The conversion section of the cross passage and shaft is a priority concern in the stress transformation of a tunnel structure during subway underground excavation. In the construction of Subway Line 5 in Xi'an, China, the main line in the loess layer was constructed through the cross passage from the subway shaft of the Yue Deng Pavilion–San Dian Village Station tunnel section. Numerical simulation and field measurement were adopted to study the construction stability of the cross passage and shaft under two possible construction methods: the “shaft followed by cross passage construction” method and the “cross passage parallel shaft construction” method. The results showed that the surface deformation and plastic zone of the surrounding rock are similar under the two construction methods. However, of the two, the “cross passage parallel shaft construction” method was more advantageous in controlling the structural deformation of the original shaft and the stress distribution of the horsehead structure. The field monitoring data showed that the surface settlements and the deformation of the original shaft structures meet the requirement of control standards under the “cross passage parallel shaft construction” method.
To reduce the impact of subway construction on ground transportation, especially in the busiest areas of urban cities, subway engineering is often accomplished using the mining method or covered excavation method [
Therefore, keeping the construction of the subway shaft and cross passage safe, especially the horsehead between the shaft and cross passage, is the key and the basis to successful underground construction. There are two commonly used construction methods through a cross passage from a subway shaft. The first method is the “shaft followed by cross passage construction” method, in which the construction sequence is first to excavate the shaft to the design elevation and then excavate the cross passage through the temporary construction platform above the shaft nest. The second method is the “cross passage parallel shaft construction” method, in which the construction sequence is first to excavate the shaft to the temporary inverted arch elevation of the top heading of the cross passage, next to excavate the top heading of the cross passage to the design footage, and then excavate the remaining part of shaft and cross passage.
Wang and Zhang [
The aforementioned research on specific engineering techniques has positive guiding significance for controlling deformation and safety of subway shaft crossing passage construction. However, there has been little research on the determination of tunnel construction methods through the cross passage from a subway shaft, or on the influence that different construction methods make to the deformation of the ground surface and shaft-cross passage structure. In practice, the shaft structure is under homogeneous soil pressure before the horsehead is excavated. After excavation of the horsehead, because of the free face of the shaft structure near the horsehead, the shaft is in a biased state, and the stratum’s bias pressure is balanced by the friction from the earth pressure on both sides of the horsehead and by the combined effect of the horsehead support force and soil arching. If the force state does not reach equilibrium, the construction may produce a collapse. The different intersection construction methods between a shaft and cross passage create different force states in the horsehead section. To assure a safe balanced state of the horsehead section, optimal analysis of the different construction methods of the horsehead section is necessary.
In this study, we examined the construction of Subway Line 5 in Xi’an, China; the main line loess tunnel of which was constructed through the cross passage from the subway shaft at the Yue Deng Pavilion–San Dian Village Station tunnel section. Numerical simulation and field measurement were adopted to study the ground surface deformation and the deformation and stress distributions of the original shaft structure, as well as the plastic zone of the surrounding rock characteristics under two construction techniques: the “shaft followed by cross passage construction” method and the “cross passage parallel shaft construction” method. The results were intended to provide a theoretical basis and technical support for the design and construction of shaft and cross passages in loess areas.
Subway Line 5 is a very important traffic thoroughfare from east to west in the Xi’an subway network. The line has a total length of 45.37 km and a total of 34 sites, and the whole line is divided into two projects. The length of the first project is 25.24 km, traversing west to the Peace Industrial Park and east to the Textile City Railway Station. The tunnel section between Yue Deng Station and San Dian Village Station in the first project was designed to be constructed using the mining method. At location ZDK44+310, the main regional tunnel was excavated through the service gallery, a shaft, and a cross passage. The relative positions of the subway and the construction shaft are shown in Figure
Study area.
The engineering site was in the third grade terrace of Xi’an, along the Chan River, and the site terrain was flat. The elevation of the exploration site varied from 450.67 m to 455.4 m (Figure
Profile of soil layers, tunnel from Yue Deng Pavilion Station to San Dian Village Station (units: m; vertical scale: 1 : 100; horizontal scale: 1 : 1,000).
