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When traversing a complex and fractured geological formation, a deep-buried highway tunnel in Yunnan Province encountered a significant uplift problem in the invert. The causes of the tunnel’s uplifted section were analyzed through on-site observations and monitoring of the lining structure’s deformation, combined with numerical simulation methods. The results indicate that the primary factors leading to the invert uplift are the softening of the surrounding rock at the invert base due to water seepage and the high water pressure in the fractured zone. The softening of the surrounding rock is crucial to the safety of the inverted uplift structure. Based on the tunnel’s engineering characteristics and the causes of the invert uplift, remediation measures such as “enhancing the drainage system, grouting with steel flower pipes, and demolishing and replacing the invert” were adopted. These measures effectively controlled the invert uplift. After the remediation, the convergence rate of the secondary lining decreased, and the deformation stabilized, indicating that the invert uplift remediation plan was reasonable and effective
Article highlights
Analysing the causes of tunnel uplift through numerical simulation.
Propose measures to deal with the uplift of the tunnel arch.
The treatment plan can effectively control the damage to the uplift arch.
Introduction
With the continuous expansion of the highway network in southwestern China, tunnel construction has increasingly involved large cross-sections, high in-situ stress, complex geological conditions, and abundant groundwater. These factors have made tunnel construction more challenging and have led to a higher incidence of engineering issues, such as invert uplift, lining cracking, and tunnel leakage [1, 2, 3–4]. Invert uplift, in particular, can cause damage to the floor structure and reduce the tunnel cross-section, thereby affecting both the construction speed and the tunnel operation [5].
The factors influencing tunnel uplift are numerous and complex, and studying these key factors is crucial for optimizing structural design and remediation [6]. Much research has been conducted on the problem of tunnel uplift. Using theoretical analysis methods, researchers have derived expressions for calculating uplift based on the main influencing factors, such as the compression and curvature of the base rock layer, expansion, and rheology [7, 8–9]. Studies have also explored the relationship between inverted deformation and various factors, such as moisture content and ground stress [10, 11]. Zhu et al. [12] analyzed the relationship between the amount of bottom drumming and the supporting pressure of the surrounding rock, the width of the roadway, and the modulus of elasticity of the rock in the bottom plate by establishing a mechanical model. Lee and Wang [13] summarized the classical deformation patterns associated with invert uplift and identified possible causes. Wang et al. [14] studied the mechanism of bottom drum generation under different rock conditions through model tests, and the bottom drum deformation of contact cemented surrounding rock is the largest.
Jiang et al. [15] analyzed the mechanisms behind invert uplift through model tests, categorizing the causes into four basic types and identifying the primary influencing factors. Du et al. [16] combined model tests and numerical simulations to study the failure modes of invert uplift under different loading conditions. Zhou et al. [17] analyzed the time-dependent changes in invert uplift through model tests, evaluating various factors that impact the uplift degree. Du et al. [18] identified the three stages of stress development in the invert and analyzed the dominant factors at each stage through field tests. Zheng et al. [19] explored the sensitivity of invert deformation to different influencing factors based on engineering practice. Fang et al. [20] investigated the effect of different groundwater levels and waterlogging levels on the tunnel bottom drum through 3D printing model tests. Hong et al. [21] studied the effect of soft-plastic loess on the displacement and deformation of the surrounding rock at different parts of the tunnel. Singer et al. [22] conducted an analysis based on field surveys and monitoring to investigate the causes of the invert arch heave.
Shi et al. [23] investigated the causes of surrounding rock deformation in tunnels through model tests and studied the feasibility of using chemical reagents to inhibit rock swelling. Huang et al. [24] studied the effects of tunnel floor load and deformation, and suggested that controlling invert deformation could be achieved by increasing the span-to-depth ratio and enhancing invert support. Other treatment measures include grouting reinforcement, U-shaped steel support, improved drainage systems, and arch replacement [25, 26, 27, 28–29]. Kong et al. [30] analyzed the effects of different tunnel shapes and support forms on invert uplift. Some scholars have also found through numerical simulations that a reasonable construction sequence can reduce the damaged area [31, 32]. Zhao et al. [33] proposed a new tunnel lining structure with reserved slots, effectively reducing tensile stress on the floor to prevent tensile failure. Chang et al. [34] used high-pressure jet piles and friction piles to reduce invert uplift. Although these studies provide extensive research data for the study and treatment of invert uplift, tunnels passing through water-rich and fractured strata present complex and variable uplift causes, necessitating further research into the mechanisms and remediation measures for invert uplift.
