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.

原位主应力的方向和开口边界的曲率在决定地下厂房的破坏模式方面起着重要作用。考虑到双江口水电站地下厂房的主要地主应力σ ₁近似平行于轴线，该厂房的破坏模式与中间地主应力σ 2_相关，而不是σ ₁. 对地下厂房（位于完整花岗岩中）进行现场勘察发现，拱形顶板上游弹簧线处形成V形缺口，下游直壁处出现竖向劈裂。采用伺服控制系统进行真三轴压缩试验以区分这些脆性失效模式。大曲率拱形屋盖的上游弹簧线发生剪切破坏。然而，压缩引起的拉伸破坏出现在具有无穷小曲率的下游直壁处，边界的直立性质很容易通过径向卸载变形导致应力松弛。此外，开挖后的中间主应力可以增强峰后脆性、裂纹萌生应力、裂纹损伤应力、和坚硬岩石的峰值强度。II类峰后应力应变曲线呈现峰后阶段大量能量剧烈释放。通过考虑峰值后应力下降时能量释放的大小和速率，提出了一个评估脆性断裂可能性的指标。该指标可以区分 I 类和 II 类峰后应力-应变曲线，这些曲线已在实验室通过真三轴压缩试验得到验证。