Confinement effect of dynamic failure of red sandstone under impact

To investigate the disturbance caused by blasting in the excavation process of tunnel and coal mine surrounding rock, it is urgent to clarify the mechanical response, failure mode and energy dissipation characteristics of red sandstone under dynamic load under confining pressure. In this study, the split Hopkinson pressure bar (SHPB) test system with a self-developed active confining pressure control device was used to carry out dynamic compression tests on red sandstone specimens under different confining pressure levels, to explore the dynamic mechanical response, failure mode and energy dissipation mechanism of red sandstone under impact load. The test results show that the stress-strain curve presents a “two stages” characteristics under unconfined condition. and the stress-strain curve changes from a “two stages” to a “three stages” pattern with the increase of confining pressure. The confining pressure significantly enhances the dynamic compressive strength and peak strain of red sandstone, both of which show significant strain rate effect and confining pressure effect. In terms of failure mode and energy dissipation, the rock specimen is crushed when subjected to higher strain rate at unconfined condition. Under confining pressure, the damage degree of the sample is significantly reduced, and finally resulting in compression-shear failure. Under the same confining pressure, the reflection energy and reflectivity increase with the increase of strain rate, while the transmission energy increases with the increase of strain rate and the transmittance decreases with the increase of strain rate. Under the same strain rate, with the increase of confining pressure, the rock reflection energy and reflectivity decrease, the transmission energy and transmittance increase. When the specimen is dynamically damaged, the dissipation energy is regulated by strain rate and confining pressure. When the confining pressure is constant, the dissipation energy and dissipation rate increase with the increase of strain rate. When the strain rate is constant, both the dissipation energy and dissipation rate decrease with the increase of confining pressure.