Investigation of the Influence of Cutter Geometry on the Cutting Forces in Soft–Hard Composite Ground by Tunnel Boring Machine Cutters

Abstract

Tunnel Boring Machines (TBMs) are integral to modern underground engineering construction, offering enhanced safety and efficiency. However, TBMs often face challenges in complex geological conditions, such as composite strata, resulting in reduced advancement speed and increased cutter wear. This study investigates the rock-breaking characteristics of TBM disc cutters in composite strata through numerical simulations using the Particle Flow Code (PFC) 5.0 software. Focusing on the Jinan Metro Line 6, the research analyzes cutter forces, rock crack propagation, and the impact of cutter edge shapes on rock-breaking efficiency. The discrete element method (DEM) is employed to simulate microscopic behaviors of rocks, providing insights into crack formation, expansion, and failure. This study’s findings reveal that cutter design and operational parameters can significantly influence cutter lifespan and efficiency. By modifying cutter spacing and penetration depth, enhancing rock-breaking efficiency, and grouting softer layers, TBMs can maintain effective excavation in composite strata. The study establishes a comprehensive understanding of the interplay between TBM cutters and complex geological conditions, offering actionable strategies to enhance TBM performance and mitigate cutter damage.

1. Introduction

In underground engineering construction, Tunnel Boring Machines (TBMs) are extensively utilized for tunnel construction due to their high efficiency and safety [1,2]. However, as engineering projects increasingly confront complex geological conditions, including fault zones, frequent stratigraphic changes, and composite strata of soft and hard layers, the difficulties encountered during TBM construction have become increasingly pronounced [3,4]. These complex strata conditions often lead to reduced advancement speed, intensified cutter wear, decreased stability of the excavation face, and extended construction periods [5]. Therefore, studying the rock-breaking characteristics of TBMs in these special strata holds significant practical importance for improving machine performance and tunnel construction efficiency.

During the advancement of a TBM, disc cutters come into contact with rock and apply pressure, thereby generating zones of stress concentration in the rock [6]. When the stress reaches the rock’s critical strength, a zone of plastic damage forms, accompanied by the generation of cracks. The rock-breaking process of TBMs and the load of disc cutters are shown in Figure 1. In composite strata of soft and hard layers, this rock-breaking mechanism becomes even more complex. Existing studies, such as those by Yang et al. [7], have indicated that the rock’s transverse isotropy and the interface inclination angle significantly affect the rock-breaking behavior. Munoz et al. analyzed the stress–strain characteristics of rock through uniaxial compression tests and explored the rock-breaking behavior under different interface conditions [8]. Zhang et al. focused on the force and failure modes of disc cutters under composite ground conditions, noting that changes in geological conditions could lead to increased eccentric moment of the disc cutters and a higher risk of tool damage. Zhao explored the impact of penetration depth on cutting composite strata with disc cutters by establishing a comprehensive three-dimensional disc cutter rock-breaking model and analyzed the causes of abnormal cutter damage [9]. Rostami et al. summarized the main concepts of cutterhead design and offered a better solution for cutterhead design, which must ensure the balance of the cutterhead forces. This can significantly improve the excavation efficiency and reduce tool maintenance costs [10,11].

Nevertheless, full-scale tests involving Linear Cutting Machines (LCMs) or Rotary Cutting Machines (RCMs) conducted under laboratory conditions cannot adequately mimic the complexities of on-site conditions. Moreover, such tests often prove to be both costly and time-intensive [5,12,13]. Therefore, scholars have shifted their focus to numerical simulation techniques to investigate the dynamics of rock fragmentation under disc cutter cutting conditions [14,15,16]. Although commercial software such as ABAQUS and ANSYS have gained popularity in simulating the rock-cutting process with disc cutters, these finite element method (FEM)-based models typically rely on element removal to simulate rock breakage, which does not offer a precise representation of actual crack formation and propagation. Specifically, while FEM software excels in providing comprehensive mechanical insights, including force distributions on the cutter and stress patterns within the rock, they often lack the capability to offer the intuitive and detailed visualization of fracture progression that discrete element methods can provide [17,18].

