Mechanical behavior and failure mechanisms of cylindrical and prismatic rock specimens under various confining stresses

Although cylindrical and prismatic specimens are often used in laboratory tests, the differences between their deformation and failure characteristics remain unclear. Cylindrical and prismatic specimens of two rocks (marble and granite) were analyzed under various conventional triaxial conditions. The failed prismatic specimens exhibit multiple fractures intersecting in the cross section. With increasing confining pressure, the failure mode of the prismatic specimen changes from tensile failure to shear failure. In the progressive failure process of prismatic specimens, the two lateral strains ε₂ and ε₃ are the same before the peak, but some differences arise after some degree of post-peak failure, where the acoustic emission (AE) activity clearly increases. The mechanical parameters (peak strength, internal friction angle, cohesion, Young's modulus, and Poisson's ratio) determined for the cylindrical and prismatic specimens are consistent. However, a significant difference in the fracture morphology exists among them, even with similar change trends in the tensile-shear failure mode with confining pressure.