Study on Drill–Rock Interaction During Joint-Surface Crossing Considering Longitudinal-Torque Effect

Investigation of the drill–rock interaction mechanism is fundamental to the design and optimization of drilling engineering. Existing theoretical studies have primarily focused on intact and homogeneous rock formations, whereas analytical studies of drilling in jointed rock masses remain limited. To address this issue, this study examines the drilling behavior of jointed rock masses and analyzes the forces acting on the drill bit during joint crossing. It is proposed that the non-uniform distribution of rock at the borehole bottom generates an additional longitudinal torque on the drill bit within the vertical plane. Based on whether this torque is considered, both static and dynamic mechanical models for drilling across a joint interface are established. Using parametric analyses, the evolution of weight on bit (WOB) and torque under the two modeling approaches is investigated, together with their sensitivity to differences in mechanical properties of rocks on either side of the joint. The results show that when the drill bit penetrates from hard rock into soft rock across a joint interface, both WOB and torque continuously decrease if longitudinal torque is neglected. In contrast, when longitudinal torque is considered, WOB and torque first increase and then decrease. The longitudinal torque increases both drilling parameters, with maximum increments in WOB and torque reaching 56.0% and 2.8%, respectively, indicating a more pronounced influence on WOB. As the mechanical-property differences between rocks on either side of the joint increase, the relative increments of WOB and torque, which characterize load fluctuation magnitude, initially increase and then gradually stabilize. The critical increments at stabilization are 0.73 for the internal friction angle and 7.17 for cohesion. These findings indicate that WOB and torque variations during joint crossing increase with increasing strength contrast across the joint interface, and that differences in internal friction angle exert a greater influence than cohesion. This study is primarily theoretical, and the proposed models are preliminarily validated through comparison with results in the literature. The developed analytical models reveal the drill–rock interaction mechanisms during drilling in jointed rock masses, clarify the influence of longitudinal torque on drilling parameters, introduce quantitative indices characterizing WOB and torque fluctuations, and establish their relationships with mechanical-property contrasts across joint interfaces. The findings provide a theoretical basis for interpreting drilling responses induced by geological discontinuities, evaluating jointed rock masses, and optimizing drilling parameter design.