Abstract: The minimum tunnel overburden thickness is a critical parameter in underwater tunnel construction. To calculate the minimum tunnel overburden thickness considering the influence of grouting, an analytical solution for the seepage field of an underwater tunnel influenced by grouting is derived using the mirror method and seepage mechanics theory. The solution is refined using the Japanese minimum seepage method. The accuracy of the theoretical solution is verified through formula simplification and laboratory simulation tests. Using this solution, this paper investigates the influence of parameters including the relative permeability coefficient of grouting, grouting range, and water depth. The findings show that, at different depth ratios, water pressure outside the tunnel lining increases linearly with head height. A lower depth ratio increases the sensitivity of external water pressure to water depth variations. Improving the permeability of the grouting ring reduces tunnel seepage more effectively than enlarging the grouting range, especially within a range of 1 m to 3 m, where the effect is most pronounced. In the absence of grouting, the tunnel overburden thickness is highly sensitive to changes in water depth, and enhancing the permeability of the grouting material can significantly mitigate the impact of water depth. For high-risk tunnels, high-permeability grouting materials with a thickness exceeding 5 m are recommended. In conventional projects, a grouting range of 1–3 m provides an optimal balance between safety and cost.

Key words: tunneling, seepage field, overburden thickness, grouting ring, indoor model tests