Dynamic mechanical properties and failure mechanism of saturated coal-measure sandstone in open pit mine with damage under real-time low-temperature conditions

Many large open-pit coal mines in China are located in the western alpine-cold regions such as Xinjiang Province. The freezing of free water in the rock mass of the slope under low temperature environment will have a significant impact on the macro and microstructure of the damaged rock mass, causing further deterioration of the mechanical properties of the rock mass and thus inducing large-scale landslides and other geological disasters. Therefore, the study of the dynamic mechanical properties and damage mechanisms of saturated coal measures sandstone containing initial damage under low temperature environment is of great significance to the stability of slopes and safe mining of open-pit coal mines in the cold regions. In this paper, using the real-time low-temperature separated Hopkinson pressure bar(LT-SHPB) test system, the low-temperature impact loading test was carried out on the water-saturated open-pit coal measures sandstone with initial damage. Combined with the observation of SEM, its dynamic mechanical properties and meso fracture mechanism were systematically studied. The results show that(1)with the increase of impact velocity, the dynamic compressive strength, elastic modulus and peak strain of sandstone increase rapidly, showing an obvious impact strengthening effect. At the same time, under the fixed impact strength, the compressive strength and elastic modulus gradually decrease with the increase of initial damage, showing an aboriginal damage weakening characteristics, while the peak strain shows some damage strengthening characteristics.(2) Under the impact load, the failure degree of sandstone gradually increases with the increase of initial damage and impact velocity, and the failure mode presents the transformation characteristics from tensile failure to comminuted composite failure.(3) The low temperature environment will have an obvious frost heaving effect on the water-saturated damaged sandstone, resulting in an increase in the scale and number of defects inside sandstone, which is the fundamental reason for the deterioration of the dynamic mechanical properties of sandstone.(4)The dynamic failure mode of sandstone is mainly brittle failure. Under high initial damage, the meso-scale ductile fracture morphology appears in the sandstone, and the local ductile fracture is obvious. When the damage value of sandstone increases from 0 to 50.26%, the brittle ductility coefficient of sandstone shows a rapid increase, indicating that the dynamic fracture mode presents a trend from brittle failure to ductile failure.