This paper addresses the issue of invert uplift in a highway tunnel, employing on-site monitoring and numerical analysis to explore the causes of uplift and proposing corresponding remediation measures. These findings provide technical support and a reference for addressing similar uplift issues in tunnels traversing mud-rich strata.
Engineering background
Engineering geological conditions
The tunnel is a unidirectional, two-lane separated tunnel. The left tunnel is 10,235 m long with a maximum depth of 1,210.85 m, while the right tunnel is 10,210 m long with a maximum depth of 1,199.53 m. The primary stratigraphic lithology of the tunnel consists of granitic schist from the Lancang Group of the Lower Proterozoic boundary. The geological conditions are complex, with the tunnel passing through several fracture zones, as shown in Fig. 1. According to survey data and laboratory analysis, the rock formation primarily comprises quartz, feldspar, mica, and other minerals. The rock is strong to moderately weathered, grey to brownish-grey in color, with a sheet-like structure, and features well-developed joints and fissures, resulting in a highly fragmented rock mass. Additionally, the Yanshanian biotite granite found in the area is grey to grey-black in color, with a blocky structure and hard rock.
Fig. 1 [Images not available. See PDF.]
Geological longitudinal section of the tunnel
According to geophysical exploration data for the right tunnel, the uplift section lies at a depth of 140 to 180 m, where the rock body exhibits a high degree of weathering. The lithology is predominantly strongly weathered granite, with some areas of granitic schist, both of which are influenced by fault fracture zones. The fissures are well-developed, and the rock mass is fragmented, exhibiting a block-crushed structure. The fissure water is well-developed but not highly water-rich. Bedrock fissure water is primarily found in the joints and fissures of the granitic schist and granite. The groundwater table is shallow at the site, with a significant water influx, making sudden mud and water inrush accidents more likely during tunnel construction.
Tunnel support design parameters
The tunnel is entirely located within Class V soft rock. Using the excavation of the bench method, complemented by composite lining.selecting the right-hand invert heave section as the analysis segment. This section uses SF5a and SF5c support systems. The SF5a support is designed for the shallow-buried or V2 sections within the Class V surrounding rock, while the SF5c support is intended for the faulted and fractured zones and reinforced sections of the Class V surrounding rock. As shown in Table 1, the reserved deformation is 15 cm, with a tunnel excavation height of 10.05 m and a span of 12.5 m.
Table 1. Support design parameter list
Lining type | SF5a | SF5c |
|---|---|---|
Steel arch frame | I18 Steel Arch, spaced at 60 cm | I20a Steel Arch, spaced at 60 cm |
Shotcrete | C25 Shotcrete, 25 cm, ø8 Steel Mesh, 15 × 15 cm grid | C25 Shotcrete, 27 cm, ø8 Steel Mesh, 15 × 15 cm grid |
Grouting bolt | ø25 Hollow Grouting Anchor, L = 300 cm, with 100 × 60 cm spacing | ø25 Hollow Grouting Anchor, L = 400 cm, with 100 × 60 cm spacing |
Thickness of secondary lining | C30 Waterproof Reinforced Concrete Lining, 50 cm thick | C30 Waterproof Reinforced Concrete Lining, 60 cm thick |
Reserved deformation | Reserved deformation 15 cm | Reserved deformation 15 cm |
Tunnel damage conditions
The tunnel face exposed surrounding rock consisting of granitic metamorphic schist, characterized as a soft rock with strong weathering, a broken rock mass, water-induced softening, poor bonding between rock layers, and low strength. The tunnel was constructed using the step method. Initial water seepage appeared on the right side of the tunnel face, and after sealing the tunnel face, a second stream of water emerged in the arch. Despite the implementation of drainage measures, the tunnel face experienced six small mud bursts of varying magnitudes within two days, with the largest mud burst measuring 7,310.4 m3 and a cumulative total of 9,461.7 m3. Before the secondary lining was installed, the steel arch frame in the initial support structure of the mud burst section became deformed, and the concrete of the initial support cracked and fell off.
