The influence of penetration and cutting speed of pipe-jacking disc cutter on its cutting force and rock-breaking efficiency

The disc cutter, which is essential to pipe-jacking to break rock, is the main tool used in this crucial method of tunnel building. The present research is focused on the force of the disc cutter and how well it breaks rock. This paper uses the medium-weathered granite of the Xinjing Mountain Substation - Beifeng Substation as an example to study the effects of penetration and cutting speed on the force and rock-breaking efficiency of the disc cutter. It does this by using ABAQUS to analyze the disc cutter’s operating state under various penetration and cutting speeds and to find the ideal rock-breaking efficiency. The findings demonstrate that: (1) Breaking rock is a discontinuous process that advances continuously. The rock unit beneath the disc cutter ring will change from an elastic to an elastic condition as the disc cutter moves. The rock unit beneath the disc cutter ring will undergo continuous extrusion as the disc cutter moves, changing from an elastic to a plastic state until failing as a result of damage. (2) All of the cutting forces rise as penetration and cutting speed increase, but the rolling force is more affected by penetration and vertical force by cutting speed. When the disc cutter breaks in a straight line, the side force is little. (3) The rock-breaking efficiency increases with a 4 mm penetration and a faster cutting speed.


Introduction
Advanced tunnel construction tools like pipe jacking are frequently utilized in large-scale projects like river-sea tunnels and underground urban developments.The equipment is considered a significant national asset.The effectiveness and expense of the construction process are greatly impacted by the disc cutter, a crucial part of pipe jacking for tunneling [1].But disc cutter rock-cutting is a very complicated process, and engineering study on cutter rock-cutting mechanisms has long been a contentious and challenging topic.To increase the effectiveness and economy of pipe-jacking excavation, it is crucial to understand the force law and rock-breaking law of the disc cutter.To increase the effectiveness and economy of pipe-jacking excavation, it is crucial to understand the force law and rock-breaking law of the disc cutter.The cutting forces over a disc cutter have typically been broken down into three orthogonal directions in previous studies: the side force, which is perpendicular to the disc cutter's cutting track (FS), the rolling force parallel to the disc cutter's rolling direction (FR), and the vertical force perpendicular to the rock surface (FV) [2].The majority of research in this field was theoretical.
This study proposed a new model to predict the rolling and normal forces occurring on disc cutters of the CCS type [3].The reference [4] addressed the interpretation of the cutting forces of the open pipe-jacking on contract H33 of the Brenner Base Tunnel from a geological and geotechnical perspective.The geological documentation is examined and analyzed with the spatially distributed disc forces.
Many academics are currently conducting experimental studies and numerical simulations.For example, Reference [5] conducted linear disc-cutting experiments utilizing full-size disc cutters that had grooves.To shed light on the mechanism and process of rock-breaking, reference [6] established a two-dimensional planar mechanical model of rock-breaking with twin cutters to ascertain the stress distribution law in the rock mass.The validity of the existing mechanical model is confirmed by contrasting its outcomes with the findings of two-arc indenter indentation experiments.
Large instruments and a long enough disc cutter running distance are necessary for doing experimental research; theoretical calculations are more involved and time-consuming.The research is time-consuming, expensive, and challenging.Consequently, ABAQUS is used in this study for numerical simulation.In a short amount of time, this technology can more realistically and conveniently recreate the motion process of the disc cutter rock-breaking operation.

Geometric modeling and meshing
The disc cutter is chosen as a typical 18-inch cutter with rock specifications of 1000x400x300 (length x width x height), as shown in Figure .1(a).Using C3D8R (linear reduced integration) for the disc cutter (1606 elements and 2409 nodes), the rock was divided into 15000 elements and 17136 nodes from Figure. 1 (a).mainly a section of the shield excavation face was chosen to replicate and analyze the rock sample since rock-breaking mainly impacts the area around the contact surface during the cutting operation.
For the motion constraint equation, the penalty contact approach was chosen, and for the slip equation, the finite slip method was chosen.The friction factor of tangential behavior was found to be 0.2 in the interaction module where the contact interaction attribute was defined.Furthermore, the friction formula was derived from the penalty function, and typical behavior was determined by hard contact.The equivalent stress is represented by σy, the stress at the beginning of the rock's damage, the equivalent plastic strain corresponding to σy is represented by ε0pl, and the equivalent plastic strain is represented by ε0pl when the rock is shattered.Inside the rock, microcracks start to show up when the material reaches the stress of the original damage.The material is consequently softer due to deterioration, that is, (1) The software modifies the boundary conditions for recalculation and deletes the rock unit from the grid when D1 = 1, destroying the rock entirely.This process continues until a new equilibrium state is obtained.For assessing rock materials, ABAQUS's Drucker-Prager model is a great option (Table 1).We utilized Xinjing Mountain Substation -Beifeng Substation Medium weathered granite as the research object for rock-breaking, assuming that the rock is homogenous and isotropic.Its specifications are stated in Table 1, and the disc cutter's modulus of elasticity is 210 GPa.

