Determining a Suitable Reinforcement Strategy for TBM Advance in a Gully Fault Zone Without Jamming—A Numerical Analysis

Abstract

This study aims to identify the most suitable pre-reinforcement support measures to prevent TBM jamming when passing through the fractured zone of a gully fault. Given the high likelihood of jamming in such areas, the research focuses on selecting the most effective support system by considering factors such as surrounding rock stability, strata displacement, support structure stress, and cost-effectiveness. Theoretical analysis is employed to predict TBM jamming risks, based on design data, a 10 m gully unit and fractured rock mass were established at 75 m in the excavation direction with assigned parameters. Support effects of pipe curtains, grouting, anchors, and arch supports were analyzed under four conditions: chemical grouting, conduit installation, advanced pipe grouting, and double-layer pipe grouting. On-site verification reveals that TBM jamming occurs when the resisting torque on the cutter exceeds the maximum torque the cutter can generate. For the gully fault, pre-reinforcement measures are essential to stabilize the surrounding rock. Among the different methods, surface drilling reinforcement is the most effective. It significantly improves the surrounding rock’s stability, reducing the plastic zone’s depth by approximately 52.3% compared to the advanced pipe shed method. The axial force on the anchors decreases by 77.9–83.8%, arch stress is reduced by 68.9–90.8%, and tunnel deformation is minimized by 2.13–50.78%, all of which contribute to enhancing the safety of the initial support structure. On-site coring results, TBM boring parameters, and deformation monitoring data confirm that the surface drilling pre-reinforcement method outperforms the grouting pre-reinforcement for the pipe shed, ensuring the safe excavation of TBM in the gully fault conditions. These findings provide valuable insights for TBM tunnel construction in similar geological environments.

1. Introduction

Major water conservancy projects are the backbone and key components of the water conservancy infrastructure system, and the construction of hydraulic tunnels is an important part of water conservancy engineering. Currently, the TBM (Tunnel Boring Machine) method is commonly used for construction. Compared to traditional drilling and blasting methods, TBM construction has significant advantages regarding excavation speed, surrounding rock stability, and working environment [1,2,3]. However, complex geological conditions often lead to frequent machine jamming, greatly affecting construction efficiency and project progress [4,5,6]. The geological conditions of fault zones with fractured zones are complex, with poor formation stability, loose rock mass structure, and limited self-bearing capacity. When the TBM passes through the fault-fractured zone, multiple risks, such as collapse, water inflow, and machine jamming, may occur. These potential threats not only seriously jeopardize the safety of construction personnel but may also lead to delays in construction progress [7,8,9]. To address the adverse geology of fault-fractured zones, grouting reinforcement methods are commonly used to improve the physical and mechanical properties of the rock mass. By injecting grout into the rock mass to fill fractures, the integrity and stability of the rock mass are enhanced, significantly increasing its strength and resistance to deformation, thereby providing reliable protection for the safe and efficient passage of TBMs through poor geological sections [10,11,12].

Research on grouting reinforcement for tunnels in fault-fractured zones is divided into two categories based on the grouting area: grouting inside the tunnel and grouting outside the tunnel. For grouting inside the tunnel, the effect of pre-grouting reinforcement on tunnel passage through the fault-fractured zone can be studied using three-dimensional finite element numerical simulation methods with an optimal grouting range. At the same time, it is necessary to analyze the mechanisms of flow velocity and pressure evolution in the cross-section during the grouting sealing process of the fault-fractured zone [13,14]. In the case of a TBM with an open face encountering machine jamming while passing through a fault-fractured zone, grouting plans should be developed based on the actual situation to ensure the TBM is successfully freed. Additionally, for water-rich fractured zones, research on full-face grouting technology should be conducted. By setting different grouting parameters for the curtain grouting, the patterns of tunnel crack initiation and propagation can be analyzed, with the results ultimately applied to the field [15,16]. In the case of grouting outside the tunnel, the porosity and permeability of the grouting area can be calculated through theoretical analysis and numerical simulation, thereby determining the appropriate grouting scheme. Additionally, based on hydraulic fracturing theory, the minimum grouting pressure for surface pre-grouting at a specific depth can be determined. It is important to note that for fault-fractured zones, surface pre-grouting reinforcement should still be combined with on-site monitoring techniques to analyze the diffusion patterns of the grouting area and assess the grouting effect. For extremely fractured regions, a key technical system for directional borehole protection of tunnels through faults can be adopted, which is based on drilling construction techniques and high-pressure pre-grouting technology [17,18,19]. The computational modeling of mechanized tunneling, particularly in long and deep tunnels through challenging ground conditions, has been the subject of numerous studies. Notably, Zhao et al. [20] conducted extensive numerical investigations into TBM tunneling using various computational methods. In addition, Ramoni and Anagnostou [21] developed dimensionless design nomograms, which were derived from computational analysis results. These nomograms provide a rapid and effective means for the preliminary assessment of the thrust force required to overcome shield skin friction and prevent shield jamming.

