Large Deformation Mechanism and Support Countermeasures of Deep-Buried Soft Rock Tunnels Under High Geostress State

Abstract

To address the problem of large deformation in deep-buried high geostress soft rock tunnels, the Yuelongmen Tunnel was selected as the research subject and adopting the methods of on-site measurements, laboratory experiments and theories, the characteristics of large deformation and its mechanism in high geostress soft rock tunnels are studied in depth, and based on the mechanism of large deformation in tunnels and the concept of active and passive synergistic control, an optimized support scheme that dynamically adapts to the deformation of the surrounding rock is put forward. The results show that (1) the deformation volume and rate of tunnel surrounding rock is large, the duration is long, and the deformation damage is serious; (2) the main factors of tunnel surrounding rock deformation damage are high geostress and stratum lithology, followed by geological structure, groundwater and support scheme; (3) the tunnel deformation hierarchical control scheme effectively controls the deformation of surrounding rock, and reduces the deformation of steel arch and the risk of sprayed concrete cracking, which verifies the applicability of this scheme to the project. It verifies its engineering applicability. The research results provide important technical reference and theoretical support for the design and construction of similar projects.

1. Introduction

Due to the compression of the Qinghai-Tibet, Yangtze, North China, and Tarim plates in the western plateau of China and its surrounding areas, there are widespread adverse geological conditions, such as high ground stress and weak surrounding rock. Under these geological conditions, tunnels often encounter issues such as cracking of the initial support, twisting and deformation of steel arches, structural instability due to intrusion into the clearance envelope, and cracking of the secondary lining during construction [1,2,3,4,5]. These engineering hazards pose significant threats and constraints to the standard construction and subsequent operation and maintenance of tunnel projects [6,7,8].

Although many studies have focused on the problem of large deformation in high geostress soft rock tunnels, there are still some shortcomings in the existing studies. On the one hand, most of the existing studies focus on the characteristics and mechanisms of tunnel large deformation from the mechanical and geological perspectives [9,10,11]; for example, Fan et al. [12] analyzed the mineral composition and prominent deformation characteristics of the surrounding rock in the Minxian Tunnel, identifying the failure mechanism as the swelling of carbonaceous slate due to water absorption, which leads to significant deformation of the surrounding rock. Zhao et al. [13] concluded that the formation lithology, ground stress, groundwater and rock structure were the main factors causing large deformation of the surrounding rock of tunnels; through the study of the microstructure of the thousand mafic rocks, the mineral composition, and the mechanical properties of the rocks. However, these studies are mostly theoretical analyses or single-factor explorations, which are difficult to comprehensively reflect the deformation mechanism under complex geological conditions in actual projects.

In order to avoid large deformation disasters during the construction of high geostress soft rock tunnels, effective control measures need to be taken during tunnel excavation to improve the mechanical properties of the surrounding rock, to ensure the stability of the tunnel’s surrounding rock and the safety of the tunnel construction stage. The large deformation control methods of high geostress soft rock tunnels can be divided into the surrounding rock reinforcement, rigid support, let pressure support joint support and other methods [14]. In peripheral rock reinforcement, to give full play to the anchoring role of anchors in high geostress soft and broken peripheral rock, usually the combination of grouting and anchoring technology is an effective method to solve the problem of high geostress soft rock support. Zhang et al. [15] studied the mechanical properties of pre-fractured sandstone before and after grouting through triaxial compression tests, concluding that grouting can significantly improve the integrity and mechanical properties of fractured rock masses. Lu et al. [16] conducted experiments to study the shear strength of soft rock with well-developed joints after grouting reinforcement, proving that grouting can effectively enhance the shear properties of rock masses. Regarding rigid support, high-strength steel arches, steel pipe concrete, and restrained concrete arches are usually used for rigid restrained support [17,18,19]. However, this method cannot effectively solve the impact of increasing surrounding rock pressure on the tunnel support structure. It is difficult to control the large deformation of the tunnel effectively, and the construction is difficult and costly [20,21]. In terms of letting pressure support, in order to improve the ability to resist the deformation of the surrounding rock in high geostress soft rock tunnels, various types of yielding units or yielding support techniques have been proposed to absorb the more considerable surrounding rock deformation and maintain the stability of the support structure, such as yielding bolts/cables, U-shaped yielding steel arches, letting pressure type lining structure, dampers and multilayered steel arch support techniques. Some compressible/deformable new materials for extruded large deformation letting support, such as compressible/deformable concrete [22,23,24]. Rai et al. [25] developed a novel energy-absorbing inflatable airbag resistance limiter for solving the problem of large deformation in high-stress soft rock tunnels. He et al. [26] and Tao et al. [27] developed energy-absorbing anchors/cables with high constant resistance and elongation for the disaster of large deformation of the surrounding rock under deep, high stress and complex geological conditions. Salazar et al. [28] added corrugated metal plates to fiber concrete to address the problem of large deformation in tunnels, thus improving the compressibility of the concrete and guaranteeing the stability of the tunnel lining structure. Although yielding support technology is an effective support method for addressing the problem of large deformations in soft rock, key issues such as the “amount of yielding” and “timing of yielding” still primarily rely on engineering experience and need to be complemented by other support technologies to meet actual support requirements fully.