Particle Flow Code 3D (PFC3D), on the other hand, utilizes the discrete element method (DEM), which simulates the mechanical behavior of rocks by modeling the interactions between individual rock particles. This approach is particularly suitable for simulating complex phenomena like rock fracture and granular flow, offering a realistic representation of rock fracture behavior during the disc cutter rock-cutting process [19,20]. PFC3D not only excels in its ability to visualize crack development in detail but also provides efficient and convenient parameter adjustment, making it a valuable tool for researching disc cutter design, optimization, and tunneling parameter adjustments.

In this study, PFC3D 5.0 software is employed to numerically simulate the challenges encountered in the composite strata of the Jinan Metro. The aim is to analyze the main issues and their causes in the cutting process of composite strata, specifically focusing on cutter cutting force and rock crack expansion. Furthermore, by comparing the rock-breaking effects of disc cutters with different tip shapes, this research offers references and guidance for shield tunneling through composite strata, aiming to fill the current gaps in the literature and provide feasible solutions to problems encountered in actual construction.

2. Engineering Challenges

The characterization and classification of composite strata conditions lack a unified definition and systematization. From a geological perspective, composite strata conditions refer to tunnel excavation ranges where two or more geological structures with distinct geomechanical and hydrogeological properties, or structures with the same weathering grade but different characteristics, coexist. For simplicity, some researchers have suggested using the uniaxial compressive strength (UCS) as a direct reference for defining mixed-face conditions, where the UCS ratio between the weakest and the strongest material is less than 1/10. However, this approach overly simplifies the complexity, neglecting critical operational and performance parameters of the TBM. Toth et al. considered TBM penetration, operational parameters, and the assessment method for the support system installed behind the TBM [21].

Three principal factors affect tunnel excavation efficiency: (a) rock compressive strength, (b) rock integrity, and (c) rock abrasiveness [22]. When TBMs operate in composite strata, the variability in rock strength significantly increases the difficulty of tunnel excavation. Taking the Jinan Metro Line 6 as an example, the strata mainly include silty clay, strongly weathered diorite, and moderately weathered diorite. In the tunnel crossing area, these rock layers exhibit undulating characteristics, forming a composite geological layer with soft upper and hard lower sections—such geological conditions are quite common in the Jinan Metro projects. Figure 2 presents the geological overview of the Jinan Metro Line 6 area with relevant details, while the main strata mechanical parameters are shown in

Table 1. Main-formation mechanics parameters.

Stratum	Stratum Designation	Rock Quality Designation (%)	UCS (MPa)	Cerchar Abrasivity Index
Highly weathered diorite	9	0~14	4~20	0.2~1.6
Moderately weathered diorite	10	30~75	21~152	1.7~3.9

and the tunnelling parameters in different sections are shown in

Table 2. TBM operational parameters under different geological conditions.

Stratum	Advance Rate (mm/min)	Cutterhead Rotational Speed (r/min)	Thrust (t)	Torque (kN·m)
Highly weathered diorite	20~45	1.5~2.0	800~1300	1200~1400
Highly and moderately weathered diorite mixed-face ground	10~25	1.0~1.6	1500~2050	1400~1900

.

Compared to homogeneous geological conditions, the TBM’s excavation speed in composite strata may decrease, with a wider range of fluctuations in operational parameters leading to instability. The principal causes of such instability during excavation in composite strata include the following: (a) significant differences in the strength of the upper and lower rock layers, resulting in substantial variations in TBM tunneling speed, and (b) an uneven distribution of forces on the disc cutters within the composite strata, leading to the generation of substantial overturning moments [23].

Figure 3 displays the cutterhead layout of the Earth Pressure Balance (EPB) TBM used in a Jinan Metro tunnel. In this project, occurrences of abnormal wear, such as flat wear, cutter blade chipping, and cutter ring cracking, were frequently observed, as shown in Figure 4. Compared to the wear conditions of TBM cutters in traditional single-layer strata, flat wear is more pronounced, and the proportion of cutter damage is higher in composite strata. Typically, the causes of abnormal cutter wear include the following: (a) a diminished rolling force of the cutter in soft rock, resulting in inadequate rotational torque and failure to rotate properly; (b) a low advancement rate, with spoil clogging the cutterhead chambers, forming “cakes” under temperature and pressure, preventing normal cutter rotation and leading to severe abnormal wear; and (c) cutters frequently colliding with hard rock at the interface with soft rock, which may lead to cutter overload, resulting in ring cracking and damage to bearings and cutter holders.