In some sections, the ground surface at the center showed varying degrees of bulging and cracking. For example, in the K1section, no cracks or encroachments were observed in the vault or side walls. However, arch cracks in this section were significant, with the maximum crack width reaching 7 cm. The K1 section contained 8 cracks, primarily extending from the lower mileage to the higher mileage along the centerline of the lining, with other cracks extending from the centerline to the ditches on both sides. The filling layer in this section exhibited a noticeable bulge, with a maximum height of about 50 cm, and some cracks showed significant misalignment. The cracks varied in size, with the largest measuring 20 cm in width and extending approximately 110 m in length. The tunnel damage is shown in Fig. 2.
Fig. 2 [Images not available. See PDF.]
Tunnel damage. a External conditions of the section with sudden mud and water gushing out. b Deformation of the initial arch structure and concrete cracking. c Cracks in the road surface. d Water seepage in the drainage ditch. d Water seepage in the drainage ditch
Numerical analysis of invert heave damage
Calculation model
Combined with the site conditions, a section within the K1 segment is selected as the calculation section, with a depth of 150 m. To eliminate the influence of model boundary effects, the calculation range is set to 3–5 times the tunnel diameter, which in this case is taken as 120 m. The invert of the tunnel is positioned 65 m below the top of the model, as shown in Fig. 3. The boundary conditions of the model are as follows: vertical self-weight stress from the surface to the top of the model is applied at the top, horizontal constraints are applied on both sides, and fixed constraints are applied at the bottom.
Fig. 3 [Images not available. See PDF.]
Computational model
Calculation conditions and material parameters
In-situ stress measurements were conducted near the tunnel face, revealing that the in-situ stress field is dominated by horizontal stress. The maximum horizontal principal stress measured ranges from 3.45 to 5.28 MPa. Based on these results, a lateral pressure coefficient of K = 1.6 is used in the numerical simulation. Additionally, a goaf was detected approximately 30 m above the tunnel roof, with significant water accumulation. The groundwater level is set 25 m below the model's top, considering the field conditions.
The initial support, secondary lining, invert, and filling layer of the tunnel are all modeled using an elastic model. The steel arches, steel mesh, and similar components are converted into the corresponding concrete structures using the equivalent stiffness method. The softened portion of the surrounding rock at the arch bottom is modeled using the Hoek–Brown model, while the remaining surrounding rock is modeled using the Mohr–Coulomb model.. Its physical and mechanical parameters are obtained based on on-site exploration and engineering analogy. Flac3D is used to simulate the working conditions. The physical and mechanical parameters of the materials used in the numerical calculations are shown in Table 2.
Table 2. Physical and mechanical parameters
Material | Density /(kg/m3) | Elasticity modulus / GPa | Poisson’s ratio | Cohesion /MPa | Internal friction angle /(°) |
|---|---|---|---|---|---|
Surrounding rock | 22 | 1.0 | 0.3 | 0.21 | 28 |
Initial support | 22 | 23 | 0.2 | 3.0 | 35 |
Secondary lining | 25 | 32.2 | 0.2 | 4.3 | 35 |
Invert | 25 | 30 | 0.2 | 4.3 | 35 |
Filling layer | 22 | 21 | 0.2 | 2.6 | 35 |
Numerical simulation analysis of the causes of inverted arch uplift
Borehole sampling of the bedrock of this section of the tunnel was carried out, and it was found that the softening of the bedrock was more serious. According to the relevant literature and sampling results, the thickness of the bedrock softening in the model analysis was 2 m, and the GSI = 20 [35, 36]. To analyze the factors affecting the deformation of the uplift arch, two different working conditions were simulated, and the results are shown in Table 3.
