Dynamic mechanical properties and fracture characteristics of red sandstone under multiaxial confining pressure and impact loading

During rock blasting at deep depths, the rock is subjected to high geostress and explosion impact. Understanding the dynamic mechanical behaviour of rock induced by the combined static and dynamic loads is crucial for revealing the mechanism and improving the fragmentation of deep rock blasting. A true triaxial split Hopkinson pressure bar (SHPB) was used in this study to investigate the dynamic mechanical properties and fracture behaviour of red sandstone under multiaxial confining pressure and impact loading. Sieving tests and fractal dimension were used to quantitatively assess the effects of confining pressure and impact loads on the dynamic fracture characteristics of rock. The test results show that the dynamic peak stress, elastic modulus, incident energy and dissipated energy of rock fracture decrease with an increase in the axial confining pressure (6x-st) along the impact loading direction. While these quantities associated with the dynamic mechanical properties and fracture increase with increasing the lateral confining pressure (6y-st and 6z-st). The enhancing effect of 6y-st and 6z-st is dominated by the minimum pre-stress component. The dynamic strength and elastic modulus under an isobaric confining pressure condition are higher and more energy is required to dynamically fracture rock in comparison to anisobaric pre-stress states with the same average pressure. For red sandstone, the dynamic strength under multiaxial confinement is also strain rate dependent. Moreover, the strain rate effect is more significant under triaxial and anisobaric confining pressure. The size distribution and fractal dimension of rock fragments based on the sieving tests show that 6x-st promotes the dynamic rock fracture, while 6y-st and 6z-st have an inhibiting effect. As the impact load increases in a constant pre-stress state, the fractal dimension of rock fragments is positively and linearly related to the energy dissipated in the rock fracture. From the perspective of the promoting effect of 6x-st on dynamic rock fracture, rock fragmentation by blasting can be improved and accordingly explosives used in the blasting can be reduced if geostress conditions are well utilized. In this regard, it is recommended that the blastholes detonated simultaneously are arranged along the direction of the maximum geostress on the blasting work face.