Crack coalescence behavior of sandstone specimen containing two pre-existing flaws under different confining pressures

Flaws widely exist in engineering rock masses, which significantly influence the overall mechanical behaviors (strength, elastic modulus and failure, etc.) of rock masses. The study of crack coalescence behavior of rock specimen containing pre-existing flaws enhances the understanding of failure mechanism of rock. In this study, a series of PFC3D simulations on numerical specimens were carried out to investigate the crack initiation, propagation and coalescence process under conventional triaxial compression. First, numerical model for cylindrical sandstone specimen was constructed and a set of micro-parameters were calibrated by comparing with the experimental results of intact sandstone with different confining pressures. Then, two pre-existing open flaws which were the same as the tested sandstone were created by deleting balls. Triaxial compression simulations were conducted on pre-flawed specimens for a range of confining pressures (10-40 MPa). The numerically simulated results were compared with the experimental results, which showed that the simulations were in agreement with the laboratory tests. The processes of crack initiation, propagation and coalescence for pre-flawed specimens were investigated by program in PFC3D. The relationships among the cracking process, stress-strain curve and number of micro-crack were analyzed in detail. Finally, the internal crack patterns in pre-flawed specimens were revealed by analyzing the cross-sections from X-ray CT scanning and PFC3D.