Brittle failure modes of underground powerhouses: an insight based on true triaxial compression tests

The directions of in situ principal stresses and curvature of the opening boundary play a major role in determining the failure modes of underground powerhouses. Considering the major in situ principal stress σ1 acting approximately parallel to the axis of the underground powerhouse of the Shuangjiangkou Hydropower Station, the failure modes of this powerhouse are related to the intermediate in situ principal stress σ2, rather than σ1. An onsite investigation of the underground powerhouse (which is located in intact granite) revealed that V-shaped notches formed at the upstream spring line of the arched roof while vertical splitting occurred at the downstream straight wall. True triaxial compression tests with a servo-controlled system were undertaken to distinguish these brittle failure modes. Shear failure occurs at the upstream spring line with the large-curvature arched roof. However, compression-induced tensile failure appears at the downstream straight wall with infinitesimal curvature, where the upright nature of the boundary readily leads to stress relaxation via radial unloading deformation. Moreover, the intermediate principal stress after excavation could enhance the post-peak brittleness, crack initiation stress, crack damage stress, and peak strength of the hard rock. Class II type post-peak stress–strain curves present such that huge amount of energy violently releases in the post-peak stage. By considering the magnitude and rate of energy release at the post-peak stress drop, an index is proposed to assess the potential for brittle fracture. The index can differentiate between Class I and II types of post-peak stress–strain curves which has been verified in the laboratory via true triaxial compression tests.