Research on the Mechanism of Loose Deformation in Weak Fracture Zone Tunnel Surrounding Rock and Support Control

Abstract

In the fractured weak fault zone, rock mass exhibits low strength and poor self-stability. The geological conditions are complex, and when tunnels cross through fractured zones, significant deformations and collapses are prone to occur, leading to geological hazards. This paper investigates the in situ stress and deformation patterns of the Dongmachang Tunnel No. 1, proposing support solutions for addressing tunnel deformations through field experiments and numerical simulations. The on-site monitoring results indicate that despite implementing measures such as grouting reinforcement and temporary steel supports to control surrounding rock deformation, significant structural damage still occurred in the tunnel support system. The manifestations included severe sinking and cracking of the arch crown, strong inward deformation of the sidewalls, widespread cracking, crushing, and spalling of shotcrete, slight arching uplift, and severe distortion and twisting of steel arches forming a “Z” or “S” shape. To ensure tunnel safety and control the stability of excavations in weak fault zones, a comparison of tunnel deformation support schemes is conducted through field experiments and numerical simulations, indicating that replacing the upper tunnel structure and invert can effectively prevent tunnel deformations. These measures are vital for the sustainable development of tunnel.

1. Introduction

Geological characteristics [1,2] and construction conditions [3,4] are crucial factors influencing the sustainable development of underground engineering. During tunnel construction, traversing through soft rock fault zones [5,6] inevitably leads to support structure deformations [7,8], cracking [9,10,11], and other issues, significantly impacting tunnel construction safety, timeline, and quality, thereby posing challenges to tunnel engineering. To mitigate the damage caused by soft rock fault zones to tunnel structures, studying the deformation patterns of tunnel surrounding rock when crossing through such zones and implementing protective support measures has become an urgent engineering issue that needs to be addressed.

In recent years, scholars have increasingly emphasized the deformation issues of tunnel surrounding rock when crossing fault zones and have conducted extensive research on support measures [12,13,14]. Qin et al. [15] used the Guqiao coal mine in China as an engineering case study. They employed pilot industrial tests, laboratory experiments, and field measurements to analyze the deformation and failure characteristics of the surrounding rock in the mudstone fault zone. They identified factors influencing excavation stability and proposed measures to control excavation stability. Li et al. [16] studied the wetting mechanism of the surrounding rock in fault fracture zones of tunnels through experiments and modeling, proposing corresponding control measures. Based on monitoring data and engineering geological conditions, Ma et al. [17] analyzed the surface deformation patterns caused by segmented collapse mining in the Dahongshan iron mine fracture zone. They also proposed a regional characterization method for lateral support forces in loose media. Yang et al. [18] conducted physical model experiments to study the deformation mechanism of carbonaceous slate tunnels in the Mudongling fault zone under asymmetric stress conditions. By considering the tunnel’s macroscopic failure characteristics and patterns, they explored the impact mechanism of asymmetric stress on carbonaceous slate tunnels and proposed control measures. Finally, they put forward control measures for carbonaceous slate tunnels under asymmetric stress conditions. Xu et al. [19] established the deflection curve equation for the critical floor strata in the Zhaogu Lane fault-crossing tunnel. They analyzed the characteristics of sudden water influx in the fractured zone behind the floor heave in the tunnel and summarized the variations in floor heave and plastic zone when pressurized water acts on different rock layers. Zhou et al. [20] used the Xiangjunshan Tunnel in the high-stress fault zone of Sanmenxia as a case study. Through on-site monitoring and testing, they analyzed the deformation patterns and stress responses of six sections in the tunnel construction process within the fourth and fifth classes of rock formations. The experimental results indicate that the deformation of fault fracture zone tunnels during excavation is asymmetric. The stress in the surrounding rock exhibits a spatially discrete distribution, and there is a lack of coordination in the stress variations between shotcrete and steel frames. Huo et al. [21] investigated the deformation and failure characteristics of the surrounding rock and support structures in the Tabaiyi Tunnel faulted fractured zone, introducing a novel NPR anchor–truss active-passive coupled support system. Wang et al. [22], utilizing a severe deformation case in the main roadway of a western coal mine, analyzed the deformation characteristics of loose and fractured roadway rock and their influencing factors. Cui et al. [23] using the 333 return airway of the Gaokeng Coal Mine as a case study, analyzed the deformation characteristics and failure mechanisms of the composite roof rock in loose and weak coal roadways. They discussed the reasons for significant deformations, compared the superiority of prestressed truss anchors with conventional anchors from a mechanical perspective, and proposed a targeted coal roadway support approach that addresses the deformations. They discussed the reasons for significant deformations, compared the superiority of prestressed truss anchors with conventional anchors from a mechanical perspective, and proposed a targeted coal roadway support approach that addresses the deformations. Their study delves into the mechanical advantages of prestressed truss anchors over regular anchors, presenting an effective roadway support strategy. Through these analyses, a comprehensive understanding of the deformation and failure mechanisms in various geological conditions is attained, offering valuable insights for enhancing tunnel stability and support systems in similar environments. Previous studies on tunneling through fault zones have primarily focused on single fault zones or shallow fault zones. However, there is limited research on the stability of tunnel excavation in narrow clearances in fractured soft rock within large fault zones in high-stress regions. Currently, there are no successful engineering cases reported on the safety and long-term stability of excavating tunnels with a minimum clearance of 24 m under a newly active fault zone wider than 90 m and extending 180 km.

