Extension Strain and Rock Strength Limits for Deep Tunnels, Cliffs, Mountain Walls and the Highest Mountains

Brittle rock can fail in tension even when all principal stresses are compressive. The culprit is Poisson’s ratio, but marked stress anisotropy due to a neighbouring free surface, and due to a raised principal tangential stress is also necessary. Extension strain-induced failure causes fracture initiation in tension. Propagation in unstable shear may occur if the tunnels or mine openings are deep enough, and if they are located in hard, brittle,...