Study on the Asymmetric Failure Characteristics and Failure Mechanisms of Surrounding Rock During Excavation of a Deep Buried Small-Clearance Tunnel

Abstract

In deeply buried, small-clearance tunnels, the failure of the surrounding rock is profoundly influenced by the superposition of stresses and the cumulative disturbance effects from multiple blasting events. Consequently, the failure characteristics and mechanisms of the surrounding rock are highly complex. Through a comprehensive analysis encompassing failure investigations, geological assessments, and surrounding rock pressure monitoring, this study systematically examines the spatio-temporal failure characteristics and geological discrepancies across 3 parallel tunnels (namely, a pilot tunnel, a left tunnel, and a right tunnel). The analysis reveals the asymmetric failure behavior of the surrounding rock and offers a detailed discussion of the underlying mechanisms. The temporal and spatial evolution of the surrounding rock pressure in these tunnels is carefully analyzed, with an emphasis on uncovering the asymmetric failure mechanisms during the excavation of deep, small-clearance tunnels. The results demonstrate that the failure of the surrounding rock exhibits significant asymmetry during excavation, with the damage being more pronounced on the valley side compared to the mountain side. Furthermore, the degree of damage in the advance tunnel is substantially greater than that in the backward tunnel, particularly in sections following the excavation of the backward tunnel. Additionally, the distribution of the surrounding rock pressure in the advance tunnel also exhibits pronounced asymmetry. The asymmetric failure of the surrounding rock is primarily attributed to the stress concentration in the deep valley and the disturbances introduced by the excavation process, which induces tangential stress concentrations in the surrounding rock mass. The findings of this study hold considerable significance for the design and optimization of tunnel support systems, as well as for disaster prevention strategies in deeply buried, small-clearance tunnels.

1. Introduction

With the improvement in the world economy and the level of science and technology, the need to develop underground space and resources is increasing rapidly. The scale of the construction of transportation and hydropower infrastructure continues to expand, with hydropower diversion tunnels, railway tunnels, and highway tunnels becoming increasingly longer, larger, and deeper [1,2,3,4,5,6]. The southwest region, characterized by significant topographical relief, intense tectonic activity, and widespread construction projects, is particularly affected by high tectonic stress due to the squeezing of mountains by plate tectonics [7]. The complex geological environment, varied lithology, and high ground stress conditions prevalent in this region pose significant risks, leading to engineering disasters such as rockbursts, severe compressive deformations, and time-dependent failures of support structures [8,9,10,11,12,13,14]. These challenges severely impact the design, construction, and long-term operation of tunnels in this area [15].

Both domestic and international scholars have extensively studied the deformation and failure characteristics of shallow-buried, small-clearance tunnels. Tyagi [16] found that the effects of failure of a nearby tunnel on an otherwise stable tunnel in improved soil surround is sensitive to the center-to-center spacing between the two tunnels. Huang [17] explored the effects of blasting during the construction of shallow-buried urban tunnels and found that blasting had the most significant impact on the arch of existing tunnel sections. Lei [18] identified the failure mode of shallow-buried, offset small-clearance tunnels as an inverted conical fracture, emphasizing the critical role of micro-fracture surfaces and tensile fractures in the tunnel surrounding rock. Zhang [19] analyzed stress and deformation in small-clearance tunnels in composite strata and concluded that arch deformation and ground settlement are more pronounced in the backward tunnel when the soil–rock interface is located below the arch foot. Wu [20] compared the deformation of double-arch tunnels under asymmetric loads, identifying greater surrounding rock deformation on the shallow-buried side. Chen [21] indicated that asymmetric loading is a primary cause of local excessive deformation and failure in shallow-buried small-clearance tunnels. Other studies have focused on tunnel spacing, deformation monitoring, and model simulations to optimize construction sequencing and prevent failure [22,23,24,25,26].

In the study of the failure characteristics and mechanisms of deep tunnels and caverns, extensive research has been conducted across various projects. He [27] investigated the characteristics and causes of floor uplift in deep mine roadways through field failure investigations, theoretical analysis, and field testing, revealing the mechanism of asymmetric floor uplift. Wang [28], combining numerical simulations and field investigations, analyzed the instability, deformation, and failure modes of deep roadways. His research focused on the displacement, stress, and plastic expansion processes, elucidating the evolution of damage and failure in the surrounding rock, while also uncovering the temporal and spatial characteristics of surrounding rock deformation and failure. Jiang [29] analyzed 70 marble spalling samples from the second phase of the China Jinping Underground Laboratory (CJPL-II), examining their microstructural morphology and failure mechanisms. His findings clarified that the failure characteristics of the samples are primarily indicative of brittle fracture. Liu [30] used numerical simulations to study failure mechanisms in deep-buried hydropower tunnels, emphasizing the influence of bedding angles and principal stress directions. Sun [31] explored the deformation of tunnels in inclined strata, noting significant asymmetry in failure patterns. In summary, while research on the failure characteristics and mechanisms of deep-buried powerhouses and tunnel surrounding rock is relatively well established, there remains a paucity of studies focused on the failure characteristics of deep-buried small-clearance tunnels, with the underlying mechanisms yet to be fully understood.