This paper focuses on the Dongmachang Tunnel No. 1 in the Chenghai fault zone in southwestern China. It investigates the characteristics of in situ stress distribution in mountainous weak and fractured rock layer tunnels, identifies and analyzes the types of defects that occur during construction, and proposes corresponding remedial measures. To assess the effectiveness of these treatments, long-term monitoring of the mechanical behavior of the surrounding rock and support structures is conducted. This paper compares support solutions for addressing tunnel deformations through field experiments and numerical simulations, demonstrating that replacing the upper tunnel structure and invert can effectively prevent tunnel deformations. This provides a reliable reference for future similar engineering projects.

2. Background and In Situ Stress

2.1. Engineering Background

The Dongmachang Tunnel No. 1 has a total length of 5098 m on the left line and 5205 m on the right line, and is situated on the western side of Yongsheng County, Lijiang City, Yunnan Province, China (Figure 1). It serves as a crucial project for the Huaping–Lijiang Expressway. The longitudinal slope of the tunnel section is −2.000%, with a maximum depth of 613 m. The tunnel section lies in a region characterized by tectonic erosion block high and medium mountain landforms, featuring steep terrain and developed karst basins between valleys. The Dongmachang Tunnel No. 1 consists of Grade IV and Grade V rock masses, which account for 68.9% of the total tunnel length. The surrounding rock mainly comprises metamorphic fractured soft rocks, such as mudstone and limestone, with low strength and significant rheological characteristics. Notably, there is a significant difference in lithology between the two sides of the fault. The entrance is composed of Jurassic mudstone and siltstone, while the exit is composed of Devonian dolomitic limestone and limestone. These sections experience strong tectonic stress, leading to the development of fractures and a fragmented rock mass structure. The groundwater comprises three types: quaternary porous water, structural fissure water, and carbonate rock solution water. The maximum water inflow in the tunnel is approximately 40,765 m³/day.

In particular, the Chenghai Fault (F3) intersects with the right line of the tunnel at K72 + 164 (left line ZK72 + 150) at a 41° angle (Figure 2). The Chenghai Fault is an active fault in the Holocene, a seismic fault with a fault attitude of 310° ∠ 72°, classified as a thrust reverse fault. The rock type of the fault zone is dolomite, which is highly fractured due to fault influence, indicating intense weathering of rock layers, well-developed joint fractures, karst development, low rock mass rating, poor stability, high water content, good permeability, and significant dynamic changes. Influenced by the fault, there are notable differences in rock types at the entrance and exit, predominantly sandstone and mudstone at the entrance, and dolomite at the exit.