Although many scholars have conducted much research on the mechanism of large deformation and control technology of soft rock tunnels and obtained some useful results, they are mostly for specific engineering geological conditions and lack of universality and dynamic adaptability. The surrounding rock deformation in complex high geostress soft rock tunnels has significant time effects and spatial variability. A support technology that can be dynamically adjusted to adapt to different deformation stages and degrees is needed to solve the limitations of the existing support technology in high-geostress soft rock tunnels and improve the support’s economy and reliability [29,30].

In this paper, using the Yuelongmen Tunnel as a relying project, the deformation monitoring, in-situ stress measurement, strength test and mineral composition analysis of the tunnel’s large deformation section were evaluated, and the damage characteristics and large deformation mechanism of the tunnel were assessed. In addition, according to the tunnel large deformation mechanism and the concept of active-passive synergistic control, a hierarchical control scheme for tunnel large deformation is proposed, and its engineering applicability is verified through field tests. This study theoretically deepens the understanding of the large deformation mechanism of high-stress soft rock tunnels. It provides more targeted and dynamic adaptive support technology for the design and construction of tunnel engineering, which has important scientific significance and engineering value.

2. Project Background

2.1. Project Overview

The Yuelongmen Tunnel is in Sichuan Province, China, spanning Mianyang City and the Aba Tibetan and Qiang Autonomous Prefecture. It traverses the Longmen Mountain Range and is a critical control project of the Sichuan-Qinghai Railway. The entrance of the Yuelongmen Tunnel is in Gaochuan Township, Anzhou District, Mianyang City. At the same time, the exit is located in Tumen Town, Mao County, within the Aba Tibetan and Qiang Autonomous Prefecture. The tunnel is a double-line railway tunnel, with the left line tunnel from D2K91 + 004 to DK110 + 985 and the right line tunnel from YD2K90 + 999 to YDK111 + 041. The left line tunnel is 19,981 m long, and the right line tunnel is 20,042 m long. The tunnel is designed to accommodate runs at a speed of 200 km/h rate, with a maximum depth of 1445 m. Figure 1 shows the tunnel’s geographical location, and Figure 2 shows its geological longitudinal section.

2.2. Engineering Geology

Based on the comprehensive analysis of the advanced geological forecast report and the on-site excavation findings, the surrounding rock of the Yuelongmen Tunnel belongs to the Sinian Qiujihe Formation (Zbq) and consists of carbonaceous slate, shale, and sandy slate, with carbonaceous slate being the predominant rock type. The rock is characterized by thin layers and a foliated structure, and it is relatively soft. Influenced by regional tectonic movements, small folds between the rock layers are common, and the bedding orientation is chaotic. Joints are highly developed, making the rock mass extremely fractured, soft, and of low strength. The tunnel face shows poor stability, with significant block fall occurrences at the arch. Groundwater is weakly developed, appearing moist, with localized occurrences of water gushing. The tunnel face exposes carbonaceous slate, as shown in Figure 3.

2.3. Tunnel Excavation and Support Design

The construction scheme design for the D2K99 + 700 to the D2K99 + 941 tunnel section is based on Class III surrounding rock, using the New Austrian Tunneling Method (NATM) for design and construction. The excavation method employed is full-face tunneling, and the support design adopts a composite lining structure with “advanced support + primary support + secondary lining”. The advanced support uses Φ42 advanced small pipe. The primary support includes Φ22 mortar bolts and a φ8 wire mesh. The secondary lining consists of C30 concrete. The supporting parameters of the lining structure are presented in

Table 1. Tunnel Support Design Parameters.

Items	Primary Support				Secondary Lining	Ancillary Construction Measures
	System Bolt	Wiremesh	Shotcrete	Deformation Tolerance
III surrounding rock	Φ22 mm × 2.5 m Mortar bolt (@1.2 × 1.5 m)	Single-layer Φ6 × Φ6 (@20 × 20 cm)	C25 Concrete 8 cm	3~4 cm	C30 Concrete 35 cm	Φ42 advance small pipe @35 mm × 300 mm

. The original supporting design of the Yuelongmen Tunnel is shown in Figure 4.

3. Characteristics and Mechanisms of Large Deformation in Tunnels

3.1. Characteristics of Large Deformation

In the tunnel section from D2K99 + 775 to D2K99 + 918, severe deformation and damage occurred to the surrounding rock after excavation, especially D2K99 + 850 section and D2K99 + 905 section. So, the surrounding rock deformation of D2K99 + 850 and D2K99 + 905, two sections of the on-site monitoring, can be derived from the deformation characteristics of the tunnel initial support through the analysis of the monitoring data and the on-site observation.

