The safety of subway tunnel construction using the PBA construction is the focus of attention at present. In this paper, based on the underground excavation project of Xin-cao section of Beijing Metro Line 19, the PBA construction process is studied by numerical simulation. The influence laws of surface settlement, structural deformation and internal force change after six construction procedures are compared and analyzed. The research results show that: after the excavation of small pilot tunnel, the extreme value of pavement settlement appears above the secondary lining vault, and the extreme value of safety factor of primary support structure of small pilot tunnel appears in the middle and both sides of the pilot tunnel vault; in order to ensure the construction safety, considering the displacement and internal force, it is suggested to excavate the middle pilot tunnel first, then construct the pile crown beam, and then excavate the two sides of the pilot tunnel Compared with other working conditions, this working condition can effectively control the pavement settlement and structural deformation, and the structural safety factor is the largest. The research results can provide a reference for the excavation of shallow buried multi pilot tunnel with tunnel pile-beam-arch approach.
Research Article
Optimization of PBA construction process in Xinfadi-caoqiao section of Beijing Metro Line 19
https://doi.org/10.21203/rs.3.rs-1608923/v1
This work is licensed under a CC BY 4.0 License
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Subway tunnel
shallow tunnel by excavation
pile-beam-arch approach
process optimization
Nowadays, the level of economic construction in China has been greatly improved, and the city has developed rapidly. The subway has become the main mode of transportation in modern cities (Bao and Xuidng 2018). Due to the complex conditions of urban underground space, subway lines inevitably cross existing structures. Existing structures include underground pipelines, main roads, houses, etc. (Ding Z et al. 2019; Lai H et al. 2020). In order to reduce the impact of tunnel construction on the surrounding environment, choosing appropriate construction methods has become the key to safe operation. At present, the subway tunnel excavation methods include shield approach (Hu B and Wang C 2019), the CRD approach (Annan J et al. 2011), the pipe jacking approach (Liu G et al. 2019), the pile-beam-arch approach and other construction methods. As a new construction approach, the pile-beam-arch approach can be effectively applied to the complex urban environment and the surrounding settlement and deformation control projects with higher requirements (Liu Jiazhu et al. 2018), which is gradually favored by people in the subway construction project.
At present, some experts and scholars have done some research on the construction of tunnel pile-beam-arch approach. Based on the measured surface settlement data of 30 subway stations in Beijing after PBA construction method, Yu L and others summarized the surface settlement characteristics of different geological conditions, different sections and different excavation stages (Yu L et al. 2019). Xu X and others carried out relevant research on PBA construction method relying on Beijing metro line 14. The field monitoring data show that the surface settlement is mainly caused by the construction of side pilot tunnel and middle pilot tunnel (Xu X et al. 2020). Yang Zixuan and others, relying on the West Extension tunnel of Beijing Metro underpass the existing line 5, optimized the construction scheme of PBA method through numerical simulation, and proposed that pouring the second lining buckle arch after excavating 1 / 4 of the total length of the tunnel in the middle pilot tunnel can effectively control the surface settlement (Yang Zixuan et al. 2020). To sum up, there are deep researches on the influence of tunnel pile construction on stratum deformation law, surface settlement stage and lining deformation, but there are relatively few researches on the optimization of tunnel excavation and pile construction process. Based on the underground excavation section of Xinfadi-caoqiao section of Beijing Metro Line 19, through the finite difference numerical calculation, this paper studies the process optimization of pilot tunnel excavation and pile construction. The research results can provide a reference for similar construction process design.
FLAC3D software is a simulation software developed by Itasca company. Developed from FLAC2D software. FLAC3D software mainly through the establishment of three-dimensional mathematical model of soil, rock and other materials for Mechanical Properties Simulation and plastic flow analysis. FLAC3D adopts explicit Lagrangian algorithm and hybrid discrete partition technology, which can simulate the plastic failure and flow of materials very accurately (An et al. 2020).
For example, relying on Harbin Metro Line 2, Yi Q et al. Established a three-dimensional mathematical model through FLAC3D software to conduct numerical simulation analysis on the excavation of the new tunnel. The calculation results are consistent with the field monitoring data, which verifies the effectiveness and applicability of FLAC3D software (YI Q et al. 2019). Although FLAC3D software has advantages in data processing and calculation methods, it also has disadvantages: complex modeling and long operation cycle. So we can combine the third-party tools to make up for the lack of FLAC3D software, to play its advantage in data processing.
