Evaluation of drillability and fracture mechanisms for different rock types under the penetration of spherical and conical inserted teeth

The efficient fragmentation of deep hard rock presents a critical challenge in shaft sinking by the drilling method. To investigate the performance differences in rock-breaking between two fundamental insert tooth types, spherical and conical teeth, sedimentary rocks (limestone, green sandstone), igneous rocks (granites A, B, and C), and metamorphic rocks (marble, slate) were investigated. A self-developed high-stress single-insert-tooth penetration test system was used to evaluate drillability and analyze the fragmentation mechanism. First, a brittleness index (B_I) prediction model applicable to both tooth types was established based on the uniaxial compressive strength (σ_c), tensile strength (σ_t), and specific gravity (γ) of the rocks. Subsequently, quantitative correlations were established between the B_I and key metrics, including the specific energy of rock breaking (S_E), the average fragment size (d_m), and the fractal dimension (D_m). The fragmentation mechanism was further elucidated through integrated macro-, meso-, and micro-scale observations. The main findings are: (1) Drillability is jointly influenced by rock type and tooth shape. The brittleness index ranks as: igneous rocks (B_I= 6.05~8.73) > metamorphic rocks (B_I= 3.90~9.50) > sedimentary rocks (B_I= 3.47~5.22). For a given rock type, the B_I under spherical tooth action is, on average, approximately 21.5% higher than that under conical tooth action. (2) The specific energy (S_E) exhibits an exponential increase with rising B_I. Spherical teeth generally yield higher S_E values; for instance, their S_E in slate was measured to be 87% higher than that of conical teeth. (3) A concave functional relationship is observed between the average fragment size (d_m) and B_I. The B_I corresponding to the minimum d_m is higher for spherical teeth (9.33) than for conical teeth (4.49). When B_I exceeds 5, the fractal dimension (D_m) for conical teeth becomes significantly lower than that for spherical teeth, indicating a tendency towards coarser fragmentation. (4) The rock fragmentation mechanisms are fundamentally distinct. Spherical teeth primarily generate radial tensile stress, forming symmetrical and extensive crushing pits with an average fracture angle of 141.6°. In contrast, conical teeth induce stress concentration, readily promoting asymmetric fragmentation that propagates along weak planes, with an average fracture angle of 136.4°. (5) The rock fracture angle (ranging from 126° to 161°) initially decreases and then increases with the compressive-to-tensile strength ratio (σ_c/σ_t). An inflection point is identified within the σ_c/σ_t range of 18~19. This variation is governed by a transition in the failure mode, which is intrinsically controlled by the rock's microstructure. These results provide a theoretical basis for the selection and combination of insert teeth based on lithological characteristics, the optimization of rock-breaking parameters, and the achievement of high-efficiency, low-energy-consumption shaft sinking.