Lake in Hefei, China. It was found that the Japanese minimum seepage water volume (JMSWV) method and the mechanical equilibrium (ME) method are effective, and fulfill the upper and lower limits of the reasonable minimum cover depth. Afterwards, the reasonable minimum cover depth was determined by performing three-dimensional finite difference analysis considering the fluid-mechanical interaction of the ground and tunnel lining responses at different cover depths estimated by the conventional methods. The analysis indicates a consistent pattern of the cover depth effect on ground surface settlement, subsurface vertical displacement, pore water pressure, tunnel lining horizontal displacement, bending moment, and major principal stress, when the cover depth increases from 2.3 m to 6.8 m. However, for the tunnel lining vertical displacements at the tunnel vault and the tunnel bottom, the pattern of the cover depth effect reverses at an intermediate cover depth (5 m). Based on the analysis, a new procedure combining the appropriate conventional methods and numerical analysis was proposed to estimate the reasonable minimum cover depth of an underwater shield tunnel.