(1)

Significant Deformation and Rate of Surrounding Rock: The results of the surrounding rock deformation monitoring curves are shown in Figure 5. As seen in Figure 5a, at section D2K99 + 850, the accumulated subsidence of the tunnel vault is 822.0 mm, with a maximum deformation rate of 50.8 mm/d. The accumulated convergence at the sidewall is 791.4 mm, with a maximum deformation rate of 44.5 mm/d. In Figure 5b, at section D2K99 + 905, the accumulated subsidence of the tunnel vault is 457.4 mm, with a maximum deformation rate of 23.8 mm/d, and the accumulated convergence at the sidewall is 423.3 mm, with a maximum deformation rate of 24.6 mm/d. The surrounding rock deformation at tunnel section D2K99 + 850 is severe, with a high deformation rate that has significantly exceeded the reserved deformation tolerance, severely encroaching the limits.

(2)

Long Duration of Surrounding Rock Deformation: The surrounding rock deformation in the section from D2K99 + 775 to D2K99 + 918 lasted long. After the initial support deformation occurred, the surrounding rock deformation did not stop immediately but continued to develop, exhibiting significant progressive and time-dependent effects. For instance, the surrounding rock deformation at section D2K99 + 850 lasted approximately 70 days before gradually stabilizing and converging.

(3)

Severe Deformation and Damage to Surrounding Rock: The surrounding rock in the section from D2K99 + 775 to D2K99 + 918 exhibited varying degrees of deformation and damage, primarily manifested as cracking of the sprayed concrete, spalling, and distortion or breakage of the steel framework. Among these, the deformation and damage at section D2K99 + 850 were the most severe, with signs of collapse. Over time, the deformation area from D2K99 + 775 to D2K99 + 918 gradually extended to the section from D2K99 + 700 to D2K99 + 941. According to on-site monitoring and measurement data, the initial support of the D2K99 + 700~D2K99 + 941 section is almost all infringed, which indicates that there is a significant safety risk in the subsequent construction of the tunnel, so it is necessary to dismantle and replace the initial support that has already been constructed, as shown in Figure 6.

To control the further development of surrounding rock deformation, tunnel residue backfill was applied to the collapsed section from D2K99 + 840 to D2K99 + 890. For the remaining section from D2K99 + 700 to D2K99 + 941, temporary support using “square timber + large-diameter steel pipes” or steel frames was adopted, as shown in Figure 7.

3.2. Large Deformation Mechanism

(1)

Geological Structure

The tunnel site is located within multiple fault zones, exhibiting complex and diverse geological structures. The construction site mainly traverses an “A”-shaped block composed of the Longmen Mountain fault zone, the West Qinling fold belt, and the Minjiang fault zone. Intense geological tectonic processes have promoted the development of structural surfaces such as joints, fractures, unconformities, and contact surfaces within intrusive bodies, thereby reducing the strength of the rocks. Influenced by high ground stress, blasting vibrations, and tunnel construction activities, the self-stabilization capability of the surrounding rock mass has decreased, leading to shearing and sliding along structural surfaces and causing large deformation disasters, resulting in severe damage to initial support systems.

(2)

High Geostress

During the tunnel construction process, an in-situ stress test was conducted at the D2K100 + 059.4 section of the Yuelongmen Tunnel, as shown in Figure 8. The ground elevation at the measurement point is approximately +2133 m, and the tunnel track elevation is approximately +1134 m, giving a burial depth of 999 m. The overlying rock layer’s average unit weight is 27 kN/m³. The results of the in-situ stress test are presented in

Table 2. Measurement results of geostress in Yumengmen Tunnel.

Test Position Depth (m)	Maximum Horizontal Principal Stress (MPa)	Minimum Horizontal Principal Stress (MPa)	Vertical Stress (MPa)	Direction of Maximum Horizontal Principal Stress
29.1	24.62	13.69	27.76	N35.0°E
27.9	21.49	11.67	27.73
26.8	20.38	10.83	27.70
25.7	20.17	10.61	27.67	N14.7°E
23.2	19.62	10.32	27.60	N43.5°E

.

At the location of D2K100 + 059.4, the maximum horizontal principal stress in the vicinity of the in-situ stress test ranges between 20 and 25 MPa, with the predominant direction of the maximum horizontal principal stress being NNE. According to the test results, the average direction of the maximum horizontal principal stress is N31°E, and the vertical stress is approximately 27 MPa. Based on these findings, the stress regime in the area is preliminarily classified as ${\sigma }_{\mathrm{V}} > {\sigma }_{\mathrm{H}} > {\sigma }_{\mathrm{h}}$, with vertical stress being the maximum principal stress.

According to the Engineering Rock Classification Standard (GB/T500218-2014) [31], there is extremely high geopathic stress when ${\mathrm{R}}_{\mathrm{c}}{/\sigma }_{\max }$ ≤ 4, and according to the test results in Table 2, it can be concluded that ${\mathrm{R}}_{\mathrm{c}}{/\sigma }_{\max }$ is 1.47 in the Yuelongmen Tunnel, which belongs to extremely high geopathic stress.