The horizontal dimension of the shaft was 5.0 m × 6.5 m. The depth of the shaft was 22.451 m. The cross passage had a total length of 27.932 m, a net width of 4.5 m, and a net height of 7.35 m. The cross passage was equipped with a horsehead. A shaft wellhead set up the locking ring beam, and the size of the ring beam section was 2 m × 1 m. Shaft support parameters are shown in Table
Shaft support parameters.
Item | Main materials and specifications | Structure size | |
---|---|---|---|
Support | Leading conduit | Φ42 |
Spacing: 1.0 |
Grid steel frame | C25, C14 steel | Vertical clearance: 0.75 and 0. 5 m | |
Mesh reinforcement | Φ8 150 |
Surrounded by laying, double | |
Vertical reinforcement | C22 steel bar | Ring spacing: 1.0 m, double | |
Shotcrete | C25 early strength concrete | Thickness: 0.4 m |
Cross passage support parameters.
Item | Main materials and specifications | Structure size | |
---|---|---|---|
Support | Big pipe shed | Φ108 |
Length: 10 m; ring spacing: 0.5 m |
Leading conduit | Φ42 |
Ring spacing 0.3 m | |
Vertical clearance: 1.5 m | |||
Grid steel frame | C25, C14 steel bar | Vertical clearance: 0.5 m | |
Mesh reinforcement | Φ8 150 |
Surrounded by laying, double | |
Vertical reinforcement | C22 steel bar | Ring spacing: 1.0 m, double | |
Shotcrete | C25 early strength concrete | Thickness: 0.3 m | |
Secondary lining | C35 waterproof reinforced concrete seepage resistance grade p10 | Thickness: 0.6 m |
The intersection construction between the shaft and cross passage was described in a numerical model. Two different construction methods (Section
The two construction methods for the tunnel section between Yue Deng Station and San Dian Village Station were the “shaft followed by cross passage construction” method (hereinafter referred to as Method 1) and the “cross passage parallel shaft construction” method (hereinafter referred to as Method 2).
In both methods, the intersection construction between the shaft and crossing passage was divided into three phases: locking section construction, shaft well construction, and top heading and bench of the horsehead construction. According to the design layout, a draining well was to be arranged to advance the site drainage before excavating the shaft so that the construction of the shaft and cross passage could be accomplished in dry soil. The construction sequence of Method 2 is shown in Figure
Construction sequence of Method 2 (unit: mm).
In the numerical analysis, the
Finite element model of the shaft and cross passage.
Based on the geological survey report of the tunnel section between Yue Deng Station and San Dian Village Station, the surrounding rocks of the shaft and cross passage were mainly composed of miscellaneous fill, new loess, paleosol, and old loess (designated Q2eol). The physical and mechanical parameters are shown in Table
Physical and mechanical parameters of model.
Material | Unit weight (kN/m3) | Elastic modulus (MPa) | Poisson’s ratio | Angle of internal friction (°) | Cohesion (kPa) | Thickness (m) |
---|---|---|---|---|---|---|
Miscellaneous fill | 16.5 | 9.0 | 0.43 | 16.0 | 5.0 | 2.0 |
New loess | 15.8 | 13.5 | 0.375 | 21.0 | 25.0 | 11.0 |
Paleosol | 17.3 | 24.0 | 0.29 | 20.0 | 35.0 | 8.0 |
Old loess | 15.9 | 18.0 | 0.35 | 20.0 | 30.0 | 17.0 |
Ring beam | 25 | 30,000 | 0.2 | — | — | 1.0 |
Early support | 22.0 | 23,500 | 0.2 | — | — | 0.25 |
Temporary support | 22.0 | 17,500 | 0.2 | — | — | 0.1 |
The yield criterion commonly used in geotechnical engineering includes Tresca criterion, Mises criterion, Drukle–Plager criterion, and Mohr–Coulomb criterion, and so on. Among them, the Mohr–Coulomb criterion can well reflect the strength effects of soils and the sensitivity to hydrostatic pressure and is simple and practical. The physical and mechanical parameters, cohesion (
Because the preliminary support and temporary support of the shaft and cross passage included steel arches, metal mesh, and shotcrete, the analysis included an equivalent model based on the parameters of a steel arch, metal mesh, and shotcrete (Tables
According to the analysis and calculation, the physical and mechanical parameters of the equivalent support are shown in Table
The shaft and cross passage construction process was simulated using a step-by-step approach. A total of 90 steps were used in each of the two construction methods. The excavation was simulated by removing the elements in front of the tunnel face, and the supporting structure was simulated by activating the structural elements. The supporting structures were installed immediately after the excavation of the shaft and cross passage. The length of the excavation at each step in the construction was 1 m. The two construction processes were as follows. Method 1 The shaft locking section was excavated using the full-face excavation method, and then the ring beam was set up. The total length of the step was 3 m. The shaft well was excavated, and shaft supporting structure was constructed until the bottom of the shaft was reached; then the permanent closed bottom construction of the shaft was completed. The total length of the step was 18 m. The top heading excavation of the cross passage was completed, and the supporting structure was constructed. The total length of the step was 6 m. The bench excavation of the cross passage was completed, and the supporting structure was constructed. The total length of the step was 3 m. The top heading and bench excavation of the cross passage were constructed to the designed length. The entire construction simulation was completed.
