Drainage design combining drain holes and pinholes for tunnel boring machine segments subject to high water pressure

Balance of the groundwater and ecology is crucial for controlled discharge. However, regarding the segments of tunnel boring machines (TBMs) under high water pressure, the stability of the lining structure is often reduced by excessive drain holes required to achieve this balance. The large discharge of pinholes can easily have severe consequences, such as the lowering of the groundwater table, drying of springs, and vegetation wilting. Thus, in this study, according to the fluid-structure coupling theory, a new drainage design for TBM segments was developed by considering a mountain tunnel subject to a high water pressure as a case study. The evolution characteristics, including the external water pressure of the lining, discharge volume of the segment, and groundwater-table drawdown, were investigated via numerical modeling with drain holes and pinholes. The results indicated that the optimal design parameters of drainage segments for the project case were as follows: a circumferential spacing angle and longitudinal number on one side of a single ring of 51° and 2, respectively, for the drain holes and an inclination angle and length of 46.41° and 0.25 times the grouting thickness, respectively, for the pin holes.