2.2. In Situ Stress

In tunnels with significant depth and complex geological conditions, the influence of self-weight stress and horizontal tectonic stress on the tunnel is substantial. To investigate the characteristics of the in situ stress distribution in the Dongmachang Tunnel No. 1, hydraulic fracturing is employed during the geological exploration phase to measure the in situ stress. The measurement points are located at K71 + 288 and K73 + 860, with horizontal boreholes arranged along the tunnel sidewalls and vertical boreholes placed in the floor. The measurement results revealed maximum principal stresses of 14.61 MPa and 16.24 MPa for the horizontal and vertical boreholes, respectively, while the minimum principal stresses are 10.90 MPa and 11.18 MPa (Figure 3). Furthermore, considering the influence of stress redistribution caused by tunnel excavation on the measurement results, we averaged the stress values from the deepest points of three horizontal and vertical boreholes to obtain the in situ stress values of the rock mass.

Table 1. The in situ stress and direction of rock mass.

Position	Hole Depth (m)	Maximum Principal Stress (MPa)	Minimum Principal Stress (MPa)	Direction of Breaking (°)
Horizontal hole	34	13.12	9.78	46
Vertical hole	42.6	16.09	10.88	NE75

shows that the maximum and minimum in situ stresses for the horizontal boreholes are 13.12 MPa and 9.78 MPa, respectively, while for the vertical boreholes, the maximum and minimum in situ stresses are 16.09 MPa and 10.88 MPa. Moreover, the maximum horizontal principal stress has a prevailing direction of NE75°, with an angle range of 75° to 85° relative to the axis orientation of the unexcavated tunnel section. It intersects the axis of the unexcavated tunnel section at a large angle. The construction of the Dongmachang Tunnel No. 1 faces numerous challenges such as high in situ stress, traversing fractured soft rock, and active fault zones. Many scholars’ research [8,24,25,26] results show that high geo-stress and poor rock mass quality are the main reasons for the large deformation of the tunnel. Additionally, asymmetric deformation may occur during tunnel construction, which is often attributed to tilted contact between soft and hard rocks or the inclined distribution of stratified rocks.

3. Damage Law of Tunnel Lining and Support System

3.1. Tunnel Damage Patterns and Regulations

Due to its crossing of the Chenghai Fault Zone, the surrounding rock of the Dongmachang Tunnel No. 1 is fragmented and weak, with significant lithological differences on either side of the fault. Moreover, the tunnel is subjected to a complex stress environment. Therefore, extensive deformation and damage occurred within the tunnel near metamorphic mudstone, limestone, and fault zones. In particular, post-excavation, the initial single-layer support of the tunnel experienced severe deformation, manifested by substantial arch crown subsidence and cracking, strong inward displacement of the sidewalls, widespread cracking and spalling of the concrete layer, radial and longitudinal fissures, and severe torsional deformation of the steel arches, forming “Z” or “S” shapes (Figure 4A). Additionally, significant cracking and uplifting are observed in the invert, with the appearance of longitudinal cracks at the center of the top surface after invert casting. These cracks continued to propagate towards the tunnel entrance, with the widest crack reaching 12 cm. The maximum uplift of the arch reaches an astonishing 1.5 m (Figure 4B). Moreover, secondary lining exhibited notable damage, primarily characterized by collapse and dislodgment of the arch crown and sidewalls, localized spalling, and the presence of radial and oblique cracks (Figure 4C). The cumulative length of cracked and damaged sections on the left and right tunnel walls amounted to 614 m.

Post-construction monitoring of the tunnel is essential to prevent potential collapse accidents. The monitoring points for the arch roof settlement and surrounding displacements are illustrated in Figure 4D. G1, G2, and G3 are three monitoring points used for measuring the arch roof settlement, while BC and DE are monitoring points for measuring the displacements around the tunnel. A total of 225 and 182 monitoring sections are established in the tunnel entrance and exit directions, respectively. The cumulative displacement curves for the tunnel’s arch roof settlement and surrounding displacements are depicted in Figure 4E–H.