Steps 1–24 were used to simulate the construction of the shaft, steps 25–37 were implemented to model the construction of the horsehead, and steps 38–90 were used to simulate the construction of the cross passage. Method 2 The shaft locking section was excavated using the full-face excavation method, and then the ring beam was set up. The total length of the step was 3 m. The shaft well was excavated, and the shaft supporting structure was constructed until the temporary inverted arch elevation position of top heading of the cross passage was reached; then the temporary closure of the bottom of the shaft was constructed at the design height of the top heading. The total length of the step was 14 m. The top heading excavation of the cross passage was completed, and the supporting structure was constructed. The total length of the step was 6 m. The temporary closure of the bottom of the shaft was dismantled. The remaining part of the shaft was constructed, and the permanent closing support of the shaft bottom was constructed. The total length of the step was 4 m. The bench excavation of the cross passage was first excavated to 3 m. Then the top heading and bench excavation of the cross passage were constructed to the design length. The entire construction simulation was completed.
Steps 1–20 and steps 28–33 were used to simulate the construction of the shaft. Steps 21–27 and steps 34–38 were implemented to model the construction of the horsehead, and steps 39–90 were used to simulate the construction of the cross passage.
When a subway is constructed through an environmentally sensitive area, the settlement deformation of the ground surface is an important index of construction safety [
Ground surface settlement resulting from construction methods 1 and 2 (units: mm). (a) Method 1. (b) Method 2.
The plastic zone of the surrounding rock mass is an important index for the stability of the material surrounding the tunnel [
Plastic zone distribution in surrounding rock. (a) Method 1. (b) Method 2.
To compare and analyze the most dangerous position of plastic deformation of surrounding rock resulting from the two construction methods, a plastic deformation value ≥100
Distribution of maximum plastic deformation. (a) Method 1. (b) Method 2.
In the excavation process of the underground structure, the deformation and stress distribution of the supporting structure can directly reflect the stability of the surrounding rock. Furthermore, different construction programs can produce different changes in the shaft support structure.
According to the coordinate system shown in Figure
Maximum deformation of shaft (unit: mm).
Method | Directions | Absolute value | Position |
---|---|---|---|
Method 1 | X | 17.48 | The shaft wall near the invert of the horsehead |
Y | 25.46 | The shaft wall near the two haunches of the horsehead | |
Z | 38.15 | The bottom of the shaft | |
Method 2 | X | 13.65 | The shaft wall near the invert of the horsehead |
Y | 23.10 | The shaft wall near the two haunches of the horsehead | |
Z | 38.35 | The bottom of the shaft |
Figure
Deformation of shaft at different depths. (a) Deformations away from the horsehead. (b) Deformations parallel to the cross passage axis.
As shown in Figure
As shown in Figure
The maximum stresses of the shaft support after construction under the two construction methods are given in Table
Maximum stress of shaft (unit: kPa).
