Do joint geometrical properties influence the fracturing behaviour of jointed rock? An investigation through joint orientation

Abstract

Failure of brittle rock is often progressive and understanding the fracture development process within rock masses is important to identify the precursors of failures. Joints in rock masses influence the fracture progression behavior and we studied the influence of joint orientation on fracture progression in jointed rock. Undrained triaxial tests were conducted on 54 mm diameter and 108 mm high intact and singly-jointed sandstone specimens. The joints in jointed specimens were made rough (joint roughness coefficient = 10–12) and embedded in six different orientations—0°, 30°, 45°, 60°, 75° and 90°—and three different combinations of confining and initial pore-water pressures were considered (confining pressure/pore-water pressure = 4/1, 10/4 and 25/10 MPa). An acoustic emission (AE) monitoring system was employed during all tests to monitor the fracture development within the test specimens. The post-failure patterns of specimens displayed three different failure mechanisms—shearing through intact material, sliding along the joint and a mixture of both shearing and sliding—depending on the joint orientation and confining pressure. The fracture progression behavior of the specimens showed a strong correlation with these failure mechanisms. Intact specimens and specimens with joints oriented at 0°, 30°, 90° failed by shearing. Nevertheless, the AE patterns of intact specimens and specimens with 90°-oriented joints contrasted with those of specimens with joints oriented at 0° and 30°. Specimens with joints oriented at 60° failed by sliding with a constant rate of acoustic event generation starting from the beginning of deviatoric loading. A mixed failure mechanism was observed for specimens with joints oriented at 45° and 75°. While the characteristics of both sliding and shearing failure were observed from the AE patterns of both 45° and 75° orientations, the shearing failure component of the 75° case was found to be a result of experimental limitations rather than real behavior. Finally, we propose a family of typical AE curves to assist with the characterization of the fracture progression behavior of jointed rock by taking confining pressures and failure mechanisms into consideration.