This study is devoted to exploring the asymmetric failure mechanism of surrounding rock during the excavation of a deep-buried small-clearance tunnel. In this paper, the damage investigation, geological analysis, and surrounding rock pressure monitoring are carried out on three deep-buried small-clearance parallel tunnels. Firstly, the temporal and spatial failure characteristics and geological conditions of each tunnel are compared, and the asymmetric failure characteristics of the surrounding rock are expounded. Then, the evolution characteristics of the surrounding rock pressure in these tunnels are analyzed. Finally, the causes of the asymmetric failure of the surrounding rock during the excavation of the deep-buried small-clearance tunnel are discussed. These findings offer valuable insights for the design, construction, and disaster prevention of similar tunnel projects.

2. Engineering Background and in Situ Monitoring Methods

2.1. Engineering Background

The deep-buried small-clearance tunnel is designed to be excavated using the drilling and blasting method along three parallel alignments. The distance between adjacent tunnel walls is approximately 11.7 m, with a maximum burial depth of around 2080 m. The terrain traversed by the tunnel is characterized by steep slopes, deeply incised gullies, and highly developed “V”-shaped valleys. A partial section of the tunnel is depicted in Figure 1. The primary external forces influencing the tunnel are ice, snow, and cold weathering. The geological structure of the tunnel area is complex, with tectonic movements predominantly exhibiting compressive stress in the north-northeast direction. The lithology in the area is highly varied, with significant metamorphic variation in the local gneiss. The rock formations primarily consist of gneiss (90.2%) and granite (9.8%), with interspersed layers of schist and quartzite.

The rock mass in this section is relatively fractured, with well-developed joints and a f5–8 fault. The fault is initially speculated to exhibit a left-lateral strike-slip motion, with a fault strike orientation of N35°–50° W and a dip direction of 70°–85° N. The fault is approximately 120 m wide and intersects the tunnel alignment at a steep angle of around 87°, and the intersection mileage is DK193+135~DK193+255. The fault is predominantly composed of crushed rock and fault breccia, with localized altered rock formations. The location plane of the fault and tunnel is shown in Figure 2.

2.2. In Situ Monitoring Method of Surrounding Rock Pressure

The in situ monitoring scheme primarily involves the selection of appropriate equipment, the layout of sensors, and the design of data acquisition protocols.

2.2.1. In Situ Monitoring Scheme

The detailed scheme is as follows:

(1).

Equipment selection.

To ensure the effectiveness of the monitoring system, a comprehensive comparison of existing monitoring equipment and methods was conducted, taking into account their applicability, technical feasibility, and compatibility with the specific conditions of the tunnel environment. The selection criteria included factors such as accuracy, durability, and the ability to perform under challenging conditions. A summary of the comparative analysis is presented in

Table 1. Comparison of monitoring equipment and methods [32,33,34,35].

Monitoring Methods	Principle	Precision	Application Scenario	Limitations
Vibrating string pressure box	String vibration frequency–pressure conversion relationship	±0.1% FS	High humidity environment and long-term stable monitoring.	Slow dynamic response
Fiber grating sensor	Grating wavelength offset-strain relationship	±0.01% FS	Distributed monitoring and anti-electromagnetic interference.	High cost and complex installation.
Strain type bolt sensor	Elastic mechanics strain–stress relationship	±0.5% FS	Deep surrounding rock, local stress concentration area.	Affected by anchoring agent.
Displacement inverse analysis	Displacement-pressure numerical inversion	Depends on the accuracy of inversion model	The sensor cannot be deployed directly under complex geological conditions.	High calculation complexity and uncertainty.
Vibrating string pressure box	String vibration frequency-pressure conversion relationship	±0.1% FS	High humidity environment and long-term stable monitoring.	Slow dynamic response

.

Considering the geological and environmental conditions of the surrounding rock, including high temperature and water-rich environments, the selected sensors must exhibit high temperature resistance and corrosion resistance, while meeting the demands of long-term monitoring and high accuracy. Therefore, a vibrating string pressure sensor was chosen for this purpose, as it satisfies these critical requirements. The sensor used in this paper is the JMZX-3001 type intelligent vibrating string pressure box of Changsha Jinma Measurement and Control Technology Co., Ltd. in Changsha City, China. The pressure accuracy is ±0.1% FS and the temperature accuracy is ±0.5 °C.

(2).

Sensor layout.

The sensor layout design was guided by the following principles:

(1)

Given the potential blasting disturbances and unloading effects due to the excavation of adjacent tunnels, the entire section of the tunnel may be subjected to negative impacts. It has been shown that the pressure sensor should be evenly distributed in the tunnel section to ensure the accuracy of monitoring the key parts of each section [36]. When Yang [37] carried out the study of large tunnel deformation, the layout scheme of the surrounding rock pressure monitoring points involved arranging eight sensors uniformly in the tunnel section. As a result, eight pressure sensors are evenly distributed along the tunnel section, with each monitoring section arranged in a single row (Figure 3).

