Extremely hard and massive rock mass is preferable environment for tunnel construction; however, some engineering problems may occur at greater depths where the rock mass is under very high stresses. Rockbursts and spalling typically occur in high strength rock mass (greater than 100 MPa) with few or no discontinuities at depths of more than 1000 m. Rock popping commonly happens from the side or the roof of the tunnel where the rock is in a state of intense elastic deformation. Spalling refers to rock formations from which slabs of rocks are suddenly detached after the rock has been exposed in a tunnel. Tensile cracks typically initiate at 40%-50% of the uniaxial compressive strength of most massive rock. The occurrence of cracks can be estimated using the relationship between the brittle crack initiation stress and uniaxial compressive strength of massive rock (Hoek and Marinos, 2009).
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