## 4 Discussion: Analysis of Factors Affecting Drilling Forces
### Influence of Rock Mass Structural Characteristics on Drill Bit Forces
Since the structural plane separates the rock mass symmetrically, as the drilling depth increases, the drill bit and the structural plane are always in contact at the middle part, and thrust force and torque decrease by basically the same extent. Clearly, the presence of structural planes significantly influences the mechanical properties of the rock mass. Therefore, utilizing real-time drilling monitoring methods can effectively identify the existence of rock structural planes and provide insights into characteristics such as structural plane inclination and thickness. Figure 16: Maximum principal stress cloud image with rock mass structure plane. (**a**) Structural surface inclination angle 45\({}^{\circ}\) (**b**) Structural surface inclination angle 90\({}^{\circ}\). Figure 15: Finite element mesh model with rock mass structure plane. (**a**) Structural surface inclination angle 45\({}^{\circ}\) (**b**) Structural surface inclination angle 90\({}^{\circ}\). The finite element mesh model for the rock mass with soft-hard interlayers is depicted in Figure 18. In this model, the upper rock layer has a thickness of 3 mm, while the lower rock layer has a thickness of 17 mm. The drilling process involves initial contact with the upper rock layer, followed by penetration through it and then contacting the lower rock layer. The distribution of soft and hard layers is considered in two scenarios: upper soft/lower hard rock layers and upper hard/lower soft rock layers. The same 75# drill bit is used, with drilling and rotation speeds set at 2 mm/s and 200 rpm, respectively. The drilling duration is set at 2.5 s. For the physical mechanical parameters, values are selected based on Table 1, with the physical parameters for the hard rock layer taken from rock mass quality Level II and those for the soft rock layer from rock mass quality Levels III and IV. Figure 17: Curve of thrust force, torque and thickness of the structural surface containing the rock mass structural plane. (**a**) Structural surface inclination angle 45\({}^{\circ}\) (**b**) Structural surface inclination angle 90\({}^{\circ}\). Figure 18: Finite element mesh model of alternatively distributed soft and hard rock layers. (**a**) Soft at the top, hard at the bottom (**b**) Hard on top, soft on bottom. Figure 19 shows the thrust force and torque curves of soft and hard interbedded rock mass. Drilling analysis of the upper soft/lower hard rock layer and upper hard/lower soft rock layer were carried out, respectively. The hard rock layer is assigned properties corresponding to rock mass quality Level II, while the soft rock layer properties are selected from rock mass quality Levels III and IV. Regardless of the upper soft/lower hard rock layer distribution or the upper hard/lower soft rock layer distribution, the thrust force and torque values will change suddenly at the interface of the two rock layers, and there is an obvious boundary. During the drilling process, owing to the recurrent cycle of adhesion, slippage, and re-adhesion between the drill bit and the rock, the thrust force and torque consistently oscillate within a specific range during the stable phase. As depicted in Figure 19a,b, when drilling in a rock formation with an upper layer of soft rock and a lower layer of hard rock, the drill bit initially comes into contact with the soft rock layer. After penetrating the upper soft rock layer, it reaches the lower hard rock layer, and drilling continues within the hard rock until completion. Upon the initial contact between the drill bit and the soft rock surface, significant impact pressure and torque are generated. Likewise, when the drill bit encounters the interface between the soft and hard rock layers, there is a notable increase in impact pressure and torque. This phenomenon is a result of the continuous action of drilling speed and rotational velocity during the transition from the soft rock surface to the hard rock layer. Once drilling stabilizes, both thrust force and torque remain within a certain range.