Study on the Reasonable Cover Thickness of a Subsea Tunnel with the Numerical Calculation Criterion Method

The minimum rock cover thickness is a significant parameter during longitudinal section’s design of subsea tunnel by means of drilling and blasting. The thickness will impact the cost and safety when subsea tunnel is constructed. On the basis of numerous elasto-plasticity numerical tests and theoretical analysis, the minimum rock cover thickness’ criterion is provided, including minimum displacement method and factor of safety (FOS). For a certain section of subsea tunnel, with the growth of rock cover thickness, vault displacement will decrease first and then increase. There always exists a minimal value. At the same time, the same section is calculated and analyzed using a strength reduction method, and some clear regularities of subsea tunnel’s FOS are summarized. That is, for any section of a subsea tunnel, with the increasing of rock cover thickness, FOS increases first and then decreases, always with a maximum value. There is a close correlation between FOS and vault displacement’s variation tendencies. Accordingly, it can be thought that the technology feasible and economic reasonable minimum rock cover thickness is a rational and effective thickness, considering the maximum FOS and minimum vault displacement at the same time. Under this numerical method, the determined minimum rock cover thickness can give great reference values to the planning of tunnel’s vertical routes.