According to the Mohr-Coulomb criterion, the higher the bias stress and the lower the strength of the rock mass, the easier it is for the rock mass to undergo deformation and damage. In a high-stress environment, the rock bias stress increases significantly, accelerating the deformation damage of the rock mass [32,33,34]. Therefore, high stress is the key factor leading to large deformation of tunnel-surrounded rock.

(3)

Formation Lithology

The lithology of the strata is a significant factor influencing the deformation of the surrounding rock. The surrounding rock at the tunnel face mainly consists of carbonaceous slate, shale interbedded with sandy slate from the Sinian Qiujihe Formation (Zbq), with carbonaceous slate being the predominant type. Thin layers or sheet-like structures characterize these rocks, and they are relatively soft. Due to the influence of regional tectonic movements, small folds are standard between the rock layers, resulting in a chaotic arrangement of the strata. The joints are highly developed, leading to extremely fragmented rock masses. The tunnel face exhibits poor stability, with severe block falling from the arch, indicating poor engineering performance and a tendency for large deformations in the surrounding rock. The weathering resistance is weak, and after excavation, the rock is prone to crumbling into fragments and debris, even in the absence of water, leading to pulverization.

(4)

Rock Mass Mineral Composition and Microstructure

The mineral composition and microstructure of rock masses have a decisive impact on their mechanical and failure characteristics [35]. To study the mineral composition of carbonaceous slate, representative samples collected from the field were ground into fine powder and made into thin sections for analysis. X-ray diffraction (XRD) analysis was conducted using a German D8 ADVANCE X-ray diffractometer. Figure 9 shows the results of the composition and relative content of the carbonaceous slate’s whole rock and clay minerals. As seen in Figure 9, the primary minerals in the carbonaceous slate from the Yuelongmen Tunnel are quartz and clay minerals, with clay minerals accounting for 45.2% of the composition. These clay minerals mainly include chlorite, illite, kaolinite, and feldspar, with proportions of 20.3%, 12.6%, 9.1%, and 3.2%, respectively. The high content of illite and chlorite in the rock leads to softening and swelling when exposed to water, which reduces the rock’s strength and makes it prone to shear-slip failure along the bedding planes.

The microstructure of the carbonaceous slate samples was observed using a ZEISS Sigma 300 scanning electron microscope, and the results are shown in Figure 10. As shown in Figure 10, the microscopic structure of the carbonaceous slate is dense, with an uneven surface, exhibiting flaky and layered structures, as well as developed pores and solution holes. These characteristics make the carbonaceous slate prone to shear slip failure along the layering planes under external forces. Additionally, the presence of pores and solution holes can reduce strength after water absorption, thereby affecting the stability of engineering projects.

(5)

Groundwater Influence

The Yuelongmen Tunnel has well-developed groundwater, with localized water leakage and water spraying occurring at the tunnel face. Groundwater indirectly affects the deformation of the surrounding rock in the tunnel. The surrounding rock is susceptible to water, absorbing and dissolving in water. When exposed to water, especially under pressure, it can soften, disintegrate, and turn into mud, reducing its mechanical properties [36].

To study the effect of moisture content on the mechanical properties of carbonaceous slate, uniaxial and triaxial compression tests were conducted in the laboratory on its natural and saturated moisture states. The stress-strain curves of carbonaceous slate in both natural and saturated states are shown in Figure 11.

As shown in Figure 11, the strength of the carbonaceous slate is relatively high, and there are significant differences in the mechanical properties of the carbonaceous slate in its natural state and saturated state under different confining pressures. Under low confining pressure conditions, carbonaceous slate shows no prominent yield characteristics, exhibiting linear elastic deformation before the peak. When the confining pressure reaches 20 MPa, carbonaceous slate enters the elastic-plastic deformation stage, particularly in the saturated state, where this behavior is more pronounced. This indicates that under high confining pressure and high moisture content conditions, carbonaceous slate’s plastic and ductile characteristics are more significant, reflecting the mechanical characteristics of soft rock.

Figure 12 shows the test results of the relationships between the peak strength, elastic modulus of carbonaceous slate, and confining pressure. As shown in Figure 12a, under natural states, the peak strengths of carbonaceous slate in uniaxial compression and triaxial compression tests at confining pressures of 10, 20, and 30 MPa are 55.04 MPa, 98.87 MPa, 152.48 MPa, and 215.65 MPa, respectively. Under saturated states, the peak strengths are 40.79 MPa, 77.89 MPa, 116.39 MPa, and 163.95 MPa, showing reductions of 25.89%, 21.2%, 23.6%, and 22.4%, respectively. As shown in Figure 12b, the elastic moduli under natural states are 20.35 GPa, 33.59 GPa, 48.16 GPa, and 72.78 GPa. In contrast, under saturated states, they are 6.79 GPa, 16.56 GPa, 31.13 GPa, and 52.11 GPa, reflecting reductions of 66.63%, 50.69%, 35.36%, and 28.40%, respectively. It can be concluded that, under natural and saturated states, carbonaceous slate’s peak strength and elastic modulus increase significantly with confining pressure, exhibiting a robust linear relationship. Additionally, the compressive strength and elastic modulus of carbonaceous slate in the water-saturated state are notably lower than those in the natural state.

