Simulation test research and application of blasting and penetration improvement in the soft coal seam floor

Aimed at the problems of difficult drilling construction of blast penetration, undeveloped fractures produced by blast penetration, and the fractures is easy to recompact in the soft coal seam, the blasting operations carried out in the floor was proposed to achieve the purpose of increasing the penetration of soft coal seams and thus improve the effectiveness of gas extraction. In order to monitor the cross-interface stress wave propagation pattern and the damage state of the coal seam, a physical model was constructed in the laboratory and a similar simulation test of coal seam floor blasting was carried out. Concurrently, the numerical simulation research method was employed to supplement the evolution of damage and fractures within the coal and rock during the blasting process of the coal seam floor. The results shown that, the blast cracks expanded along the blast hole to the surrounding rock mass in soft coal seam floor, causing more serious damage at the location of blast hole, the coal-rock interface of the floor, and within the coal seam, producing cross-interfacial damage cracks. When the explosive stress wave propagated from the soft coal seam base rock layer to the soft coal seam, the interface between the rock and the soft coal generated transmitted compressive stress wave and reflected tensile stress wave. Transmission waves acted on the soft coal to increase the fractures in the coal seam. Meanwhile, the reflected waves reacted on the rock, forming cross-cracks in the coal-rock interface, making the fractures of the rock floor and the soft coal seam penetrated, which was conducive to the vertical transport flow of gas in the soft coal seam and the extraction of pressure-relief gas. Field application at Pan Yidong coal mine shown that, the pure volume of gas extraction and its concentration increased rapidly after blasting the coal seam floor to improve the penetration. The pure volume of gas extraction increased from 0.06 m³/min to 1.46 m³/min, an increase of 23.33 times. Gas concentration surged from 10.46% before blasting to approximately 45.5%, an increase of 3.34 times, and remained at a high level for a long time. The research results can provide theoretical basis and technical support for efficient gas extraction from deep soft coal seams.