Figure 4E,F show that as the tunnel approaches the Chenghai Fault Zone, the deformation and damage to the tunnel become more pronounced. The deformation is greater in the right tunnel compared to the left tunnel, with the side of softer rock exhibiting more significant deformation. Specifically, at the tunnel entrance, the deformation is mainly concentrated in the damage to the arch crowns on both sides, while at the tunnel exit, deformation occurs in both the arch crown and the sides. Additionally, the settlement displacement of the arch roof is greater than the convergence of the surrounding rock, and the horizontal convergence deformation in the upper part (BC) of the tunnel is generally larger than in the middle and lower parts (DE). The left tunnel, constructed earlier, shows more severe damage compared to the right tunnel constructed later. Moreover, after implementing the original design’s double-layer initial construction scheme, significant improvements in displacement are observed. However, there are still issues of uplift and cracking in the arch crowns at the entrance and significant deformation problems at the exit of the tunnel.

3.2. Internal Force Analysis of Typical Section

To investigate the internal forces within the tunnel section, we conducted internal force monitoring on the left line entrance mudstone section of Dongmachang Tunnel No. 1. This monitoring included testing the initial support pressure, lining pressure, and internal forces, as well as the internal forces of the steel arches. The tunnel’s mechanical behavior monitoring primarily entailed utilizing a 2 MPa pressure gauge to monitor the initial support’s contact pressure with the surrounding rock and a 300 MPa pressure gauge to monitor the stress in the steel arch. Pressure elements are placed on the contact surface using a tripod stand; the stand is welded in place. The steel stress gauge installation method involved placing a pair of steel stress gauges parallel within flanges on either side of the steel arch and welding the steel bars at both ends of the stress gauges to the flanges. Figure 5 shows that the surrounding rock pressure and internal forces of the first-layer initial support and second-layer initial support both experienced sudden increases before stabilizing after 150 days. Specifically, the maximum surrounding rock pressure on the right shoulder of the first-layer initial support reached 1000 kPa, while the maximum surrounding rock pressure on the left shoulder of the second-layer initial support reached 500 kPa. Additionally, the maximum surrounding rock pressure in the center of the second-layer initial support invert reached 1200 kPa, and the maximum surrounding rock pressure on the left shoulder of the second-layer lining reached 680 kPa. Furthermore, the maximum internal force at the crown of the first-layer steel arch reached 24 kN, while the maximum internal force on the inner side of the left shoulder of the second-layer steel arch reached 80 kN. The internal force at the center of the second-layer steel arch invert reached a maximum of 37 kN. Moreover, the internal force on the outer side in the center of the invert of the second-layer lining reached 22 MPa, and the internal force on the left side of the arch waist of the second-layer lining reached a maximum of 20 MPa.

In conclusion, the initial support exhibits significant deformation, high deformation rates with slow convergence, prolonged deformation duration, and non-uniform deformation leading to potential structural failure. The tunnel instability may be attributed to the high stresses in the geological structure induced by the Chenghai Fault Zone, as well as the uneven distribution of soft and hard rock layers at the tunnel site. Furthermore, the high stress in the Chenghai Fault Zone’s strata leads to rapid deformation of the early support structures. Even after the completion of the second-layer lining, the deformation persists. Therefore, it is imperative to carry out immediate remediation at the damaged sections to maintain the stability of the tunnel.

4. Tunnel Deformation Treatment

Although the Dongmachang Tunnel No. 1 implemented measures such as increasing the I-beam dimensions, adding extra deformation allowances, reinforcing anchor bolts, and strengthening radial grouting during construction, severe deformation, arching, cracking, and upheaval occurred in the initial single-layer support, with significant damage to the secondary lining. To address these issues, we improved the existing support scheme with a new approach and conducted a 100-day monitoring of the initial tunnel support displacements post-improvement. Furthermore, numerical simulations provided additional validation for the rationality of the new support scheme. The parameters of the support scheme before and after optimization are shown in

Table 2. The parameters of the support scheme before and after optimization.

