Seepage field and drainage system in the connection part of underwater tunnel based on conformal mapping method

Composite construction methods may be adopted for some underwater tunnels due to complex geological conditions. However, the seepage field and the drainage system at connection part characterized by irregular cross-section are rarely reported. This study used the Zhujiangkou underwater tunnel as an engineering case, where the non-circular cross-section of the connection part was converted to the circular shape through the combination of the conformal mapping method and genetic algorithm. Additionally, the longitudinal seepage law in geotextile was developed, allowing for the theoretical deduction of the seepage field of tunnel with irregular cross-section while considering the role of the drainage system. The analytical results were validated through degradation tests and compared with numerical simulations and practical monitoring data. Moreover, the impacts of waterproofing and drainage system parameters on water inflow and water pressure were discussed. The results indicate that the water inflow increases as the distance between circumferential drainage pipes decreases and both the thickness and permeability coefficient of the geotextile increase, while the water pressure on lining decreases conversely, highlighting the significant influence of the drainage system. These findings can guide the design of the waterproofing and drainage system through parameter optimization for the consideration of safety and economy.