Abstract:  High water pressure in underwater tunnel connection sections constructed using combined methods leads to structural diseases, such as arch cracking and track uplift. Therefore, a case study is conducted on the Zhujiangkou tunnel of the ShenzhenJiangmen railway. An external drainage system is applied to the connection section to release the water pressure, and the analytical solution of the seepage volume of the tunnel and the water pressure on the lining is deduced, which is validated via degradation tests and numerical simulations. The sensitivity analysis of parameters is conducted to reveal the working mechanism of the external drainage system. The results are summarized as follows: (1) The deduced analytical solution is highly accurate. The seepage volume and pressure on the lining decrease with increasing radius of the external drainage hole, which is beneficial to water release and pressure reduction. (2) 〖ＪＰ３〗The external drainage hole allows the direct discharge of seepage water in the grouting circle, which shortens the original seepage path, thus reducing the requirement of the grouting circle and achieving safe construction and good economic benefit.

Key words: underwater tunnel, combined method, connecting section, ultrahigh water pressure, external drainage, water and pressure , reduction