A B S T R A C T The initiation and propagation ofmicrofractures and their contribution to material failure in compression are examined. The early part of the fracture process, the lateral and the axial yield points of the stress-strain curves, are identified with the onset of microfracture, respectively at the tensile and the compressive stress concentrations of the elastic flaw boundary. Later stages, including the initiation of inclined shear fractures, the mobilization of total resistance and the reduction of strength to the residual level, are discussed in terms of a modified Coulomb model.

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SYNOPSISThe strength of discontinuous rocks under direct shear loading is examined analytically and experimentally.In the direct shear test a homogeneous and isotropic block of rock may fail by three mechanisms: failure in tension, failure in shear and failure at ultimate strength. The strength parameters include the tensile strength, fundamental shear strength (cohesion), angle of internal friction, the angle of friction at ultimate strength and the normal stress on plane of enforced shear.The direct shear strength of planes of weakness, orientated in the plane of enforced shear, is derived from two sources: strength of solid rock where no separation exists and joint friction along the separated parts of the shear surface.Maximum shear strength develops only if solid strength and joint friction are mobilized simultaneously, that is at the same deformation.

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