Laboratory investigation of rock fracture around boreholes

Theoretical failure of the inner hole of a thick-walled cylinder is analysed. The occurrence of failure is estimated using the Mohr-Coulomb criterion and the Shear Strain Energy criterion. The failure surface is shown by means of its intersections in three coordinate spaces: the principal stress space; the cylindrical elastic stress space; and the space of pressures applied to the surfaces of the thick-wall cylinder. Both criteria globally predict the same stability zone. The Shear Stress Energy criterion predicts a smoother surface than the Mohr-Coulomb criterion and shows the confining effect of the intermediate principal stress on the specimen stability.; Fracture of a thick-wall cylinder of artificial rock was investigated with laboratory tests. The properties of the rock-like material were determined using standard uniaxial compressive, triaxial, and Brazilian tests. Two specimen were failed: one under an excessive hoop stress and the other under an excessive axial stress. Acoustic emissions and inner hole volume change recorded during both tests proved to be equivalent fracture indicators.; Five specimens of inner diameter ranging from 1 to 3.5 in. were brought to fracture under an excessive hoop stress. The cores resulting from drilling the specimen were also tested in uniaxial compression. Ultimate stresses were plotted with respect to diameter for both the solid and the hollow cylinders. The fitted curves show that the maximum hoop stress of a thick-wall cylinder with a given inner diameter is 1.9 times as high as the maximum uniaxial stress that can be applied to a solid cylinder of the same diameter and twice as long.; Twelve specimens of the same dimensions were brought to failure so that the elastic cylindrical stresses at the inner wall of the cylinder could be ranked in all twelve types of stress combinations, including intermediate states for which two stresses are equal. Breakout related failure are reported in four cases, namely A1 ({dollar}sigmasb{lcub}rm r{rcub}