This paper primarily aims to investigate the mechanisms behind TBM cutter head jamming when tunneling through fault fracture zones, particularly during the passage through fault zones with erosion-induced lower layers. The goal is to identify the most effective pre-support reinforcement measures to prevent cutter head jamming. This requires a comprehensive consideration of various factors, including the stability of the surrounding rock, ground displacement, the stress on the support structure, and cost. This paper proposes a procedure for selecting optimal reinforcement techniques based on these factors to ensure smooth TBM operations in challenging geological conditions. On-site geological prognosis indicates a high risk of TBM jamming when traversing fault fracture zones beneath gullies. While existing studies suggest that pre-reinforcement with pipe roofs can ensure smooth TBM passage, this method proves less effective for gully fault conditions. This study focuses on analyzing pre-reinforcement measures to prevent TBM jamming in gully fault fracture zones, providing scientific and technical support for TBM construction in such complex geological conditions.

The structure of this paper is as follows: Section 2 introduces the background of the engineering problem and provides a brief overview of the fault fracture zones encountered by the TBM. Section 3 discusses the mechanism of cutter head jamming when an open-type TBM tunnels through fault fracture zones and presents a method for identifying cutter head jamming. Section 4 evaluates the reinforcement effects of four advanced treatment methods: chemical grouting at the tunnel face combined with deep-hole pipe umbrella grouting and shield tail small conduit grouting, pipe umbrella grouting, artificial excavation of guiding tunnels with double-layer pipe umbrella grouting, and surface pre-grouting. Section 5 presents the on-site application results. Section 6 provides the conclusions and summary of the paper.

2. Project Overview

A water conservancy project tunnel in Xinjiang is located in the denuded, structurally complex hilly region on the southern slope of Mount Taishan, with elevations ranging from 650 to 800 m above sea level. The tunnel, with a diameter of 7.8 m, is being constructed using an open-type TBM. The total excavation length of the TBM is 26.5 km, divided into three sections: Section 1, Section 2 and Section 3. The surrounding terrain is relatively flat, with minor height variations (10–20 m, locally up to 70 m), and the area features a desert landscape with exposed bedrock.

The primary lithologies in the exposed strata include gneiss, tuffaceous sandstone, biotite plagioclase gneiss, biotite plagioclase schist, and Late Variscan (γ33b) granite. During the design phase, exploration data revealed four significant erosion gullies (referred to as Gully 1 through Gully 4) on the tunnel’s roof in Section 2 and Section 3. Notably, when the TBM passed through Gully 1, the surrounding rock was highly fractured, leading to the cutter head becoming stuck. This situation pointed to the presence of fault zones where the cave roof forms a gully, posing significant safety hazards, including the risk of TBM equipment damage and the jamming of the cutter head and shield body.

Additionally, the section of the tunnel in the erosion gully area (Gully 1) is located within an erosion-affected low hill region of the Altai Mountains, where bedrock is largely exposed. The section is characterized by faults that can potentially impact tunnel stability. A total of ten faults (f120, f56, f121, f122, f158, f123, f124, f125, f126, and f159) were identified, with the most significant being fault f56. This fault exhibits a fracture zone width of 6–12 m and a strike of 55°NW∠80°. The fault extends over a distance of 15–18 m and has a considerable impact on tunnel stability. Other smaller faults, ranging from 0.3 to 5 m in fracture zone width, have moderate effects on stability.

In Sections Gully 2 and Gully 3, nine additional faults (f43, f106, f107, f108, f109, f110, f111, f112, and f113) were identified. Among these, faults f111 and f55 are of particular concern, with fracture zones 5–10 m wide, a strike of 55°NW∠70°, and orientations 85° to the tunnel axis. These faults extend over 12–19 m, contributing to tunnel stability issues. Other smaller faults, with fracture zones 2–3 m wide and lengths between 10 and 36 m, also affect stability. To ensure construction progress and mitigate the risks associated with these fault zones, detailed explorations of Gully 2 through Gully 4 were conducted, with the results summarized in

Table 1. Cave Top Washout Statistics.

Name	Gully Alignment	Lithology	Compressive Strength of Rock (MPa)	Gully Area (m2)	Crossing Length with Tunnel (m)
Gully 2	116 + 158–119 + 495	Granitic Gneiss	5–65	27.3	54
Gully 3	119 + 495–119 + 769	Granitic Gneiss	5–60	15.8	140
Gully 4	119 + 769–120 + 427	Granite	5–51	21.3	130

. The geological profile of the erosion gullies and fault zones is illustrated in Figure 1. Crossing these fault zones remains a critical challenge to maintaining the safety and efficiency of the TBM operation.

3. Analysis of the Mechanism of Cutterhead Jamming in Open-Type TBM Traversing Fault Fracture Zones

Based on the case of TBM passing through a fault fracture zone in the position of gully 1, this paper analyzes the mechanism of cutter head jamming in an open TBM passing through a fault fracture zone and provides a basis for determining the jamming in subsequent gullies 2–4.

Based on the research by Wen Sen et al. [22] on cutter head jamming, this paper further analyzes the mechanism of cutter jamming. The normal operation of the TBM cutter head requires overcoming the cutting resistance torque of the tool, the contact friction torque in front of the cutter head, and the friction torque at the circumferential edge of the cutter head. To ensure the normal excavation of TBM, the total torque mentioned above cannot exceed the maximum torque that the cutter head can release, otherwise, there will be a risk of machine jamming. Based on this theory, the mechanism of machine jamming is analyzed, and the analysis process is shown in Figure 2. This paper conducts theoretical research on the frictional resistance generated by the friction between the cutting tool and the surrounding rock on the cutting tool shaft and finally obtains the maximum torque T1 for TBM cutter head disengagement in Figure 2. The following identification process can provide a basis for TBM jamming determination.