3.1 Geological conditions
According to the field survey, the formation lithology and physical properties of each layer include artificial fill layer, recent sediment layer, quaternary Holocene alluvium and diluvium (silty soil ③ layer, silty clay ③ 1 layer, silty fine sand ③ 3 layer) and quaternary late Pleistocene alluvium and diluvium (Pebble ⑤ layer, pebble ⑦ layer) from top to bottom.
2.2 construction scheme
The underground excavation section of Xinfadi-caoqiao section of Beijing Metro Line 19 is constructed by pile-beam-arch approach (PBA), and the excavation section is shown in Fig. 1 In the side of the upper pilot tunnel, there are tunnel piles and crown beams with large cross-section piles. The total length of the upper span section is about 70 m. The section of the section adopts the form of flat top and straight wall. The section of the large tunnel is 18.75 m, 7.3 m high, the width of the small pilot tunnel is 3.6 m, the height is 3.9 m, and the thickness of the arch top soil is about 4 m.
2.3 structural reinforcement and deformation control
In order to ensure the safety of the upper trunk road and municipal pipelines, pipe sheds and advance small tremie grouting are constructed within 180 ° of the top to enhance the support stiffness of the top.
4.1 Calculation model
Taking Xinfadi-caoqiao section of Beijing Metro Line 19 as the research background, the calculation model is established. The constitutive model adopts elastic-plastic model and the yield criterion adopts Mohr-Coulomb criterion. The range of stratum simulated by the whole model is 3 ~ 5 times of the tunnel diameter at both ends, about 35 m, and the total width is 70 m; the height of the model is based on the actual overburden thickness, and the buried depth is about 4 m; the tunnel bottom is 36 m, and the total height of the model is 40 m; the total longitudinal length is 70 m. As shown in Fig. 2 The boundary conditions of the model are as follows: the horizontal constraint is imposed on the surrounding boundary, the vertical constraint is imposed on the lower boundary, and the top is a free surface.
4.2 Calculation parameters
According to the actual geological exploration data, the calculation parameters of the model are shown in Table 1.
| project | h/m | E/GPa | µ | C/kPa | φ/° | γ/kN·m-3 |
|---|---|---|---|---|---|---|
| ① Plain fill | 1.35 | 0.005 | 0.35 | 5 | 10 | 18 |
| ② Silt | 0.9 | 0.023 | 0.26 | 27.9 | 23.9 | 18.9 |
| ② 3 fine sand | 3.2 | 0.03 | 0.2 | 0 | 26 | 20 |
| ② 5 round gravel | 1.3 | 0.06 | 0.25 | 0 | 45 | 22 |
| ② 3 fine sand | 5.0 | 0.03 | 0.2 | 0 | 26 | 20 |
| ⑤ Pebble | 10.0 | 0.06 | 0.2 | 0 | 45 | 22 |
| ⑨ 3 pebbles | 27.75 | 0.09 | 0.2 | 0 | 45 | 22 |
| Primary lining C20 | 0.3 | 25.5 | 0.2 | - | - | 25 |
| Second lining C40 | 0.45 | 32.5 | 0.2 | - | - | 25 |
| pile | - | 32 | 0.2 | - | - | 25 |
| Pipe shed | 0.105 | 32.0 | 0.2 | - | - | 25 |
4.3 Calculation conditions
The small pilot tunnel excavation and pile construction process are divided into six working conditions for analysis, and the specific excavation methods are as follows:
Working condition 1: first excavate No. ① and No.⑤ small pilot tunnels on both sides, and then excavate No. ③ small pilot tunnel in the middle. After the pilot tunnel is excavated, the pile foundation and crown beam are constructed.
Working condition 2: first excavate No. ① and No. ⑤ small pilot tunnels on both sides, then construct the pile foundation and crown beam of No.① and No. ⑤ small pilot tunnels after excavation, and then excavate No.③ small pilot tunnel in the middle, and construct the pile foundation and crown beam of No. ③small pilot tunnel after excavation.
Working condition 3: first excavate both sides of No. ①small pilot tunnel, and then construct the pile foundation and crown beam of No. ① small pilot tunnel after excavation, then excavate both sides of No. ⑤ small pilot tunnel, and then construct the pile foundation and crown beam of No. ⑤ small pilot tunnel after excavation, finally excavate middle No. ③ small pilot tunnel, and construct the pile foundation and crown beam of No. ③ small pilot tunnel after excavation.
