Study on vertical cross loading fracture of coal mass through hole based on AE-TF characteristics

Abstract

With the gradual deepening of coal mining, under the action of stress concentration, coal and rock in deep stopes are prone to dynamic disasters such as rock burst and large area pressure. Therefore, using the fracture characteristics of the rapid fracture stage of coal rock mass, the evolution process of damage and fracture of coal rock mass under stress concentration conditions can be effectively analyzed. The experiment takes engineering coal and rock mass as the research object, and uses ultrasonic testing to select test blocks with the same wave velocity and perform central drilling on the test blocks to ensure the same initial damage in the rock mass, avoiding the dispersion of the final loading data of the rock mass caused by various differences. In the test, vertical block loads were applied to test blocks with different apertures, and the coal masses were monitored in real time while loading. Based on the monitoring results, the key failure locations are obtained, and the characteristics of acoustic emission-time–frequency (AF-TF) characteristics and damage failure laws of coal and rock bodies with different pore sizes in the fast rupture stage are analyzed. The experimental results show that: (1) After drilling, the compressive strength, number of acoustic emission events, and accumulation of coal rock masses have been significantly reduced, but after the hole diameter has been excessively increased, the number of events and energy at the final fracture of coal rock masses have increased abnormally; (2) During the rupture stage of the cave coal rock mass, the frequency spectrum changes from cliff-like attenuation to continuous step-down attenuation with increasing aperture. At the same time, the waveform suddenly becomes continuous in the time domain, and the characteristic signal appears concentratedly; (3) Under high-frequency vibration, the non-porous coal rock mass cracks, and finally at the peak, the accumulated stress is quickly released in the manner of tensile failure. However, the low-frequency continuous vibration of the cavern coal rock mass causes the rock mass to be step-graded to release stress, and the key damage guidance leads to the formation of conjugate shrinkage and fracture of the rock mass. It is because with the increase of the hole, the degree of deterioration of the rock mass increases and stress builds up at the hole wall, and finally a rock burst occurs at the hole wall. The study on the fracture of the coal mass in the hole can provide effective theoretical support for the coal rock stress grooming during the excavation of the deep coal mass and the search for key fractures to avoid the occurrence of dynamic disasters.