To analyze the impact of aberrant parameters on cutter wear, this study takes the average operational parameters of each tunneling cycle without considering the variability during the TBM tunneling process. These parameters are used to establish a three-dimensional PFC model of rock fragmentation by cutters in composite strata, analyzing the influence of abnormal parameters on wear from the perspectives of cutter mechanics and rock fracture propagation.

3. Description of the PFC Model

3.1. Bonded Particle Model (BPM) Implementation

The PFC3D software allows adjacent particles to bond at contact points through a model known as the Bonded Particle Model (BPM). Among its implementations, the parallel bond model is particularly effective in transmitting forces and torques, as well as in capturing the mechanical behavior of rock under load, including the initiation, propagation, and fracturing processes [24]. Therefore, this study utilizes the parallel bond model to simulate rock behavior. As illustrated in Figure 5, the parallel bond model is effective within a rectangular region at the interface of touching particles. In this model, stiffness is determined by both the linear elastic interface and the linear elastic bond interface. The bond exhibits resistance to both rotation and shear, effectively transmitting forces; however, when the load exceeds the critical strength threshold, the parallel bonds break under stress, rendering the load un-transmittable and reducing model stiffness, consistent with the mechanical characteristics of rock.

The PFC modeling environment facilitates the creation of virtual representations of rock, capturing its natural heterogeneity and anisotropy. Particles are generated within a predefined space following a specified size distribution to mimic the granular nature of rock. These particles are then bonded using the parallel bond model, which provides normal and shear stiffness at contact points, facilitating the transmission of forces and torques between particles.

The PFC program is grounded in Newton’s Second Law and the force–displacement law, continually updating particle displacement and velocity during computations based on the Second Law. The contacts between particles, or between particles and walls, are updated using the force–displacement law, as shown in Figure 6. There are two types of motion: (1) particle rotation, as described by Equation (1), and (2) particle translation, as described by Equation (2).

$$M=I\alpha$$

(1)

$$F=ma$$

(2)

Herein, M represents the torque applied to the particles, I denotes the moment of inertia, α signifies the angular acceleration, F stands for the force applied to the particles, m indicates the mass of the particles, a refers to the acceleration, i and j are the indices of the particles, S_n represents the normal stiffness of the bonds, S_t denotes the tangential stiffness of the bonds, c is the cohesion, and φ is the internal friction angle.

3.2. Calibration and Validation

Prior to embarking on the principal simulations within the PFC environment, a rigorous calibration and validation process is essential for the micro-properties of the rock model. Given that direct, simplistic correlations between the microscopic parameters of the particle flow model and the macroscopic physical and mechanical properties of materials do not exist, an iterative process is employed to adjust the model’s microscopic parameters [25]. This adjustment aims to ensure that the macroscopic behavior of the simulated material aligns with the actual macroscopic behavior observed in rock samples. The calibration involves matching the outcomes of numerical simulations with laboratory test results, refining the properties of particles and bonds until a satisfactory degree of conformity is achieved.

In this study, laboratory physical tests were utilized to obtain the rock’s elastic modulus, uniaxial compressive strength (σ_c), and tensile strength (σ_t), with specific parameters detailed in

Table 3. Mechanical parameters of soft and hard rocks from laboratory tests.

Rock	Young’s Modulus (GPa)	Uniaxial Compressive Strength (MPa)	Tensile Strength (MPa)
Slightly weathered diorite (hard rock)	35.2	137.1	7.53
Highly weathered diorite (soft rock)	0.75	7.48	1.29

. Subsequently, by fine-tuning the model’s microscopic parameters and comparing the simulation outcomes with experimental data, the model’s predictive capability is verified. Once calibrated and validated, the BPM becomes a potent tool for investigating the complex mechanics of rock fragmentation by TBM cutters in composite strata.

In the PFC framework, the size of the model’s “spherical particles” significantly influences the calibration of macroscopic parameters. Therefore, the calibration model should select the diameter of “spherical particles” judiciously based on the overall model size. This was accomplished through uniaxial compression tests with a diameter of 400 mm and a height of 800 mm, as well as Brazilian split tests with a diameter of 300 mm and a height of 300 mm, thereby achieving a correlation calibration between macroscopic and mesoscopic parameters. Figure 7 presents the stress–strain curves and failure modes for strongly weathered diorite and moderately weathered diorite.