Table 3. Calculation conditions
Calculating operating conditions | Calculation conditions |
|---|---|
Case 1 | Does not consider stratum softening; only considers water pressure |
Case 2 | No water pressure, only ground softening |
As shown in Fig. 4a, under the conditions of Case 1, the maximum tensile stress is only 0.67 MPa, with higher values concentrated on both sides of the invert's center. While most of the invert is in a tensile state, and the stress does not reach the allowable tensile stress of 1.35 MPa for C30 concrete, it still impacts structural safety. This indicates that high water pressure is one of the contributing factors to invert cracking.
Fig. 4 [Images not available. See PDF.]
Contour of maximum principle stress of inverted arch (MPa): a Case 1, b Case 2 shows the cloud map of the maximum principal stress on the invert obtained from the calculations. The maximum principal stress reflects the tensile stress acting on the invert. When the tensile stress is significant, approaching or exceeding the allowable tensile stress of the concrete, it can adversely affect the structure. The tensile stress on the invert varies greatly between Case 1 and Case 2
In contrast, as shown in Fig. 4b, under the conditions of Case 2, the invert is subjected to more unfavorable stress, and the tunnel as a whole experiences greater tensile stress. The maximum tensile stress reaches 2.41 MPa, with the highest values concentrated within 5 m of the invert's center. In this area, the tensile stress exceeds 1.0 MPa, and at the center of the invert, it surpasses 2.0 MPa. The concrete in this region has already been damaged by tensile stress, suggesting that the softening of the surrounding rock base is not only a factor contributing to invert cracking but also has a more significant impact on invert safety than high water pressure.
The vertical displacement curves of key parts of the tunnel under different conditions are shown in Fig. 5, where positive values indicate uplift and negative values indicate subsidence. Overall, the tunnel's upper part shows subsidence. At the same time, the invert structure exhibits significant uplift, with a distribution pattern along the tunnel's radial direction that is larger in the middle and smaller on the sides, consistent with the basic characteristics of tunnel invert heave. The invert uplift values for Cases 1 and 2 are 46.16 cm and 32.51 cm, respectively, indicating that both water pressure and surrounding rock softening can influence invert uplift. The tunnel's lower structure is more sensitive to surrounding rock softening, as the deformation of the invert when considering rock softening is 1.42 times that of the case where only high water pressure is considered.
Fig. 5 [Images not available. See PDF.]
Displacement curves of key parts of the tunnel (cm)
As shown in Fig. 6, the entire area below a certain depth of the invert is in a plastic zone. The expansion of the plastic zone in the tunnel base area is a key factor contributing to the increase in inverted deformation. The figure also shows the maximum tensile stress in the filling layer when considering the softening of the surrounding rock. The tensile stress at the center of the filling layer reaches 1.89 MPa, indicating that the deterioration of the base rock causes the expansion force of the surrounding rock to directly act on the invert and transfer to the filling layer. The softening of the surrounding rock thus accelerates the process of invert heave in the tunnel.
Fig. 6 [Images not available. See PDF.]
Distribution of Plastic zone and maximum tensile stress of filling layer (MPa)
Influence of softening of the basement perimeter rock
Influence of different softening degrees
In order to investigate the Influence of different softening degrees of surrounding rock on the deformation of the back arch, a numerical simulation method is used for calculation and analysis, and the surrounding rock in the trapezoidal range of 2 m below the back arch is set as the region that produces strength deterioration. The degree of softening of the surrounding rock is simulated by setting different softening parameters. In the numerical simulation process, only the softening degree of the perimeter rock at the base is changed, and other parameters are taken according to the original design of the arch structure parameters. The softening parameters of the surrounding rock at the base of the arch are shown in Table 4 below. The simulation results are shown in Fig. 7 Influence of softening degree on deformation and stress of inverted arch.
Table 4. Model Surrounding Rock Parameters under Strain Softening Constitutive Model
Mechanical parameters | Unsoftened | Degree of softening I | Degree of softening II |
|---|---|---|---|
Density /(kg/m3) | 22.0 | 22.5 | 23.0 |
Elasticity modulus / GPa | 1.0 | 0.7 | 0.5 |
Poisson's ratio | 0.3 | 0.32 | 0.34 |
Cohesion /MPa | 0.21 | 0.11 | 0.07 |
Internal friction angle /(°) | 28 | 26 | 24 |
Fig. 7 [Images not available. See PDF.]
