Abstract
The realization of two-bore complementary ventilation in construction tunnels is a new challenge. This paper takes two-way four-lane, six-lane and eight-lane tunnel as the research object, and studies the possibility of two-bore complementary construction ventilation and the influence of construction parameters on the effect of complementary ventilation when when there is a difference in wind demand of double-bore. The results show that: two-bore six-lane tunnel is in line with the characteristics of the complementary construction ventilation, when the distance from the wind pipe outlet to the working surface is 20 m, the height of the upper step is 3.5 m, and the safety distance between the working face of the twin holes is 50 m, the ventilation effect of double-hole complementary construction is enhanced. This kind of ventilation can introduce the surplus airflow from the tunnel with small air demand to the tunnel with large air demand, reduce the energy consumption of the tunnel ventilator in the tunnel with large air demand, and enrich the theory of the tunnel construction of the long double-cavity construction.
Subject terms: Engineering, Civil engineering
Introduction
The long tunnels are divided into two types: double-hole single-lane tunnels and single-hole double-lane tunnels. During the construction, factors such as construction grade, construction safety and post-operation should be fully considered to determine the construction plan comprehensively. In comparison, the double-hole single-lane tunnel has the following advantages: (1) during construction, the double-hole tunnel section area is small, the probability of collapse is low, and the construction risk is small1,2; (2) during operation, piston wind can be utilized for the complementary ventilation3, (3) for disaster relief, when a disaster occurs in one tunnel, the other tunnel can be used for evacuation and disaster relief4. Therefore, the construction of long double-bore single-line tunnels has become a new development trend. At present, there are a large number of extra-long double-hole single-lane highway tunnels in Sichuan, Yunnan, Guizhou and Northwest China5–7.
In recent years, China’s infrastructure construction has been advancing rapidly, relying on a large number of tunnel engineering sites, domestic experts and scholars have carried out research on optimization of construction ventilation system. Relying on the Xinliangfengya Tunnel, Liu et al. studied the effects of different wind pipe locations on the change of wind speed and gas concentration distribution under the press-in ventilation, and finally determined that the best gas control effect is achieved when the wind pipe is 2.5S1/2 (30 m) away from the Changzifang8. Wang et al. relied on the Jiming gas tunnel to simulate the influence of factors such as the diameter of the press-in wind pipe, the humidity in the tunnel, and the location of the press-in air outlet on the ventilation efficiency of the gas tunnel, and the results showed that the ventilation efficiency was increased when the humidity in the tunnel was 43.5%, and the diameter of the press-in wind pipe was 1.1 m, and the air outlet to the face of the palms was 15 m9.Liu and Li et al. designed orthogonal tests on the structure and operating parameters of the press-in wind turbine by combining field tests and numerical simulations, and clarified the priority ranking of the importance of each factor and the optimized arrangement scheme10,11. Zeng et al. analyzed the influence of different factors on the flow field of a large-section tunnel under the double-sidewall guided pit method of excavation, and used the average concentration of CO as a measure to obtain the priority ranking of the importance of each factor12. Relying on Jinjing Tunnel, Chen et al. analyzed the influence of construction parameters of the step method on the flow field characteristics of tunnel ventilation and obtained an optimal combination of parameters by using the section average wind speed and gas concentration as evaluation indexes13. All of the above tunnel ventilation methods are based on the pressure-entry ventilation, and a clear evaluation index has been directed to design orthogonal tests to read out the priority of the importance of each factor, so as to optimize the construction ventilation system. However, some geographically special tunnels, such as Sichuan-Tibet and Yunnan-Tibet, tend to be super-long, super-large and have curvature, how to design and optimize the ventilation of these super-long, super-large tunnels is the main problem at present. In this regard, Wang et al. optimized the ventilation effect by relying on the Dabashan Tunnel and using different ventilation methods for each stage in combination with the existing construction plan14. Yang used modeling analysis and calculation to explore whether to apply jet fans in the compartmentalization method of inclined shafts, and suggested that no jet fans should be installed, and tried to ventilate the main fan compartment, and the ventilation efficiency was improved15. Relying on a railroad tunnel, Li et al. preferred the tandem press-in ventilation method by comparing the wind flow transportation law under different ventilation methods, and matched the selection of the wind pipe in the work area16. Xie, Gao, Xu, et al., for spiral tunnels, different curvature radius tunnels along the resistance, local resistance coefficients, and wind flow transport law research, optimized the design parameters of the indentation ventilator, the wind tube layout program, improve the spiral tunnel construction ventilation effect17–20.