(2)

The positioning of the sensors was determined based on the geological conditions of the surrounding rock encountered during excavation. Pressure sensors are strategically placed at locations corresponding to the most unfavorable structural planes.

(3)

The first set of eight pressure sensors in a single row is arranged within the first borehole, while the second and third sets of pressure sensors are installed sequentially in subsequent boreholes within the same section.

The excavation sequence of the three tunnels is that the parallel pilot tunnel is used as the first tunnel excavation, the right line starts to follow after the excavation of more than 200 m, and the left line starts to advance after the excavation of more than 150 m. Due to the small clearance between the tunnels, the surrounding rock of each tunnel has different degrees of damage after the excavation of the left line, and the parallel pilot tunnel is the most serious. In order to facilitate the monitoring of the changes in the surrounding rock pressure when the face of the adjacent tunnel is close to and away from the section, the preliminary design is to install the first group of pressure sensors in the parallel pilot tunnel to determine the mileage of the monitoring section. When the two backward tunnels of the left and right lines follow up, the second group and the third group of pressure sensors are installed in turn at the monitoring section. The relative diagram of the installation process and position of the pressure sensor is shown in Figure 4.

2.2.2. Basic Theory of in Situ Monitoring of Surrounding Rock Pressure

By embedding or installing sensors, real-time monitoring of the mechanical interactions between the tunnel surrounding rock and its supporting structure is facilitated. The surrounding rock pressure is converted into measurable signals, enabling the evaluation of the stability and engineering safety of the surrounding rock through data analysis and processing. As the surrounding rock deforms, it induces a corresponding deformation in the pressure surface of the vibrating wire pressure sensor. This deformation results in strain on the steel wire within the pressure sensor. The theoretical relationship between the vibration frequency of the steel wire and the tensile stress is given by [38]:

$$f={\displaystyle \frac{1}{2l}}\sqrt{{\displaystyle \frac{{\sigma }^{\prime }}{\rho }}}$$

(1)

In Formula (1), f is the natural vibration frequency of the steel string (Hz), l is the length of the steel string (mm), ρ is the density of the steel string, and σ′ is the tensile stress borne by the steel string (MPa).

According to the calculation theory of axisymmetric bending of elastic thin plate, it can be obtained:

$$tg\theta ={\displaystyle \frac{pl}{128D}}{(l}^2-L^2)$$

(2)

In Formula (2), p is the uniform load acting on the bearing plate (MPa), l is the steel string length (mm), L is the diameter of the bearing plate (mm), D is the bending stiffness of the bearing plate, and $D=E·t^3/12(l-{\mu }^2),$ where E is the elastic modulus of the bearing plate (MPa) and μ is the Poisson ratio of the bearing plate. t is the thickness of the bearing plate (mm).

As the deformation of the bearing plate is very small, tg θ ≈ sin θ. Based on Hooke’s law, Equation (2) can be substituted to deduce:

$$\sigma =E{\displaystyle \frac{∆l}{l}}=E{\displaystyle \frac{2bsin\theta }{l}}=E{\displaystyle \frac{bp}{64D}}{(l}^2-L^2)$$

(3)

In Formula (3), b is the height of the column (mm).

By combining Formulas (1) and (3) and considering the stress sign in elasticity, it can be obtained:

$$p={\displaystyle \frac{256l^2\rho ·D}{Eb{(L}^2-l^2)}}{(f}^2-f_0^2)$$

(4)

In Formula (4), f is the frequency of the steel string under the action of external force p of the pressure box, and f₀ is the initial frequency of the steel string.

In order to meet the needs of the project, the measured stress of the pressure box can be corrected.

2.2.3. Principles for in Situ Monitoring

During the in situ monitoring of the surrounding rock pressure, the following requirements must be met:

(1).

Sensor installation.

The pressure membrane of the pressure sensor must be in close contact with the rock surface. If the rock surface is uneven, leveling pretreatment should be carried out prior to installation. The steel frame should establish good contact with the pressure box to ensure proper stress transfer. The installation effect is shown in Figure 5.

(2).

Calibration and protection.

After the installation of the pressure sensor, its identification numbers should be calibrated. Additionally, protective measures must be implemented for the data transmission lines to ensure uninterrupted data transmission, as illustrated in Figure 6.

(3).

Pre-installation debugging.

Prior to installation, the pressure sensor should be thoroughly calibrated and debugged. Following installation, the first set of pressure data should be recorded to confirm the system’s functionality.

(4).

Data collection protocol.

During data collection, special attention should be given to ensuring that the transmission wire is properly inserted into the acquisition instrument interface. Data should only be read once the values have stabilized, ensuring accuracy and reliability.

(5).

Data integrity.

Data collection should occur once daily to maintain continuity and ensure the integrity of the dataset.

