## 4 Discussion: Analysis of Factors Affecting Drilling Forces
### Effect of Rock Mass Confining Pressure on Drill Bit Stress  
Due to limitations in model size and computational capacity, it is not possible to directly simulate the entire drilling process. Therefore, it is necessary to apply confining  
Figure 19: Curve of thrust force and torque in drilling of alternatively distributed soft and hard rock layers. (**a**) Soft at the top (Level III), hard at the bottom (Level II) (**b**) Soft at the top (Level IV), hard at the bottom (Level II) (**c**) Hard on top (Level II), soft on bottom (Level III) (**d**) Hard on top (Level II), soft on bottom (Level IV). pressure at the model boundaries to simulate the in situ stress field in the rock mass. The study considers the variations in thrust force and torque when the drill bit penetrates under different Levels of static hydrostatic pressure and deviatoric stress. Consider the changes in the bit pressure and torque of the rock mass when drilling under different hydrostatic pressures and partial confining pressures. The effect of lateral confining pressure considers two different ways of drilling--vertical and horizontal. The finite element grid model with applied rock confinement pressure is shown in Figure 20. Normal confinement pressure is applied to the upper part of the rock, and radial confinement pressure is applied at the lateral boundaries. The bottom of the rock is subjected to fixed constraints. A 75# drill bit is used for drilling, and the drilling process is controlled by setting the drilling speed and rotation speed to 2 mm/s and 200 rpm, respectively, with a drilling duration of 2.5 s. In the case where the normal confinement pressure (\(\sigma_{z}\)) equals the radial confinement pressure (\(\sigma_{r}\)), the rock is in a state of hydrostatic pressure. Different static water pressures of 0 MPa, 2 MPa, 10 MPa, and 20 MPa are applied to the rock. Figure 21 depicts the thrust force and torque for the rock under various static water pressures. When drilling vertically, the normal confining pressure is equal to the vertical stress, and the radial confining pressure is equal to the horizontal stress. The rock is subjected to normal pressures of 0 MPa, 2 MPa, 10 MPa, and 20 MPa in the vertical direction and radial pressures of 0 MPa, 0.6 MPa, 2.9 MPa, and 5.8 MPa in the radial direction. Figure 21: Curve of thrust force, torque and confining pressure of rock drilled under hydrostatic pressure. (**a**) Thrust force-Confining pressure (**b**) Torque-Confining pressure. Figure 20: Finite element mesh model of rock mass for having confining pressure. When performing horizontal drilling, the normal pressure \(\sigma_{z}\) is equal to the horizontal stress \(\sigma_{h}\), and the radial pressure \(\sigma_{r}\) is equal to the vertical stress \(\sigma_{v}\). The rock is subjected to normal pressures of 0 MPa, 0.6 MPa, 2.9 MPa, and 5.8 MPa in the vertical direction and radial pressures of 0 MPa, 2 MPa, 10 MPa, and 20 MPa in the radial direction. With the increase in confining pressure, the thrust force and torque during the drilling process also increase accordingly. The wear experienced by tools and equipment in the course of technological processes within geotechnology and mining carries substantial economic repercussions. On one hand, there are material costs stemming from the replacement of worn-out tools and equipment. On the other hand, there are expenses related to work process downtime [45]. Abrasivity is intricately linked to tool wear, rendering it imperative to ascertain the abrasivity rate specific to individual geological materials, contingent upon the nature of their interaction with the tools [46; 47]. Numerous abrasivity tests have been meticulously developed within laboratory settings, establishing abrasivity as one of the most widely recognized index methods for evaluating the abrasive nature of rocks [45; 46; 47]. Abrasivity describes the ability of rocks to wear the surface of solid materials, primarily, but not limited to, metal.