Most of the above literature studies focus on the construction ventilation of single-tube and two-lane tunnels, and few scholars focus on the construction ventilation of double-tube and single-lane tunnels. The research on the construction ventilation mechanism, flow field characteristics, and the impact of optimization of construction ventilation parameters on the flow field in twin-bore tunnels by domestic and foreign scholars is almost in a void.In the construction organization of single-lane highway tunnels with two holes, the cycle of geological exploration, drilling, blasting, slagging, initial support, first lining and second lining of the left and right holes are not synchronized, and the actual instantaneous air demand is not the same, with the complementary characteristics of the instantaneous air demand of the two holes, therefore, it is of great significance to carry out research on the optimization of the construction parameters of the two-hole tunnels and their complementary characteristics, in order to reduce the energy consumption of the tunnel machinery in the side of the air demand, as well as to improve the ventilation effect. Therefore, it is important to study the optimization of construction parameters and complementary characteristics of twin-tunnel construction to reduce the energy consumption of tunnel machinery on the air-demand side and improve the ventilation effect.
Project overview
Take the two-way four-lane Baima Tunnel, the two-way six-lane Tingshe Tunnel and the two-way eight-lane Xishan Tunnel for example, we analyzed the stability of the wind network structure of the double-hole-bore construction tunnels. The basic conditions of the above three tunnels are shown in Table 1.
Table 1.
Basic information of the tunnels.
| Tunnel name | Type | Length/(m) | Section size/(m) | Perimeter rock grade |
|---|---|---|---|---|
| The Baima tunnel |
Two-way four-lane road |
Left 13,013, Right 13,000 | 11.06 wide, 7.15 high | V |
| The Tinsel tunnel |
Two-way six-lane road |
Left 3350, Right 3335 | 15.02 wide, 7.5 high | IV |
| The Xishan tunnel | Two-way eight -lane road | Left 2602, Right 2616 | 18.50 wide, 5.0 high | V |
The Baima tunnel adopts a combination of single-head forced ventilation and roadway ventilation. Among them, the first section is mainly forced ventilation, and the maximum ventilation length is 1489 m; the Tingshe Tunnel and Xishan Tunnel are excavated on both sides, and the forced ventilation can meet the demand. The maximum ventilation lengths are 1675 m and 1308 m respectively. The above tunnel construction mainly includes drilling, blasting, slag discharge, support and lining, and the on-site construction process is shown in Fig. 1.
Fig. 1.
Schematic diagram of on-site construction.
To comprehensively determine the maximum air volume required, forced ventilation should consider factors such as the discharge of blasting smoke from the working face, the maximum number of workers in the tunnel, the allowable minimum wind speed, and the dilution and discharge of exhaust gas from the internal combustion engine21,22. The calculation is as follows :
The air volume required to exhaust the blasting smoke from the tunnel Q1 :
 |
1 |
Q1 is air volume required to exhaust the blasting smoke from the tunnel m3 /min; A is working face area, m2 ; G is the explosive required for simultaneous blasting, kg; t is time of ventilation, taking 20 ~ 30 min; Lp is blasting smoke throwing length, m.
The air flow required for the maximum number of construction workers in the tunnel Q2 :
 |
2 |
Q2 is air volume required by maximum number of construction workers in the tunnel, m3 /min; kf is reserve coefficient; m is the maximum number of construction workers, people; qf is fresh air required per worker per minute, m2/min;
The air volume required by the allowable minimum wind speed in the tunnel Q3 :
 |
3 |
Q3 is the air volume required by the allowable minimum wind speed in the tunnel, m3 /min; A is the full cross-section area of the tunnel, m2 ; v is the allowable minimum wind speed in the tunnel, m/s;
The air volume required to dilute the exhaust of the internal combustion engine Q4 :
 |
4 |
Q4 is the required air volume to dilute the exhaust of the combustion engine, m3 /min; q is the required air volume of the combustion engine, m3/(kW·min); Σw - the total power of simultaneous operation of the combustion engine in the tunnel, kW.