2.2.4. In Situ Monitoring Results

The data acquisition interface is illustrated in Figure 7. The collected data include key parameters such as the pressure sensor model, identification number, measured pressure values, ambient temperature, and steel string frequency. The data results are archived to establish a comprehensive dataset. This dataset is then processed through calculations and statistical analysis to determine the real pressure values for each monitoring point on a daily basis. Subsequently, the spatiotemporal variation trends of the surrounding rock pressure are analyzed.

3. Investigation Results

In the process of tunnel construction, there are many serious deformation and damage phenomena of the surrounding rock, mainly in the left wall of the tunnel and the left arch shoulder. The specific damage phenomena are as follows: concrete spray layer swelling, cracking, falling off, leakage, steel frame buckling, and side wall concrete spray layer sheeting. Therefore, the damage spatial position of three tunnels parallel to each other is investigated, and the damage types and spatial damage characteristics are analyzed.

3.1. Failure Characteristics of the Parallel Pilot Tunnel

The surrounding rock damage during the excavation of the parallel pilot tunnel is shown in Figure 8. The main failure phenomena include floor uplift, cracking and local dislocation, bulging and cracking of the concrete spray layer, steel frame buckling and deformation, and slab slope of the concrete spray layer.

As can be seen from Figure 8, there are multiple failures in the parallel pilot tunnel, the damage degree is relatively serious, and the spatial location of the damage is relatively dispersed. Within a few days after excavation, no obvious cracks were observed in the concrete spray layer and no obvious deformation was observed in the steel arch. With increase in time, the face of the palm continues to advance, and the concrete spray layer 60–80 m away from the rear of the face exhibits many cracks and peels. Further surrounding rock cracks extend, and the concrete spray layer is stretched to form deep cracks (Figure 8a). In more serious cases, steel frame buckling occurs, resulting in floor dislocation and through cracks (Figure 8b). After the initial concrete shotcrete support, the slab wall occurs in the parallel section after the excavation of the backward tunnel (Figure 8c); the thickness of the slab wall of the concrete shotcrete layer is generally 20–30 cm, with the maximum thickness reaching 50 cm. After the initial support is destroyed, concrete supplementary spraying is adopted in time, and cracks and spalling of concrete still occur several days after supplementary spraying. Although the phenomenon is manifested as the deformation and failure of the concrete spray layer, it actually reflects the damage to the internal surrounding rock. The internal surrounding rock exhibits tensile or dilatancy failure, resulting in the cracking or falling off of the concrete surface.

In terms of space, the main failure characteristics of the arch are the spalling of the initial concrete spray layer and the exposure of the steel frame, while the main failure characteristics of the bottom plate are cracking of the arch, with a crack length of 3–5 m. The damage characteristics of the side walls on both sides are more complex, and the damage degree of the side walls on both sides is more serious than that of the arch. The damage degree of the left side wall is more serious than that of the right side, and the main damage characteristics of the side walls on both sides are the sheet wall caused by tensile stress. Swelling of the concrete spray layer is common in the left arch shoulder and arch, and the swelling and cracking of the concrete spray layer has obvious segmenting characteristics (Figure 8a,c). The cracks are generally 0.5–2.0 m long and up to 9 m long, and the opening width is generally 0.2–2.0 cm and up to 8 cm. The distribution patterns of cracks are different—mainly developing horizontally and extending in a zigzag pattern of intermittent bending (Figure 8a). The shedding phenomenon of the concrete spray layer is widely distributed, the shedding area varies, and the shape is irregular, including long strip, sawtooth, groove, trapezoidal plane, etc. The bending of the steel frame can be seen at the part of the concrete spray layer falling off (Figure 8d), which reflects the large tangential stress of the surrounding rock at this part.

3.2. Failure Characteristics of the Right Line Tunnel

The failure of the surrounding rock on the right line tunnel is shown in Figure 9. The failure phenomena mainly include cracking, spalling, and leakage of the concrete spray layer. As can be seen from Figure 9, the damage degree of the right-side large-mileage tunnel is slightly higher than that of the plane-oriented large-mileage tunnel, and the damage area is concentrated in the left arch shoulder. Within one week after excavation, a small number of cracks and leakage occurred in the concrete spray layer (Figure 9a), and no obvious deformation was observed in the steel arch. With increase in time, the face of the palm continued to advance, and the left arch shoulder concrete spray layer 80–120 m away from the face of the palm exhibited several cracks and spalling. The cracking and spalling failure of the concrete spray layer showed regional and zonal characteristics (Figure 9b).

In terms of space, the main failure characteristics of the arch in the early stage of surrounding rock stability are the cracking of the initial concrete spray layer due to convergence deformation and water leakage in the surrounding rock fissure. The cracks are mostly 0.5–1.5 m in length and up to 2.5 m in length and less than 0.5 cm in width. The cracks and spalling of the concrete spray layer appear in the left part of the wall, while the cracks in the concrete spray layer rarely occur in the right-side wall. The damage degree of the left-side wall is more serious than that of the right-side wall.

