Experimental research into the drilling cutting method of rockburst-prone coal under water-rich conditions

Abstract

The drilling cuttings weight index fails to reflect coal’s stress conditions under water-rich environments, potentially undermining the drilling cutting method’s reliability for monitoring and early warning of rockburst risks. We integrated theoretical analysis with triaxial drilling tests to investigate the mechanical evolution, the spatio-temporal dynamics of absolute energy, and the failure modes during the drilling of water-bearing coal specimens. Moreover, we established the critical drilling cuttings weight index for these specimens by analyzing test outcomes and associated weakening coefficients. Results show that: (1) as the moisture content increases, the damage and rupture mode of coal changes from punctate frictional damage between microstructures to interlayer slip, dislocation, and friction of weak structural planes; macroscopically, it is shown as the increasing number of cracks in the horizontal and vertical directions in specimens; (2) The dynamic response frequency of rockburst-prone coal has a positive correlation with the moisture content. The elastic energy storage capacity of water-bearing coal is reduced, dynamic events develop in a shorter period while their frequency increases. In addition, the level of single energy release decreases; (3) With increasing moisture content, damage to boreholes in coal can be divided into two stages: convergence and crack initiation; (4) The deformation and damage range of boreholes is positively correlated with the moisture content. When the moisture contents are 1.4%, 2.3%, and 3.7%, the damage radii of specimens along the center of the boreholes are 2.03, 2.98, and 3.36 times the drilling radius; (5) The weight of drilling cuttings for coal with the moisture content of 3.70% is 2.78–3.48 times that of natural coal; for coal containing 5.60% of water, the weight of drilling cuttings is 1.50–3.20 times that of natural coal. The research conclusions provide reference for optimization of the index for the weight of drilling cuttings and effective rockburst monitoring of rockburst-prone coal under water-rich conditions.