Method | Method 1 | Method 2 | ||
---|---|---|---|---|
Stress | Maximum principal stress | Ultimate shear stress | Maximum principal stress | Ultimate shear stress |
Absolute value | 10,216.00 | 10,618.21 | 8,746.61 | 8,539.54 |
Position | The shaft wall near the arch of the horsehead |
To compare the influence of the two construction methods on the intersection between the shaft and cross passage in the horsehead section (Figure
Horizontal deformation of shaft at different construction steps.
As shown in Figure
Figure
Maximum principal stress distribution in the horsehead. (a) Method 1. (b) Method 2.
Figure
Maximum shear stress distribution in the horsehead. (a) Method 1. (b) Method 2.
The “shaft followed by cross passage construction” method (Method 1) has been used for a long time and is a mature technology in China. However, the construction sequence in this method is complex and causes great disturbance between each stage. In contrast, the relatively newer “cross passage parallel shaft construction” method (Method 2) depends only on a simple platform in the temporary back cover of the shaft before the top heading of the crossing passage can be constructed. Thus, in Method 2, construction is simpler and easier; furthermore, the time required for scaffolding erection and removal is shorter, which reduces the workload and makes the entire construction sequence quicker. Importantly, in Method 2, construction workers do not need to stand on scaffolding during construction; therefore, the overall construction operations do not need to consider the risk of working at height. Thus, Method 2 also is good at avoiding construction safety hazards and ensuring the safety of construction workers.
In addition, the comparative analysis of the numerical simulation results for the two construction methods showed that Method 2 can control the deformation and force of the original shaft structures more effectively than Method 1. Considering the actual demands of the subway construction, especially the need to shorten the construction period, it was concluded that the construction of the cross passage from the subway shaft at the Yue Deng Pavilion–San Dian Village Station tunnel section should be carried out using Method 2, namely, “cross passage parallel shaft construction” method, and that support of the horsehead should be strengthened during construction to ensure that the position remained stable.
A large number of numerical simulation results show that when the size of the numerical model was relatively small, the size of the boundary has serious influence on the results; when the size of the boundary exceeded a specific value, the influence of the boundary effect can be negligible. In order to verify the validity of the selected boundaries of the numerical model in Section
Figure
Deformations of the selected points in the horsehead section with different boundaries. (a) Deformations with different side boundaries. (b) Deformations with different bottom boundaries.
As shown in Figure
Based on the results of numerical simulation analysis, the construction of the cross passage from the subway shaft on the Yue Deng Pavilion–San Dian Village Station tunnel section was actually carried out using the “cross passage parallel shaft construction” method (Method 2). Considering the actual situation of the site, the construction sequence and construction control points of the shaft and cross passage were as follows. First, the designed shaft position was ensured. Then excavation of the locking region of shaft was begun, and shaft support for the locking part was set up. To prevent flooding by surface water, the height of the shaft lock was 0.3 m higher than that of the ground surface. To expand the import force area, the concrete pour around the well was extended outward by 0.2 m. Finally, a safety barrier and door were constructed around the wellhead at a height exceeding 1.2 m. The excavation of the standard section of the shaft was accomplished using the top-down excavation method in which the footage was 0.5 m. The construction was carried from top to bottom, and the supporting structures were installed immediately after the excavation. In view of the complexity of the loess stratum, full-section excavation of the shaft was avoided by using blocking and slicing excavation. The excavation sequence is shown in Figure When the shaft excavation was approximately 1.5 m from the groundwater level elevation, excavation was continued only after exploring the precipitation effect on the groundwater level. According to the groundwater situation, appropriate measures were taken to ensure that work proceeded without water ingress to ensure the stability of the bottom of the shaft during construction. When each footage was constructed in the shaft blocking and slicing excavation, it was inspected and 40 mm of sprayed concrete was placed. The Φ6.5 @ 150 mm × 150 mm steel was welded into a double-reinforced grid. Finally, the steel frame was constructed and sprayed with C25 concrete to a thickness of 400 mm. In the consolidation grouting construction and the back-filling construction, the grouting steel pipe with Φ42 × 3.5 and When the shaft was excavated to the horsehead position, a steel frame was strengthened on the top heading of horsehead, and the shaft was continuously excavated to the bottom of the top heading of the horsehead. The C25 spray concrete with a thickness of 300 mm was set to fill the bottom of the shaft, and the construction direction transferred to the cross passage. The horsehead construction was the key to the intersection construction between the shaft and the cross passage. According to the numerical simulation analysis, the stress state of horsehead structure was complicated, so the construction process of this component became the focus of attention. To ensure safety, the support replacement used the I-20 steel at the top heading excavation of the cross passage before breaking over construction of the horsehead; this was then removed after completing the construction of the hole. The leading conduit of the dome of the cross passage with Φ42 × 3.5, and The ring-like drift heading method with preformed core soil was carried out in the construction of the cross passage. The remaining part of the shaft was constructed after the cross passage construction reached 4 m. The shaft was excavated to the design depth for permanent back cover construction. The I-20a I-beams spaced on 0.5 m centers were placed, and the net was sprayed with C25 concrete to a thickness of 350 mm to complete the back cover construction. After the completion of the back cover, the bench excavation of the cross passage was carried out. Depending on setting the temporary ladder and other ancillary construction facilities, the intersection construction was completed.
