Effective control of pore water pressures on tunnel linings using pin-hole drain method

Leakage and pore water pressure are the main hydraulic factors to be considered in the design of a tunnel below the groundwater table. In particular, pore water pressure, which is the penetrating pressure, may accelerate structural deterioration and consequently increase leakage. Avoiding high water pressure is, therefore, one of the main concerns in the design of underwater tunnels. Reduction of pore water pressure is often necessary to secure structural safety, and is generally achieved by adopting peripheral filter drainage systems. The drainage system is, however, no longer valid for large and/or non-circular tunnels in deep water conditions. In addition, it is not possible to install such a drainage system for single shell tunnels. As an alternative measure, a pin-hole drain system has been increasingly used. However, there is not a great deal of information about the design of the system in the literature examined for this study. In this study, the hydraulic behavior of a pin-hole drain is investigated using the numerical method, and the applicability of the pin-hole method is evaluated by performing a numerical parametric study for various design parameters. Based on the analysis results, some design comments for the pin drainage system are made.