Analysis procedure, load setting, and working condition setting
The excavation velocity influences the disc cutter's penetration depth during a shield pipe-jacking excavation to some degree.A greater excavation velocity causes the disc cutter to penetrate deeper in shorter amounts of time.Nonetheless, the disc cutter's ability to crush rock varies depending on the depth of penetration.illustrates how the cutting force of the disc cutter varies within a specific range when the rock breaks.This is because the rock-breaking process is a discontinuous process that advances rapidly.As the disc cutter moves, the rock unit beneath it will change from an elastic to a plastic state as a result of continuous extrusion, and eventually fail as a result of damage developing.Through comparison, it is not difficult to find that, on the one hand, with the increase of the penetration degree of the disc cutter's cutting force average also increases, penetration degree of 2mm when the vertical force of the disc cutter is 210.567kN,penetration degree of 4mm and 6mm when the average value of the vertical force were 350.305kN and 500.934kN;and when the penetration degree of 8mm when the average value of the vertical force is greater than 2mm when the triple, up to 700.219kN, and the average value of the vertical force is more than 2mm when the disc cutter is in the vertical force.This is because, as the disc cutter penetration increases, the contact area between the cutter ring and the rock increases.Consequently, the vertical force increases as well as the vertical projection area.Conversely, as penetration grows, so does the vertical force's peak value, and the degree of variation increases in intensity.As a result, the single penetration in this project needs to be kept under 4 mm.Because this work assumes that the disc cutter is used for straight-line cutting without inclination, rolling power, and vertical force change laws are the same and increase with penetration.In contrast, lateral force is essentially equal to 0. It is simple to compare and see that, on the one hand, the average vertical force of the disc cutter grows with an increase in the cutting element, albeit at a very slow rate.Of course, the rolling force is more directly influenced by the cutting speed; at 200 mm/s, the rolling force of the disc cutter is 41.568 kN, and at 600 mm/s, it is 175.895kN.The average vertical force at 1000 mm/s is approximately 6 times that of the 2 mm setting, reaching 330.289 kN.This is primarily because of the increased cutting speed, which will cause the tangential resistance to cut more quickly and produce a significant increase in rolling force.Since the lateral force in this instance is still very small, it may be concluded that the disc cutter's lateral force cannot be taken into account in the context of the rock-breaking straight line.As a result, it's crucial to avoid using excessive cutting rates in the project since this could harm the disc cutter.

Rock-breaking efficiency index
Another crucial metric for assessing the pipe-jacking cutter's effectiveness in cutting rock is the cutting coefficient (CC).Cutting coefficient (CC) is the ratio of a disc cutter's rolling force to vertical force [9]

Conclusion
This study uses the finite element program ABAQUS to simulate the medium-weathered granite breaking process at the Xinjing Mountain Substation and Beifeng Substation.It provides information on the penetration, cutting speed, cutting force, and rock-breaking efficiency of the law's influence.
(1) The cutting force of the disc cutter varies continuously during the rock-breaking process within a specific range.This is because the rock-breaking process is continuously discontinuous and advances.The rock unit beneath the disc cutter ring will undergo continuous extrusion as the disc cutter moves, changing from an elastic to a plastic state until failing as a result of damage.
(2) The single-turn penetration degree of the pipe-jacking <=4mm must be controlled because it has the biggest impact on the vertical force.The disc cutter's vertical force increases significantly when the penetration degree exceeds 4mm.The rolling force of a disc cutter is mostly determined by its cutting speed; at 1000 mm/s, the rolling force is six times greater than that of a cutter operating at 200 mm/s.The rolling is six times faster than the disc cutter's cutting speed, which is equal to 1000 mm/s.
(3) At a 4 mm penetration, the computed rock-breaking efficiency index (CC) reaches its maximum and rises as the cutting speed increases.

Figure 1
Rock-breaking Model 2.2.Physical parameters and constitutive model Rock constitutive model [7][8]: The failure and deletion function of the unit in ABAQUS can be used to simulate rock-breaking and falling off during the rock-breaking process by the disc cutter.The introduction of the plastic damage-destruction model of rock was necessary for the accomplishment of this function.The damage parameter in Figure.3 is represented by D1.

Figure 4
Figure 4 Cutting force diagram Working condition 1: Because the linear cutting velocity has little effect on the forces acting on the disc cutter and rock-breaking efficiency, this study maintained a constant disc cutter cutting velocity at 1000 mm (1 m)/s to investigate the variation law of the vertical force, rolling force, and side force (see in Figure.4) of the disc cutter under different penetration degrees.For one second, the rock was chopped linearly by the disc cutter.The following were the working circumstances for the calculation: as shown in Figure.5(a) the rock was cut in a linear motion by the disc cutter at penetration depths of 2, 4, 6, 8, and 10 mm.Working condition 2: This study's primary set penetration at 4 mm to investigate the variation law of the disc cutter's side, rolling, and vertical forces at varying cutting speeds.For one second, the rock was chopped linearly by the disc cutter.The disc cutter was used to cut the rock in a linear motion at speeds of 200, 600, and 1000 mm/s.These were the operating conditions for the calculation (see Figure 5(b)).

Figure 5 Figure 6 .
Figure 5 Disc cutter penetration -rolling process setting

Figure 7
Figure 7 Plot of the effect of cutting speed on rock-breaking force

Figure 8
Figure 8 Calculation of rock-breaking efficiency