The actual required air volume for the three tunnels is shown in Table 2.The Baima Tunnel adopts a combination of single-head forced ventilation and roadway ventilation, of which the first section is mainly forced ventilation, with a maximum ventilation length of 1,489 m .
Table 2.
Actual air demand for each tunnel.
| Tunnel name | Q1/(m3/min) | Q2/(m3/min) | Q3/(m3/min) | Q4/(m3/min) | Q/(m3/min) |
|---|---|---|---|---|---|
| The Baima tunnel | 382.4 | 151.8 | 1103.8 | 1433.6 | 1433.6 |
| The Tinsel tunnel | 458.9 | 151.8 | 1324.5 | 1433.6 | 1433.6 |
| The Xishan tunnel | 428.9 | 151.8 | 1237.9 | 1433.6 | 1433.6 |
The Tingshe Tunnel and Xishan Tunnel are excavated on both sides, and the forced ventilation can meet the demand. The maximum ventilation lengths are 1675 m and 1308 m respectively.
Numerical simulation
Physical model
The numerical model was established by Design Model software and the grid file was generated. The length of the tunnel model was 200 m, and the size of the cross-section was consistent with that of Baima, Tingshe and Xishan tunnels. In order to study the influence of the size of the cross-section, the wind demand difference and the distance from the cross passage to the working face on the fluid velocity field in the double-hole tunnel, nine models were established, some of which were shown in Fig. 2. 18 working conditions were set up as shown in Table 3.
Fig. 2.
130 m from the cross passage to the working face (Tingshe tunnel).
Table 3.
Physical model of each working condition.
| Serial number | Tunnel name | LC /(m) | Differences in wind requirements | Serial number | Tunnel | LC /(m) | Differences in wind requirements |
|---|---|---|---|---|---|---|---|
| 1 | Baima tunnel | 80 | Q L > Q R | 10 | Baima tunnel | 80 | Q L < Q R |
| 2 | Baima tunnel | 130 | Q L > Q R | 11 | Baima tunnel | 130 | Q L < Q R |
| 3 | Baima tunnel | 180 | Q L > Q R | 12 | Baima tunnel | 180 | Q L < Q R |
| 4 | Tingsel tunnel | 80 | Q L > Q R | 13 | Tingsel tunnel | 80 | Q L < Q R |
| 5 | Tingsel tunnel | 130 | Q L > Q R | 14 | Tingsel tunnel | 130 | Q L < Q R |
| 6 | Tingsel tunnel | 180 | Q L > Q R | 15 | Tingsel tunnel | 180 | Q L < Q R |
| 7 | Xishan tunnel | 80 | Q L > Q R | 16 | Xishan tunnel | 80 | Q L < Q R |
| 8 | Xishan tunnel | 130 | Q L > Q R | 17 | Xishan tunnel | 130 | Q L < Q R |
| 9 | Xishan tunnel | 180 | Q L > Q R | 18 | Xishan tunnel | 180 | Q L < Q R |
In Table 3, LC indicated the distance from the cross passage to the working face of the left hole, QL indicated the air demand of the left hole, and QR indicated the air demand of the right hole. When QL > QR, the forced ventilator opened the high speed gear at the entrance of the left hole, and the forced ventilator opened the low speed gear at the entrance of the right hole, and vice versa.
The quality of the grid affects the accuracy of the simulation results, so it is crucial to verify the grid independence. Wind speed is the primary indicator to verify the grid independence. In the case of other conditions remain unchanged, gradually increase the number of grids (pay attention to the proportional increase) to observe the trend of the numerical solution, if the error of the two adjacent solutions is between 5 and 10%, the general view is that the grid on the results of the impact of the acceptable range of validation is complete. Taking the model in Fig. 2 as an example, under three different meshing schemes, the velocity distribution of human respiratory height (− 17.5, 1.6, n) on the central axis of the left and right tunnels was shown in Fig. 3. From Fig. 3, the simulation results of the low-quality grid were quite different from those of the medium-quality grid and high-quality grid. Considering the computer performance and simulation error, the medium-quality grid scheme was adopted, and a total of 1,909,446 grids are divided. The average grid size is 0.83, the maximum is 1, and the minimum is 0.15.