Working condition 4: first excavate the middle No. ③ small pilot tunnel, and then excavate both sides ① and No. ⑤ small pilot tunnels. After the completion of pilot tunnel excavation, pile foundation and crown beam construction are carried out.
Working condition 5: first excavate the middle No. ③small pilot tunnel, and then construct the pile foundation and crown beam of No. ③ small pilot tunnel after excavation, and then excavate both sides No. ① and No. ⑤ small pilot tunnels, and then construct the pile foundation and crown beam of No. ① and No. ⑤ small pilot tunnel after excavation.
Condition 6: first excavate the middle No. ③ small pilot tunnel, then construct the pile foundation and crown beam of No. ③ small pilot tunnel, then excavate both sides No. ① small pilot tunnel, then construct the pile foundation and crown beam of No. ①small pilot tunnel, finally excavate both sides No. ⑤ small pilot tunnel, and then construct the pile foundation and crown beam of No. ⑤ small pilot tunnel.
The design contents of calculation conditions are summarized in Table 2.
| working condition | Excavation sequence of small pilot tunnel |
|---|---|
| 1 | ①⑤③②④(Pile driving shall be carried out after the completion of three pilot tunnels) |
| 2 | ①⑤③②④(Pile driving after completion of small pilot tunnels on both sides) |
| 3 | ①⑤③②④(Pile immediately after the completion of small pilot tunnel) |
| 4 | ③①⑤②④(Pile driving shall be carried out after the completion of three pilot tunnels) |
| 5 | ③①⑤②④(Pile driving after completion of middle pilot tunnel) |
| 6 | ③①⑤②④(Pile immediately after the completion of small pilot tunnel) |
| Note: pile-beam-arch approach (PBA) is adopted. | |
4.4 Layout of measuring points
4.4.1 Pavement measuring points
One pavement monitoring point is set on the pavement directly above the vault of five small pilot tunnels, and one pavement monitoring point is set every 4 m along the road directly above the vault of No. ① and No. ⑤ pilot tunnels, with a total of 11 pavement monitoring points. The layout of the monitoring points is shown in Fig. 3.
4.4.2 Measuring points in tunnel
The monitoring points are arranged at the vault, side wall and arch foot of each small pilot tunnel to record the displacement and stress changes of the model, and calculate the safety factor of each monitoring point according to the internal force data. The distribution of measuring points (represented by "●") in the monitoring section is shown in Fig. 4.
5.1 Displacement analysis
5.1.1 Pavement settlement value
The pavement settlement values of each working condition after the construction of the calculation model are extracted and summarized in Table 3. Extract the pavement settlement values of each monitoring point of the calculation model under each working condition, and draw the settlement map, as shown in Fig. 5.
|
Measuring point position |
Maximum settlement of pavement /mm |
|||||
|---|---|---|---|---|---|---|
|
working condition 1 |
working condition 2 |
working condition 3 |
working condition 4 |
working condition 5 |
working condition 6 |
|
|
L1 |
1.7 |
1.3 |
0.9 |
1.8 |
1.1 |
0.5 |
|
L2 |
0.7 |
0.9 |
0.7 |
0.8 |
0.4 |
-0.3 |
|
L3 |
-4.3 |
-3.1 |
-2.8 |
-5 |
-4.7 |
-5.7 |
|
L4 |
-13.1 |
-12.2 |
-12 |
-14.1 |
-13.5 |
-14.5 |
|
L5 |
-19.9 |
-20.8 |
-20.9 |
-19 |
-17.8 |
-18.8 |
|
L6 |
-22.3 |
-24.5 |
-24.7 |
-20.5 |
-19.2 |
-20.2 |
|
L7 |
-20.1 |
-22.1 |
-23.2 |
-19.3 |
-18.7 |
-19.7 |
|
L8 |
-13.9 |
-14.9 |
-17.3 |
-14.4 |
-15.3 |
-16.6 |
|
L9 |
-5.3 |
-5.6 |
-7 |
-5.2 |
-6.3 |
-7.3 |
|
L10 |
0.2 |
-0.2 |
-1 |
0.4 |
-0.5 |
-1 |
|
L11 |
1.2 |
0.8 |
0.2 |
1.2 |
0.4 |
0.1 |
It can be seen from Fig. 5 and Table 3 that the pavement settlement value increases with the decrease of the distance from the center of the tunnel section. The settlement extreme values of the six working conditions all appear directly above the second lining arch crown of the tunnel of line 19, which are − 22.3 mm, -24.5 mm, -24.7 mm, -20.5 mm, -19.2 mm and − 20.2 mm respectively. Because the pile foundation construction is carried out after the excavation of small pilot tunnel in condition 1 and 4, the settlement map is symmetrically distributed along the central axis of the tunnel; while the pile foundation construction is carried out in different stages after the completion of small pilot tunnel in condition 2 ~ 6, the settlement value of the small pilot tunnel on the right side after excavation is significantly greater than that of the small pilot tunnel on the left side at the symmetrical position in the same condition.
