Discrete element modeling on fracture coalescence behavior of red sandstone containing two unparallel fissures under uniaxial compression

Particle flow code (PFC2D) was adopted to carry out a discrete element modeling (DEM) for the fracture coalescence behavior of red sandstone specimens containing two unparallel fissures under uniaxial compression. The numerical micro-parameters of red sandstone were firstly validated from the experimental results of intact specimen, and were then checked with the experimental results for red sandstone containing two unparallel fissures under uniaxial compression. Four key factors (i.e. the axial stress–axial strain curve, the peak strength and elastic modulus, the ultimate failure mode and the crack coalescence process) were put forward to discriminate the rightness and reasonability of numerical simulated results. A systematic simulation for red sandstone specimens was performed to compare quantitatively the numerical results with the experimental results, which showed that the numerical simulated results were in good agreement with the experimental results. Numerical simulated results showed that the peak strength and elastic modulus of red sandstone containing two unparallel fissures were increased at the fissure angle α2 = 90°, before decreasing up to our maximum fissure angle of 180°. However, the ratio of lateral stiffness of red sandstone containing two unparallel fissures firstly decreased and then increased with the increase of α2, and had a lowest value at α2 = 90°. The shear and tensile crack numbers of red sandstone reaching the peak strength with respect to α2 were also discussed. Through the numerical simulation, we made a detailed summarization for the crack initiation, propagation and coalescence in the entire deformation process of red sandstone, which was found to be dependent to α2. The simulated cracks replicated most of the phenomena observed during the experiment. Finally, the stress field in the red sandstone specimens containing two unparallel fissures was obtained, which revealed the fracture coalescence mechanism of flawed red sandstone under uniaxial compression.