Fig. 3.
Grid independence verification.
Mathematical model
The wind speed in the tunnel is not large and the pressure change is small, so the compressibility of the air can be ignored. Therefore, the air flow in the tunnel is regarded as a three-dimensional incompressible and stable viscous turbulent flow in the calculation. The turbulent flow The turbulent flow model adopts the high Reynolds number k-ε model. The mathematical model includes continuity equation, momentum equation and k-ε model Eqs23,24 .
Incompressible continuity equation:
 |
5 |
Incompressible momentum equation:
 |
6 |
Realizable k-ε turbulence model:
 |
7 |
k-equation:
 |
8 |
ε-equation:
 |
9 |
Among Eq. (5) ~ (9) ρ is fluid density, kg/m3; ui and uj are the velocity components of the fluid, respectively, m/s; p is the pressure on the fluid micro-element body, Pa; µ is dynamic viscosity, Pa·s; µt is turbulent viscosity, Pa·s; k is turbulent kinetic energy, m2/s2; ε is the dissipation rate, m3/s; σk and σε are the Prandtl numbers corresponding to k and ε equations, respectively. Besides, according to the relevant experimental verification, the constant values involved in the model are as follows: Cµ=0.09, Cε1=1.44, Cε2=1.92, σk=1.0, σε=1.3.
Boundary conditions
The cycle of the construction organization of the two holes is not synchronized, which shows the instantaneous difference of the wind demand in the working face in the double-hole tunnel. The inlet of right tunnel air duct is inlet1, the inlet of left tunnel air duct is inlet2; the inlet of right tunnel is outlet1, the outlet of left tunnel is outlet2, and the relative pressure at the tunnel outlet is 0 Pa. The side wall and floor of the tunnel are wall; other main selections are included in Table 4.
Table 4.
Main parameter settings.
| Setting options | Parameter settings |
|---|---|
| Timing | Stabilise |
| Solver type | Pressure-based |
| Energy equation | Cloture |
| Turbulence model | Realizable 
|
| Pressure-speed coupling scheme | SimpleC format |
In view of the difference of wind demand in double-hole tunnel, the specific initial values are as follows: (1) when the wind demand of the left tunnel QL > the wind demand of the right tunnel QR, inlet1 = 8.16 m/s, inlet2 = 16.18 m/s; (2) when the wind demand of the left tunnel QL < the wind demand of the right tunnel QR, inlet1 = 16.18 m/s, inlet2 = 8.16 m/s.
Theoretical analysis
Figure 4 Diagram of the ventilation network of the double-hole and the single-line tunnel.
Fig. 4.
shows the ventilation network of the double-hole single-cross passage.
In Fig. 4, the working face of the left hole is node 1, the working face of the right hole is node 2, the connection between the cross passage and the left tunnel is node 3, and the connection between the cross passage and the right tunnel is node 4. The air direction in the tunnel depends on the value of the pressure energy values of nodes 3 and 4. The airflow flows from the node with the high energy level to the node with the low energy level. When the energy levels of the two nodes are the same, the airflow stagnates.
Equation (10) is obtained from the loop energy balance:
 |
10 |
The pressure-energy difference between node 3 and node 4 is:
 |
11 |
When there is no wind in the cross passage, E3 − 4 = 0, that is,
 |
12 |
It can be obtained by organizing Eq. (12):
 |
13 |
When the wind direction in the cross passage is from node 3 → node 4, the pressure energy of node 3 is higher than that of node 4, and E3 − 4 > 0, that is,
 |
14 |
It can be obtained by organizing Eq. (14):
 |
15 |
Similarly, the relationship can be derived when the wind direction in the cross passage changes from node 4 to node 3.:
 |
16 |
The discriminant (17) is obtained by combining Eqs. (13), (15) and (16):
 |
17 |
Through numerical simulation, the flow Qm of A-A, B-B, C-C and D-D sections in the four branches and the resistance hm of each branch are known, and the wind resistance Rm of each branch is obtained. The wind resistance Rm of each branch is substituted into the formula (17) for theoretical calculation, and the wind flow direction of the double-hole tunnel can be determined by the K value.
