Effect of Confining Pressure and Strain Rate on Mechanical Behaviors and Failure Characteristics of Sandstone Containing a Pre-existing Flaw

In underground engineering constructions, fractured rock masses are likely to be subjected to three-dimensional static stress and dynamic loading simultaneously. In this study, via a modified split Hopkinson pressure bar (SHPB) system, a series of triaxial dynamic tests are carried on sandstone containing a pre-existing flaw under different radial confining pressures ranging from 5.7 to 22.7 MPa and different strain rates varying by 85–252 s⁻¹. The results indicate that both the dynamic strength and energy dissipation density of single-flawed sandstone feature a positive correlation with the rising radial confining pressure and strain rate; while the dynamic elastic modulus, failure strain and energy utilization efficiency all exhibit an evident radial confining pressure enhancement effect, these are insensitive to the strain rate. Moreover, the fragment distributions of single-flawed sandstone are adequately characterized by combining sieving tests and generalized extreme value function. Under a higher strain rate, the single-flawed specimen is featured by smaller average fragment size and wider fragment distribution range, while the higher confining pressure has the opposite effect. By post-mortem examination, three main failure types are classified, and the fracturing mechanism of new cracks is revealed by microscopic observation using a scanning electron microscope (SEM). The macro oblique shear crack and through shear crack are mainly induced by the shear slip mechanism, while the quasi-coplanar crack is tensile dominated.