Shaft excavation and supporting sequence.
To ensure the safety of the shaft and cross passage construction, the surface settlement and the shaft deformation caused by the “cross passage parallel shaft construction” method were monitored during construction. The shaft surface monitoring points of the Yue Deng Pavilion–San Dian Village Station tunnel section are shown in Figure
Layout of deformation measuring points at ground surface.
Figure
Comparisons of the field monitoring data and numerical simulation results. (a) Surface settlements. (b) Deformation of the shaft support.
Figure
The main line tunnel of Subway Line 5 in Xi’an, China, was placed in the loess stratum and was constructed through the cross passage from the subway shaft at the Yue Deng Pavilion–San Dian Village Station tunnel section. This research focused on the construction stability in the loess material. The ground surface deformation, surrounding rock mass plastic zone, and the deformation and state of stress of the original shaft support structure were analyzed as a result of two construction methods: “shaft followed by cross passage construction” and “cross passage parallel shaft construction.” Combining simulated responses and field monitoring data, the following conclusions can be drawn.
The ground settlement and the plastic deformation of the surrounding rock caused by both construction methods are basically the same in the loess formation, but the maximum plastic deformation position of the surrounding rock is different. Therefore, different construction methods should strengthen different locations in a construction project to prevent excessive deformation of the surrounding rock.
Compared with the “shaft followed by cross passage construction” method, the “cross passage parallel shaft construction” method has more advantages in controlling the displacement and stress of the original shaft structure, especially for the shaft structure in the cross passage excavation direction. In this study, the maximum deformation caused by the latter construction method was 14.48% (3.05 mm) less than that caused by the former, and the maximum stress and the maximum shear stress were 14.38% and 19.57% less, respectively, when construction was complete.
Under both construction methods, the local stress concentration of the shaft wall will appear near the horsehead, so this location is the key position for construction reinforcement. However, in terms of stress values, the maximum principal stress and the maximum shear stress in the “cross passage parallel shaft construction” method are 14.38% and 8.93% less, respectively, than comparable stresses caused by the “shaft followed by cross passage construction” method. The temporary back-filling measures of the shaft in the “cross passage parallel shaft construction” method can help to reduce the deformation of the shaft support and achieve better results in controlling the deformation value and velocity of the shaft support. Therefore, the “cross passage parallel shaft construction” method makes the horsehead support safer than does the other construction method.
The field monitoring data show that the deformation of the ground surface and shaft structures produced by the “cross passage parallel shaft construction” method in the loess stratum was less than 15 mm, which satisfied the safety requirements and was close to the numerical simulation results. Therefore, the “cross passage parallel shaft construction” method is safe and feasible for use in a loess stratum, and the results of this research can be reference for similar construction projects.
The authors declare no conflicts of interest.
This paper is supported by the National Natural Science Foundation of China (Nos. 51578447 and 51404184) and the Natural Science Basic Research Program of Shanxi Province (No. 2016JQ4009).