Abstract:An effective pre-drainage system in a limited area that has a high water head and huge inflow is important to prevent flooding accidents during subsea tunnelling. Most of collapses in subsea tunnels are associated with huge inrushes of water due to high water head and flows through faults. To find out the causes and countermeasures for flooding cases,a dozen of cases for TBM and NATM tunneling are studied. Case studies presented here show that if the leakage had been forecasted and pre-drained prior to the tunnel excavation,such accidents could have been prevented. In this study,a new horizontal pre-drainage system is suggested. Numerical analyses are performed to analyze the water head controlling effect on the tunnel face by drainage holes during the construction of subsea tunnels. It is supposed that the rock cover of a subsea tunnel is 100 m,and the depth of seawater is 60 m. Drainage system analyses are performed to analyze performance of the drainage system. The total head after horizontal pre-drainages,when the permeability of the ground is 0.0036 m/h,reduces to about 60% at the tunnel crown and 53% at the tunnel spring line. When the radius of the drainage pipe is 5 cm and the permeability of the ground is 0.0036 m/h,the length of the pipe should not exceed 250 m to maintain the drainage performance. Numerical analysis and a drainage performance analysis both show that the suggested horizontal pre-drainage system provides a clear drainage and water head reducing effect. The system is a new alternative to the present water controlling methods applied to subsea tunnels.