Evolutionary patterns of cracking and damage due to anti-reflection of controllable shock waves at the roof of loose and low-permeability coal seams

Controllable shockwave technology, as a new type of coal seam penetration enhancement technology, has the characteristics of safety and reliability, high energy utilisation, and the number of impacts and energy can be controlled, etc. It has good application prospects in reforming the permeability of coal seams and strengthening gas extraction. This technology has a good application prospect in transforming the permeability of coal seams and strengthening gas extraction. Aiming at the problems of poor gas extraction effect and short effective extraction time in soft and low-permeability coal seams, the technology of permeability enhancement in the roof of soft and low-permeability coal seams based on the controllable shock wave is proposed in this paper. However, the crack extension and damage evolution law of coal and rock under the cyclic impact loads of different discharge voltages are not clear, which cannot provide guidance for the determination of key process parameters. Physical models of coal-rock assemblages were cast according to the similarity criterion, and a controllable shockwave similarity simulation test platform capable of cyclic loading of the physical models with different discharge voltages was constructed independently in the laboratory. Based on the above conditions, a similar simulation test of controllable shockwave penetration enhancement in the roof of a soft and low-permeability coal seam was carried out. Various methods were used to investigate the dynamic response of the model, including analysing the crack distribution and evolution patterns under different discharge voltages, constructing the displacement field on the surface of the model using a digital camera and Matlab-Ncorr open source scattering software, and establishing a mathematical relationship between the damage factor and the ultrasonic wave speed to quantitatively characterise the three-dimensional damage evolution inside the model. The main conclusions are as follows: Discharge voltage and number of times play a key role in the process of controllable shockwave penetration of soft and low-permeability coal seams, and high voltage leads to high crack density and rapid expansion of the model. Increasing the number of discharges can promote the crack expansion, forming a ‘fan-shaped’ crack network and optimising the penetration enhancement effect. As the number of discharges increases, the vertical and horizontal displacements of the coal-rock model increase. The displacement is positively correlated with the discharge voltage. The increase in displacement promotes the crack expansion of the coal rock body, provides more space for gas transport, and contributes to the unloading and enhancing permeability of the coal seam. Coal and rock damage gradually accumulate under the action of controllable shock wave, and the damage factor is positively correlated with the number of discharges, and coal and rock damage is a cumulative process. The damage of coal and rock near the fracturing drill hole and coal-rock interface is serious, and the damage under high voltage accelerates the evolution and eventually forms a completely damaged area, which promotes the enhancement of coal seam penetration. This test elucidated the dynamic response characteristics of cracks, displacements and damages under cyclic impact loads with different discharge voltages, and provided a reference for the selection and optimisation of engineering parameters.