Hydro-mechanical behavior of sandstone with interconnected joints under undrained conditions

• Experimental study of undrained behavior of rock with interconnected joints
• Failure mechanism changes with degree of joint interconnectivity.
• Failure mechanism changes with confining pressure.
• Hydro-mechanical behavior of jointed rock strongly dependent on failure mechanism

In rock masses containing multiple joint sets the joints will intersect. The geometric nature of such joint intersections can influence the hydro-mechanical response of jointed rock. An undrained experimental study was performed on fully-saturated, doubly-jointed sandstone specimens. Testing considered a variety of joint orientations and interconnected joint configurations, and various values of confining and initial pore-water pressure. Test results revealed three failure mechanisms: (1) shearing (through intact material); (2) crushing failure in the vicinity of the joint junction point (with associated sliding on joints), and (3) sliding along a preferred joint plane. Using these results, a ‘failure mode matrix’ was developed to assist in prediction of failure mechanisms for different joint angles and intersection configurations, at different confining pressures. Greater peak induced pore-water pressures were observed for symmetric interconnected joint configurations than skew-symmetric configurations, for all confining pressures, though the difference was significantly more pronounced at higher confining pressures. Greatest peak strength values were obtained for joint geometries in which both interconnected joints were at a low angle. When both joint angles were greater than the friction angle of the joints, peak strength did not show a noteworthy difference between symmetric and skew-symmetric joint configurations for relatively low confining pressures. However, at higher confining pressures, skew-symmetric joint configurations showed considerably higher peak strength than symmetric joint configurations. These behaviors were attributed to differences in failure mechanism (between symmetric and skew-symmetric specimens) and the influence of failure mechanism and confining pressure on end friction and specimen response.