3.3. Failure Characteristics of the Left Line Tunnel

The surrounding rock damage of the left line tunnel is shown in Figure 10. The main damage phenomena are leakage of the concrete spray layer, swelling, cracking, spalling and steel frame exposure of the concrete spray layer. As can be seen from Figure 10, the damage degree of the left line tunnel is less marked than that of the right line tunnel, and the damage area is concentrated in the left arch shoulder and arch. Within a few days after excavation, no obvious cracks were observed in the concrete spray layer and no obvious deformation was observed in the steel arch frame. With the passage of time, the face of the palm continued to advance, and the left arch concrete spray layer 80–120 m away from the face of the palm exhibited several cracks and spalling (Figure 10a). The local concrete spray layer of the left arch shoulder exhibited extrusion cracks (Figure 10b).

In terms of space, the main failure characteristics of the arch are the cracking, spalling of the initial concrete spray layer, and the exposure of the steel frame due to the convergence and deformation of the surrounding rock and the leakage of fissure water. The main failure characteristics of the left arch shoulder are the swelling and cracking of the initial branch concrete spray layer due to the convergence and deformation of the surrounding rock. The cracks are scattered. As a backward tunnel, the left line tunnel has not experienced another disturbance after excavation, and the damage degree is lighter than that of the other two tunnels. In terms of spatial distribution, the damage is still concentrated on the left side of the tunnel, and there are few cracks in the right side of the tunnel.

3.4. Comparative Analysis of Spatial Failure Characteristics and Geological Conditions Across Different Tunnels

3.4.1. Comparison of Spatial Failure Characteristics

The comparison reveals that the failure locations of each tunnel exhibit spatial asymmetry, with the valley side experiencing significantly more severe damage than the mountain side, as shown in Figure 11. The causes and mechanisms of failure vary across different locations. Cracking in the tunnel arch and floor is primarily attributed to the convergence of unloading deformation and stress concentration under high ground stress conditions. The failure of the left wall and left arch shoulder is primarily caused by the stress associated with the deep valley on the left side and the stress concentration resulting from high ground stress. Conversely, the failure of the right arch shoulder is related to the presence of unfavorable structural planes extending from the right side to the arch top. The right wall is caused by excavation unloading and construction blasting disturbance after the excavation of the right line tunnel. The failure location of the right line is relatively concentrated in the left wall and left arch shoulder, which is mainly caused by the convergence of excavation unloading deformation and stress concentration under the condition of high ground stress. The failure location of the left line is relatively concentrated in the arch and the left arch shoulder, which is mainly caused by the convergence of excavation unloading deformation under high ground stress. It is worth noting that the left arch shoulder steel arch deformation is caused by excessive tangential stress concentration, while the surrounding rock deformation towards the free face makes it difficult to produce the above phenomenon. Therefore, it can be inferred that the failure of the surrounding rock in this part is caused by high stress concentration, rather than deformation, and the failure is a typical high ground stress failure.

By comparing the failure sections, it is found that the deformation and failure of the parallel pilot tunnel are longer than that of both the right and left line tunnels. The statistical table of the paragraphs with serious deformation and failure is shown in

Table 2. Statistical table of the serious deformation and damage of each tunnel.

Number	Tunnel	Severely Deformed and Damaged Sections	Length (m)	Maximum Amount of Deformation
1	The left line	DK1193+114~DK1193+126	12	Settlement of the arch: 77 mm. Convergence value of the side wall: 255 mm.
2		DK1193+265~DK1193+430	165
3		DK1193+550~DK1193+630	80
4		DK1193+690~DK1193+873	183
5	The right line	DyK1193+130~DyK1193+165	35	Settlement of the arch: 52 mm. Convergence value of the side wall: 155 mm.
6		DyK1193+600~DyK1193+675	75
7		DyK1193+706~DyK1193+870	164
8	Parallel pilot tunnel	PK1193+130~PK1193+270	140	Settlement of the arch: 122 mm. Convergence value of the side wall: 199 mm.
9		PK1193+600~PK1193+750	150
10		PK1193+760~PK1193+900	140
11		PK1194+130~PK1194+180	50

. In the statistical sections of the parallel pilot tunnel, PK1193+130~PK1193+270, PK1193+600~PK1193+750 and PK1193+760~PK1193+900, the damage phenomenon occurs again after the spray layer is damaged, and the upper damage frequency is more than two times.

In terms of failure intensity, the cracks in the parallel pilot tunnel are generally 0.5–2.0 m long and up to 9 m long, and the opening width is 0.2–2.0 cm and up to 8 cm. In the severely damaged section, the area of the damaged area accounts for about 50% of the total area. Most of the cracks in the large-mileage tunnel on the right line are 0.5–1.5 m in length and up to 2.5 m in length, and less than 0.5 cm in width. In the severely damaged sections, the area of the damaged area accounts for about 20% of the total area. Most of the cracks are less than 0.5 m in length and less than 0.5 cm in width in the left line large-mileage tunnel, and the area of the damaged area in the severely damaged section accounts for about 10% of the total area.

3.4.2. Comparison of Geological Conditions

The geological disclosure of the palm face at the same mile in the direction of the big mileage for each tunnel is compared in

Table 3. Geological conditions of each tunnel.