Table 4. Microscopic parameters of soft and hard rocks in numerical models.

Rock	Density	Emod	Kratio	pb_ emod	pb_ kratio	pb_coh	pb_ten	pb_fa	fric
Slightly weathered diorite (hard rock)	2800	16	2	16	2	98	37	30	0.5
Highly weathered diorite (soft rock)	2650	0.5	2	0.5	2	15.8	5.4	35	0.5

and

Table 5. Mechanical parameters of soft and hard rocks in numerical models.

Rock	Young’s Modulus (GPa)	Uniaxial Compressive Strength (MPa)	Tensile Strength (MPa)
Slightly weathered diorite (hard rock)	36.2	130.0	8.55
Highly weathered diorite (soft rock)	0.86	7.45	1.24

list the mesoscopic parameters of the particle motion model and the macroscopic parameters, respectively.

3.3. Modeling of Rock Specimens and Disc Cutters

Previous research has demonstrated that linear rock-cutting schemes using disc cutters can effectively predict the actual rock-cutting behavior during TBM tunnel excavation. Consequently, this study employs a rectangular specimen for the linear cutting experiment, aiming to eliminate the influence of rock boundaries on the cutting trials. Preliminary numerical simulations were conducted to determine optimal model dimensions, ensuring a thorough examination of the mechanical characteristics of disc cutters and their interaction with crack development in composite strata.

As illustrated in Figure 8, during the preparatory simulation experiments, the specimen is positioned within a walled box. This box features a bottom surface along with left and right side walls that confine the specimen, while the front and back walls maintain a 50 mm open space. This open-space accommodation is crucial for allowing the disc cutter to engage, thereby ensuring reproducibility and consistency across all simulation tests. The simulation results, depicting the crack propagation in the mixed rock specimen, are shown in Figure 9. The findings reveal notable details about crack propagation: in the hard rock, lateral cracks extend laterally by about 34 mm at the penetration point, with a vertical crack propagation depth of approximately 42 mm. In the soft rock, the lateral cracks extend laterally by about 44 mm at the penetration point, accompanied by a vertical crack extension depth of about 59 mm.

Building on these insights, the present study adopts a rectangular model with dimensions of 800 mm in length and 400 mm in both height and width to analyze the crack propagation in rock and to assess the impact of cutter spacing on mixed faces. The particle parameters are consistent with those of the calibration parameters; within the 0 to 400 mm range on the x-axis, soft rock parameters are applied, while from 400 to 800 mm, hard rock parameters are used. In tunneling projects, the common size for TBM disc cutters is a diameter of 432 mm, as shown in Figure 10. Different cutter profiles can significantly affect rock-breaking efficiency; hence, various cutter profiles are modeled, as depicted in Figure 11. In reality, disc cutters are manufactured from high-strength alloys, while in the numerical software, cutter geometry is replicated using wall elements after mesh division to simulate the action of disc cutters. As the disc cutter advances and rotates, it carves a groove into the rock surface, initiating its cutting action in the softer rock before progressing into the harder rock. To elucidate the rock-breaking conditions post-cutting, simulations were set up with various penetration depths of 2, 4, and 6 mm. This step is crucial to understand the interaction between the cutter and the rock, as well as the consequent fragmentation process, under different operational scenarios.

4. Numerical Analysis Results

4.1. Cutter Force Analysis

The force exerted by the disc cutter during rock cutting not only affects rock-breakage efficiency and cutter lifespan but is also a critical factor in achieving rapid tunneling progress. Therefore, the “history” command is utilized to record the forces acting on the disc cutter in the X, Y, and Z directions during the rock-cutting process. These forces are then translated into normal force, rolling force, and lateral force on the cutter, as illustrated in Figure 12, Figure 13 and Figure 14. The graphs reveal the following observations: (a) The triaxial forces on the cutter fluctuate significantly with the cutting distance. This fluctuation is attributed to the drop in cutter force following rock fracture and a subsequent increase as the cutter continues to break the rock. (b) The forces on Cutter 1 and Cutter 2 are similar, with the average force in the harder-rock section being significantly higher than in the softer rock. (c) When transitioning from cutting soft to hard rock, there is a sharp increase in normal force, rolling force, and lateral force, accompanied by impact.

