1528 2.4 Experimental Arrangement The experimental arrangement is shown in Table 2. Table 2 - Test design of spherical and conical double-inserted teeth penetration rock No. Stress Environment Insert Type Penetration Velocity / mm·min-1 Penetration Depth /mm 1 axial pressure 20 MPa confining pressure 20 MPa spherical-spherical dual inserts conical-conical dual inserts 0.6 2.5 2 4.0 3 0.9 2.5 4 4.0 5 1.2 2.5 6 4.0 7 1.5 2.5 8 4.0 9 1.8 2.5 10 4.0 As shown in Table 2, a total of 30 sets of experiments were conducted, in which bluestone sandstone was penetrated by spherical-spherical and conical-conical dual inserts at different penetration velocities and depths, with an insert spacing of 10 mm (i.e., twice the insert diameter). The study aimed to investigate the relationship between the rock-breaking efficiency of dual-insert combinations with different geometries under in-situ stress conditions, and the effects of penetration velocity and depth. 2.5 Analysis Methods In response to the differences brought by spherical and conical insert shapes during the rockbreaking process, this paper employs the rock brittleness index to evaluate the difficulty of insert penetration, the load stability index to assess the fluctuation during penetration, and the rock-breaking efficiency index (along with indicators such as cuttings particle size distribution) to evaluate the work efficiency during insert penetration into rock. The specific analysis methods are as follows. (1) Rock Brittleness Evaluation Index. The rock brittleness index is an important indicator for judging the difficulty of rock failure under the action of an indenter. In engineering, the uniaxial compressive strength and Brazilian splitting strength of rock are commonly used for this purpose. Equation (1) presents three different strength ratios widely used in previous studies to indirectly quantify brittleness based on strength(Yagiz, 2009). (1) In the equation, σc represents the uniaxial compressive strength of the rock specimen, in MPa; and σt represents the Brazilian splitting tensile strength of the rock specimen, in MPa. This paper adopts the Rock Brittleness Index (BIm) proposed by Saffet Yagiz(Yagiz, 2009), which is based on rock sample tests from 48 tunnels worldwide. This index is defined as the ratio of the maximum indentation load (kN) on the specimen to the corresponding penetration depth (mm), as shown in Equation (2). (2) In the equation, Fmax represents the maximum indentation load, in kN; d represents the indentation depth/displacement, in mm. (2) Rock-Breaking Efficiency Evaluation Index. Specific Energy (SE) is a key metric for studying the efficiency of cutter rock-breaking. Equation (3) defines the concept of specific energy as the energy consumed to break a unit volume of rock. Based on specific energy theory, numerous researchers have ! " # " ! ! " ! " " ! " # # # ! ! ! ! ! ! ! ! " # = = = + ! ! !"# ! " #$ B =
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