Permeability of granite from a shallow buried mountain pipe gallery tunnel under unloading confining pressure

Abstract

The rock mass within mountain tunnels undergoes complex changes in permeability, strength, and deformation during tunneling excavation. The comprehensive pipe gallery tunnel for the 2022 Winter Olympics buried in Haituo Mountain experienced water leakage because of meltwater and heavy rainfall during the summer. Permeability tests were conducted on granite samples with a low confining stress of 5.0 MPa, which was then reduced to 2.5 MPa. Granite samples were obtained by drilling a borehole into surrounding rocks classified as Class Ⅲ, IV, and V. The mechanical properties and permeability characteristics of granite were subsequently evaluated. The permeability behavior of granite can be categorized into four stages: crack closure, elastic deformation, yield, and postpeak stages. The permeability ranges from 1.943 to 0.528 nm² for fractured granite from Class Ⅲ, IV, and V surrounding rocks. The relationship between stress and permeability can be modeled by a power-law function, with the influence of unloading confining stress on permeability being greater than that of loading axial stress. The incomplete release of confining stress and varying degrees of weathering have led to diverse patterns of crack propagation and connectivity during rock fracture.