Influence of softening degree on deformation and stress of inverted arch. a Relationship between degree of softening and vertical displacement of the superelevation arch. b Relationship between the degree of softening and the axial force of the back arch. c Relationship between the degree of softening and the bending moment of the arch
According to the data in Fig. 7 Influence of softening degree on deformation and stress of inverted arch, with the increase of the softening degree of the surrounding rock at the base of the elevated arch, the deformation and force of the elevated arch structure increased accordingly. When the softening degree of the surrounding rock is II, the vertical displacement of the center of the arch reaches the maximum value of 46.16 cm. Similarly, the maximum value of the axial force of the arch occurs in the center of the arch, which reaches 2973 kN. Furthermore, the maximum value of the bending moment at the end of the arch is −180 Kn m. In the case of the surrounding rock not softening, the maximum value of the vertical displacement of the arch and axial force of the arch occurs in the center of the arch, and the maximum value of the bending moment is located at the end of the arch, which is −145 kN m. When the degree of softening of the surrounding rock increases from class I to class II, the vertical displacement of the center of the arch increases by 11.5%, and the axial force increases by 3.8%, and finally reaches 2973 kN; at the same time, the bending moment at the left end of the arch increases by 28%, and reaches 120 kN m. The softening of the surrounding rock of the arch's base has produced a significant uplift effect, resulting in a slight increase in the axial force and a significant increase in the bending moment in the part connected to the superstructure. For the vault, no significant change was observed, indicating that the surrounding rock's softening has a relatively small effect on the vault.
Influence of different softening depths
The softening depth of the surrounding rock at the base of the tunnel also affects the deformation and force characteristics of the upraised arch structure. Numerical simulation is used to analyze and study the deformation and force characteristics of the tunnel arch under different softening depths of the surrounding rock at the base of the tunnel, and the simulation is designed according to the four softening depths of 0, 1, 2, and 5 m. In the numerical simulation, only the softening depth of the surrounding rock is changed. In the numerical simulation process, only the softening depth of the base rock is changed, the softening parameter of the surrounding rock is selected as softening degree II, and other parameters are taken according to the original design of the structural parameters of the arch. The simulation results are shown in Fig. 8 Influence of softening depth on deformation and stress of inverted arch.
Fig. 8 [Images not available. See PDF.]
Influence of softening depth on deformation and stress of inverted arch. a Relationship between softening depth and vertical displacement of the superelevation arch. b Relationship between the softening depth and the axial force of the back arch. c Relationship between softening depth and arch bending moment
From Fig. 8 Influence of softening depth on deformation and stress of inverted arch, it can be seen that with the increase of softening depth of surrounding rock at the base of the tunnel, the vertical displacement and axial force of the back arch and bending moment also show an increasing trend. When the softening depth of the peripheral rock at the base of the elevation arch is 1 m, the maximum vertical displacement of the elevation arch appears in the center of the elevation arch, with a value of 38.17 cm; the maximum value of the axial force of the elevation arch also appears in the center of the elevation arch, which is as high as 2,594 kN. The maximum value of the bending moment of the elevation arch appears at the end of the elevation arch, which is −164 kN·m. When the softening depth is increased to 5 m, the displacement of the center of the arch increases by 39% to 53.12 mm, the axial force of the center of the arch increases by 22.17% to 3169 kN, and the bending moment of the left end of the arch increases by 23.7% to −203kN·m. From the comprehensive analysis of the softening degree of the surrounding rock, the deeper the softening of the surrounding rock and the greater the degree of the reduction of the surrounding rock parameters, the deformation of the elevation arch rises, and the stresses have a more significant effect on the deformation and stresses.
