Objective In high-speed railway shield tunnels under high water pressure, the segment structure is subjected to significant external loads. To reduce the bearing load on the segments, drainage holes can be installed to alleviate the water pressure. Rational placement of drainage holes helps minimize external loads on the segments and enhances the structural safety. Therefore, investigating the placement schemes of drainage holes at different circumferential positions and longitudinal spacings is of great importance for engineering construction.

Method Taking the Tanglang Mountain High-speed Railway Shield Tunnel in Luohu District, Shenzhen City, Guangdong Province as a case study, the general overview of the project is introduced and a numerical model for the case study is established. Four working conditions are defined for the circumferential arrangement of drainage holes, i.e. drainage at the vault and invert, drainage at the arch shoulders, drainage at the arch waist, and drainage at the arch foot. These are compared with a no-drainage condition to analyze the distribution patterns of the surrounding rock seepage field, external water pressure on the segments, and drainage volume under different drainage hole positions. Subsequently, compared with the no-drainage condition, five longitudinal spacings (10 m, 8 m, 6 m, 4 m, and 2 m) are selected to further investigate the above-mentioned distribution patterns under different layout schemes for circumferential positions of drainage holes.

Result & Conclusion When drainage holes are located at the arch foot, the disturbance range to the surrounding rock seepage field is the largest, the maximum external water pressure on the segments is the smallest, and the drainage volume is the largest. When the longitudinal spacing of drainage holes exceeds 8 m, adjacent drainage holes have no significant influence on each other, and there is no notable difference in the drainage effect. As the longitudinal spacing decreases, the disturbance to the surrounding rock seepage field gradually increases, the maximum external water pressure gradually decreases, and the non-uniformity of longitudinal external water pressure is progressively reduced. The maximum drainage volume is reached at a longitudinal spacing of 4 m. The optimal working condition for pressure relief effect is observed when drainage holes are located at the arch foot with a longitudinal spacing of 2 m.