Project	Initial Support									Secondary Lining		Deformation Allowance (mm)
	Shotcrete			Bolt				Reinforcing Mesh (cm)	Steel Frame Spacing (cm)	Arche/Wall (cm)	Inverted Arch (cm)
	Material	Arche/Walls (cm)	Inverted Arch (cm)	Specifications	Location	Length (m)	Distance (m × m)
Before optimization	C25	31/29	29	φ25	Arches/walls	4	1.0 × 1.2	φ8@150 × 150 (Arches/walls)	60	70	90	500
After optimization	C25	31/29	60	φ32	Arches/walls	4.5/9/12	1.0 × 1.2/1.2 × 1.2	φ8@150 × 150 bilayer (Arches/walls)	60	70	70	500

.

4.1. Implementation Effect of Support Scheme

To investigate the control effectiveness of the new treatment scheme on tunnel settlement, we designed three scenarios. These include replacing the upper structure of the tunnel with the new scheme while keeping the arch unchanged, replacing the inverted arch with the new scheme while keeping the upper structure unchanged, and replacing both the upper structure and the inverted arch with the new scheme. In order to replace the tunnel’s supporting structure, we follow the steps of excavation of the treatment site, lower step construction of the first floor, initial support construction of the second floor, anchor rod construction, and pouring concrete for the invert.

The Figure 6B shows that when only the upper tunnel support structure is replaced, after the completion of the lower step construction, the average displacement of the first-layer initial support ranges from 18.4 mm/day to 22.3 mm/day. Before anchor installation, the tunnel’s first-layer initial support had an average displacement of 14.3 mm/day to 17.3 mm/day (Figure 6D). After the concrete of tunnel arch casting and tunnel stabilization, the total cumulative displacement of the first-layer initial support ranges from 344.6 mm to 424.6 mm (Figure 6A). Similarly, after the completion of anchor installation and second-layer of initial support construction, the average displacement of the second layer of initial support ranges from 2.4 to mm/day 2.6 mm/day (Figure 6D). The average displacement of the second-layer initial support before arch construction is 2.9 mm/day to 3.4 mm/day (Figure 6D). Following arch construction, the average displacement of the second layer of initial support ranges from 2.1 mm/day to 2.3 mm/day (Figure 6D). The total cumulative displacement of the second layer of initial support ranges from 310.1 mm to 342.4 mm (Figure 6C). Despite a significant reduction in average displacement after arch construction, it still does not meet the requirement of an average displacement less than 2 mm/day. Therefore, merely replacing the upper structure does not ensure tunnel safety. To further investigate improved support schemes, we implemented an arch replacement strategy in the tunnel section located within the fault zone.

Moreover, Figure 7A,B show that after the completion of the lower step construction, the average displacement of the first-layer initial support ranged from 7.5 mm/day to 10.8 mm/day, with a cumulative displacement of 150.1 mm to 218.4 mm. Before anchor installation, the average displacement of the tunnel’s first-layer initial support was 7.2 mm/day to 11.3 mm/day (Figure 7B). The total cumulative displacement of the first-layer initial support ranged from 258.2 mm to 436.7 mm (Figure 7A). Additionally, after the completion of anchor installation and the second layer of initial support construction, the average displacement of the second-layer initial support ranged from 1.7 mm/day to 2.1 mm/day (Figure 7D). Before arch construction, the average displacement of the second-layer initial support is 1.8 mm/day to 2.2 mm/day (Figure 7D). Following arch construction, the average displacement of the second-layer initial support ranged from 0.9 mm/day to 1.5 mm/day (Figure 7D). The total cumulative displacement of the second-layer initial support ranged from 111.4 mm to 142.8 mm (Figure 7C). The average displacement after replacing the arch is less than 2 mm/day, indicating that this approach aids in providing support and protection in the tunnel within the fault zone.