Analysis of invert heave causes
1. Low Strength of Surrounding Rock at the Invert Location. The tunnel's heave section primarily consists of strongly weathered granite and granitic schist, characterized by well-developed fissures, a fractured rock mass, poor stability, and a tendency to soften upon contact with water. The strength of the rock is very low. After tunnel excavation, the surrounding rock undergoes unloading and stress redistribution, leading to an increase in tangential stress and an expansion of the plastic zone radius. This results in deformation and damage progressing from shallow to deeper areas, gradually increasing the deformation and ultimately causing invert failure.
2. Impact of Groundwater. The rock mass exhibits well-developed fissures and, influenced by nearby fault structures, has well-developed water-conducting features. Groundwater accumulates around the tunnel through these fissures. When the tunnel drainage facilities become blocked, long-term water accumulation occurs at the tunnel's base. Additionally, the invert core contains a large amount of expansive minerals, which exert expansive pressure on the invert when exposed to water. Prolonged water saturation softens the surrounding rock, reducing its strength. As the strength of the tunnel base rock mass decreases under horizontal stress, further damage occurs, making it easier for water to penetrate deeper into the rock layers. This exacerbates the hydro-mechanical effects on the deeper rock layers of the base, leading to a more extensive reduction in rock mass strength. Concurrently, the softening effect of water causes fissures in the surrounding rock to increase and expand, resulting in greater deformation and, ultimately, invert failure. The softening of the surrounding rock at the tunnel base and the impact of groundwater are the primary internal factors contributing to invert failure.
3. Issues with Invert Support. In the inverted support for this section, the low strength of the surrounding rock at the base, combined with its tendency to expand and soften when exposed to water, was not adequately addressed with enhanced support measures. Under the influence of horizontal lateral forces and vertical loads from the surrounding rock, the weak rock mass at the base is compressed. When the tunnel invert cannot withstand the pressure transmitted from the base, it results in an invert heave.
The combined effects of these factors generate significant tensile stress at the center of the invert, leading to road surface uplift, the formation of cracks, and subsequent impacts on the tunnel's construction and operation.
Invert remediation measures and effects
Invert remediation measures
Based on the above analysis of the inverted structure’s stress conditions, the severe softening of the surrounding rock due to water, the insufficient bearing capacity of the support structure, and the high water pressure from abundant groundwater are identified as the two major factors causing damage to the inverted structure. Therefore, in the process of invert remediation, it is necessary to reinforce the surrounding rock at the tunnel base and implement effective drainage measures. Through research and analysis, the scheme of 'enhancing the drainage system + grouting with steel flower pipes + replacing the inverted arch' is proposed to reduce the influence of groundwater on the surrounding rock and to demolish or reinforce the structure. (Table 5):
Drainage System. First, clear the tunnel drainage facilities and relocate the tunnel cables and fire protection systems. Drainage holes with a diameter of ϕ115 should be drilled at 5 m intervals along the cable location (approximately 50 cm above the bottom of the tunnel sidewall). The drilling depth should reach the surrounding rock behind the sidewall (≥ 1.5 m). Soft permeable pipes with a diameter of ϕ100 should be installed in the holes and connected via T-joints to a ϕ200 longitudinal drainage pipe added inside the cable trench.
Arch Foot. At the arch foot, install 6 m-long ϕ108 × 6 mm grouting steel pipes with a longitudinal spacing of 1.0 m. The grouting process will use a two-stage grouting technique. The first stage grouting pressure is set at 0.5 MPa, and the second stage fracture grouting pressure is 2–3 MPa. Grouting continues until the voids and shrinkage areas within the pile holes are fully filled, and thick cement slurry is observed returning from the borehole opening. The cement slurry mix ratio for the first stage is cement = 1:0.5, and for the second stage, it is cement = 1:1.
Inverted Arch. The original pavement shall be demolished, the inverted arch and inverted arch shall be filled, the bottom virtual slag shall be removed, and the bottom accumulated water shall be removed. Then the initial support and secondary lining of the inverted arch shall be re-applied according to the original design. At the junction of the old and new linings, stub reinforcement connections are used to ensure structural continuity and stability. Restore the road surface after the construction is completed.
