Study on Mechanical Response and Failure Characteristics of Coal Specimens Under the Coupling Effect of Joints and Drillings

To investigate how joint density and drilling diameter impact the failure features of coal specimens, a numerical simulation test was conducted using PFC 2D 5.0 software. The mechanical characteristics, failure characteristics, and energy changes of borehole coal specimens with different joint densities and different drilling diameters were analyzed, and the sensitivity of the two was compared by range analysis. The results show that (1) the increase of joint density significantly reduces the bearing capacity of coal specimens, while the drilling diameter has little effect on the peak stress, but it will significantly change the failure path of coal specimens; (2) Under the condition of low joint density, the specimen is mainly characterized by tensile brittle failure, and the fragments are large. Increasing joint density shifts the specimen’s failure mode towards shear failure and produces smaller fragments; (3) With the increase of drilling diameter, the initiation and propagation of cracks are more likely to occur around the drilling, and the acoustic emission hot spots are more concentrated around the drilling. The increase of joint density leads to more complex crack distribution, and the distribution range of acoustic emission hot spots is expanded and the number is increased; (4) The joint density has a weakening effect on the elastic energy storage of coal specimens, and this weakening effect decreases with the increase of drilling diameter. and drilling affects the way of energy dissipation.