The maximum frictional torque generated by the cutter to overcome its frictional resistance intersects with the normal direction of the tunnel face, so the effect of the cutter overcoming frictional force needs to be analyzed separately. The maximum static frictional torque before the roller cutter rolls is calculated using Equation (1).

$$T_I=n{\mu }_{1}r{f}_R$$

(1)

In the equation:

T_I: Overcoming the static frictional torque of the roller shaft before the cutter engages in cutting the surrounding rock. Number of cutters, with double cutters set to 2. Maximum static friction coefficient before the roller cutter starts rolling. Rolling radius of the cutter, in meters. Tangential force of the roller cutter, in kilonewtons (kN).

The rotational resistance torque generated by the TBM cutter head during tunneling due to friction with the surrounding rock primarily consists of the frictional torque T_a produced by the cutter cutting into the surrounding rock on the cutter head shaft, the resistive torque T_b from friction between the cutter head sidewall and surrounding rock, and the frictional torque T_c from contact between the front of the cutter head and the tunnel face.

The calculation expression for the maximum frictional torque T_a generated on the cutter head shaft before the cutter engages in cutting the surrounding rock is:

$$T_a={\displaystyle \sum _{i=1}^n{\mu }_1}{r}_2{f}_R$$

(2)

In the equation:

T_a: The maximum static frictional torque generated on the cutter head shaft before the roller cutter engages in cutting the surrounding rock. The radius of rotation of the tool around the rotary axis of the turntable, m.

The calculated expression for the resisting moment Tb resulting from the friction between the sidewalls of the cutter and the surrounding rock is given by:

$$T_b={\displaystyle \frac{\mu \pi d^{2}b}{8}}\left(f_1+f_2+f_3+f_4\right)$$

(3)

In the equation:

T_b: Maximum static friction torque at the edge of the blade. Maximum static friction resistance coefficient at the side edge of the blade. Blade diameter. Blade side edge width. Shield top pressure. Shield bottom pressure. Top lateral pressure. Bottom Lateral Pressure.

The expression for the contact friction moment Tc between the blade panel and the palm surface is calculated as:

$$T_c={\displaystyle \frac{\alpha \mu \pi d^2}{32}}\left(f_1+f_2\right)$$

(4)

In the equation:

T_c: Maximum static friction moment of contact between the blade panel and the palm surface. The ratio of the area of panels in contact with the surrounding rock.

When TBM excavation passes through fault fracture zones, a large amount of fractured rock mass on the face of the tunnel directly contacts the front of the cutter head. Part of the slag that has not entered the opening of the cutter head rotates with the cutter head, and under the axial thrust of the TBM, it rubs against the front of the cutter head, causing a sharp increase in the contact resistance torque Tc in front of the cutter head.

When the total torque T applied to the TBM cutter head exceeds its inherent disengagement torque, jamming occurs. Establish criteria for TBM jamming state discrimination:

$$T_I=n{\mu }_{1}r{f}_R<T_N\mathrm{and} T_{II}=T_a+T_b+T_c<T_M,\mathrm{non-jamming}$$

(5)

$$T_I=n{\mu }_{1}r{f}_R>T_N\mathrm{or} T_{II}=T_a+T_b+T_c>T_M,\mathrm{jamming}$$

(6)

In the equation:

T_N: The maximum torque that can be provided by the TBM tool.

T_M: The breakaway torque that can be provided by the rotation of the TBM cutter.

As shown in Figure 3, serious machine jamming occurred during the actual excavation process of on-site gully 1. To verify the accuracy of the card machine mechanism, real-time TBM excavation data of Gully 1 in front of the card machine was extracted for research.

On-site, manual excavation of pilot tunnels, double-layer advanced pipe sheds, and chemical grouting were used for advanced reinforcement. The specific method is to first set up an advanced pipe shed above the shield, excavate a small guide hole, and then set up another layer of advanced pipe shed above the small guide hole. Subsequently, grouting reinforcement was carried out on the surrounding rock in front of the cutter head and within a range of 5–8 m from the tail of the shield, which took up to 6 months. Figure 4a shows the on-site construction drawing of the first layer of the advanced pipe shed and chemical grouting, and the right figure shows the grouting effect after passing. Figure 4b shows the second layer of advanced pipe shed grouting for the excavation of the top small guide tunnel, and the right figure shows the grouting effect after passing.

As shown in Figure 5, the real-time excavation data of TBM reveals important information, where the red area indicates the warning interval where theoretically, TBM jamming may occur. The red line represents the rated torque and escape torque of the TBM. During the excavation process, when the torque of the cutter head reached 4200 kN/m, the TBM entered the predicted warning area for jamming, ultimately causing the cutter head to jam.

According to the identification process, there is a significant risk of the cutter head getting stuck at location 1 of the gully. It is urgent to conduct a comparative analysis of advanced pre-reinforcement schemes for Gully 2 and select the best one.

