Triaxial experiments were carried out on cylindrical samples to investigate the influence of confining pressure on the deviatoric axial stress at which crack initiation occurs in saturated sandstone. Samples were subjected to different confining pressures (2, 5, 8, 10 and 12 MPa) and a constant pore water pressure of 1 MPa during testing to failure. Acoustic Emission (AE) detection techniques were employed during all experiments, to identify the stress threshold values for crack initiation. The results from the testing were found to be similar, though not entirely consistent, with those from similar testing reported for dry rock of different types (granite, sandstone and marble) in the literature. Plots of cumulative AE counts against applied stress revealed that crack initiation occurs at lower deviatoric axial stress values (relative to peak deviatoric axial stress) for tests performed at higher confining pressures. This is consistent with failure occurring more suddenly following the onset of damage (i.e. a more brittle failure response) for the lower confining stress tests. Thus, increasing confinement was observed to cause progressively less brittle failure response for the tests carried out on the saturated sandstone.
Heterogeneity is an inevitable feature in many natural materials, including rock. Identification of these crack development stages is of utmost importance for understanding and characterizing expected rock behavior for various applications in civil and mining engineering. Martin and Chandler [1] used relationships between axial stress, axial strain, lateral strain and volumetric strain to identify stress thresholds for various crack development stages (i.e. crack initiation, propagation and damage) using intrinsic characteristics of the curves. In addition to the stress-strain relationships, methods for recognition of stress thresholds for different cracking stages include use of scanning electron microscopy, photoelasticity techniques and acoustic emission detection (see [2-9]).
