Mechanical Responses of Underground Unparallel-Fissured Rocks Subjected to Coupled Static-Dynamic Loading

AbstractPreexisting fissures within a natural rock mass significantly affect the mechanical responses of underground engineering rocks when subjected to coupled static-dynamic loads. Understanding the dynamic mechanical responses of prestressed fissured rocks is beneficial for optimizing the stability of rock engineering structures. Based on the split Hopkinson pressure bar (SHPB) device, a series of coupled static-dynamic compression tests were systematically performed to investigate the mechanical properties, the failure behaviors, the fragment characteristics, and the energy dissipation of unparallel-fissured rocks. Results show that the coupled dynamic strength increases with the applied strain rate for a given static prestress, while the elastic modulus does not exhibit any clear loading rate dependence. Under the given strain rate, the fissured sandstone has the highest strength under the static prestress of 0.6 uniaxial compressive strength (σs), followed by the static prestress of 0.4 σs, 0.2 σs, and 0.8 σs. The failure modes of fissured sandstone highly depend on the dynamic strain rate, and the ultimate failure pattern gradually changes from the mixed tensile-shear failure mode to the shear failure dominant mode with increasing strain rate. With an increasing dynamic strain rate, fissured rocks should absorb more energy in a short time, which results in more crack formation in the failure process. A higher static prestress can induce greater energy dissipation density and energy utilization efficiency when the static prestress is less than 0.8 σs. In addition, the relationships between energy dissipation and mechanical response of fissured sandstones under coupled static-dynamic loads are established and discussed