Keywords

1 Introduction

In order to ensure the operation safety of large tunnels and timely evacuation and rescue in case of accidents, it is generally necessary to set up vehicle cross passage and pedestrian cross passage in separate tunnels. The spacing of vehicle cross passage should be 750 m, not more than 1000 m; The setting spacing of pedestrian cross passage should be 250 m, not more than 500 m [1]. The cross traffic passage is excavated in the main tunnel of the existing tunnel, with large free surface at the junction and complex stress. If the construction process is improper and the monitoring is not in place, the lining of the main tunnel of the existing tunnel may be deformed and cracked, and the construction risk is high. At present, the research on construction mechanics of vehicle cross passage and main tunnel is mainly numerical simulation. Shi Yanwen et al. [2] used three-dimensional numerical simulation method to conduct elastic-plastic analysis of the main tunnel and vehicle cross passage; Liu Shanhong et al. [3] used finite element software to conduct numerical analysis on the tunnel crossing section of juyun mountain in Fuling, Chongqing and concluded that in-situ stress and structural stress concentration are two key factors leading to local cracking and affecting the stability of the intersection; Sun Zhijie [4] and others combined the engineering field test with finite element simulation to study the deformation law of the main tunnel during the construction of vehicle cross passage; Luo Yanbin [5] et al. carried out three-dimensional finite element numerical simulation of the tunnel of Tianheng mountain in Harbin and studied the influence of cross passage construction on the main tunnel structure of the tunnel; Liu Xiaoliang [6] conducted three-dimensional numerical simulation analysis of tunnel in Danan mountain and studied the influence of vehicle cross passage excavation on the surrounding rock displacement at the intersection of the main tunnel of long-span tunnel.The above scholars have made some progress in the research on the mechanical characteristics and deformation mechanism of the intersection between the main tunnel and the cross passage, but they are limited to the role between the normal main tunnel and a single vehicle cross passage, and rarely mention the role between the main tunnel and the vehicle cross passage when the main tunnel has been weakened. Based on an example of a highway tunnel, using the method of three-dimensional numerical simulation and theoretical calculation, this paper analyzes the deformation and stress law of the lining of the main tunnel when the cross passage is built nearby and the cross passage is excavated, and checks the safety of the lining structure at the interface between the main tunnel and the cross passage after the cross passage is completed, the results can provide reference for similar projects.

2 Project Overview

2.1 Geological Conditions

The tunnel site belongs to the tectonic denudation low mountain landform area, with large topographic fluctuation and change, the relative height difference is about 335 m, the inlet side slope is about 25–35°, and the outlet side slope is about 15–20°. The slope is covered with Deluvial cohesive soil strongly weathered siltstone. The drilling reveals that the upper part of the tunnel site is quaternary eluvial diluvium; The underlying bedrock is Permian Wenbishan formation siltstone and weathered layer, occurrence 118°∠20°; Siltstone and its weathered layer of Carboniferous woodland formation, occurrence 30°∠51°; Quartz sandstone and weathered layer of woodland formation are locally developed. The surrounding rock of the tunnel is a weak permeable layer, and various indexes measured by the groundwater and surface water in the site are slightly corrosive to the concrete and the reinforcement in the concrete.

2.2 Tunnel Overview

The total length of the tunnel is 1022m, and the left and right tunnels are arranged separately. The starting and ending mileage of the tunnel is ZK19+179-ZK20+191 for the left tunnel and YK19+183-YK20+215 for the right tunnel. The left and right tunnels are 1012 m and 1032 m long respectively. The main tunnel of the tunnel adopts Z7-3 lining structure, and the support parameters are: shotcrete C25, thickness 30 cm; 20B I-steel support, spacing 70 cm; C30 waterproof concrete for secondary lining, 50 cm thick; Hollow grouting anchor rod L = 4 m. The entrance and exit of the tunnel are located within the plane curve range. The entrance and exit curve radii of the left line are 1200 m and 1100 m respectively, and the entrance and exit curve radii of the right line are 1200 m and 1000 m respectively. Design elevation of inlet: 349.840 m for left tunnel and 349.796 m for right tunnel; Design elevation of exit: 346.80 m for left tunnel and 337.035 m for right tunnel.

