SIZE AND STRESS GRADIENT EFFECTS ON FRACTURE AROUND CAVITIES

The effect of hole size on fracture initiation around cavities in rock is examined through physical model tests on Tyndall limestone. Twenty five models, employing nine different hole sizes, ranging from 3.2 to 62 mm in diameter were tested in uniaxial compression. Strain gauges placed both on the face of the model block and inside the cylindrical cavity were used to record the state of fracture. Fracture initiation data for primary, remote and sidewall fractures were determined using the stress-strain curves from the respective strain gauge locations. For all three fracture types, the initiation stress decreases with increasing hole size and appears to approach a horizontal asymptote for large cavity sizes. The fracture mechanics formulations and the stress averaging method give the best fit to the primary fracture initiation except for very small cavity sizes. The stress averaging method also provides a very good fit to the sidewall fracture initiation data.