Comparative study of the hydromechanical behavior of intact, horizontally jointed, and vertically jointed rocks under undrained conditions

This paper investigates the influence of the presence of a joint (vertically or horizontally oriented) on the hydromechanical behavior of sandstone under undrained triaxial conditions. A series of undrained triaxial experiments was performed on intact and horizontally and vertically jointed sandstone samples under different confining pressures and initial pore-water pressures. Joints in jointed samples were artificially created with rough and irregular surface profiles. An increase in pore-water pressure with increasing deviatoric stress was observed for all tests. Maximum induced pore-water pressure values were higher and occurred later in the jointed rock samples when compared to the intact samples at all confining pressures. This may be related to the efficiency of the response of the joint pore volume to loading when compared to that of the matrix pore volume. Moreover, the significance of new pore volume creation (by microcracking during failure) on sample pore-water pressure is dependent on the initial pore volume, which itself varies with the presence and size of joints regardless of orientation. Effective peak deviatoric stresses observed from the tests were consistently higher for the intact rock samples when compared to the jointed rock. This result suggests that the compressive strengths of intact and horizontally and vertically jointed rocks are not comparable when the joint is rough and the deformation is undrained. The origin of the influence of horizontal or vertical joints on rock strength may relate to the presence of edges on the rough joints, which act as stress intensification points and allow material failure under lower applied stress for the jointed samples than for the intact (homogeneous) samples.