Geological Conditions	The Left Line Tunnel	The Parallel Pilot Tunnel	The Right Line Tunnel
Stratigraphic lithology	Gneiss, grayish white, gneiss-like structure, mainly composed of quartz, feldspar, mica and so on.	Gneiss, gray-black, gneiss-like structure, mainly composed of quartz, feldspar, mica and so on.	Gneiss, cyan-gray mixed with grayish white, gneiss-like structure, mainly composed of quartz, feldspar, mica and so on.
Rock mass integrity	Weak weathering, relatively developed joints, two groups of main joints developed; the rock mass is relatively intact.	Weak weathering, more developed joint fissure, development of Group 2 main joints; there is a slight kneading and extrusion phenomenon on the palm surface, the rock mass is wedged and blocky, the rock mass is more complete, but there is local fragmentation.	Weak weathering, the joints are more developed, and the palm surface is slightly crumpled. A group of joints runs through the whole palm surface, the rock mass is more complete, and the whole rock mass is blocky.
Compressive strength σc (MPa)	61.4	91.4	57.8
Groundwater seepage rate [L/(min·10m)]	2	4	2
Occurrence of gneiss	N39° E/80° S	N51° E/84° S	N46° E/82° N
Joint fissure	J1: N32° E/48° S, micro-tensile joint, rough joint surface, no filling, joint spacing of 0.6–0.7 m, joint extension of 3.0–4.0 m. J2: N47° E/34° N, micro-tensile joint, rough joint surface, no filling, joint spacing 0.7–0.8 m, joint extension 5.0–7.0 m.	J1: N38° E/65° S, micro-tensile joint, rough joint surface, no filling, joint spacing of 0.7–1.1 m, joint extension of 3.0–5.0 m. J2: N50° W/48° N, micro-tensile joint, rough joint surface, no filling, joint spacing 0.6–0.9 m, joint extension 2.0–4.0 m.	J1: N50° E/46° S, micro-tensile joint, rough joint surface, no filling, joint spacing of 0.6–0.8 m, joint extension of 4.0–9.0 m. J2: N47° W/39° N, micro-tensile joint, rough joint surface, no filling, joint spacing of 0.8–1.1 m, joint extension of 5.0–8.0 m.
Buried depth (m)	837	938	957

.

By comparison, it is found that the stratigraphy of the three tunnels is gneiss. However, there are obvious differences in the integrity of the rock mass. The parallel pilot tunnel rock mass is wedge-shaped and blocky, the joints and cracks are relatively developed, and two main joints are developed. The rock mass of the right line has good integrity, with one group of main joints developed, and the whole rock mass is blocky. The rock mass of the left line is complete and the joints are developed. Two groups of main joints are developed, and the integrity of the rock mass is better than that of the parallel pilot and the right line tunnel. In addition, the compressive strength of rock with the parallel pilot tunnel is higher than that of the left and right lines tunnel, the seepage flow of groundwater is twice that of the left and right lines, and the buried depth is higher than that of the right lines and far higher than that of the left lines. To sum up, the joints and cracks may be the reason why the damage degree of the parallel pilot tunnel is higher than that of the other two tunnels.

4. Discussion

4.1. Evolution Characteristics of Surrounding Rock Pressure

To further investigate the failure mechanism of the surrounding rock, the monitoring data of the surrounding rock pressure from the parallel pilot tunnel were analyzed. The pressure box was installed on 16 May 2024 to determine the initial surrounding rock pressure, with the initial supporting process completed on 17 May 2024. By 31 October 2024, a total of 169 days of monitoring data had been collected. The time-dependent variation in the surrounding rock pressure is shown in Figure 12. For the purpose of analyzing the evolution characteristics of the surrounding rock pressure, the monitoring period was divided into four distinct phases: the early increasing period, the stable pressure period, the pressure fluctuation period, and the secondary stable pressure period. Notably, from 16 May 2024 to 25 June 2024, there was a significant increase in the surrounding rock pressure following the initial support of the parallel pilot tunnel excavation. This resulted in a pre-growth phase of the surrounding rock pressure lasting approximately 40 days, during which the surrounding rock pressure steadily increased to its maximum value. Subsequently, the local surrounding rock surface was damaged, resulting in a drop in the pressure value. In the following 50 days or so, the surrounding rock pressure entered a stable period. About 20 days before the working face of the right line tunnel was about to reach the monitoring section of the parallel pilot tunnel, the surrounding rock pressure enters a change period. During this period of time, the construction of the parallel pilot tunnel was stagnant. There were pieces falling off near the monitoring section, and the data collection process was dangerous, so some data were missing. After the working face of the right line tunnel passes through the monitoring section, the surrounding rock pressure again enters a stable period.

