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Strength of non-persistent rock joints
Jamil, S. Muhammad
Doctoral Committee Chair(s)
Cording, Edward J.
Department of Study
Civil and Environmental Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
The present experimental study was conducted to investigate strength-anisotropy and failure modes of closed, persistent and non-persistent rock joints under biaxial loading. Previous investigations concerning the behavior of rock masses were conducted by simulating the rock mass as an assemblage of unit blocks. This arrangement is susceptible to rotation, imperfect contacts, and surface mismatch of the individual elements. In this study a procedure was developed to form closed, persistent and non-persistent joints in the sample, during casting, to represent the in-situ behavior of a rock mass containing joints of a single set. For joints capable of sliding, failure in rock masses with non-persistent joints can either be along the joints or stepping (sliding along joint segment and tension failure between the joint rows). The Mohr-Coulomb shear criterion is suitable for predicting failure along the joints; but failure by stepping needs another approach, which takes into account the failure in tension. The Mohr-Coulomb criterion in conjunction with the proposed expression gives satisfactory agreement with the test results.
The parameters investigated are: orientation with respect to the applied axial load, confining pressure, degree of persistence, and relative joint density. Samples with joints oriented at 22.5$\sp\circ$ to the axial stress failed along the joints under all conditions. Joints oriented at 67.5$\sp\circ$ consistently failed by stepping between joint rows. Joints oriented at 45$\sp\circ$ exhibited two modes of failure: failure along the joints (Mode 1) and stopping (Mode 2). The stepping failure mode occurred only under unconfined conditions for the degree of persistence of 0.667. Samples with joints oriented at 80$\sp\circ$ also exhibited two failure modes: an apparent stepping failure between closely spaced joints, in a zone along multiple surfaces (Mode 3), and a fracture through intact rock (Mode 4) with no significant influence for widely spaced joint. The transition from stepping to shearing across the joints occurred when the relative joint density was decreased from 2 to 1. The effect of joint orientation on the deformation modulus for non-persistent joints was found to be insignificant. The degree of persistence and relative joint density were found to influence deformation modulus, and could be described in terms of the effective joint stiffness.