As the penetration depth increases, the normal force on the disc cutter gradually intensifies. At a penetration depth of 2 mm, the maximum normal force is 167 kN, which escalates to 250 kN at a depth of 4 mm. Continuing to increase to 6 mm, the peak force reaches 314 kN, exceeding the rated load capacity of 250 kN for the cutter. This indicates that TBM tunneling in composite strata is best conducted at lower penetration depths. Meanwhile, the rolling force and lateral force also rise with increased penetration depth, although to a lesser extent compared to the normal force. During excavation in soft-rock strata, if the rolling force is too small and the cutter’s starting torque is set too high, it can lead to the continuous sliding and cutting of the soft rock on one side, causing flat wear on the cutter. The reasonable setting of cutter penetration can optimize tool life and maintain efficient and effective tunnel construction under different geological conditions.

4.2. Crack Propagation in Rock Specimens

Figure 15 depicts the distribution of transverse cracks within the rock specimen for penetration depths of 2, 4, and 6 mm. A discernible contrast exists between the extent of crack propagation in soft and hard rocks. The development of cracks in soft rock is significantly more extensive than in hard rock. At a penetration of 2 mm, cracks in the soft rock between the disc cutters have already interconnected, indicating effective rock breakage, whereas in the hard rock, the lateral expansion of cracks is limited to 17 mm, which is insufficient for rock fragmentation. As the penetration depth increases to 4 mm, the cracks in the soft rock are completely interconnected. In the hard-rock section, while the transverse cracks continue to progress with some areas interconnecting, there remain large regions where cracks do not propagate fully; this partial penetration is less efficacious for rock breaking. Further increasing the penetration to 6 mm leads to the interconnection of transverse cracks in both soft- and hard-rock sections, achieving the aim of rock fragmentation in a single cut. However, due to the force on the cutter in the hard-rock section exceeding its load limit, high penetration depths are not recommended for excavation.

Figure 16 presents an analysis of the development of vertical cracks along sections A-A and B-B, examining the depth of crack propagation along the excavation direction of the tunnel. The vertical crack depth in rock also increases with penetration depth. At a penetration depth of 2 mm, vertical cracks in soft rock measure 5.5 cm, while in hard rock, they are approximately 2.6 cm. This suggests that the soft-rock region is more significantly disturbed, increasing the likelihood of instability during tunnel excavation. As the penetration depth is increased to 6 mm, the depth of cracks in soft rock increases by 1.3 times, whereas in hard rock, it increases by 1.7 times, indicating that higher penetration depths have a marked effect on excavation in hard-rock strata. It should be ensured that higher penetration depths are maintained within the load limits of the disc cutters.

4.3. The Impact of Cutter Edge Shape on Rock-Breakage Effectiveness

To compare the impact of cutter edge shapes on rock-breaking efficiency, rock-cutting experiments were conducted using Cutters A, B, and C at a penetration depth of 4 mm. Figure 17 displays the development of transverse cracks after cutting with different cutter edge shapes. It is observed that in soft rock, crack development is extensive across all cutter types, with the cracks fully interconnecting. However, when cutting hard rock, Cutters B and C demonstrate more complete crack development, with Cutter B achieving the most comprehensive crack propagation, effectively meeting the goal of single-pass rock fragmentation. Furthermore, Cutter B exhibits a similar extent of transverse crack disturbance in both hard and soft rock, which could reduce inconsistencies in rock damage between different rock types.

Figure 18, Figure 19 and Figure 20 present the normal, rolling, and lateral forces experienced by the different cutters. Cutter B exhibits significant fluctuations in normal force, with peak forces exceeding 250 kN, whereas Cutter C’s maximum normal force is 246 kN. This suggests that while Cutter B achieves better rock-breaking performance, it also experiences increased force fluctuations, potentially exceeding the cutter’s operational lifespan limit. On the other hand, Cutter C achieves an effective rock-breaking performance while minimizing cutter force, suggesting it could be a more sustainable option. However, Cutter C experiences greater fluctuations in lateral force, which could lead to excessive imbalances in lateral forces and potentially cause the chipping of the cutter edge.