In addition, we proceeded by replacing both the upper tunnel and the arch in the tunnel section crossing the fault zone area. Following the completion of the lower step construction, the average displacement of the first-layer initial support ranged from 9.2 mm/day to 11.1 mm/day (Figure 8B), with a cumulative displacement of 505.3 mm to 612.3 mm (Figure 8A). Before anchor installation, the average displacement of the first-layer initial support was 8.2 mm/day to 9.8 mm/day (Figure 8B). The total cumulative displacement of the first-layer initial support ranged from 590 mm to 708.7 mm (Figure 8A). After the completion of anchor installation and the second layer of initial support construction, the average displacement of the second-layer initial support ranged from 1.1 mm/day to 1.5 mm/day (Figure 8D). Before arch construction, the average displacement of the second-layer initial support was 1.3 mm/day to 1.8 mm/day (Figure 8D). Following arch construction, the average displacement of the second-layer initial support ranged from 0.4 mm/day to 0.5 mm/day (Figure 8D). The total cumulative displacement of the second-layer initial support ranged from 36.6 mm to 48.7 mm (Figure 8C). The average displacement after replacing the arch is less than 2 mm/day, indicating that this strategy is superior to the previous two, helping to maintain stability as the tunnel crosses the fault zone.

4.2. Numerical Simulation of Tunnel Deformation Support Scheme

Numerical methods such as the Finite Difference Method [27], Bezier Multi-Step Method [28], and Finite Element Method [8] can all be employed for studying tunnel deformations and support schemes. To validate this hypothesis, we utilized the FLAC3D 7.0 numerical simulation software to construct three tunnel support models based on support design parameters and geological survey reports. The dimensions of the computational model are 50 m × 50 m × 50 m. Displacement constraints were applied to the left, right, front, back, and bottom boundaries. The boundary loads in the X direction are 13.25 MPa, in the Y direction are 9.16 MPa, and in the Z direction are 8.50 MPa, with a boundary shear load of 1.15 MPa. The physical and mechanical parameters of the rock mass, tunnel, and support are detailed in

Table 3. Numerical calculation parameters.

Category	Young’s Modulus (GPa)	Poisson’s Ratio	Bulk Density (KN/m3)	Cohesive Force (MPa)	Internal Friction Angle (°)
Strongly weathered mudstone sandstone	2.20	0.33	23.00	0.30	35.00
Anchor rod	200.00	0.20	78.50	N/A	N/A
The first layer of initial support	32.05	0.20	22.00	N/A	N/A
The second layer of initial support	32.03	0.20	22.00	N/A	N/A
Lining	31.50	0.20	25.00	N/A	N/A

. Figure 9A shows the initial support structure, lining, and monitoring points of the tunnel model. Figure 9B shows the numerical model of the anchor for replacing the tunnel upper structure. The new anchor is installed after the second-layer initial support construction. The numerical simulation results indicate that the maximum settlement of the arch crown for the first-layer initial support is 0.06 mm, with a maximum displacement of 0.27 mm at the arch waist (Figure 9I). For the second-layer initial support, the maximum settlement of the arch crown is 0.08 mm, with a maximum displacement of 0.19 mm at the arch waist (Figure 9J). Furthermore, after replacing the upper structure, the tunnel’s arch crown experiences settlement, the surrounding rock at the arch waist is compressed towards the tunnel’s interior, and there is bulging at the arch base (Figure 9C–H). The displacements of the second-layer initial support are consistently smaller than those of the first-layer initial support, indicating that replacing the tunnel upper structure with anchor can reduce the displacement of the surrounding rock.

Furthermore, to investigate the feasibility of new replacement schemes, we developed a numerical model of a new anchor replacement for the tunnel invert. Figure 10A shows the numerical model of the anchor replacement for the tunnel invert. The numerical simulation results indicate that the maximum settlement of the arch crown for the first-layer initial support is 0.03 mm, with a maximum displacement of 0.18 mm at the arch waist (Figure 10B). For the second-layer initial support, the maximum settlement of the arch crown is 0.02 mm, with a maximum displacement of 0.09 mm at the arch waist (Figure 10C). Further, after replacing the upper structure, the tunnel’s arch crown experiences settlement, the surrounding rock at the arch waist is compressed towards the tunnel’s interior, and there is bulging at the arch base (Figure 10D–I). Comparing the displacement of the surrounding rock in the tunnel after replacing the invert with that after replacing the upper structure, it is evident that the replacement of the tunnel invert leads to reduced displacement, making it a more favorable solution than the previous one.

