Mechanism of rockburst induced by double-roadway excavation in a steeply inclined coal seam and a control method through stress relief and energy release

With the continuous increase in mining depth of steeply inclined coal seams, rockburst in roadways during excavation has become increasingly prominent. This paper adopts a comprehensive approach combining field monitoring, numerical simulation and theoretical analysis to investigate the mechanism of rockburst induced by double-roadway excavation in a steeply inclined coal seam and a control method through stress relief and energy release. Microseismic monitoring shows that, compared with single-roadway excavation, the energy and frequency of microseismic events increase significantly during opposing or parallel double-roadway excavation in the steeply inclined coal seam. The mutual disturbance between the two roadways during excavation promotes the outburst risk. It is found that the stress and energy in the coal seam are highly concentrated under the action of high horizontal stress. The roadway excavation face is only 4.2 m away from the peak stress and energy zone, which provides the stress and energy basis for the occurrence of rockburst. Compared to single-roadway excavation, the increase in horizontal stress concentration coefficient is relatively small, about 0.03, when the two roadways are excavated in opposite directions or in the same direction. However, roadway excavation induces massive energy release from the coal seam, generating a strong dynamic load. When the strong dynamic load caused by the disturbance of roadway excavation is superimposed with the high static load of the burst-prone roadway and exceeds the rockburst strength, a rockburst will be triggered in the excavation roadway. According to the rockburst mechanism, a control method of stress relief and energy release for excavation roadways is proposed: reducing the number of simultaneous excavations or pre-fracturing the coal mass ahead of the excavation face to release accumulated energy and weaken dynamic load disturbance, and destroying the lateral coal mass of roadways to alleviate horizontal stress concentration. On this basis, measures including coal seam water infusion pre-splitting and large-diameter lateral boreholes are optimized. After the implementation of the optimized measures, the total frequency and daily total energy of microseismic events decrease by 30.43% and 58.36% respectively, which demonstrates a considerable reduction in the outburst risk and favorable effects in rockburst prevention and control.