It can be seen that the extreme value of pavement settlement in condition 5 is the smallest, and the extreme value of pavement settlement in condition 3 is the largest. The maximum settlement of working conditions 4 ~ 6 is obviously better than that of working conditions 1 ~ 3, that is, excavating the middle pilot tunnel first has a better effect on controlling the pavement settlement. After the completion of the excavation of the middle pilot tunnel, the middle pile shall be constructed immediately, and after the completion of the pilot tunnels on both sides, the pile on both sides (condition 5) shall be constructed. The pavement settlement value shall be the minimum and meet the deformation control standard.
5.1.2 displacement of primary support structure
The deformation of the primary support structure of No. ①, No. ③ and No. ⑤ small pilot tunnel under each working condition after the completion of small pilot tunnel is extracted and summarized in Table 4.
|
Working condition |
No. ① pilot tunnel /mm |
No. ③ pilot tunnel /mm |
No. ⑤ pilot tunnel /mm |
|||
|---|---|---|---|---|---|---|
|
Vault settlement |
Invert uplift |
Vault settlement |
Invert uplift |
Vault settlement |
Invert uplift |
|
|
1 |
-1.06 |
4.21 |
-1.07 |
4.83 |
-0.91 |
4.43 |
|
2 |
-1.20 |
4.22 |
-0.89 |
4.76 |
-1.13 |
4.32 |
|
3 |
-1.29 |
4.14 |
-0.96 |
4.57 |
-0.89 |
4.36 |
|
4 |
-0.95 |
4.96 |
-0.94 |
3.40 |
-0.90 |
4.79 |
|
5 |
-0.87 |
4.70 |
-0.88 |
3.63 |
-0.89 |
4.61 |
|
6 |
-1.10 |
4.46 |
-1.03 |
3.56 |
-0.88 |
3.87 |
It can be seen from Table 4 that due to the pre-reinforcement effect of the top pipe shed, the vault settlement of the three small pilot tunnels is obviously smaller than the invert uplift under each working condition, and the vault deformation meets the deformation control standard. After the completion of small pilot tunnel construction, the minimum value of invert uplift of No. ①, No. ③, No. ⑤ small pilot tunnel appears in condition 3, 4 and 6, which are 4.14mm, 3.40 mm and 3.87 mm respectively; (the maximum value of invert uplift of No. ①, No. ⑤ small pilot tunnel appears in condition 4, which are 4.96 mm and 4.79 mm respectively; (the maximum value of invert uplift of No. ③ small pilot tunnel appears in condition 1, which is 4.83 mm; the invert uplift of each pilot tunnel can be seen The difference of uplift value is small, and the uplift value of condition 4 ~ 6 is slightly small at the No. ③ small pilot tunnel excavated first.
In terms of vault settlement, the vault settlement of No. ① small pilot tunnel under condition 3 is the largest among all working conditions, which is -1.29 mm; the vault settlement of No. ③ and No. ⑤ small pilot tunnel under condition 1 is the largest among all working conditions, which is -1.07 mm and − 0.91 mm respectively; the vault settlement of No. ① and No. ③ small pilot tunnel under condition 5 is the smallest among all working conditions, which is -0.87 mm and − 0.88 mm respectively The vault settlement value of No. ⑤ small pilot tunnel is the smallest in all working conditions, and the minimum settlement value is -0.88 mm, followed by -0.89 mm in working condition 5. To sum up, the deformation control of small pilot tunnel initial support structure in condition 5 is better.