As can be seen from Figure 12, in the early growth period, the surrounding rock pressure caused by the deformation of the surrounding rock gradually increased with increase in time after mixing. In the two weeks after the completion of excavation and spraying, the surrounding rock pressure increases gently, and in the third week, the growth rate of the surrounding rock pressure increases. One month after the completion of excavation, the surrounding rock pressure increases to a higher level, and its growth rate slows down, and the surrounding rock pressure remains at a high level. From June 5 to 13, the surrounding rock pressure increased to a greater extent. It can also be seen from Figure 12 that in the early growth period, the overall growth rate of the surrounding rock pressure of the No. 4 and No. 5 sensors at the arch top was faster than that of the No. 6 and No. 7 sensors at the left arch shoulder and the left wall, while the growth rate of the surrounding rock pressure of the No. 1 and No. 3 sensors at the right wall and the No. 8 sensors at the left arch foot was slower. The spatial evolution trend of the surrounding rock pressure in the monitoring section is that the growth rate of the arch is greater than that of the two sides of the wall, and the growth rate of the left side is greater than that of the right side.

The measured distribution of the deformation pressure of the surrounding rock of the monitoring section with the parallel pilot tunnel on 23 June 2024 is shown in Figure 13. During the initial support process of concrete spraying, the data line of sensor No. 2 on the right-side wall of the monitoring section was cut off in the concrete, and the surrounding rock pressure value was 0 MPa. When calculating the pressure value of sensor No. 2 in Figure 13, this paper adopts the interpolation of the monitoring results of sensors No. 1 and No. 3. It can be seen from the monitoring results of other normal working pressure sensors that the distribution of the surrounding rock pressure is asymmetrical. The surrounding rock pressure of the left side wall of the cross-section is greater than that of the right-side wall, and the surrounding rock pressure of the arch is greater than that of both sides of the wall. It is worth noting that the surrounding rock pressure at measuring point No. 4 of the arch reaches the maximum value of 0.809 MPa.

To sum up, the spatial distribution characteristics of the surrounding rock pressure in the parallel pilot tunnel section are the highest on the left arch shoulder, second highest on the arch top, and smallest on the left wall.

4.2. Analysis of the Asymmetric Failure Mechanism

According to the regional geological structure data, the tunnel engineering area is a typical alpine canyon area, and the current tectonic stress field is mainly in the north-northeast compressive stress state. The maximum principal stress direction of the regional tectonic stress is near NE. The ground stress field is mainly tectonic stress, and the rock mass is mainly hard and relatively complete metamorphic gneiss. According to the exploration and measurement results in the early exploration stage, the maximum horizontal principal stress of the tunnel is 11.93–59.35 MPa, and the direction is N48° E–N86° E. The minimum horizontal principal stress of the tunnel is 14.8–18.18 MPa, and the direction of the minimum horizontal principal stress is N28° W. The direction of the principal stress is projected on each section of the tunnel, as shown in Figure 14. The maximum principal stress direction intersects with the tunnel axis at a small angle and with the rock surface at a large angle, which is conducive to the stability of the surrounding rock of the cavern. But the minimum principal stress direction intersects with the tunnel axis at a large angle, which is not conducive to the stability of the surrounding rock of the cavern. The maximum principal stress direction intersects with the tunnel axis at a small angle and with the rock surface at a large angle, which is conducive to the stability of the surrounding rock of the cavern. But the minimum principal stress direction intersects with the tunnel axis at a large angle, which is not conducive to the stability of the surrounding rock of the cavern.

In this paper, three-dimensional discrete element software (3DEC V7.0) is adopted for numerical simulation, and the model size is 150 × 100 × 40 m (length, height, thickness). In order to reasonably control the density of the grid elements and the fineness of the model, the dimensions of the grid elements for the tunnel, the inner surrounding rock, and the outer surrounding rock are 1, 2, and 4 m, respectively. The block model adopts the strain softening model, and the joint model adopts the Coulomb slip joint model. Normal displacement constraints are imposed on the model boundaries, and initial stresses are directly applied to the blocks and elements. Within the tunnel excavation range, the maximum principal stress value is approximately 24 MPa.

The maximum principal stress, minimum principal stress, and maximum shear stress of the surrounding rock after simulated excavation are shown in Figure 15. It can be seen from Figure 15 that the compressive stress concentration occurs on the left arch shoulder and the right arch foot of the tunnel, and the stress value can reach 39 MPa. Tensile stress concentration occurred within approximately 0.5 m of the surrounding rock on the surface of the tunnel, with the stress value reaching 1.4 MPa. Shear stress concentration also occurred on the left arch shoulder and the right arch foot of the tunnel, with the stress value reaching 35 MPa.

