Propagation and coalescence of two infilled parallel fractures in shale: laboratory testing and DEM simulations

Filling materials exist widely in natural rock fractures, which can impact rock mechanical behavior. However, only limited research has been performed to study the influence of the infill on the fracture behavior of rocks or rock-like materials. This paper investigates experimentally and numerically crack propagation in shale specimens with two infilled parallel fractures. The experiments focused on the influence of ligament length and orientation angles of two infilled fractures on shale cracking behavior under uniaxial compression. The numerical approach was based on a parametric study using the particle discrete element method (DEM) and the moment tensor theory of acoustic emissions (AE). The numerical results show that: (1) Shear strength is higher with infilled fractures than with open fractures; and (2) Stress concentration in the bridging area of specimens with infilled fracture is less evident, causing less microcracks in this area resulting in a lower possibility for the coalescence of the cracks thereby increasing the rock mass shear strength. These results agree with experimental observations and validate the numerical modeling. For both infilled and clear fractures, AE magnitudes exhibit a normal distribution, while the frequency of the number of cracks forming every AE event exhibit an exponential decay. Further DEM results show that for infilled fractures: (1) Four kinds of coalescence patterns in the bridging area were obtained, namely: non-coalescence, wing cracks coalescence, anti-wing crack coalescence and mixed cracks coalescence; and (2) The infilled fractures have a notable influence on the direction of the AE moment tensors, specifically, the extensional components of the tensile cracks near the fractures tend to be parallel to the long axis of the fractures.