Objective Traditional tunnel drainage systems have blind spots in relieving water pressure on the invert, which may lead to water-induced damages such as track heave and settlement, structural cracking, and mud pumping from the track bed during train operation. Therefore, it is necessary to optimize the tunnel drainage system.

Method Based on a subsea tunnel of Qingdao Metro, an overall research framework is established. An optimized internal-external combined drainage scheme is proposed. A fluid-solid coupled numerical model is developed to analyze the water pressure relief effect of the above-mentioned scheme and identify key influencing factors. Finally, based on the simulation results, rational design parameters for the drainage system are proposed, providing theoretical support for engineering practice.

Result & Conclusion  After optimization, the water pressure distribution pattern on the invert changes from a U-shape to a W-shape, and the maximum water pressure decreases to 40.93% of that under the existing drainage system, with a reduction coefficient of only 0.304. Taking the two longitudinal drainage pipes as boundaries, the tunnel drainage system can be divided into two completely independent and non-interfering drainage zones, facilitating independent design of each zone. When the circumferential drainage pipe spacing is 8.00 m, the water pressure reduction coefficient of the lining can be uniformly taken as 0.367. Under different water level heights at the arch crown, the water pressure reduction coefficient of the tunnel lining at any position remains consistent, which depends only on the water pressure relief capacity of the drainage system itself. The water pressure on the invert first decreases and then increases with deepening the drainage pipe burial depth, and gradually decreases with increasing pipe diameter. Compared with burial depth, the water pressure relief effect on the invert is more sensitive to pipe diameter. It is recommended to adopt a pipe diameter of 0.60–1.20 m and a burial depth of 0.55 m, which can ensure the water pressure reduction effect of no less than 61.86% and 67.14% at the arch bottom and arch waist respectively.