Effect of joint orientation on the hydromechanical behavior of singly jointed sandstone experiencing undrained loading

Abstract

The hydromechanical behavior of singly jointed sandstone under undrained triaxial conditions was studied. Maximum induced pore water pressure was observed to increase with increasing joint orientation from 0° to 45°, for all confining pressures considered. For a 60° joint orientation, a considerable drop in maximum induced pore water pressure was observed (relative to the neighboring 45° and 75° joint orientations), under 4 and 10 MPa of confinement. At 25 MPa of confinement, the drop in maximum induced pore water pressure for the 60° joint orientation was not so pronounced. These observations were explained in terms of failure mechanism (sliding versus shearing) for the different joint orientations. For sliding failure, pore water pressure at failure did not decrease significantly from the maximum value, whereas a marked pore water pressure decrease (after maximum) was observed in the process of sample loading when failure involved intact material rupture. Peak differential stress values at different joint orientations showed that the critical joint orientation was 60° and increasing confining pressure can reduce the influence of joints on the compressive strength of rock. Volumetric strain data showed that dilation-related volume increase at or close to failure is significant for joint orientations of 0°, 30°, 45°, and 90°, but not for orientations of 60° and 75° at confining pressures of 4 and 10 MPa. At 25 MPa of confinement, none of the samples showed a considerable dilation-related volume increase. The results illustrate the major influence of failure mechanisms (as governed by joint orientation, confining pressure, etc.) on the hydromechanical response of jointed rock. ©2014. American Geophysical Union. All Rights Reserved.