Hollow cylinder tests for studying fracture around underground openings

We are beginning a study of deformation and fracture around underground openings through experiments on thick-walled hollow cylinders of rock that incorporate several features: plane strain loading, the ability to impose different stress paths, and ‘freezing’ of the fracture geometry under load. The elastic moduli calculated from the measured deformations are different than the moduli reported from uniaxial compression tests. The ‘unconfined’ strength of the rock surrounding the hole is two to three times typical uniaxial strengths, and a confining stress in the hole strengthens the rock. Although the load is axisymmetric, there is a prefeιτed direction of failure. Failure is by brittle spalling, resulting in triangular failed regions with pointed tips. The extent of failure is influenced by stress path and perhaps by strain rate.