(6)

Impact of Human Factors

① Insufficient strength of primary support. In high geostress soft rock tunnels, the loosening zone of the surrounding rock is relatively large after excavation [33]. In the original support design, the primary support employed Φ22 mortar anchor bolts with a length of 2.5 m and spacing of 1.2 m by 1.5 m. The relatively short anchor bolts make it difficult to secure them into deeper, stable rock layers, which is necessary to form a reinforcing ring for the surrounding rock and utilize its self-supporting capacity. Additionally, the original design did not include steel arch supports, resulting in inadequate load-bearing capacity to withstand the significant pressure from the surrounding rock.

② Impact of construction scheme. The original design of the tunnel utilized full-section excavation. Due to the larger excavation cross-section, the stability of the surrounding rock is reduced, making it prone to deformation and collapse. Moreover, the initial support is required to bear considerable geological stress in a short period, leading to complex loading conditions for the support structure and increasing the support difficulty.

③ Impact of construction techniques. Field investigations during the tunnel construction revealed that the construction did not adhere to the design requirements. Major issues included severe over- and under-excavation, lack of grouting for advance support, absence of pads for system anchor bolts, delays in implementing primary support after excavation, and delays in secondary lining construction. The prolonged stress duration on the primary support structure led to deformation and damage due to compression.

4. Tunnel Large Deformation Optimization Support Scheme

4.1. Active-Passive Cooperative Control Technology

(1)

Active support

The principle of combined short and long anchor support is illustrated in Figure 13.

Short anchors provide support resistance, sharing the load with the later-applied long anchors and other support measures. They effectively reinforce the surrounding rock, forming a “compression arch” in the shallow surrounding rock, which fully utilizes the self-supporting capacity of the rock, reduces initial deformation, and improves the stress distribution around the tunnel. Long anchors ensure that the load-bearing effect of the initial short anchor arch is maintained. They link the shallow surrounding rock with the deeper rock layers, making the overall deformation of the surrounding rock more coordinated. The shallow “compression arch” is anchored into the deeper surrounding rock, expanding the anchoring and load-bearing zone.

Additionally, when combined with grouting, the anchors can fill the cracks in the surrounding rock, increasing the strength of geological structures (such as faults, fractures, bedding planes, and joints), sealing water sources, and enhancing the rock mass’s strength, internal friction angle, and cohesion. This process creates a layered load-bearing zone within the surrounding rock.

(2)

Passive support

From the perspective of stress state and deformation coordination, tunnel excavation is a comprehensive process involving the release of in situ stress and the coordination of surrounding rock deformation. Tunnel excavation results in the release of original rock stress (P₀). After implementing support measures, the released original rock stress P₀ is primarily composed of three components: ① elastoplastic energy released in the form of surrounding rock deformation (P_d) ② self-supporting capacity of the surrounding rock (P_r); ③ support force (P_i). This can be expressed as:

$${\mathrm{P}}_0={\mathrm{P}}_d+{\mathrm{P}}_r+{\mathrm{P}}_i$$

(1)

From Equation (1), it can be seen that releasing surrounding rock pressure, reinforcing surrounding rock, and increasing the strength of support structures are the primary strategies to prevent significant deformation and damage to surrounding rocks during tunnel construction.

The principle of surrounding rock-support interaction can be illustrated using the surrounding rock-support characteristic curve. The relationship between the displacement around a symmetrical circular tunnel and the supporting reaction force is [37,38]:

$${\mu }_0={\displaystyle \frac{\sin \varphi }{2G}}{R}_0\left(p_0+c\cot \varphi \right){\left[{\displaystyle \frac{\left(p_0+c\cos \varphi \right)\left(1-\sin \varphi \right)}{\left(p_i+c\cot \varphi \right)}}\right]}^{{\displaystyle \frac{1-\sin \varphi }{\sin \varphi }}}$$

(2)

For a symmetrical circular tunnel with a circular lining, the circular lining can be considered as a thick-walled cylinder subjected to uniform external pressure P. According to the formula for thick-walled cylinders, the radial displacement at the outer edge of the cylinder is given by [39]:

$${\mu }_0={\displaystyle \frac{p_1{R}_0^3\left(1+{\nu }_1\right)\sin \varphi }{E_1\left(R_0^2+a^2\right)}}\left(1-2{\nu }_1+{\displaystyle \frac{a^2}{R_0^2}}\right)$$

(3)

where a and R₀ are the inner and outer diameters of the cylinder, respectively; E₁ and υ₁ are the elastic constants of the material; Equation (2) represents the Ground Reaction Curve (GRC), while Equation (3) represents the Support Reaction Curve (SRC).

By solving Equations (2) and (3) simultaneously, the support-surrounding rock interaction curve for an axisymmetric circular tunnel can be obtained, as shown in Figure 14.