4. Analysis of the Effect of TBM Crossing Fault Fracture Zone Under Different Treatment Methods

4.1. Calculation Condition Setting

To ensure the safe and efficient crossing of fault zones by TBM, four treatment measures are proposed for discussion and analysis, starting from two methods of treatment inside and outside the tunnel. These four treatment measures correspond to four working conditions:

Condition I involves chemical grouting of the palm face, grouting of the deep pipe shed, and grouting of the shield tail small conduit, as shown in Figure 6a. Condition II is the grouting of the pipe shed (control group), as shown in Figure 6b. Condition III is the construction of a double-layered pipe shed for grouting in a manually expanded pilot tunnel, as shown in Figure 6c. Condition IV is surface pre-grouting, as shown in Figure 6d,e. Analyze the treatment effects of the above four working conditions through numerical analysis. The detailed parameter settings for the above operating conditions are listed in

Table 2. Working condition setting.

Pipe Shelter Grouting	Case I	Reinforcement depth	Grouting Pipe Diameter	Spacing	Note
	Shallow hole chemical grouting on palm face	5 m	40 mm	/
	Deep pipe shed grouting	20 m	100 mm	60 cm	Vault 120 degrees
	Shield tail small conduit grouting	5 m	40 mm	1.2 m	Vault 150 degrees
	Case II (control group)	Reinforcement depth	Diffusion Radius	Spacing	Note
	Pipe Shelter Grouting	20 m	0.6 m	60 cm	Vault 120 degrees
	Case III	Number of layers of pipe scaffolding	Diffusion Radius	Spacing	Note
	Hand-expanded double-layer pipe shed	2-story	0.6 m	60 cm	Vault 120 degrees
Surface Grouting	Case IV	Reinforcement area (H × L × W)	Grouting Pipe Diameter	spacing	diffusion radius
	Surface pre-grouting	16 m × 60 m × 22 m	90 mm	1 m	0.65 m

.

4.2. Computational Models

4.2.1. Establishment of Excavation Model for Gully Fault

According to the actual situation of gully 2, a pre-reinforcement grouting model was established, with a model size of 140 × 140 × 180 m (length × height × width), and the detailed dimensions of each component are shown in Figure 7. ABAQUS (https://www.3ds.com/zh/products-services/simulia/products/abaqus/ accessed on 17 February 2025) was used to divide the parts and mesh and then imported into FLAC3D for simulation and analysis. The details of each grouting and pre-reinforcement part are shown in Figure 8 and Figure 9.

4.2.2. Physical–Mechanical Parameters

In this paper, a combined failure criterion was used for the calculations, which integrates the Mohr–Coulomb criterion for shear failure and the maximum principal stress criterion for tensile failure. Both sections of the gully pass through Class V surrounding rock, and the parameters of the surrounding rock should be selected according to the actual situation on site. To select the surrounding rock parameters that match this project and reduce numerical calculation errors, FLAC3D was used to calculate the final settlement of the arch crown under the condition of pre-grouting reinforcement of the gully 1 pipe shed using a plane strain model. The final settlement obtained corresponds to the measured settlement value on site, and a reasonable set of surrounding rock parameter values can be obtained by recursively deducing gully 2. The schematic diagram of the plane strain model and the final settlement calculation of the arch crown under plane strain are shown in Figure 10.

According to the calculation in Figure 10, the actual settlement of the surrounding rock arch is about 1.78 cm, which is more consistent with the numerical calculation result of adjusting the surrounding rock parameters to 1.6 cm. Therefore, the physical and mechanical parameters of the rock and soil mass simulated by this model can be used as the required parameters for the three-dimensional numerical model, as shown in

Table 3. Physical and mechanical parameters of the surrounding rock. (Mohr-Coulomb).

Geologic Situation	Weight Capacity γ (kN/m3)	Modulus of Elasticity E (GPa)	Cohesion (MPa)	Friction Angle ψ (°)	Tensile Strength (MPa)	Poisson Ratio
Geologic fault	19.30	1.30	0.30	25.00	1.92	0.31
Gully II	23.50	1.67	0.37	30.00	2.21	0.32

. The physical and mechanical parameters of the initial support material are shown in

Table 4. Calculation of geological conditions of cross sections.

Structural Component	Strength Grade	Weight Capacity γ (kN/m3)	Modulus of Elasticity (GPa)	Poisson Ratio
Mix by Spraying	C30 concrete	25.00	30.00	0.25
Anchor	Q235	78.50	210.00	0.30
Steel arch	Q235	78.50	210.00	0.30
Cement paste	C25	22.00	25.00	0.20

.

4.2.3. Division of Construction Stages

The TBM construction method exhibits significant spatiotemporal characteristics, mainly reflected in the differences in the implementation time of support measures and the time characteristics of the material hardening process. Therefore, when conducting theoretical analysis and numerical simulation, it is necessary to reasonably divide the construction stages to reflect the excavation process of TBM throughout the entire process. For the pre-reinforcement of the pipe shed, the construction is divided into six stages:

