Dynamic Mechanical Response and Fracture Characteristics of Multi-flawed Rocks Exposed to Hydrostatic Confinements

Flawed rocks subjected to hydrostatic confinements in deep underground engineering are vulnerable to dynamic disturbances. Understanding the dynamic mechanical responses and fracture characteristics of flawed rocks is crucial for the design and construction safety of deep underground structures. In this research, using a triaxial split Hopkinson pressure bar device, the dynamic triaxial compression tests are performed on multi-flawed sandstone specimens containing different flaw angles and distances. The experimental results revealed the influence of the flaw angle, flaw distance, and hydrostatic confining pressure on the dynamic strength characteristics, deformation properties, failure modes and fracture mechanisms of rocks. Both the dynamic strength and elastic modulus increase with rising hydrostatic confining pressure, while the dynamic strength depicts a positive correlation with strain rate. The dynamic strength fluctuates zigzag with the flaw angle, and decreases first and then increases slightly with flaw distances. Specimens with different flaw angles all demonstrate conjugated “X” shape failure, while the single shear band failure is shown for certain flaw distances. Seven crack categories are identified based on the basis of different trajectories and the fracture characteristics of new cracks are analyzed in multiscale perspective. The scanning electron microscope observation indicates numerous shear traces on the fracture surface while brittle fracture features are relatively few. Furthermore, the fracture surface roughness is quantified by the 3D scanning technique and fractal dimension. Rocks containing higher flaw distance or under higher hydrostatic confining pressure are featured by lower fractal dimension and smoother fracture surfaces.