As shown in Figure 14, under the same support conditions, its self-bearing capacity can be fully utilized if the surrounding rock is reinforced in advance. The required support force and rock deformation in a balanced state are significantly less than before reinforcement. The above analysis indicates that appropriate reinforcement of the surrounding rock can reduce the stiffness required for the support structure and the deformation of the surrounding rock, thereby lowering the support costs.

After tunnel excavation, if stiff support (ABB’) is used, a large support force P_i is required, while the surrounding rock only bears a small pressure P_r corresponding to the elastic deformation μ₀. This scheme results in a high-strength support system with high construction costs and poor economic efficiency, indicating that adopting a support scheme with high stiffness is unreasonable. If no support or delayed support (FG) is provided after tunnel excavation, the displacement around the tunnel reaches its maximum value μ_limit, with minimal deformation pressure. In this situation, the surrounding rock may relax or collapse, necessitating shotcrete linings for support. Suppose the support structure is not strong enough. In that case, significant deformation or collapse of the surrounding rock may quickly occur during construction in environments with high geostress and weak surrounding rock (ACDFG). The optimal support point should be on the left side of point F’ and near point F’, such as point E, where the surrounding rock can undergo significant deformation, bearing more rock pressure. The support structure bears less deformation pressure, ensuring the surrounding rock does not undergo large deformation and failure.

In response, a variable stiffness support method (ACDE) is proposed:

After tunnel excavation, the surrounding rock undergoes a small deformation μ₀. At this time, the first layer of initial support with high deformability (AC) is applied, allowing controlled “pressure relief” (energy release) without destabilizing the surrounding rock.

The deformation of tunnels in high geostress and weak surrounding rock has a significant “time effect.” Therefore, it is necessary to increase the support resistance promptly, ensuring that the support characteristic curve intersects with the surrounding rock characteristic curve and maintaining the long-term stability of the tunnel’s surrounding rock. When the cumulative deformation of the surrounding rock reaches μ_c, the second layer of primary support (CD) is applied, with both support layers working together. At this point, the stiffness of the support system is at its maximum.

When the deformation of the first layer of primary support reaches μ_d and enters the plastic stage, where the deformation increases but the load it bears remains unchanged, the second layer of primary support provides the support force (DE), limiting excessive deformation of the surrounding rock, stabilizing it, and ultimately preventing large deformation and failure of the tunnel’s surrounding rock.

Anchor and grouting reinforcement as active support can repair the surrounding rock, fully mobilize the self-supporting capacity of the surrounding rock, reduce the maximum principal stress of the surrounding rock, and inhibit the internal deformation of the surrounding rock; double-layer initial support as passive support, the first layer of rapid stabilization of the surrounding rock to control the deformation, and the second layer of the further enhancement of the support effect, resistance to the surface of the surrounding rock deformation, both synergistically share the external load, and jointly inhibit deformation from the inside and outside of the enclosing rock in terms of deformation control.

At the same time, the reserved deformation volume in the first and second layers of initial support can cause the double-layer initial support to play out the effects of “stress release” and “active resistance”, jointly guaranteeing the stability of the tunnel’s surrounding rock.

4.2. Tunnel Large Deformation Grading Control Program

Based on the deformation failure characteristics and mechanisms of the surrounding rock in the tunnel section from D2K99 + 700 to D2K99 + 941, and considering the economic viability of the support scheme, the original support plan (Level III composite lining support scheme) has been optimized as follows:

(1)

For sections with slight deformation, use the Type I support scheme.

(2)

For sections with moderate deformation, use the Type II support scheme.

(3)

For sections with severe deformation, use the Type III support scheme.

Tunnel D2K99 + 700~D2K99 + 941 section support design scheme is shown in

Table 3. Tunnel large deformation section supports the design scheme.

Start and Ending Mileage	Length (m)	Working Drawing		Dynamic Design
		Surrounding Rock Level	Deformation	Surrounding Rock Level	Support Measures
D2K99 + 700~ + 892	192	Class III	severity	Class V	Circular section, long and short anchors + grouting, double-layer initial support
D2K99 + 892~ + 918	26		medium	Class V	Circular section, plain composite lining, single layer initial support
D2K99 + 918~ + 941	23		minimal	Class IV	Ordinary composite lining, single-layer initial support

.

(1)

The Type I support scheme is shown in Figure 15.

The support parameters of this support scheme are as follows:

The shape of the tunnel section is adjusted from the original egg-shaped section to a round one so that the tunnel force is more uniform;

The tunnel arch was constructed using 42 mm diameter, 4.5 m long, small foreboding pipes with a ring spacing of 40 cm;

System anchors using “long and short anchor combination” layout, tunnel arch using 22 mm diameter hollow grouting anchor bolts, length 3.0 m, spacing 1.2 m × 1.0 m, sidewalls using 22 diameter mortar anchor bolts, length 3.0 m, spacing 1.2 m × 1.0 mm, tunnel arch using 32 diameter resin anchor bolts, length 6.5 m, spacing 1.2 m × 1.0 mm, resin anchors bolt with a length of 6.0 m and a spacing of 1.2 m × 1.0 mm, and sidewalls and elevation arches are made of diameter 32 mm self-improving anchor bolts with a length of 8.0 m and a spacing of 1.2 m × 1.2 m;

