Strength of a Model of Jointed Rock

TLDR: In this paper, triaxial compression tests were carried out on idealized samples containing sets of preformed discontinuities inclined at various angles to the sample axes, and four major modes of failure were: axial cleavage fractures at low confining pressures, shear failure through the plaster and across joint planes, slip on joint planes and ductile failure.

Abstract: In an attempt to extend the presently limited state of knowledge of the strength of jointed rock masses, triaxial compression tests were carried out on idealized samples containing sets of preformed discontinuities inclined at various angles to the sample axes The 4 in by 4 in by 8 in samples were built up from one in cubes of a high strength gypsum plaster Triaxial tests were carried out on 5 sample types at each of 5 confining pressures in the range of 0 to 2000 psi The four major modes of failure were: (1) axial cleavage fractures at low confining pressures, (2) shear failure through the plaster and across joint planes, (3) slip on joint planes, and (4) ductile failure The strengths of block-jointed samples failing by mode (2) were less than those of the corresponding unjointed samples At the higher confining pressures of mode (4), the strengths were higher than those of the unjointed samples and were unaffected by joint orientation Friction parameters determined from tests in which mode (3) applied were the same as those determined for the plaster in direct shear tests Mohr's circle envelopes were curved, and could be described by a power law in which the normal stress index varied with joint orientation