The influence of double-hole tunnel type on the stability of wind network structure
Two-way four-lane tunnel
The wind speed cloud diagram of the central axis of the left hole of the Baima Tunnel and the wind speed of the human breathing height of the double hole are shown in Figs. 5 and 6 respectively.
Fig. 5.
Cloud map of wind speed on the central axis of the left tunnel (Baima Tunnel) (remark: Fig. 5 is processed using Tecplot 360 EX 2024 R1 software).
Fig. 6.
Comparison of human respiratory height and wind speed on the axial plane( Baima Tunnel).
It can be seen from Figs. 5 and 6 that there are significant differences in the wind velocity distribution of the tunnel due to the instantaneous wind demand difference of the working faces and the different distance LC from the cross passage to the working faces. Taking model 1, 2 and 3 as examples, the wind speed in the left hole of model 1 is significantly lower than that of model 2 and model 3, indicating that the wind flow in the left hole may flow to the right hole through the cross passage.
According to the simulation results of model 1, 2, 3, 10, 11 and 12, combined with Fig. 4, the Rm and K of each working condition branch of Baima tunnel can be obtained, as shown in Table 5. It can be seen from Table 5 that when there is an instantaneous wind demand difference between the two holes, changing the distance LC from the cross passage to the working face will affect the wind resistance Rm of each branch and the stability of the wind network. However, the K values are all less than 1, indicating that the airflow flows from the left tunnel excavated in advance to the right tunnel under excavation.
Table 5.
Comparison of Rm and K of each branch (Baima Tunnel).
| Model | R 1 | R 2 | R 3 | R 4 | K |
|---|---|---|---|---|---|
| 1 | 1.2E− 04 | 1.2E− 03 | 7.3E− 05 | 1.6E− 03 | 0.57 |
| 10 | 4.3E− 04 | 8.3E− 03 | 1.0E− 02 | 2.0E− 03 | 0.01 |
| 2 | 1.6E− 05 | 2.1E− 04 | 1.0E− 05 | 4.5E− 05 | 0.31 |
| 11 | 5.9E− 06 | 2.9E− 05 | 1.4E− 05 | 8.4E− 07 | 0.01 |
| 3 | 6.2E− 06 | 1.3E− 05 | 1.4E− 05 | 1.3E− 05 | 0.46 |
| 12 | 5.1E− 07 | 3.1E− 06 | 9.8E− 06 | 2.1E− 06 | 0.04 |
Two-way six-lane tunnel
The wind speed cloud diagram of the central axis of the left hole of the Tingshe tunnel and the wind speed of the human breathing height of the double hole are shown in Figs. 7 and 8, respectively. It can be seen from Figs. 7 and 8, it can be seen that when QL> QR, different from Baima tunnel, the wind speed fluctuation of human respiratory height in Tingshe tunnel is larger; when QL< QR, the closer the distance between the vehicle cross passage and the working face, the smaller the wind speed after the left hole is stabilized, and the larger the wind speed after the right hole is stabilized. The airflow may flow from the left hole to the right hole.
Fig. 7.
Cloud map of wind speed on the central axis of the left tunnel (Tingshe Tunnel) (remark: Fig. 7 is processed using Tecplot 360 EX 2024 R1 software).
Fig. 8.
Comparison of human respiratory height and wind speed on the axial plane (Tingshe Tunnel).
According to the simulation results obtained from models 4, 5, 6, 13, 14 and 15, combined with Fig. 4, the Rm and K of each working condition branch of Tingshe Tunnel can be obtained, as shown in Table 6. As shown in Table 6, when LC = 80 m, the airflow flows from the left hole to the right hole; when LC = 130 m, the airflow flows from the right hole to the left hole; when LC = 180 m, the airflow flows from the side of the tunnel with small air demand to the side with large air demand. Increasing the wind speed of the tunnel with large air demand can realize the ventilation of double-hole complementary construction.
Table 6.
