Effect of localized water pressure on mountain tunnels crossing fracture zone

Localized Water Pressure (LWP) on tunnel means water pressure concentrating on partial areas of tunnel periphery instead of distributing evenly all around tunnels. High LWP generally occurs in tunnels crossing mountain areas with faults and fracture zones or results from clogging of the water drainage system of tunnels. High LWP on tunnels could lead to severe structural cracks and leakages. A mountain tunnel in fracture zones with LWP of nearly 2 MPa was taken as an example to investigate the acting mechanism and effect of LWP on the safety of the tunnel lining during the whole process of the tunnel crossing through the fracture zone by a 3D Ground-Structure modeling. The main conclusions drawn from the research work are as follows: (1) The LWP acting on the tunnel lining dynamically changes in both magnitude and distribution (longitudinal and crosssectional) with the advance of the tunnel face. The magnitude of LWP in the fracture zone is up to 17.9 times higher than that outside the fracture zone and can lead to damage to the tunnel structure. The main distribution scope of the LWP on the lining structure is within 8 m from the boundary of the fracture zone. (2) The actual internal forces of lining near the tunnel face are highest, and the tunnel structure in the fracture zone and within 16 m outside the fracture zone is at risk of crushing induced by high LWP. (3) Cutoff grouting is effective in reducing the LWP and the axial force remarkably with a decrease of 52.4% and 76.8%, respectively. The necessary thickness of the grouting ring for the tunnel crossing fracture zone is set to 6 m. (4) A novel drainage network was proposed to improve the reliability and sustainability of the drainage system of the tunnel in the fracture zone with high water pressure. The results of this study provide practically useful suggestions for the design and construction of mountain tunnels crossing fracture zones of high LWP.