5.2 Internal force analysis
5.2.1 Maximum principal stress analysis
Extract the cloud diagram of the maximum principal stress after the construction of each working condition of the calculation model, as shown in Fig. 6. The maximum principal stress value of the model is extracted, and the variation of each working condition relative to working condition 1 is calculated and summarized in Table 5.
|
Working condition |
Maximum principal stress /MPa |
Variation / MPa |
|---|---|---|
|
1 |
0.821 |
- |
|
2 |
1.288 |
0.467 |
|
3 |
1.606 |
0.785 |
|
4 |
1.024 |
0.203 |
|
5 |
1.039 |
0.218 |
|
6 |
0.984 |
0.163 |
It can be seen from Fig. 6 and Table 5 that the maximum principal stress of the six working conditions occurs at the vault. On the premise that the pile foundation and crown beam are constructed after the pilot tunnel is excavated, the maximum principal stress of working condition 1 (first edge and then middle) is slightly smaller than that of working condition 4 (first middle and then edge), and the maximum principal stress of working condition 1 is the smallest, which is 0.821 MPa; after a certain stage of small pilot tunnel is completed, the maximum principal stress of working condition 1 is 0.821 MPa On the premise of pile foundation and crown beam structure construction, the maximum principal stress of condition 2 and 3 (first edge and then middle) is slightly larger than that of condition 5 and 6 (first middle and then edge). The maximum principal stress of condition 6 is the smallest, which is 0.984 MPa, and the maximum principal stress of condition 3 is the largest, which is 1.606 MPa. It can be seen that except for the second and third working conditions, the maximum principal stress values of the other four working conditions are almost the same, and the maximum difference is only 0.218 MPa.
5.2.2 Minimum principal stress analysis
Extract the cloud chart of minimum principal stress after construction under various working conditions of the calculation model, as shown in Fig. 7. The minimum principal stress value of the model is extracted, and the variation of each working condition relative to working condition 1 is calculated and summarized in Table 6.
|
Working condition |
Minimum principal stress /MPa |
Variation / MPa |
|---|---|---|
|
1 |
-1.828 |
- |
|
2 |
-2.236 |
-0.408 |
|
3 |
-2.274 |
-0.446 |
|
4 |
-1.441 |
0.387 |
|
5 |
-1.917 |
-0.089 |
|
6 |
-1.874 |
-0.046 |
It can be seen from Fig. 7 and Table 6 that the minimum principal stress of condition 1 ~ 3 appears at the inner arch foot of small pilot tunnel on both sides, while the minimum principal stress of condition 3 ~ 5 appears at the arch foot of middle small pilot tunnel. Among them, the minimum principal stress value of condition 3 is the largest of the six conditions, which is -2.274 MPa; the minimum principal stress value of condition 4 is the smallest of the six conditions, which is -1.441 MPa, which is significantly less than other conditions; the minimum principal stress values of condition 2 and 3 are significantly greater than other conditions.
To sum up, the minimum principal stress values of working conditions 1 ~ 3 constructed in the sequence of first side then middle side are larger than those of working conditions 4 ~ 6 constructed in the sequence of first middle then middle side, and the minimum principal stress extreme values of working conditions 5 and 6 (first middle then side) are smaller on the premise of pile foundation and crown beam structure construction after a certain stage of small pilot tunnel completion.
5.2.3 Safety factor analysis
The internal force of the structure is calculated according to the material mechanics formula (1) and (2), and then the safety factors of each monitoring point of the primary support structure of the small pilot tunnel are calculated by the formulas (3) and (4) are summarized in Table 7(Code for design of railway tunnel 2005). Draw the bending moment broken line diagram of each monitoring point of the initial support structure, as shown in Fig. 8.
|
Pilot tunnel monitoring point |
Condition 1 |
Condition 2 |
Condition 3 |
Condition 4 |
Condition 5 |
Condition 6 |
|---|---|---|---|---|---|---|
|
C1 |
21 |
21 |
337 |
170 |
194 |
209 |
|
C2 |
1299 |
322 |
264 |
1050 |
314 |
471 |
|
C3 |
475 |
372 |
207 |
486 |
1582 |
1536 |
|
C4 |
46 |
76 |
62 |
37 |
35 |
33 |
|
C5 |
166 |
34 |
26 |
150 |
155 |
161 |
|
C6 |
10 |
10 |
11 |
62 |
42 |
41 |
|
C7 |
60 |
102 |
79 |
116 |
152 |
49 |
|
C8 |
47 |
44 |
141 |
34 |
35 |
38 |
|
C9 |
517 |
278 |
524 |
1497 |
204 |
478 |
|
C10 |
736 |
367 |
1215 |
813 |
442 |
1156 |
|
C11 |
67 |
147 |
251 |
208 |
192 |
246 |
It can be seen from Table 7 and Fig. 8 that the safety factor at the vault of No. ③ pilot tunnel under condition 1 ~ 3 constructed in the sequence of side first and middle second is obviously smaller than that under condition 4 ~ 6 constructed in the sequence of side first and middle second. At this position, the safety factor of condition 1 is the smallest, which is 10, and the safety factor of condition 4 ~ 6 is the largest, which is above 41.