According to the above analysis, after the initial concrete injection, the pressure caused by the deformation of the surrounding rock gradually increases with increase in time. As a whole, the surrounding rock pressure of the tunnel valley side is greater than that of the mountain side, but the surrounding rock pressure of the arch is greater than that of the side wall on both sides. The tunnel passes through an area characterized by steep terrain and deep valleys, where a “V”-shaped valley intersecting with the tunnel body at large angle is extremely developed. This geological configuration has resulted in greater stress concentration on the left side of the tunnel compared to the right side. Therefore, when it is not affected by other geological structures, the surrounding rock pressure on the left side of the tunnel is relatively greater. Under the action of this uneven and asymmetric pressure, the steel arch is easy to deform and distort. In addition, due to the development of two groups of main joints in the surrounding rock of the monitoring section, the joints and rock layers cut each other, and the interlayer is generally combined. The two groups of main joints develop into a network cutting system, which divides the intact rock mass into diamond or wedge blocks, significantly reducing the overall compressive strength and self-stability of the surrounding rock. There is an unfavorable intersection relationship between the joint plane and the occurrence of rock strata, and it easily forms a penetrating structural plane combination, which provides a structural foundation for the block sliding. The combination of structural planes developed from the right waist to the top of the arch forms an “X”-type cross-joint system, whose intersection lines tend to the inner direction of the cave, which constitutes the spatial geometry conditions of the potential sliding surface. The secondary stress field of the surrounding rock forms a tensile stress concentration area in the arch roof, which combines with the original structural plane to produce a superposition effect, resulting in tensile shear compound failure. Therefore, at the beginning of monitoring, the surrounding rock pressure of the right arch is relatively large, which indicates that the deformation and failure of the right arch is relatively large under the influence of the unfavorable structure. After tunnel excavation, under the influence of excavation unloading, the rock mass around the tunnel changes from the state of three-way compression before excavation to the state of unconfined two-way compression or even tension. Under this action, the surrounding rock mass with joint development around the tunnel eliminates or weakens the slip along the structural plane due to lateral restriction and moves to the free plane.

In the short time after excavation, the pressure value of the surrounding rock changes little in the process of existing stress adjustment, and the steel arch does not show obvious deformation. Under the influence of deep valley stress and excavation disturbance, the tangential stress is concentrated on the left side of the tunnel, the surrounding rock pressure increases constantly, and the steel arch in some sections of the tunnel is exposed and even buckling. However, when the tunnel is excavated close to the monitoring section, the deformation and damage of the surrounding rock are aggravated by the blasting effect. In particular, the surrounding rock pressure from the left arch shoulder to the arch and the right arch shoulder increases further, and the dynamic disturbance and large surrounding rock pressure cause the buckling deformation of the steel arch in some areas.

Affected by the slow slip of the fault zone, there may be a potential risk of aging failure near it. In the later construction process, the deformation monitoring of the support and the surrounding rock should be strengthened, and the risk of local collapse should be prevented. The excavation activities of the parallel pilot tunnel and the right tunnel have fully released the energy and transferred the stress near the surrounding rock. Therefore, the left-line tunnel, as a backward tunnel, will not be affected by the excavation disturbance of the adjacent tunnel again after the excavation. The damage degree of the surrounding rock is slighter and relatively more stable than that of the parallel pilot tunnel and the right-line tunnel.

5. Conclusions

In view of the surrounding rock deformation and failure problems, such as spray layer cracking, sheet slope buckling, and steel arch buckling, in a deep buried high stress and small-clearance tunnel in southwest China, the asymmetric failure characteristics of the surrounding rock are clarified through failure investigation, geological analysis, and surrounding rock pressure monitoring. The evolution characteristics of the surrounding rock pressure of different tunnels are analyzed, and combined with geology, excavation and surrounding rock pressure, the asymmetric failure mechanism of the surrounding rock during the excavation of deep buried small-clearance tunnel is revealed. The following four conclusions are presented:

(1) A monitoring method for the surrounding rock pressure in a deep buried small-clearance tunnel is proposed. A group of single-row pressure sensors is arranged in the advance tunnel to determine the mileage of the monitoring section. When the two left and right lines are followed up by the backward tunnels, the second group and the third group of pressure sensors are installed in turn at the monitoring section. Considering the stress of the deep valley, the excavation effect, and the adverse structural influence of the rock mass, the surrounding rock pressure monitoring method with eight sensors in each monitoring section and local dense arrangement is determined.

(2) In the process of excavation of the deep buried small-clearance tunnel, the surrounding rock failure is asymmetrical, which shows that the surrounding rock pressure of the tunnel is more serious on the valley side than the mountain side. In the backward tunnel, the damage degree of the left line is the least, followed by the right line. In the advance tunnel, the parallel pilot tunnel pressure is the most serious, especially in the sections after the excavation of the left and right lines.

(3) The surrounding rock pressure in the excavation process of the deep-buried small-clearance tunnel can be divided into five periods. The spatial evolution trend of the surrounding rock pressure on the monitoring section is that the growth rate in the arch is greater than that in the side wall, and the growth rate in the valley is greater than that in the mountain side. The pressure distribution characteristics of the surrounding rock are asymmetrical.

(4) High ground stress is the main cause of failure of the surrounding rock during excavation of the deep buried small-clearance tunnel. Under the influence of the deep valley stress, the stress concentration of the surrounding rock on the valley side is greater than that on the mountain side. In the follow-up process of the backward tunnel, tangential stress concentration occurs on the valley side of the advance tunnel under the influence of the deep valley stress and excavation disturbance. Since the energy and stress around the surrounding rock are fully released and transferred under the excavation of the first tunnel, the backward tunnel is relatively stable.