Furthermore, we develop a new anchor replacement scheme for the tunnel arch and invert. Figure 11A shows the numerical model of the anchor replacement for the tunnel arch and invert. The numerical simulation results show that the maximum settlement of the arch crown for the first-layer initial support is 0.007 mm, with a maximum displacement of 0.11 mm at the arch waist. For the second-layer initial support, the maximum settlement of the arch crown is 0.007 mm, with a maximum displacement of 0.05 mm at the arch waist. Furthermore, after replacing the upper structure, the tunnel’s arch crown experiences settlement, the surrounding rock at the arch waist is compressed towards the tunnel’s interior, and there is bulging at the arch base (Figure 11D–I). Moreover, the anchor bolt support scheme proposed by Cui et al. [23] resulted in arch crown settlement of 0.049 mm, whereas our proposed support scheme reduced settlement by 85.71%. In the support scheme suggested by Zhang et al. [3] the maximum arch crown settlement was 0.03 mm, while our proposed support scheme decreased settlement by 76.67%. Comparing the displacement of the surrounding rock in the tunnel after replacing the upper structure and invert with that after replacing the upper structure, it is evident that the replacement of the tunnel upper structure and invert leads to reduced displacement, making it a more favorable solution than the previous one. The potential reason for this could be that the tunnel itself is situated within a complex stress environment where both the arch and invert have undergone significant deformation. Merely replacing one of these components is insufficient to withstand the complex stress. Only by replacing the upper tunnel structure and the invert can the system effectively resist the complex stress.

5. Conclusions

This paper focuses on the underground tunnel in the Chenghai fault zone in southwestern China. In the fractured weak fault zone, the rock mass exhibits low strength and poor self-stability. The geological conditions are intricate, leading to significant deformations and collapses when the tunnel traverses the fault zone, resulting in geological hazards. This research delves into the deformation characteristics and mechanical behavior of the tunnel, proposing a new support scheme to address tunnel deformations. The main conclusions are as follows:

(1)

Considering the influence of stress redistribution caused by tunnel excavation on the measurement results, the maximum principal stresses obtained through in situ stress testing from horizontal and vertical boreholes are determined to be 13.12 MPa and 16.09 MPa, respectively. The maximum horizontal principal stress has a prevailing direction of NE75°, with an angle range of 75° to 85° relative to the axis orientation of the unexcavated tunnel section. The tunnel is situated in an environment of extremely high in situ stress.

(2)

Extensive deformation and damage occurred within the tunnel near metamorphic mudstone, limestone, and fault zones. The initial support exhibits significant deformation, high deformation rates with slow convergence, prolonged deformation duration, and non-uniform deformation leading to potential structural failure.

(3)

To further verify the cause of tunnel deformation, we can obtain the internal force of tunnel support by monitoring the internal force of the tunnel. The maximum surrounding rock pressure on the right shoulder of the first-layer initial support reached 1000 kPa, while the maximum surrounding rock pressure on the left shoulder of the second-layer initial support reached 500 kPa. Additionally, the maximum surrounding rock pressure in the center of the second-layer initial support invert reached 1200 kPa, and the maximum surrounding rock pressure on the left shoulder of the second-layer lining reached 680 kPa.

(4)

A comparison of tunnel deformation support schemes is conducted through field experiments and numerical simulation calculations, indicating that replacing the tunnel upper structure and invert can effectively prevent tunnel deformations.

The study primarily focuses on the in situ stress distribution characteristics of mountainous weak and fractured rock layer tunnels, identifying and analyzing the types of defects encountered during construction while proposing corresponding remedial measures. It is essential to conduct further comparative analyses of the deformation patterns of tunnels in different geological layers. Additionally, future research will integrate numerical modeling techniques to investigate tunnel deformations, failures, and support strategies under various influencing factors such as rock layer angles and lithology.