The safety factor at the vault of No. ① pilot tunnel under working condition 1 ~ 3 is larger than that under working condition 4 ~ 6. The safety factor of condition 6 is the smallest, which is 33, and that of condition 2 is the largest, which is 75. The minimum safety factor of condition 5 appears at this position, which is 35. The safety factor of No. ⑤ pilot tunnel vault under working condition 1 ~ 3 constructed in the order of side first then middle is larger than that under working condition 4 ~ 6 constructed in the order of side first then middle. Among them, the safety factor of condition 4 is the smallest, which is 34, and that of condition 3 is the largest, which is 140.99.
It can be seen that in the construction stage of the primary support structure of small pilot tunnel, the parts with smaller safety factor are concentrated in the arch foot position of No. ① and No. ⑤ small pilot tunnel and the vault position of No. ① ~ No. ⑤ small pilot tunnel, and the safety factor of No. ① ~ No. ⑤ small pilot tunnel is smaller; in the working condition 1 ~ 3 of side first then middle sequence construction, the safety factor of the arch foot position of both sides of pilot tunnel excavated first is smaller, and the safety factor of No. ③ small pilot tunnel in the middle is smaller The safety factor at the vault of the tunnel decreases obviously. In conclusion, the safety of the initial support structure is higher when the excavation sequence is from the middle to the back.
(1) The settlement extremum of the six working conditions all appear above the second lining vault of the tunnel of Line 19. The pavement settlement control effect of working condition 5 is the best, and the settlement extremum is − 19.2 mm, which is within the allowable range of deformation control.
(2) The vault settlement values of No. ① and No. ③ small pilot tunnels under condition 5 are the smallest among all conditions, with the minimum settlement values of -0.87 mm and − 0.88 mm respectively. The vault settlement values of No. ⑤ small pilot tunnel under condition 6 are the smallest among all conditions, with the minimum settlement value of 0.88 mm, followed by 0.89 mm of condition 5. The invert uplift values of each pilot tunnel are similar, and the uplift values of small pilot tunnel No. ③ in working conditions 4 ~ 6 are slightly smaller.
(3) After the completion of pilot tunnel excavation, the maximum principal stress of the construction of pile foundation and crown beam under condition 1 (first edge, then middle) is slightly smaller than that under condition 4 (first middle, then middle); while the maximum principal stress of the construction of pile beam structure under condition 2 and 3 (first edge, then middle) is slightly larger than that under condition 5 and 6 (first middle, then middle) at a certain stage after the completion of small pilot tunnel; the construction is carried out in the order of first edge, then middle The results show that the minimum principal stress values of condition 1 ~ 3 are larger than those of condition 4 ~ 6. The maximum and minimum principal stress values of the other four working conditions are almost the same except for working conditions 2 and 3.
(4) In the excavation stage, the extreme value of the safety factor of the initial support structure of the small pilot tunnel appears at the vault of No. ①, No. ③ and No. ⑤, and the safety factor value at the vault of No. ③ small pilot tunnel is the smallest. The minimum safety factor of condition 4 ~ 6 is generally larger, and the minimum safety factor of condition 5 is 35.
(5) Considering the changes of surface settlement, structural displacement and internal force, the excavation sequence of mode 5 (excavation of middle No. ③ small pilot tunnel, construction of pile foundation and crown beam of No. ③ small pilot tunnel after excavation, excavation of both sides No. ① and No. ③ small pilot tunnels, construction of pile foundation and crown beam of No. ① and No. ⑤ small pilot tunnel after excavation) can ensure the safety of the project All in order.。
Funding:National Natural Science Foundation of China (51408008)
Conflicts of interest/Competing interests: The authors declare that there is no conflict of interest regarding the publication of this paper.
Ethics approval : This article does not deal with the relevant remarks about morality.
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Availability of data and material: The data part of this paper is the data of Chinese codes, and the calculation results are the results of numerical simulation analysis. The calculation results can provide reference for related projects.
Code availability:FLAC3D
Authors' contributions: Conceptualization and methodology, Guang-yao Cui and Bo-han Song, investigation, Ji-hua He, data curation, Guang-yao CUI and Ji-hua He, writing—original draft preparation, Guang-yao Cui and Bo-han Song, writing—review and editing, Bo-han Song, project administration, Guang-yao Cui, funding acquisition, Guang-yao Cui and Ji-hua He.
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