High-pressure air blasting experiments on concrete and implications for enhanced coal gas drainage

High-pressure air blasting is one type of destress blasting techniques to enhance gas drainage in coal seams and thus to prevent coal and gas outbursts. In this work, firstly, high-pressure air blasting experiment is conducted on concrete specimen with a size of 600 mm × 600 mm × 600 mm, the blasting crack distribution is observed and the particle vibration acceleration at the outer boundary of the sample is monitored. Then a damage model for high-pressure air blasting is proposed when the loading induced by high-pressure air blasting is considered as two consecutive stages, i.e. the dynamic loading stage induced by the stress wave and the quasi-static loading stage induced by quasi-static air pressure. Based on the experimental curve of particle vibration acceleration and corresponding numerical simulation, the stress wave distribution with Gaussian form around the hole is inversed. In addition, the blasting induced damage evolution in the concrete specimen is numerically simulated, and the numerical result of final cracking pattern compares favorably with the experimental observation. During the high-pressure air blasting on concrete block, the blasting stress wave initiates primarily radial cracks and the crushed zone, and the quasi-static air pressure results in the extension of existing cracks and the expansion of crushed zone. Compared to the hydraulic fracturing with water, the high-pressure air blasting induces more fractures, which would be of great benefit to the enhanced gas drainage in coal seam. Finally, by considering the effect of coal seam damage on permeability, the gas drainage that may be enhanced by high-pressure air blasting is quantified based on the numerical simulations. It is demonstrated the damage induced by high-pressure air blasting around the hole enhances the gas drainage dramatically; however, the in-situ stress in coal seam may restrain the blasting-induced fractures and subsequent gas drainage, which should be paid attention to during the design of enhanced gas drainage with high-pressure air blasting.