The first stage is advanced pipe shed support. Due to the obvious time-dependent hardening characteristics of grouting in the pipe shed, when the TBM excavates to the shield to support the surrounding rock, the pipe shed and shield jointly bear the pressure of the surrounding rock. At this time, the support resistance is P1, and its main function is to control the advanced deformation of the surrounding rock and prevent equipment jamming. The second stage is palm face support, during which the surrounding rock provides maximum stiffness to the palm face when it is first exposed. The mortar of the pipe shed still needs to be further hardened, and the support resistance is P2 when the shield tail is dragged out. The third stage is the timely construction of arches, steel bars, and anchor rods. The arches and steel bars can immediately provide maximum stiffness, and the support resistance is P3. Until the anchor rod mortar is completely hardened, at which point the support resistance is P4. In the fourth stage, when the support resistance reaches its maximum until the concrete spraying bridge is constructed, there is still a small amount of deformation in the surrounding rock. The fifth stage is to carry out spraying and mixing at the spraying bridge position. As the concrete hardens, its stiffness gradually increases to stability, and the maximum support resistance at this time is P5. The sixth stage is when the sprayed concrete is completely hardened, and the deformation of the surrounding rock tends to stabilize. The relationship between the above stages and their supporting stiffness is shown in Figure 11. Because surface pre-grouting reinforcement belongs to advanced reinforcement outside the tunnel, the TBM will remain the same as the normal excavation section after reinforcement is completed. The schematic diagram of the support stiffness at each stage is shown in Figure 11b.

4.2.4. Arrangement of Monitoring Points

For gully 2 (K116 + 113~K119 + 490), monitoring will be conducted at K116 + 148, K116 + 440, and K119 + 460, respectively, with corresponding measurement points set up at 30 m, 75 m, and 100 m positions in the tunnel excavation direction of the numerical model. The monitoring project includes the displacement of the strata at a distance of 1 D~5 D (tunnel diameter) from the center of the tunnel (limited to the fault fracture zone section). Deformation and settlement at the arch crown and left and right arch waist positions of the tunnel. The schematic diagram of the monitoring project is shown in Figure 12.

4.3. Analysis of Calculation Results

Calculate the displacement deformation of the tunnel arch crown, arch waist, and arch bottom at y = 30 m, 75 m (fault fracture zone), and 100 m, respectively. Calculate the displacement settlement of the fault location and the distance from the tunnel excavation axis 1 D~5 D strata.

4.3.1. Study of the Distribution Pattern of Plastic Zones

The depth of plastic zone development under four operating conditions is calculated and compared. Figure 13a shows that the plastic zone depth at different positions is compared visually. The results indicate that in the three pipe shed support conditions, the plastic zone is more pronounced at the arch waist and mainly occurs near the interface between the pre-grouting range and the soil. In contrast, under the surface pre-grouting condition, the surrounding rock plastic zone is less developed, with a significantly smaller depth compared to the other conditions. This suggests that tunnel excavation disturbance to the surrounding rock is effectively reduced, and surface pre-grouting provides better reinforcement.

Additionally, the results show that condition II, involving pipe shed grouting, caused substantial soil damage, especially at the bottom of the arch, where a large plastic zone formed. This suggests that the single-layer pipe shed support is insufficient to prevent plastic deformation of the surrounding rock in fault fracture zones due to soil collapse. Therefore, enhanced support is necessary for smooth TBM passage. In condition I, by adding chemical grouting at the palm face and installing inclined small conduits at the shield’s tail, the bearing capacity of the support structure was improved. This limited the collapse and damage of surrounding rock in the fault fracture zone, yielding favorable results.

However, it should be noted that the grouting area increases the load on the lower soil, leading to some expansion of the plastic zone at the bottom of the arch. This indicates that, when designing support schemes, the combined effects of pipe sheds and grouting must be considered to optimize the support structure’s performance.

Comparing the four working conditions, surface pre-grouting results in the smallest plastic zone, with a development depth between 1.54 and 1.9 m. In contrast, the use of single-layer pipe shed support causes the greatest damage, with a maximum plastic zone depth of 4.55 m. After double-layer pipe shed and chemical grouting treatment, the geological conditions improve, and the plastic zone development is less pronounced, with a depth between 3 and 4 m.

4.3.2. Analysis of Tunnel Deformation and Ground Displacement

At a depth of 30 m (y = 30 m), the tunnel deformation under four working conditions was calculated, and the deformation cloud map is shown in Figure 14a. The deformation trends of the surrounding rock are similar across all four conditions before reaching the fault fracture zone. Specifically, the arch crown deforms by 5 mm to 7 mm, the arch waist by 4 mm to 7 mm, and the arch bottom lifts by about 7 mm. Preliminary analysis suggests that this similarity is due to the consistent geological conditions at this depth, which minimizes the impact of different grouting methods.

Before conducting further calculations and analysis, we first standardize the model by unifying the grid and normalizing other variables to eliminate their influence on the results. This ensures the analysis focuses on the core research variables, improving the accuracy and reliability of the model. The deformation calculation results validate the model’s effectiveness in assessing the stability of surrounding rock at fault locations.

We calculate the deformation of the tunnel at the position of y = 75 m (tunnel model excavation 75 m, fault fracture zone) for four working conditions, and calculate the deformation cloud map as shown in Figure 14b. It is not difficult to see that the first three working conditions all use the pipe shed grouting model, while the fourth working condition uses the surface pre-grouting model. According to the calculation, the deformation of the surrounding rock at the left and right arch waist positions is relatively obvious, about 14–17 mm. Due to the grouting reinforcement effect, the load is evenly distributed on the initial support structure of the upper surrounding rock of the tunnel. The deformation at the arch crown is 2–3 mm smaller than that at the arch waist, and the uplift at the bottom of the arch in condition II is higher than in other conditions. It is not difficult to see that using the surface pre-grouting scheme results in a much smaller overall deformation of the tunnel surrounding rock.

