Mechanism of Reducing the Bursting Liability of Coal using Liquid Nitrogen Cyclic Fracturing

The liquid nitrogen (LN2) cyclic fracturing technology offers a novel approach to rockburst prevention. To study the effectiveness and feasibility of LN2 cyclic fracturing to reduce coal bursting liability, LN2 cyclic fracturing, ultrasonic velocimetry tests, and uniaxial compression experiments were carried out. Changes in P-wave velocity, porosity, and bursting liability indices, namely, dynamic fracture duration time (DFDT), uniaxial compressive strength (UCS), bursting energy index (BEI), and elastic strain energy index (ESEI), were studied. Moreover, acoustic emission (AE) and failure characteristics of coal were analyzed. Finally, the mechanism of strength weakening for reducing burst liability by LN2 cyclic fracturing is discussed. The results showed that LN2 cyclic fracturing can effectively cause the deterioration of coal samples, resulting in decrease of P-wave velocity by 25.92% and increase of porosity from 1.42 to 9.1%. After LN2 cyclic treatment, the bursting liability of coal samples was lowered considerably; coal DFDT rose significantly, whereas the post-peak phase of load-time curves dropped in a stepped pattern. After four times of LN2 fracturing, the UCS and BEI of coal samples decreased by 73.5% and 83.2%, respectively, and the ESEI declined from 2.85 to 1.06. The coal DFDT increased rapidly with decrease in P-wave velocity, while the UCS, BEI, and ESEI correlated positively with P-wave velocity. The AE activities of various LN2 cycle coal samples can be classified into fracture compaction stage, fracture steady growth stage, fracture unstable growth stage, and post-peak stage. Moreover, the cumulative AE energy declined with increase in LN2 cycles, while the proportion of post-peak AE energy increased in cycles. The failure mode of coal samples transformed from dynamic failure to static failure after four LN2 fracturing cycles.