The whole ring is grouted with orifice pipe diameter direction;

Reinforcing steel mesh adopts 8 mm diameter double-layer reinforcing steel mesh, with a grid spacing of 20 × 20 cm;

The tunnel ring set double initial support, the first layer of initial support using HW200 steel frame reinforcement support, longitudinal spacing of 60 cm, spraying thickness of 30 cm of C25 concrete, the reserved deformation amount of 30 cm, the second layer of initial support using HW175 steel frame reinforcement support, longitudinal spacing of 60 cm; spraying thickness of 25 cm of C25 concrete; reserved deformation amount of 15 cm;

The second lining adopts reinforced concrete lining with a thickness of 60 cm.

(2)

The Type II support scheme is shown in Figure 16.

The shape of the tunnel section is adjusted from the original egg-shaped section to a round one so that the tunnel force is more uniform;

The tunnel arch was constructed using 42 mm diameter, 4.5 m long, small foreboding pipes with a ring spacing of 40 cm;

System anchors complete ring playing set, using the use of diameter 32 mm self-improving anchor bolts, length of 6.0 m, spacing of 1.2 m × 0.8 m; the whole ring using the orifice tube diameter to the grouting;

Reinforcement mesh adopts 8 mm diameter single-layer reinforcement mesh, with a grid spacing of 20 × 20 cm;

The whole ring of the tunnel is set up with single-layer initial support, and HW200 steel frame is used to strengthen the support, with a longitudinal spacing of 60 cm; C25 concrete with a thickness of 27 cm is sprayed, and the reserved deformation amount is 30 cm;

The secondary lining adopts reinforced concrete lining with a thickness of 55 cm.

(3)

Type III support scheme is shown in Figure 17.

The tunnel section is egg-shaped;

The tunnel arch was constructed using 42 mm diameter, 4.5 m long, small foreboding pipes with a ring spacing of 40 cm;

The tunnel arch adopts 22 mm diameter hollow grouting anchor bolts with a length of 4.0 m and a spacing of 1.2 mm × 0.8 mm, and the side walls adopt 22 mm diameter mortar anchor bolts with a length of 6.0 m and a spacing of 1.2 mm × 0.8 mm;

The whole ring is grouted in the diameter of the orifice pipe;

Reinforcement mesh adopts 8 mm diameter single-layer reinforcement mesh, with a grid spacing of 20 × 20 cm;

The whole ring of the tunnel is set up with single-layer initial support, adopting the I20b steel arch with a longitudinal spacing of 80 cm; C25 concrete with a thickness of 27 cm is sprayed; the reserved deformation volume is 25 cm;

The secondary lining adopts reinforced concrete lining with a thickness of 45 cm.

4.3. Construction Method

In the construction of the Yuelongmen Tunnel, to avoid too many divisions and multiple interferences in the traditional method and to improve the mechanized construction efficiency and the overall bearing capacity of the steel frame connection of the circular cross-section, the “Mechanized construction method with two steps up and down” is proposed in section III of severe deformation from D2K99 + 700 to +892 and the section II of medium deformation from D2K99 + 892 to +918, as shown in Figure 18.

① The initial support is applied immediately after the initial spraying of the upper step excavation to reinforce the initial loosening ring of the arch;

② Stop digging after 6 m of the upper step, then carry out the construction of the lower step with the tilting arch, and immediately complete the initial support of the tunnel bottom and sidewalls;

③ To be 6 m length of the lower and upper steps into a ring, then the second layer of the tunnel bottom initial support steel frame reserved for construction and temporary backfill with slag, and so on according to the 6 m section of the formation of a “short step” fast into a ring to promote the construction;

④ In about 30~35 m of lagging digging palm surface, or before the first layer of support deformation convergence failure, timely construction of the second layer of arch wall initial support steel frame;

⑤ After lagging the second layer of initial support for a certain distance and after the second layer of support is closed into a ring and the convergence deformation data of the section is evaluated and stabilized, reasonably push forward the “secondary excavation and transportation” of the cave slag at the bottom of the back end of the tunnel, and steadily push forward the construction of the back arch and fill casting and secondary lining.

This method can decompose the deformation of large deformation into as few as possible in the process of transferring sequence, step-by-step control, and rapid closure to ensure that the overall excavation to the time of forming a ring is controlled in 5~10 days; timely support and layer by layer support can effectively improve the distribution of stress around the hole and reduce the development of the range of peripheral rock fragmentation, and realize the effective control of the mechanization of large deformation construction.

5. Verification of Countermeasures

Field tests were conducted in the severely deformed and moderately deformed sections of the tunnel to verify the reliability of the above hierarchical control scheme for large deformation in tunnels. Firstly, the original support structure was completely dismantled, and the optimized support scheme was adopted for support. At the same time, D2K99 + 850 (severely deformed section) and D2K99 + 905 (moderately deformed section) were selected as monitoring sections to monitor the deformation of the surrounding rock. Monitoring point A is set up at the location of the tunnel arch as the arch settlement monitoring point, and monitoring points C and D are set up at the sidewalls on both sides as the sidewall convergence measurement line.