We calculate the deformation of the tunnel at the position of y = 100 m (100 m in the excavation direction of the tunnel model) for four working conditions, and the deformation cloud map is shown in Figure 14c. The difference between this calculation section and the position of y = 30 m is that the surface pre-grouting range of working condition IV still includes the position of the 100 m section. So, analyzing the deformation cloud map shows that using surface pre-grouting can still effectively reinforce the surrounding rock when the tunnel passes through ordinary strata. Compared with the other three working conditions, the deformation of the tunnel surrounding rock can be reduced by 52.3%.

The deformation cloud map helps visualize the differences In surrounding rock deformation caused by various grouting schemes. Comparative analysis is conducted on the deformation at different positions—arch crown, arch waist, and arch bottom—under four working conditions (Figure 15), with different line styles used to distinguish between conditions I–IV.

First, the deformation at the arch positions under different working conditions is analyzed. As shown in Figure 15a, the deformation curve at the fault fracture zone (y = 75 m) is significantly higher than at other positions, indicating greater overall deformation at the fault. Among the four working conditions at the fault fracture zone, Condition II (pre-grouting reinforcement with a pipe shed) results in the highest surrounding rock deformation, reaching 14.2 mm. The grouting schemes of double-layer pipe shed, chemical grouting + shield tail small pipe grouting + advanced pipe shed, and surface pre-grouting reduced the deformation by 2.12%, 3.97%, and 50.78%, respectively, compared to Condition II. This shows that the surface pre-grouting scheme has the best control over surrounding rock deformation.

Next, the deformation differences at the arch waist position under different working conditions are analyzed. As shown in Figure 15b, Condition II still results in the largest surrounding rock deformation (16.8 mm), which is a 1.78% increase compared to the arch position. Grouting schemes such as double-layer pipe shed grouting, chemical grouting + shield tail small pipe grouting + advanced pipe shed, and surface pre-grouting reduced the horizontal deformation at the arch waist by 11.97%, 10.33% and 56.21%, respectively, compared to Condition II. Notably, the double-layer pipe shed grouting method effectively controlled the horizontal convergence of the surrounding rock. Compared to the arch crown, the grouting scheme in Condition III reduced deformation by 1.64% more than in Condition I. This is because the double-layer pipe shed provides additional support, reducing the load on the lower structure and allowing it to be more evenly distributed.

Finally, the deformation differences at the arch bottom position under different working conditions are analyzed, as shown in Figure 15c. In the double-layer pipe shed grouting scheme, the arch bottom uplift reached 16.78 mm, an increase of 22.6%, 23.2%, and 61.8% compared to Conditions I, II, and IV. This is primarily because the surface pre-grouting significantly improves the formation’s integrity, while the pipe shed grouting reinforces the surrounding rock locally. However, this method does not provide comprehensive reinforcement, leading to stress concentration in certain areas. The double-layer pipe shed transmits the least load downward, resulting in a more pronounced uplift at the arch bottom.

In summary, surface pre-grouting enhances soil stability, strength, and stiffness, reducing the risk of collapse and deformation. The injected slurry improves the soil’s physical and mechanical properties, increasing its bearing capacity. This method accelerates construction, improves efficiency, and enhances safety. Three-dimensional numerical simulations show that surface pre-grouting maintains good surrounding rock integrity with minimal deformation, small relative displacement, and effective control over geological settlement. Therefore, surface pre-grouting is an effective technique for managing fault fracture zones.

4.3.3. Analysis of Stratigraphic Displacement Patterns in Fault Fracture Zones

The geological displacement within a distance of 0–5 D (tunnel diameter) from the tunnel center, specifically in the fault fracture zone section, is shown in Figure 16. Tunnel excavation in the fault fracture zone primarily affects ground displacement within 0–2 D. The maximum displacement at the tunnel center under four working conditions is as follows: Condition I: 5.98 mm, Condition II: 7.93 mm, Condition III: 6.53 mm, Condition IV: 2.23 mm. Beyond 3 D from the center, the displacement drops below 2 mm, and disturbance to the strata decreases with distance. Based on the curve amplitudes, the displacement intensity for each condition is ranked: Condition II > Condition III > Condition I > Condition IV. This suggests that surface pre-grouting effectively reduces disturbance to the surrounding strata in the fault fracture zone.

4.3.4. Support Force Analysis

To further analyze the stress on the supporting structure, axial force distribution maps of elements like cables and beams were examined. From Figure 17a, it can be seen that the initial support structure of the TBM in non-fault areas experiences a nearly uniform force with no concentration. However, at the fault location, the axial force on the pipe shed, conduit, and anchor rod increases significantly—by 52% to 68% compared to the non-fault area. For the cable unit, the axial force on the anchor rod in the surface pre-grouting zone ranges from 1.5 to 8.9 kN. In working conditions I–III, the axial force on the anchor rod increases by 77.9% to 83.8%, indicating that surface pre-grouting enhances the surrounding rock stability in the fault fracture zone.

As shown in Figure 17b, for the beam unit, the stress on the arch frame within the surface pre-grouting range is about 56.7–81.2 kN. In working conditions I–III, the stress on the arch increases by 68.9–90.8%, indicating that surface pre-grouting significantly enhances the surrounding rock’s bearing capacity, helping to share the arch’s stress. In working conditions I-III, significant stress concentration occurs in the arch of the fault zone, suggesting that the support force required to reinforce fractured rock is much higher than for conventional geological sections.

