Experimental study on failure behaviour of deep tunnels under high in-situ stresses

Geomechanical model tests are conducted in this study to investigate the failure behaviour and instability of the ‘‘large, deep, long and in-group’’ tunnels constructed in the Jinping II hydropower station. On the basis of the experimental results, deformation, stress and failure analyses are conducted for the tunnels under high in-situ stresses. It is concluded that: (1) The tunnels should be stable and the majority of the rock mass remains intact even if the applied load is 1.3 P₀, where P₀ is the in-situ stress. The rock mass around the excavations starts to crack when the applied load is 0.5–0.6 P₀, and presents nonlinear behaviour when the applied load is equal to the in-situ stress. (2) The displacement analysis indicates that the maximum displacement is 108 mm at the crown along the excavation profile and 81.2 mm in the rock mass 3 m far from the excavations. The obtained maximum displacements from the physical model test are close to those later monitored in the field, although the physical model test is actually conducted well before the construction of the tunnels. (3) The stress and failure analyses reveal that local failures occur in the rock mass around the excavation forming local rockfalls but these local failures have not accumulated to result in failures in massive area of the rock mass. There are no obvious cracking and yield in the rock mass between the two tunnels, which indicates that the designed span of 60 m may be feasible. (4) The geomechanical model test provides a means to understand the deformation, stress, cracking and potential failure of the tunnels constructed in the Jinping II hydropower station, which forms a basis for the subsequent 3D finite element modelling and construction process.