The stake number of the left tunnel of the vehicle cross passage is ZK19+440, 261 m away from the tunnel entrance; The chainage of the right tunnel is YK19+449, 266 m away from the entrance. Z8-2 lining structure is adopted for the vehicle cross passage, and the support parameters are: shotcrete C25, thickness 20cm; Grating steel support, spacing d = 1 m; C30 waterproof concrete for secondary lining, 35 cm thick; Hollow grouting anchor rod L = 2.5 m. The main tunnel and pedestrian cross passage of the tunnel have been completed, and the pedestrian cross passage has been backfilled. The outer contour of the vehicular cross passage is only 3 m away from the outer contour of the pedestrian cross passage. The construction of vehicle cross passage needs to destroy the lining of the original main tunnel, and the surrounding rock will inevitably be disturbed during the construction process. The positional relationship between the main tunnel and the cross passage of the tunnel is shown in Fig. 1.

Fig. 1.
figure 1

Position relationship between main tunnel and cross passage of tunnel.

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3 Numerical Simulation

3.1 Model Establishment

The three-dimensional model is established by Midas finite element software. The model takes the pedestrian cross passage as the core area, X direction as the cross passage direction and Y direction as the main tunnel direction. Generally, when the boundary range of 3 times the hole distance is taken, the analysis results meet the accuracy requirements [7]. The span of the main tunnel of the project is 14.9 m, the outer contour interval of the main tunnel is 15 m, and the span of the vehicle cross passage is 7.5 m. The model is taken as 110 m in X direction, 70 m in Y direction and 110 m in Z direction. The three-dimensional finite element mesh is shown in Fig. 2.

Fig. 2.
figure 2

3D mesh generation.

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3.2 Model Calculation Parameters

The solid element is selected for the surrounding rock, and the Drucker Prager (DP criterion) is adopted for the constitutive model. The plate element is used for the initial support and secondary lining of the main tunnel and vehicle cross passage of the tunnel, and the material calculation parameters are shown in Table 1.

Table 1. Main physical and mechanical parameters.
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4 Analysis of Calculation Results

4.1 Analysis of Surrounding Rock Deformation

When the left tunnel lining is removed, the displacement of the supporting structure of the main tunnel is shown in Fig. 3. Due to the poor self stability of class V surrounding rock, when the lining at the junction is removed, the surrounding rock deforms into the tunnel, with the maximum value of 55.3 mm. The horizontal deformation of the arch waist lining at the junction of the main tunnel and the vehicle cross passage is 12.1 mm, and the horizontal deformation of the lining between the vehicle cross passage and the pedestrian cross passage is 3.7 mm. After the excavation of rock and soil mass, the surrounding rock pressure borne by the lining is released, the vault at the junction of tunnel and vehicle cross passage sinks by 23.3 mm, and the arch bottom bulges by 19.4 mm.

When designed according to the bearing capacity, the deformation of the initial support of the composite lining shall not exceed the design reserved deformation [8]. The deformation of the cross passage is much less than the reserved deformation of 10 cm. However, due to water gushing in the tunnel and poor surrounding rock, the left tunnel shall be reinforced with steel support in advance to reduce the exposure time [9,10,11,12].

Fig. 3.
figure 3

Deformation of support structure of main tunnel of tunnel after lining removal at junction.

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After the construction of the vehicle cross passage is completed, the displacement of the supporting structure of the main tunnel is shown in Fig. 4. During the excavation of the vehicle cross passage, the excavation and support have little impact on the surrounding rock and left and right tunnels. The maximum displacement in the horizontal direction of the lining is 13.4 mm, which is only 1.3 mm higher than the previous construction step. The arch bottom bulges upward by 22.6 mm, an increase of only 3.2 mm compared with the previous construction step. As the excavation unloading causes the surrounding rock on both sides to squeeze into the vehicle cross passage, resulting in the upward uplift of the arch bottom, the vehicle cross passage lining shall be constructed in time to reduce the deformation of surrounding rock.

Fig. 4.
figure 4

Deformation of support structure of main tunnel after completion of vehicle cross passage.

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4.2 Stress Analysis of Lining

The axial force of tunnel lining after the removal of left tunnel lining is shown in Fig. 5. When the lining of the main tunnel is removed and the cross passage is excavated, the original arching effect around the main tunnel is destroyed, and the stress concentration occurs at the same time. The maximum axial force of the lining along the X-axis direction is −15244 kN, which is located at the arch waist at the junction of the vehicle cross passage and the tunnel. The maximum axial force along the Y-direction is also located at the arch waist at the excavation edge, which is −10242 kN, which is less than the axial force in the X-axis direction. (Positive values in the figure indicate tension and negative values indicate compression.)

