Effect of intermediate principal stress on the failure characteristics of an underground powerhouse

The failure modes of deep excavation absolutely relate to three-dimensional stresses. The effect of the intermediate principal stress (σ2) on the failure of the granite surrounding an underground powerhouse was examined using true triaxial compression tests. As the initial major principal stress (σ1o) is parallel to the axis of the Shuangjiangkou Underground Powerhouse, for the cross-section the tangential stress around the excavation depends on the initial intermediate (σ2o) and minor (σ3o) principal stress components. Once the excavation is created, the radial stress becomes zero, and the tangential stress increases dramatically. The axial stress in the excavating direction remains almost unchanged. Slabbing will develop towards the excavation space that is parallel to the excavation axis. The stress σ2 after excavation can help to improve the failure brittleness, crack initiation strength, crack damage strength, and peak strength of the rock when it falls within the range of stresses encountered in engineering practice. The ratio of the crack damage strength to crack initiation strength is approximately constant as σ2 varies. More specifically, the crack damage strength is about three quarters of the value of the peak strength, and the crack initiation strength is about half the value of the peak strength. As σ2 increases, the microcracks coalesce to form a major through-going crack. Meanwhile, the Poisson's ratio in the direction of strain ε2 declines steadily and that in the direction of strain ε3 increases moderately, so that the rock exhibits anisotropy. The fracture angle (the angle between the fracture plane and the plane normal to σ1) is proportional to σ2. The results of the true triaxial compression tests allow the brittle failure events in the Shuangjiangkou Underground Powerhouse to be adequately explained. It could help facilitate an optimization of the direction of the excavation axis and efficient supporting measures for deep underground powerhouse.