Transition from fracture propagation to coalescence in Westerly granite during the preparation process of macroscopic failure

Fracture development controls the conditions of catastrophic failure of rocks and other heterogeneous materials in the brittle regime. To characterize fracture development during the preparation process of failure, we calculate the volume of fractures propagating in isolation and fractures coalescing throughout eight triaxial compression experiments with in situ synchrotron X-ray microtomography acquisition. We track this behavior in experiments on Westerly granite at ambient temperature with varying confining pressure (5-20 MPa), and fluid pressure (0-10 MPa). In all of the experiments, the coalescing fracture porosity rapidly increases preceding macroscopic failure, on average at 97% of the failure stress. Fractures become increasingly parallel to the maximum compression direction with loading, and increasing volumes of coalescing fractures. The orientation of groups of fractures is more oblique to the maximum compression direction than the orientation of individual fractures, consistent with the development of faults from arrays of en echelon opening mode fractures. The volume of coalescing fractures at each stress increment is more strongly correlated with the volume of fractures with lengths greater than the mean grain size, i.e., the critical fractures, than with the volume of fractures with lengths greater than the spacing, L/S>1, between fracture pairs. Using a grain size of 1025 µm and the condition L/S>3.5 to identify the critical fractures produces the strongest correlations with the volume of coalescing fractures. The mean grain size is thus a more robust metric of fracture network behavior than the spacing between fractures.