An automated monitoring system was used to monitor the deformation of the surrounding rock (as shown in Figure 19). The monitoring system consists of wireless terminals, base stations, front-end data acquisition and control stations, and multiple data collection devices. The system employs laser phase ranging technology, precision leveling sub-millimeter pressure differential measurement technology, ultra-low power wireless communication technology, as well as function conversion indirect measurement methods and quick assembly and disassembly connection methods, ensuring high measurement accuracy, low power consumption, and ease of assembly and disassembly.

The results of the surrounding rock deformation monitoring are shown in Figure 20. It can be seen that the surrounding rock deformation can be divided into three stages: (1) Rapid deformation stage. After the initial excavation of the tunnel, constructing the first primary support leads to a sharp increase in surrounding rock deformation, releasing a certain amount of surrounding rock stress. (2) Slow growth stage. The surrounding rock deformation increases slowly after constructing the second primary support. However, the deformation rate significantly decreases, indicating that the second layer of primary support effectively enhances the stiffness and strength of the first layer. (3) Stable deformation stage. After the commencement of secondary lining construction, surrounding rock deformation is further controlled, and the surrounding rock essentially ceases to deform.

At the same time, it can be seen that at the D2K99 + 850 section (severely deformed section), the maximum values of arch settlement and peripheral convergence are 324.9 mm and 286.1 mm, respectively. The maximum values of deformation are reduced by 60.4% and 63.8% compared with the original support scheme, the maximum values of arch settlement rate and peripheral convergence rate are 28.5 mm/d and 24.9 mm/d respectively, and the maximum values of deformation rate are reduced by 43.8% and 43.3% compared with the original support scheme. The maximum value of the deformation rate is 28.5 mm/d and 24.9 mm/d, and the maximum value of the deformation rate is reduced by 43.8% and 43.3%, respectively, compared with the original support scheme; the maximum value of the arch settlement and peripheral convergence at section D2K99 + 905 (a moderately deformed section) is 234.8 mm and 191.3 mm, and the maximum value of the deformation rate is reduced by 48.6% and 54.8%, respectively, compared with the original support scheme; the maximum value of the arch settlement and peripheral convergence is 19.7 mm, respectively, compared with the original support scheme. The maximum values of the arch settlement rate and peripheral convergence rate are 19.7 mm/d and 16.1 mm/d, respectively, and the maximum values of the deformation rate have been reduced by 17.2% and 34.5%, respectively, compared with the original support scheme. After adopting the tunnel large deformation hierarchical control scheme, the perimeter rock control effect in this paper is similar to the results reported in the literature [40,41], which further verifies the scheme’s effectiveness in terms of perimeter rock stability.

The on-site investigation found that the steel arch did not exhibit abnormal deformation, and the shotcrete showed no signs of cracking or spalling. The support effect was good, effectively addressing the problem of sizeable initial deformation in tunnel support. The on-site support effect is shown in Figure 21.

6. Conclusions

Through field measurements, laboratory tests and theoretical analyses, this study thoroughly explores the characteristics of large deformation in high geostress soft rock tunnels, reveals the mechanism of its large deformation, and puts forward a grading control scheme for large deformation in tunnels. The specific conclusions are as follows:

(1)

Under the original support scheme, the deformation of tunnel peripheral rock has the characteristics of large deformation volume, large deformation rate and long duration. It is accompanied by the signs of sprayed concrete cracking, falling blocks, twisted and broken steel frames, and even peripheral rock collapse. These characteristics show that the traditional support method makes it difficult to effectively deal with the large deformation of high-geostress soft rock tunnels.

(2)

The large deformation of the tunnel’s surrounding rock is the result of a combination of factors such as high ground stress, ground lithology, geological structure, groundwater, and support scheme. High geostress and ground lithology are the main factors, while the presence of groundwater further weakens the mechanical properties of the surrounding rock and aggravates the development of deformation.

(3)

In terms of large deformation control, according to the different conditions of large deformation damage of surrounding rock, it is divided into three grades: severe large deformation, medium large deformation and slight large deformation, and corresponding support programs are designed for different grades to dynamically adapt to the deformation of surrounding rock and the “Mechanized construction method with two steps up and down” is also proposed, which ensures the safety of the construction process and realizes the mechanized construction method. Safety and effective control of mechanized construction.

(4)

The on-site monitoring results show that, after adopting the tunnel large deformation grading control scheme, the maximum values of arch settlement and peripheral convergence in section D2K99 + 850 (severely deformed section) and section D2K99 + 905 (moderately deformed section) have been reduced by 60.4%, 63.8%, 43.8%, and 43.3%, respectively, compared with that of the original support scheme. There is no abnormal deformation of the support structure. The maximum values of settlement and peripheral convergence were reduced by 60%, 63.8%, 43.3% and 43.3%, respectively, compared with the original support scheme, and there was no abnormal deformation of the support structure.