Comparison of Rm and K of each branch (Tingshe Tunnel).
| Model | R 1 | R 2 | R 3 | R 4 | K |
|---|---|---|---|---|---|
| 4 | 3.0E− 03 | 5.4E− 03 | 5.5E− 05 | 6.9E− 05 | 0.69 |
| 13 | 4.7E− 04 | 1.7E− 02 | 3.6E− 03 | 1.2E− 03 | 0.01 |
| 5 | 4.4E− 05 | 8.9E− 06 | 5.6E− 06 | 2.6E− 05 | 22.5 |
| 14 | 1.4E− 04 | 4.6E− 04 | 9.1E− 06 | 3.8E− 05 | 1.22 |
| 6 | 6.5E− 06 | 8.4E− 06 | 1.7E− 06 | 9.3E− 06 | 4.19 |
| 15 | 8.8E− 06 | 1.7E− 05 | 9.9E− 06 | 1.5E− 05 | 0.77 |
Two-way eight-lane tunnel
The wind speed cloud diagram of the central axis of the left hole of Xishan Tunnel and the wind speed of the human breathing height of the double hole are shown in Figs. 9 and 10, respectively.
Fig. 9.
Cloud map of wind speed on the central axis of the left tunnel (Xishan Tunnel) (remark: Fig. 9 is processed using Tecplot 360 EX 2024 R1 software).
Fig. 10.
Comparison of human respiratory height and wind speed on the axial plane (Xishan Tunnel).
From Figs. 9 and 10, it can be seen that when QL>QR, the closer the distance from the vehicle cross passage to the working face, the smaller the wind speed after the left hole and the right hole are stabilized; when QL<QR, when LC = 80 m, the wind speed is the smallest after the left hole is stabilized, the wind speed is the largest after the right hole is stabilized, and the airflow flows from the left hole to the right hole.
According to the simulation results obtained from models 7, 8, 9, 16, 17 and 18, combined with Fig. 4, the Rm and K of each working condition branch of the Xishan Tunnel can be obtained, as shown in Table 7. As shown in Table 7, when LC = 80 m, the air flow flows from the left hole to the right hole; when LC = 130 m, 180 m, the wind flow is from the right hole to the left hole, and there is no double-hole complementary characteristics.
Table 7.
Comparison of Rm and K of each branch (Xishan Tunnel).
| Model | R 1 | R 2 | R 3 | R 4 | K |
|---|---|---|---|---|---|
| 7 | 2.9E − 03 | 6.4E − 03 | 1.7E − 03 | 2.6E − 05 | 0.67 |
| 16 | 1.2E − 04 | 2.4E − 03 | 1.3E − 03 | 1.7E − 03 | 0.07 |
| 8 | 1.0E − 03 | 2.9E − 05 | 7.3E − 04 | 1.0E − 04 | 4.86 |
| 17 | 1.1E − 05 | 8.6E − 06 | 2.1E − 06 | 1.9E − 05 | 11.1 |
| 9 | 2.4E − 05 | 2.1E − 05 | 8.1E − 06 | 2.3E − 05 | 3.41 |
| 18 | 3.4E − 05 | 6.8E − 04 | 3.0E − 05 | 1.1E − 03 | 1.82 |
Double-hole complementary ventilation and parameter optimization
Analysis of factors affecting the flow field
From the above analysis, the Tingshe tunnel has the ventilation characteristics of double-hole complementary construction when LC = 180 m. As shown in Fig. 11, Deng huan et al. proposed that the arrangement position L1 of the outlet of the air duct, the height L2 of the upper step and the safe distance L4 of the working faces will have different effects on the airflow direction, and the length L3 of the cross passage will not affect the airflow direction25. The orthogonal test was designed to optimize the parameter combination of complementary construction with K as the evaluation index, and the parameter combination optimization in engineering application was verified.
Fig. 11.
Construction ventilation parameters affecting the flow field of the double-hole tunnel.
Orthogonal experiments and results analysis
The influencing factors, levels and scheme design of orthogonal test were shown in Tables 8 and 9.
Table 8.
Factors and levels of orthogonal tests.
| Level | Factors | ||
|---|---|---|---|
| L1 /(m) | L2 /(m) | L4 /(m) | |
| 1 | 30 | 3.0 | 30 |
| 2 | 20 | 3.5 | 50 |
| 3 | 40 | 2.5 | 40 |
Table 9.
