Experimental study and evaluation on the weakening of bursting liability of coal with boreholes

Abstract

Borehole pressure relief technology is the most commonly used method for stress control in the prevent of coal burst. Because coal burst is a dynamic failure phenomenon of coal driven by energy, compared with stress transfer, the effect of pressure relief should also be evaluated from the perspective of energy storage and dissipation. Uniaxial compressing tests of coal specimens with different boreholes are carried out. With increase of the borehole diameter and number, the peak elastic strain energy, energy release velocity, fragments kinetic energy and ejection velocity of coal all decrease. Compared with intact coal specimen, the kinetic energy of the coal specimen with one 14 mm borehole is droping to 39.91 %, the ejection velocity decreases from 9.74 m·s−1 to 6.07 m·s−1. The uniaxial compressive strength, bursting energy index, and modified bursting energy index of the coal specimen with boreholes show a negative exponential correlation with the diameter and number of boreholes, while the dynamic fracture duration shows a positive exponential correlation. In order to evaluate the effect of borehole pressure relief, energy pre-release index, energy dissipation-increasing index, and bursting liability weakening index are proposed. These three indexes are positively correlated with the diameter and number of boreholes, which can better reflect the effect of pressure in the perspective of energy storage and dissipation.