Investigation on mechanical behavior and crack coalescence of sandstone specimens containing fissure-hole combined flaws under uniaxial compression

This study focuses on the effect of pre-existing flaw geometry on mechanical behavior and crack coalescence modes of sandstone specimens containing combined flaws with different fissure angle, ligament length and fissure length under uniaxial compression. The flaw geometry is a combination of a single hole and an inclined fissure underneath, which is generated by a high pressure water-jet cutting machine and is different from that reported in previous studies. The effect of flaw geometry on mechanical behavior of sandstone specimens is analyzed. Basically, mechanical parameters including the peak strength, peak axial strain, elastic modulus and secant Young's modulus for the flawed specimens are lower than those for the intact specimens, with the reduction extent related to the fissure angle, ligament length and fissure length. Variation trends of the crack initiation stress for all tested cases are studied. Initiated crack types and cracking modes also depend on the combined flaws geometry. For the flawed specimens with a small fissure angle, ligament length or fissure length, cracking modes are generally characterized by cracks initiated from the hole-wall and evolved to the specimen boundary. However, when the fissure angle, ligament length or fissure length is increased, cracks initiated from both the hole-wall and fissure tips produce the main failure planes, accompanied by a free-standing "triangular prism structure" within the specimens. Numerical simulations using RFPA(2D) (Rock Failure Process Analysis in two dimensions) are carried out on the flawed sandstone specimens and agree well with the experimental results in the peak strength and overall cracking behavior.