Experimental design.
| Test number | Factors | ||
|---|---|---|---|
| L1 /(m) | L2 /(m) | L4 /(m) | |
| 1 | 30 | 3.0 | 30 |
| 2 | 30 | 3.5 | 50 |
| 3 | 30 | 2.5 | 40 |
| 4 | 20 | 3.0 | 40 |
| 5 | 20 | 3.5 | 50 |
| 6 | 20 | 2.5 | 30 |
| 7 | 40 | 3.0 | 50 |
| 8 | 40 | 3.5 | 30 |
| 9 | 40 | 2.5 | 40 |
As can be seen from Tables 10 and 11, working condition 5 is the optimal design scheme in the orthogonal test. At this time, the distance from the wind pipe outlet to the palm face L1 is 20 m, the height of the upper step L2 is 3.5 m, and the safe distance between the palm faces of the double holes L4 is 50 m.
Table 10.
Numerical results.
| Test number | L1 /(m) | L2 /(m) | L4 /(m) | K |
|---|---|---|---|---|
| 1 | 30 | 3.0 | 30 | 0.2346 |
| 2 | 30 | 3.5 | 50 | 0.0199 |
| 3 | 30 | 2.5 | 40 | 0.1990 |
| 4 | 20 | 3.0 | 40 | 1.2438 |
| 5 | 20 | 3.5 | 50 | 2.6112 |
| 6 | 20 | 2.5 | 30 | 0.4424 |
| 7 | 40 | 3.0 | 50 | 1.4004 |
| 8 | 40 | 3.5 | 30 | 1.3020 |
| 9 | 40 | 2.5 | 40 | 0.7088 |
Table 11.
Orthogonal experimental design scheme and numerical calculation results.
| Term | Level | L1 /(m) | L2 /(m) | L4 /(m) |
|---|---|---|---|---|
| K value | 2.5 | - | 1.35 | - |
| 3.0 | - | 2.41 | - | |
| 3.5 | - | 3.93 | - | |
| 20 | 4.30 | - | - | |
| 30 | -0.02 | - | 1.51 | |
| 40 | 3.41 | - | 2.15 | |
| 50 | - | - | 4.03 | |
| K avg | 2.5 | - | 0.45 | - |
| 3.0 | - | 0.80 | - | |
| 3.5 | - | 1.31 | - | |
| 20 | 1.43 | - | - | |
| 30 | -0.01 | - | 0.50 | |
| 40 | 1.14 | - | 0.72 | |
| 50 | - | - | 1.34 | |
| Best | 20 | 3.5 | 50 | |
| R | 1.44 | 0.86 | 0.84 |
Engineering application verification
The optimized construction ventilation parameters were applied to the project site at the exit work area of the Tingshe Tunnel, as shown in Fig. 12.
Fig. 12.
Engineering application after parameter optimization.
The distance from the air duct to the working face was 20 m, the height of the upper step was 3.5 m, and the safe distance between the working faces of the two holes was 50 m. Before and after the optimization of construction ventilation parameters, the wind velocity distribution of the human respiratory height on the central axis face of the left hole was shown in Fig. 13.
Fig. 13.
Wind speed distribution of human respiratory height on the central axial plane of the left hole before and after parameter optimization.
From the above analysis, when the distance LC from the vehicle cross passage to the working face of the right tunnel is 80 m, the complementary ventilation effect was obvious. With the continuous advance of the tunnel, the distance from the vehicle cross passage and the working face is increasing, and it is not yet known whether the double-hole tunnel complementary ventilation is still applicable. Table 12 shows the comparison of Qm and K of each branch in the tunnel under different LC conditions after parameter optimization.
Table 12.
Comparison of Qm and K of each branch under different LC conditions after parameter optimization.
