DEFORMATION AND FRACTURE AROUND CYLINDRICAL OPENINGS IN ROCK .1. OBSERVATIONS AND ANALYSIS OF DEFORMATIONS

Elastic and inelastic deformation, fracture and failure around underground openings were investigated through experiments on thick-walled hollow cylinders of Berea sandstone and Indiana limestone, incorporating plane strain loading, the application of different stress paths, transference of the external pressure to infinity, and "freezing" of the fracture geometry under stress through metal saturation. This first of two parts discusses elastic and inelastic deformations on both an experimental and theoretical level. Most elastic (recoverable) deformations exhibit non-linearity and hysteresis. Hole deformations due to internal (in the hole) pressure are controlled by different overall moduli than those due to external pressure. These overall deformations are successfully explained by a radially anisotropic moduli model, which allows different overall moduli for different stress paths. A radial-pressure-dependent modulus model cannot adequately explain these measurements. There is some evidence, however, that the stiffness of these rocks is greater with higher pressures. Evaluation of non-constant modulus models reveals that the rate of modulus increase near the hole is critical in determining whether the maximum tangential stress will occur at the hole wall or away from the wall. The apparent strength of the rock adjacent to the unsupported holes is two to three times the uniaxial compressive strength. Significant raadial dilation occurs in the failing material, as evidenced by hole closure measurements both during loading and after test completion. Proper modelling of this failure requires the incorporation of extreme dilation and extreme strength loss. A support pressure in the hole greatly strengthens and stabilizes the rock, and also reduces the amount of hole closure and dilation upon failure. Hole closure is time-dependent, especially for the unsupported holes. © 1990.