Methods to cope with external water pressure of hydraulic tunnel linings in high-pressure groundwater-rich strata

To solve the problems of lining design and safety caused by external water pressure in water-rich strata, the monitoring results of external water pressure in several practical projects are analyzed. Based on the model test results, the relationship among the external water pressure acting on the hydraulic tunnel linings, the groundwater activity in rock mass and the drainage capacity of linings is discussed. A new type of lining structure is proposed to solve the eccentric pressure problem of the traditional composite linings which may have a higher external water pressure applying on the lining bottom. The seepage control design standard of hydraulic tunnel in water-rich strata is discussed from the perspective of seepage flow and external water pressure control. The seepage control design criterion of hydraulic tunnel in water-rich strata is then discussed from the perspective of seepage control and external water pressure control. The results show that the external water pressure of hydraulic tunnel linings is mainly controlled by the drainage capacity of lining drainage system, and is related to the groundwater activity in the surrounding rock mass. To determine the external water pressure according to the method recommended in the specification in China is not accurate. The new proposed lining structure can effectively solve the eccentric pressure problem of linings because of the limited pressure reduction range of drainage holes and the larger water pressure on the lining bottom. It is suggested that the allowable discharge of hydraulic non-pressure tunnels with the traditional composite lining structure should be 3.0 m³/(md), and the external water pressure of lining should be below 0.5 MPa. The corresponding parameters of lining drainage and shutoff structures can be determined according to these two values. The research conclusion can provide technical support for the selection of coping method for the external water pressure problem of hydraulic tunnels in water-rich strata.