| Working conditions | Differences in wind requirements | Q1 /(m3/s) | Q2 /(m3/s) | Q3 /(m3/s) | Q4 /(m3/s) | K |
|---|---|---|---|---|---|---|
| LC =80 m | QL>QR | 27.9878 | 14.0778 | 36.8070 | 5.0817 | 2.6112 |
| QL<QR | 14.1216 | 28.0515 | −9.4218 | 51.1536 | 0.0145 | |
| LC =105 m | QL>QR | 27.6843 | 14.2413 | 31.8439 | 9.4894 | 1.9234 |
| QL<QR | 14.2820 | 28.3633 | 11.5336 | 31.2824 | 0.3229 | |
| LC =130 m | QL>QR | 27.8361 | 14.0640 | 28.4594 | 13.8551 | 1.6387 |
| QL<QR | 13.4774 | 28.3030 | 11.3205 | 30.1222 | 0.6273 | |
| LC =155 m | QL>QR | 27.9318 | 14.1497 | 28.3440 | 13.7691 | 1.0812 |
| QL<QR | 14.4572 | 27.3426 | 13.0678 | 28.8231 | 0.8606 | |
| LC =180 m | QL>QR | 28.2177 | 14.1497 | 28.3931 | 13.9691 | 1.0358 |
| QL<QR | 14.1872 | 27.5726 | 12.9950 | 28.7771 | 0.9734 |
According to Table 12; Fig. 14, when QL > QR, under the conditions that he distances from the cross passage to the left working face were 80 m, 105 m, 130 m, 155 m and 180 m respectively, the K values obtained were 2.6112, 1.9234, 1.6387, 1.0812 and 1.0358, respectively, which are all greater than 1, indicating that the airflow flowed from the right hole to the left hole; with the increase of the distance from the cross passage to the working face, the air volume from the right hole to the left hole was gradually reduced, and the new air volume in the left hole is 8.82 m3/s, 4.16 m3/s, 0.62 m3/s, 0.41m3/s, 0.17 m3/s, respectively. When QL< QR, under the conditions that the distance from the cross passage to the left working face was 80 m, 105 m, 130 m, 155 m and 180 m, respectively, K values were 0.0145, 0.3229, 0.6273, 0.8606 and 0.9734, respectively, which were all less than 1, indicating that the wind flow flowed from the left hole to the right hole; with the increase of the distance, the air volume from the left hole supplemented to the right hole gradually decreased, and the new air volume of the right hole was 23.10 m3/s, 4.18 m3/s, 1.82 m3/s, 1.48 m3/s and 1.20 m3/s respectively. From the above analysis, within the range of 180 m( the distance from the working face of the right to the vehicle cross passage), the optimization of the construction ventilation parameters can realize the double-hole complementarity. However, with the increase of the distance, the air volume supplemented to the tunnel with large air demand is less and less.
Fig. 14.
Fitting curves of K values under different LC conditions after parameter optimization.
Conclusions
Based on the two-way four-lane Baima Tunnel, the two-way six-lane Tingshe Tunnel and the two-way eight-lane Xishan Tunnel, the dynamic analysis of the structural stability of the double-hole ventilation network is carried out. The results show that under the existing construction ventilation parameters, the Tingshe Tunnel meets the ventilation characteristics of the double-hole complementary construction, and the Baima Tunnel and the Xishan Tunnel are not satisfied.
After confirming that the Tingshe tunnel has the characteristics of double-hole complementary construction ventilation, the orthogonal test is carried out on the layout position of the air outlet of the forced air duct, the height of the upper step and the safe distance of working faces in the double-hole tunnel. Taking K as the evaluation index, the combination of ventilation parameters for double-hole complementary construction is optimized. Finally, the optimal parameter combination is obtained. The distance between the air duct outlet and the working face L1 is 20 m, the height of the upper step L2 is 3.5 m, and the safe distance of the working face in the double-hole tunnel L4 is 50 m. The above parameter combinations are verified in the Tingshe tunnel, and the double-hole complementary effect is remarkable.
This paper proposes that when there is an instantaneous difference in air demand in the double-tunnel tunnel, the “surplus air flow” in the tunnel on the side with small air demand can be introduced into the tunnel with large air demand to form a double-tunnel complementary construction ventilation, which reduces the energy consumption of the ventilator in the tunnel with large air demand, enriches the construction theory of the long double-hole construction tunnel, and also provides new technical support for the construction of difficult tunnels such as the Yunnan-Tibet Railway and the Sichuan-Tibet Railway, which has broad application prospects.
Author contributions
All authors contributed to the writing of the paper and provided critical input that helped shape the research, analysis, and paper. XM.W.: Conceptualization, formal analysis, writing original draft, study conception and design, analysis and interpretation of results, draft manuscript preparation. Y.Z.: designed the research, supervised the work. XY.Z.: carried out the research. D.L.: carried out the research. HL.W.: carried out the research. SQ.C.: carried out the research. H.D.: carried out the research.All authors have read and agreed to the published version of the manuscript.
Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
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Associated Data
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Data Availability Statement
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.