Research on optimization of critical index of drilling powder method in water-rich working face of Xiaojihan Coal Mine

Aiming at the problem that the sandstone aquifer overlying the No.2 coal seam roof in Xiaojihan Coal Mine and the long-term water-rich condition of the coal body lead to distortion of monitoring results by the traditional drilling powder method in local areas, failing to truly reflect the stress state of the coal-rock mass and the risk of rock burst, this paper systematically investigates the main controlling factors of drilling powder amount and their variation patterns under different water-bearing conditions by means of theoretical analysis, field measurement, and laboratory triaxial tests. By comparing the geological and mining conditions of the 11221 working face with those of the 13220 and 13218 working faces, it is identified that coal seam moisture content is the key factor causing differences in drilling powder amount, and its influence significantly exceeds that of burial depth, coal thickness, geological structure, and mining technology. Based on a theoretical model of drilling powder amount and sensitivity analysis of coal physical and mechanical parameters, it is clarified that parameters such as elastic modulus, compressive strength, and internal friction angle are negatively correlated with drilling powder amount, while Poisson’s ratio and loosening coefficient are positively correlated. The contribution of each parameter is quantified using a normalization method. Through triaxial drilling tests on specimens with different moisture contents, combined with acoustic emission energy tomography, the study reveals the mechanism that under water-rich conditions, the weakening effect of water leads to an enlarged damage radius and an increase in theoretical drilling powder amount, but the actual measured drilling powder amount is difficult to discharge due to cohesion. Based on the different damage radii corresponding to specimens with different moisture contents, a correction coefficient H for the critical index of drilling powder amount is derived: H = 1.00 when the moisture content ranges from the natural state to 1.7%; H = 1.97 when the moisture content is in the range of 1.7%, 3.7%）; H = 3.69 when the moisture content is in the range of 3.7%, 5.6%）; and the method is deemed invalid when the moisture content is ≥ 5.6%. The engineering application shows that the corrected drilling powder monitoring results are significantly more consistent with the distribution of microseismic events, effectively improving the reliability of rock burst early warning. This study establishes a correction model and method for drilling powder amount applicable to water-rich coal seams, providing theoretical support and practical guidance for accurate monitoring and prevention of rock burst under similar hydrogeological conditions.