Fig. 5.
figure 5

Axial force of lining after removal of left tunnel.

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After the construction of the vehicle cross passage, the axial force of the tunnel lining is shown in Fig. 6. The maximum axial force of the lining along the X-axis direction is located at the arch waist of the boundary edge between the tunnel and the vehicle cross passage, with the maximum value of −9072 kN, and the maximum axial force of the lining along the Y-axis direction is −6421 kN. The stress concentration is obvious here. After the construction of vehicle cross passage lining, the axial force of the main tunnel lining is still large, so the monitoring and support should be strengthened.

Fig. 6.
figure 6

Axial force of lining after completion of vehicle cross passage.

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4.3 Calculation of Safety Factor

4.3.1 Calculation of Safety Factor of Secondary Lining

The secondary lining of the main tunnel is an eccentrically compressed reinforced concrete member. After the cross passage is completed, the stress concentration at the junction of the main tunnel and the cross passage is the highest. The secondary lining structure at the junction is taken for safety checking calculation. The bending moment and axial force are shown in Fig. 7–8 [12,13,14,15,16,17].

Fig. 7.
figure 7

Bending moment diagram of secondary lining.

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Fig. 8.
figure 8

Axial diagram of secondary lining.

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The secondary lining of the tunnel is an eccentric compression rectangular member, and the safety factor is calculated according to the comprehensive safety factor method according to article 10.4.25 of Guidelines for Design of Highway Tunnel [1] (JTG/TD70-2010). The results are shown in Table 2. It can be seen that the safety factors of representative positions meet the specification requirements.

Table 2. Internal force value statistics and safety factor of structural control points.
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4.3.2 Calculation of Safety Factor of Initial Support

The initial support of the main tunnel belongs to eccentric compression or tension concrete structure, and the compressive strength or tensile strength of the structure should be checked. Similarly, the initial support structure at the junction is selected for safety checking calculation. The bending moment and axial force are shown in Fig. 9–10.

Fig. 9.
figure 9

Bending moment diagram of initial support.

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Fig. 10.
figure 10

Axial diagram of initial support.

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The initial support of the tunnel belongs to eccentric compression or tension rectangular structure, and the safety factor is calculated according to the comprehensive safety factor method according to 10.4.16 or 10.4.17 of Guidelines for Design of Highway Tunnel [1] (JTG/Td70-2010), as shown in Table 3. It can be seen that the safety factors of representative positions, except the inverted arch, meet the specification requirements, and the results are relatively safe as a whole.

Table 3. Internal force value statistics and safety factor of structural control points.
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According to the calculation results in Table 2 and Table 3, except that the safety factor of the inverted arch area of the initial support of the main tunnel is low, the checking calculation of the safety factor of other parts meets the requirements of Code for Design of Road Tunnel [8], and the overall structure is relatively safe. The inverted arch and wall bottom shall be strengthened in combination with the site conditions.

5 Conclusion

Through the combination of numerical simulation and theoretical checking calculation, the influence of vehicle cross passage construction on the main tunnel is comprehensively analyzed, and the following conclusions are drawn:

  1. (1)

    Class V surrounding rock has poor self stability. After removing the lining at the junction of the main tunnel and the vehicle cross passage, the surrounding rock will displace into the tunnel, and the surrounding rock at the junction shall be reinforced in advance. After the tunnel lining is excavated, the surrounding rock pressure is released, resulting in a large displacement of the lining at the arch bottom and arch crown of the main tunnel to the free face, with a maximum of 55.3 mm. After the lining of the vehicle cross passage is constructed, the displacement change is small.

  2. (2)

    After removing the lining of the main tunnel at the junction, the axial force at the arch waist of the interface is large, which is −15244 kn. After the lining of the vehicle cross passage is applied, it is −10242 kn, which is weak compared with other positions.

  3. (3)

    After the construction of the vehicle cross passage is completed, the safety checking calculation shall be carried out for the initial support and secondary lining of the main tunnel at the junction. Most of the safety factors meet the requirements of the code for design of highway tunnels, and certain strengthening measures shall be taken for weak links.