In conclusion, surface grouting reinforcement (working condition IV) effectively reduces rock fragmentation and increases surrounding rock-bearing capacity. Compared to the other methods, it provides the best reinforcement effect.

5. Field Application Cases

To prevent the recurrence of the same machine jamming accident in Gully 1, surface pre-grouting reinforcement should be carried out promptly for the rock mass exposed by the TBM in Gully 2. To test the grouting effect, core inspection holes were arranged around the original drilling core location after grouting was completed. The depth of the inspection holes was consistent with the grouting depth. By taking cores from the inspection holes, it was found that a complete stone body with a length of about 10 cm could be obtained in the grouting area. The core sampling results indicate that defects such as cracks and voids have been effectively filled, and the overall integrity of the grouted stone body has been significantly improved. The compression test was conducted on the complete core sample, and the results showed that the consolidation strength of the slurry exceeded 0.4 MPa. The average compressive strength of the rock mass after 28 days reached 7.8 MPa, meeting the requirement of not less than 5.8 MPa in the specifications and achieving the purpose of grouting. The schematic diagram of the grouting filling effect during construction is shown in Figure 18a.

To assess the impact of grouting on TBM excavation performance, excavation parameters from the grouting section were compared with those from adjacent sections with favorable geological conditions. The main comparison between cutterhead speed and cutterhead thrust for the two sections is shown in Figure 19.

Figure 19 shows that in the normal section with intact surrounding rock, the TBM cutter head speed is maintained at 5–7 r/min, with cutter head thrust between 14,000 and 18,000 kN. In the surface pre-grouting section, the cutter head speed increases significantly, ranging from 3 to 6 r/min, while thrust drops to 6000–9000 kN. To prevent TBM jamming from excessive thrust, both speed and thrust are reduced when entering the pre-grouting section. As excavation progresses, both speed and thrust generally increase, positively correlating with the surrounding rock’s strength and integrity, indicating effective grouting reinforcement.

To validate the numerical simulation, three test sections (K116 + 148, K116 + 440, and K119 + 460) were set up in the YE tunnel. Deformation monitoring of the arch crown, waist, and bottom was conducted to verify the numerical model’s calculation results, as shown in

Table 5. Schematic of field application case zones.

Name	Gully Mileage	Crossing Length with Tunnel/m	Tunnel Depth/m	Lithology	Rock Compressive Strength/MPa	Note
SY-II	K116 + 148	108	72	granite gneiss	5–51	Test I
SY-II	K116 + 440	54	68	granite gneiss	5–65	Test II
SY-II	K119 + 460	140	76	granite gneiss	5–60	Test III

.

Real-time monitoring of arch settlement and peripheral convergence was conducted in three sections: K116 + 148, K116 + 440, and K119 + 460. The monitoring data, shown in Figure 20, indicate no sudden deformation in the surrounding rock during excavation, with the deformation data eventually stabilizing, confirming a good pre-grouting effect on the tunnel surface. A comparison of the data reveals that the numerical simulation predicted an arch settlement of 7.32 mm, while the measured value was 9.14 mm, resulting in a 19% error. For the arch waist deformation, the error was 15%, and for the bottom slab uplift, it was 13%. Overall, the errors were all below 20%, demonstrating that the numerical simulation design is reasonably accurate.

6. Economic Benefit Analysis of Pre-Grouting

6.1. Economic Benefits of Grouting in Pipe Shed

During TBM excavation, jamming incidents are common when passing through poor geological zones, causing delays. Multiple methods are often required for TBM recovery. Specific construction details are shown in Figure 14 and Figure 15, with investment information provided in

Table 6. TBM pipe shed grouting investment table.

Project	Unit	Quantity/Quantity of Works	Total Price/10,000 RMB
Drill depth	m	14,580	29.2
Cement paste	m3	132.3	75.4
Drilling and injection integrated machine		4	260.32
Number of boreholes		486	21.87
Grouting pipe		486	5.23
Total			392.02

.

A drilling and grouting integrated machine is installed on the TBM’s main beam. When encountering fault zones, advanced drilling and grouting are immediately performed to ensure safe passage. Each cycle involves drilling 27 advanced holes, each with a diameter of 89 mm. Advanced grouting inside the tunnel requires the TBM to stop. Each hole takes 3 h for drilling and grouting and 81 h per cycle (27 holes × 3 h). Considering material preparation and equipment installation, each cycle takes 7 days. A total of 18 cycles were performed in the section where the tunnel intersects a gully.

Due to the drilling-grouting machine’s limitations, only the tunnel crown (120° range) can be grouted. The support boots and floor require additional backfilling. Based on construction experience, the average excavation speed for this section was 2.5 m/day. The support and grouting work took 12 h for excavation, 20 h for cleaning and concreting, and 16 h for curing. The fault section, spanning 432 m, caused a delay of 173 days. With a standard excavation rate of 470 m per month, the total delay for this section was 318 days (173 days for grouting and 145 days for excavation).

6.2. Economic Benefits of Surface Pre-Grouting

Surface pre-grouting is performed before the TBM passes through shallow, poorly consolidated ground to strengthen the surrounding rock, facilitating the TBM’s safe passage without significantly affecting the excavation time. The main tasks of surface pre-grouting include ground clearing, drilling quantity and depth, and cement slurry preparation, with specific investment details provided in

Table 7. Surface grouting investment table.