Specific energy (SE) is an important index to evaluate the rock-breaking efficiency of disc cutters [26,27,28]. SE is defined as the energy consumption per unit volume of rock broken by the cutter. The calculation formula is shown in Equation (3). During the simulation process, a particle contact detection module was established through the FISH language to judge the contact force of particles. When the particle contact force is 0, it is determined that the rock particles are peeled off, and the cumulative peeling particles are recorded as the rock damage quality V. Combined with the rolling force F_r and rolling distance l recorded by the software, the SE of rock breaking of disc cutters with different blade forms is calculated, as shown in Figure 21. It can be seen that when the penetration is 4 mm and the ratio of cutter spacing to penetration is 20, the SE is the lowest, and the rock-breaking efficiency is higher. Disc Cutter C has the lowest specific energy and the highest rock-breaking efficiency, which is also consistent with the law of crack development. Disc Cutter C has the most abundant crack development, and its completed rock-breaking volume is the highest, which improves the rock-breaking efficiency.

$$SE={\displaystyle \frac{F_{r}l}{V}}$$

(3)

5. Discussion

5.1. Causes of Abnormal Damage to Disc Cutter

The occurrence of the severe wear and chipping of disc cutters is prevalent in composite strata characterized by softer material above and harder material below. When tunneling through hard-rock strata, disc cutters must overcome the startup torque and blockage caused by the stratum. In layered strata with soft clay on top, the clay tends to adhere inside the cutterhead, leading to the formation of mud cakes, which prevent the cutters from rotating. When the cutter rotates into the harder rock below, the mechanism of rock-cutter interaction shifts from rolling friction to sliding friction, resulting in rapid cutter wear. As shown in Figure 22, when the penetration rate increases, not only does the normal force increase, but the volatility of the force on the cutter also increases, which leads to frequent changes in the effective stress of the blade and the probability of fracture of the cutter blade.

Moreover, to prevent the occurrence of cutter blade chipping in such layered ground conditions, penetration depths are reduced. This adjustment necessitates multiple rotations to achieve crack interconnection in hard rock, thereby increasing tool wear. The challenge lies in managing the transition between these vastly different materials, which requires a delicate balance between force application to break the rock and tool preservation to avoid excessive wear or damage.

5.2. Measures to Reduce Damage to Disc Cutter

From the perspective of lateral crack propagation in rock, Cutter A, while ensuring that the force exerted is below the rated load, is unable to achieve complete crack interconnection when cutting in mixed-face conditions. In this case, switching to different cutter designs, such as Cutters B or C, may enhance rock-breaking efficiency. Additionally, reducing the spacing between the disc cutters can facilitate crack propagation, allowing effective rock fragmentation at lower penetration depths while keeping the force on the cutters below their rated load. However, it can be seen from Figure 23 that the force stability of Cutter A is better, but its average force is higher. Cutter B has the largest fluctuation, and its maximum value fluctuates more, which is not suitable for construction in mixed formations.

To mitigate the adverse effects of soft-top and hard-bottom strata on construction, it can be beneficial to reinforce the softer upper layer through grouting. This reduces the continuous impact on cutters at the interface between soft and hard materials. On this foundation, selecting a cutter design that is better suited for breaking hard rock can lead to more efficient rock fracture, address stability issues caused by the variation in strata hardness, and ultimately extend the life of the cutters.

6. Conclusions

This study addresses the complex challenges faced by TBM disc cutters when tunneling through the composite strata of Jinan Metro Line 6. A detailed investigation of the rock fragmentation process was conducted using numerical simulation methods. The research calibrated and validated numerical models against the physical properties of rocks and simulated the rock-breaking behavior of disc cutters under complex geological conditions. The main findings of this study are as follows:

(1)

When disc cutters cut through rock in mixed-face conditions, the triaxial forces experienced by cutters in hard rock are significantly greater than in soft rock. Under conditions of low penetration depth, the cracks induced by cutters in hard rock do not interconnect, necessitating multiple cuts to achieve rock fragmentation. The disturbance zone in soft rock is significantly larger than in hard rock, potentially leading to excessive overturning moments on the cutterhead.

(2)

The shape of the disc cutter blades has a significant impact on rock fragmentation efficiency and the stability of the excavation face. Although Cutter B exhibits considerable force fluctuations, it demonstrates superior rock-breaking performance, whereas Cutter C offers a more balanced approach, with less force exerted on the cutter.

(3)

Measures such as optimizing cutter spacing, adjusting penetration depth, and reinforcing the ground with grouting could potentially improve TBM performance in composite strata, reduce tool wear, and maintain efficient tunneling progress.