Safety Measures. Enhance geological advance forecasting in sections prone to mud and water inrush. Based on site conditions, install a geophone at a height of approximately 1.0 m on both the left and right side walls of the same section (total of 2 geophones). The geophones should be positioned 3.0 m from the first seismic source (explosion point). Arrange 16 seismic sources at a height of approximately 1.0 m on the left and right side walls, with a spacing of 1.5 m between points. The detection area covers a length of 100 m along the tunnel axis, with a width of 20 m and a height of 30 m around the axis.
Monitoring Projects. We employ a level, leveling staff, and steel tape measure to assess the settlement of the arch crown and the heave of the invert, utilizing the CL04A electronic level, which achieves an elevation accuracy of ± 0.1 mm. The monitoring intervals for the heave of the invert are established every 5 m, with four measurement points designated for each interval. In contrast, the settlement observation of the arch crown is performed at a single point to ensure precise tracking of changes. We utilize the SWJ-IV tunnel convergence meter for horizontal convergence assessments, which has a measurement error limit of ± 0.02 mm. Each side of the tunnel's left and right walls is equipped with one horizontal convergence observation point, maintaining a distance of 5 m between measurement intervals.
Table 5. Treatment measures for floor heave
Disposal measures | Disposal principle | Specific measures | |
|---|---|---|---|
Disposal measures of floor heave | Improved surrounding rock | Waterproof and drainage | Drilling of ϕ115 drainage holes |
Drilling of ϕ115 drainage holes | |||
Reinforce tunnel | Improve the support strength | 6 m long and ϕ108 × 6 mm grouting steel flower tube with longitudinal spacing of 1.0 m m | |
Demolition and replacement of inverted arch and backfilling | Demolition and replacement of the original pavement, inverted arch and filling, and re-construction of inverted arch according to the original design |
Invert remediation effectiveness
To verify the effectiveness of the above remediation measures, the cumulative deformation on the proper tunnel line was monitored to evaluate the results. According to the monitoring data, the overall deformation of the invert is more significant in the K1 section, with less deformation observed at both ends of the tunnel. During the reinforcement of the arch foot from April to June, all sections showed stable changes, with no obvious changes, indicating that the anti-deformation tends to converge and remain stable after the reinforcement of the arch foot. Figures 9 and 10 show the cumulative deformation of the tunnel after invert remediation from April to June and from June to September, respectively, recording the sum of the monitoring data from the left, center, and right measuring points of the invert. As observed in the figures, the deformation of the invert gradually stabilized after the remediation, indicating that the remediation plan was effective and can ensure the safe operation of the tunnel.
Fig. 9 [Images not available. See PDF.]
Deformation curve of tunnel elevation arch from April to June
Fig. 10 [Images not available. See PDF.]
Deformation curve of tunnel elevation arch from June to September
According to the monitoring data, the overall deformation of the invert is greater in the K1 section, with less deformation observed at both ends of the tunnel.From April to June, during the implementation of the micropile technique, all sections showed stable changes without significant variations, suggesting that after the micropile jacking process, the invert deformation tended to converge and remained in a stable state.
From June to July, grouting was conducted, and according to the monitoring data, significant uplift deformation was observed in the K1 section of the tunnel. However, from July to August, the deformation on the left side of the invert in the K1 section began to stabilize, with the deformation amount tending to converge. This indicates that the later grouting process improved the stress condition of the invert, and the surrounding rock has become largely stable.
Discussion
Researchers have concluded that geological conditions, the nature of surrounding rocks, groundwater conditions, and support systems are the key factors leading to the uplift of tunnel arches and have proposed a series of remedial measures [37, 38]. However, these studies are only at the level of simple theoretical analyses of the possible causes of the problem and lack numerical simulations or theoretical verification to support the feasibility of their conclusions. In addition, these studies failed to identify the main causes of the uplift arch bulge. This limitation makes the relevant conclusions inadequate regarding applicability and reliability in managing similar tunnel diseases [39]. Therefore, clarifying the main factors affecting tunnel safety is an important reference value for guiding the subsequent remediation of tunnel diseases.