Project	Quantity of Work	Unit Price/RMB	Total Price/10,000 RMB
Surface cleaning	6576/m3	5.3	3.5
Deep drilling	45,055/m	110	495.6
Cement paste	4546/m3	950	431.9
Bushing		212	13.2
Drilling machine		325,700	130.28
Total			1074.48

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Advancements in drilling equipment, grouting machinery, and materials have reduced the difficulty of surface pre-grouting in tunnel sections intersecting gullies. However, issues like hole collapse and drill loss still occur in fractured zones, making it challenging to implement pre-grouting effectively. From an economic perspective, while deep tunnel grouting involves substantial drilling and grouting volumes, the technology for deep-hole grouting is still underdeveloped, leading to high labor and material costs.

6.3. Economic Rationality Analysis

Tunnel umbrella grouting saves approximately 6.82 million yuan compared to surface pre-grouting, but it leads to an extension of the construction period by about 11 months. Moreover, advanced drilling and grouting inside the tunnel can only reinforce the tunnel crown within a 120° range, leaving the sides and bottom unprotected and posing a potential risk of TBM jamming. In contrast, surface pre-grouting strengthens the formation before the TBM reaches fault zones, ensuring smooth passage and avoiding construction delays.

TBM umbrella grouting has high costs during machine unjamming, reducing its economic efficiency. As the tunnel depth increases, the grout initially consumes a large amount of slurry, with lower pressure at the start. As the voids fill, the pressure increases. At greater depths, effectively injecting grout into deep fractures becomes a technical challenge, compromising grouting quality and prolonging the grouting period.

Surface pre-reinforcement in gully sections offers significant advantages in construction time. However, umbrella grouting poses risks during TBM unjamming, potentially causing ground collapse and endangering workers and equipment. Even if the unjamming is successful, TBM operations may still face jamming risks. Considering both economic and technical feasibility, surface pre-grouting should be prioritized when the tunnel depth is less than 70 m to effectively prevent TBM jamming incidents.

7. Conclusions

This paper investigated the mechanism of TBM jamming in fault zones during excavation, focusing on fault-fractured zones in a water conservancy project in Xinjiang. The study aimed to identify effective pre-reinforcement measures for surrounding rock to prevent such incidents, considering rock stability, strata displacement, support structure stress, and cost-effectiveness. Using theoretical analysis, numerical simulations (FLAC 3D), and field monitoring, the study concludes that grouting pre-reinforcement at the surface is the most effective measure to prevent TBM jamming, particularly in trench fault zones. While the study’s simplification of strata parameter variations may introduce minor errors, these were controlled through displacement indicators, with a final model showing less than 20% discrepancy from field data. Future research should focus on refining the modeling process and utilizing advanced optical measurement technologies to improve accuracy in simulating tunnel excavation conditions. The following conclusions are drawn:

(1)

The TBM tunnel jamming mechanism was studied, incorporating tool friction into the jamming theory. Deformation calculation before shield jamming was also analyzed, improving the efficiency of jamming detection. In working condition II (control group), it was identified that jamming occurred when passing through gullies. On-site verification showed that TBM jamming and escape took up to six months, primarily relying on manual excavation of the pilot tunnel in working condition III.

(2)

By analyzing the distribution of plastic zones under four working conditions, the results show that the plastic zone at the arch waist is deep, particularly under the pipe shed support conditions, where the interface between pre-grouting and soil shows significant plastic deformation. In working condition II, the plastic zone at the arch bottom expands notably. Since a single-layer pipe shed cannot effectively prevent soil collapse in the fault zone, additional support is needed to ensure smooth TBM passage. Condition I improves the support structure’s load-bearing capacity and limits fault area damage by using chemical grouting and inclined small conduits. Pre-grouting on the surface results in minimal plastic zone development, with depth significantly lower than in other conditions, indicating a strong reduction in surrounding rock disturbance. The double-layer pipe shed grouting, chemical grouting + shield tail small pipe grouting, and surface pre-grouting reduced rock deformation by 2.12%, 3.97%, and 50.78%, respectively, with surface pre-grouting showing the best results.

(3)

Surface pre-grouting enhances soil stability, increases strength and stiffness, reduces collapse and deformation risks, and improves soil’s physical properties and bearing capacity. This method accelerates construction, boosting efficiency and safety. Three-dimensional numerical simulations show that surface pre-grouting maintains good surrounding rock integrity, with minimal deformation and relative displacement, and effectively controls geological settlement. Therefore, it is an effective technique for fault fracture zones.

(4)

From an economic perspective, umbrella grouting saves approximately 6.82 million RMB in investment; however, it presents challenges such as extended construction periods, increased technical difficulty, and potential risks, particularly at greater tunnel depths where grouting quality may be compromised and the grouting process is prolonged. In contrast, surface pre-grouting allows for early ground reinforcement before the TBM reaches fault zones, ensuring smooth TBM operation and effectively preventing delays and jamming risks. Considering both economic viability and technical feasibility, surface pre-grouting proves to be more advantageous for tunnel depths of less than 70 m, as it minimizes costs, ensures construction safety, and enhances efficiency. Therefore, in tunnel construction planning and execution, the grouting method should be selected based on specific project conditions and economic considerations to achieve optimal project outcomes.