The performance of soundless cracking agents for weakening rock roof under different notch angles

Abstract

Aiming at the problems with hard rock roof of coal seams, such as their large strength and difficulty in caving, traditional high-pressure water injection, and deep-hole blasting methods for roof weakening are mainly used to change mechanical characteristics and conditions of roof strata. However, these methods are not free from problems, such as inconvenience operation, strict sealing requirement, and potential dynamic hazards. Differing from the traditional weakening modes, this study investigated the crack propagation in materials similar to rocks and mutual influences thereon at five different notch angles (45°, 60°, 90°, 120°, and 135°) by using acoustic emission (AE) and static strain test system based on the soundless cracking technology. On this basis, the study further investigated the effect of the charging holes with different notch angles on dynamic crack propagation in coal seam rock roof The results revealed five key points: firstly, macroscopic fracture surfaces were finally formed in the specimens after successively undergoing crack initiation, propagation, and cracking phases under the effect of hydration swelling of soundless cracking agent (SCA). Secondly, the initiation angle reduced with increasing prefabricated notch angle and it was at a maximum (82°) when the notch angle was 45°. Thirdly, compared with the specimens without prefabricated notches, the specimens with prefabricated notches exhibited a larger fractal dimension. The fractal dimension was positively correlated with the density, quantity, and width of fractures. Fourthly, the cumulative ring-down counts and cumulative energies of specimens with prefabricated notches are both larger than those without prefabricated notches. It is 1–2 times. The smaller the notch angle was, the larger the accumulative ring-down counts and cumulative energy. Fifthly, the notch angle was negatively correlated with the fracture degree as evinced by specimens with prefabricated notches showing best overall cracking effect and the fragmentation of sample when the notch angle was 45°. The study can provide theoretical support for weakening coal seam rock roof to some extent.