Numerical Study of the Mechanism of Dense Drillings for Weakening Hard Roofs and its Application in Roof Cutting

The untimely collapse of the lateral cantilever structure is one of the main factors in the deformation of the surrounding rock in hard-roof mines. In this study, a method for weakening the hard roof using dense drilling is proposed to promote roof collapse. Based on the geological engineering conditions in the Tashan coal mine, an FLAC 3D numerical model was developed by considering the plastic-softening characteristics of the hard roof to investigate the dynamic evolution process of the weakening roof surface. As the load increased, failure around drillings occurred gradually, and the failure zones expanded in an x-shape from the drillings toward each other, forming a weakened macro-surface. The influence of the drilling arrangement on weakening the hard roof was investigated using numerical models of various drilling diameters and spacings. The results indicated that a large diameter or small spacing was helpful for the formation of a weakened macro-surface. However, differences were observed in the response of key parameters to the weakening effect. The peak stress, fracture stress, and energy absorbed during loading and dissipated during the rock failure process displayed a downward trend for all models with an increasing drilling diameter. In contrast, these parameters, except for the fracture stress, tended to increase with a decreasing spacing. Field monitoring revealed that the average rock strength around dense drillings decreased from 69.0 to 60.5 MPa during the advancing of the work face after roof cutting by implementing dense drillings. Furthermore, the maximum axial tension increments of the cable and bolt reached 80 kN and 10 kN, respectively. Highlights A roof-cutting method involving dense drillings to weaken hard roofs was proposed. The progressive failure mechanism of the rock around dense drillings was investigated. The influence of the drilling arrangement on weakening hard roofs was investigated. The effectiveness of the roof-cutting method was revealed by conducting a case study.