Effects of orientation, frequency, and number of sets of discontinuities on rock strength under triaxial stresses

Abstract

Effects of orientation, number of sets, and frequency of discontinuities on the rock strength under triaxial stresses have been investigated by the empirical method in this research. Twenty groups of rock specimens including intact rock, rock specimens having one, two, or three sets of discontinuities with various frequencies and orientations of 0°, 30°, 45°, 60°, and 90° have been prepared and tested under triaxial compressive stresses. The axial strength of each group has been tested under confining pressure of 0, 5, 7, 10, 14, and 20 MPa. Axial specimen strength, having one set of discontinuity, decreases a little with increase of the orientation angle from 0° to 30° under the most confining pressures; the relationship between axial strength and orientation angle between 30° and 90° has a shoulder shape under low values of confining pressures. The shoulder shape relationship also changes with increase of the confining pressure as it disappears at high values of confining pressures. The shoulder shape range of reduction axial strength under confining pressures due to the orientation of discontinuities changes similar to a twin symmetrical shoulder shape for specimens having two or more perpendicular sets of discontinuities as another minimum axial strength occurs at orientation angle of 30° on the opposite direction as well as orientation of 60°. The uniaxial compressive strength approaches approximate zero value at orientation angle of 60° for specimens having different sets and frequencies of discontinuities and it also approaches to zero at orientation angle of 30° for specimens having two or more sets of discontinuities; axial strength has considerable value under triaxial stresses. Effects of three parameters of orientation, frequency, and number of sets of discontinuities on the axial strength decreases with increase of confining pressure as minimum and maximum values of the axial strengths of specimens are between 74 and 100 % of the axial strength of intact rock under confining pressure greater than or equal to 10 MPa in this research.