In this paper, a qualitative analysis of the causes of the up-arch distress in a specific tunnel is carried out based on numerical simulation, focusing on the impacts of two key conditions, namely, high water pressure and softening of the surrounding rock, on the tunnel lining. By analyzing the maximum principal stress distribution, displacement, and extent of the plastic zone of the lining, the main factors leading to the uplift arch bulge were identified. Analysis of the results in Figs. 4, 5 and 6 clearly shows that the effect of surrounding rock softening on the deformation of the superelevation arch is significantly higher than that of the high hydraulic pressure condition. Both softening of the surrounding rock and high hydraulic pressure, when acting, induce tensile stresses in the inner side of the up-arch, where the tensile stresses in the center of the up-arch due to softening of the surrounding rock exceed the allowable tensile stresses in the concrete. Specifically, the upward displacement of the center of the upraised arch induced by the softening of the surrounding rock increased by 13.65 cm, or 41.9%, compared with the consideration of high water pressure alone. In addition, the extent of the basal plastic zone was significantly larger than that of the uplift arch plastic zone, which further indicated that the basal softening was an important factor leading to the uplift arch bulge.
Compared with the traditional theoretical analysis method [40], this paper provides a qualitative and quantitative analysis of the causes of the uplift arch bulge through numerical simulation. On this basis, this paper further investigates the deformation and mechanical response of the tunnel uplift arch under different softening conditions and explores the sensitivity of the deformation and force of the uplift arch structure to different influencing factors. Specifically, numerical simulation methods are used to simulate the surrounding rock's softening effect by discounting the surrounding rock's mechanical parameters and adjusting the range of the basal softening surrounding rock to simulate the influence of different softening depths. The results show that the extent and degree of base softening significantly impact the force and deformation characteristics of the lining structure. Especially in the foot of the arch, the base deterioration significantly increases the bending moment, which further exacerbates the overall settlement of the lining and the risk of instability of the uplift arch.
In addition, this paper provides a systematic overview of the remedial measures for the tunnel bottom drum disease. According to the force characteristics of the upward arch and the cause of the disease, the reinforcement of the surrounding rock was proposed as the main remedial measure and the monitoring of the tunnel as a whole was carried out for five months after the implementation of the remedial measure. The monitoring results show that the remedial measures effectively improve the mechanical properties and stability of the tunnel, which verifies its reasonableness and applicability.
In summary, this paper systematically analyses the causes of tunnel uplift based on numerical simulation, clarifies the main influencing factors, and deeply explores the deformation and force characteristics of the uplift arch structure. Combined with the conclusions of the study, targeted remedial measures are proposed, which provide technical support and scientific basis for the diagnosis and remediation of tunnel uplift arch disease in water-rich coal bed. This study is an important reference value for the subsequent management of similar tunnels.
Conclusions
This study takes the tunnel uplift arch bulge as the research object, aiming to explore the leading causes and influencing factors of its generation. Through numerical simulation, we found that the softening of bedrock plays a dominant role in the basement uplift, and the softening degree and softening depth of the basement surrounding rock significantly affect the deformation and force characteristics of the uplifted arch structure, especially the influence of softening depth is more prominent. In addition, we proposed a series of effective remedial measures based on the results of the study, and this comprehensive remedial method effectively controlled the deformation of the uplift arch, which provides a valuable reference for similar projects. Nevertheless, the model of this study still has some limitations. We have not considered the complex surrounding rock conditions in actual projects, mainly since the softening of the surrounding rock may not be limited to the basement area, which makes the results of the study a limitation of the scope of application in some actual working conditions. Therefore, future studies need to introduce models that are closer to actual working conditions in order to further explore the spatial distribution of surrounding rock softening and its combined effects on tunnel structures.
Acknowledgements
The authors thank the University of Liaoning Technical University for providing valuable data and technical support for this article, and thank the editors and reviewers for their contributions to this article.
Author contributions
C.L. conceived and designed the study. Material preparation, data collection, and analysis were performed by C.L. The first draft of the manuscript was written by Y.W., and C.L. provided comments on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding
No funding was received for conducting this study.
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The authors declare that the data supporting the findings of this study are available within the paper.
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