When a brittle elastic material containing a cavity is loaded in uniaxial compression, fractures may form in three basic positions around the cavity; at the tensile stress concentration (primary fracture), at positions inside the material remote from the perimeter of the cavity, and at the compressive stress concentration. Granite blocks containing a circular cavity of radius between 2.5 mm and 50 mm were tested in uniaxial compression to collect data on primary fracture propagation. The laboratory results indicate that primary crack propagation is a stable process at small scales but approaches instability at large scales. A finite width crack model is presented which is able to capture this scale dependent behavior. The model illustrates that both tensile and compressive stresses play an important role in the primary fracture process.
Laboratory testing of granite blocks containing a circular borehole was carried out with uniaxial and biaxial loading conditions, and with borehole diameters ranging from 5 to 103 mm. Borehole breakouts were also investigated in situ using borehole diameters ranging from 75 to 3500 mm. Results show that the same strength-scaling law cannot be applied to both laboratory and in situ breakouts.
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