Feng Yu Tang Shiqiang Hu Jun
CCCC Third Public Bureau Second Engineering Co., Ltd. School of Civil Engineering, Chongqing Jiaotong University
Abstract: Based on the example of Yangbajing No. 2 Tunnel project in the control project of the Naqu to Lhasa section of National Highway G109, the finite element analysis method was used to numerically simulate the excavation process of the full-section method and the upper and lower step method of the upper span liaison air duct, analyze the response law of the deformation and stress of the tunnel surrounding rock and the main tunnel support system below during the excavation of the upper span liaison channel, and compare and analyze the influence of the two excavation methods on the main tunnel. The results show that in the process of excavation of the upper span contact air duct, especially the part close to the intersection of the upper and lower tunnels, the deformation and force of the main tunnel are greatly affected. When using the upper and lower step method, the deformation can be reduced by 32% compared with the full-section method, the tensile stress at the vault of the main tunnel can be reduced by 35%, the pressure stress at the arch foot of the main tunnel can be reduced by 72%, and the influence range of the main tunnel can also be reduced when the upper and lower step method is used for construction; The up and down step method can better ensure the safety of the project and avoid excessive disturbance of the main tunnel below.
Keywords: cross-tunneling; Upper spanning liaison air duct; Excavation method; Numerical simulation;
In the design of long tunnels, in order to meet the needs of construction period and tunnel ventilation, it is necessary to set up construction inclined shafts or ventilation inclined shafts to speed up the progress of tunnel construction and meet the ventilation needs during tunnel operation [1]. With the construction of a large number of urban underground transportation, more cross-tunnel projects have emerged, and more and more numerical simulation analysis has been carried out. Gong Jianwu et al. [2] Based on a three-lane ultra-proximity highway tunnel, a three-dimensional finite element model was established to study the spatiotemporal mechanical characteristics of the tunnel construction process, and the numerical analysis results showed that the distance of the working surface of the close-in tunnel in the tunnel excavation process could be about 1.5 times the tunnel excavation span. Meng Dong et al. [3] Analyze the peak speed of existing tunnel lining caused by the seismic wave of blasting in the construction of new tunnels in the cross section by establishing different cross angles, net distances and surrounding rock conditions, and discuss the dynamic response of blasting in the construction of three-dimensional cross tunnels under different factors. Luo Zhixiang et al. [4] Combined with the Zhaomushan Tunnel Project in Chongqing, according to the blasting monitoring data and numerical simulation results of the construction site, the stability and safety of the adjacent and intersecting tunnels during the blasting construction of the new tunnel were studied, and the influence of tunnel spacing on the vibration of the adjacent and intersecting tunnel lining structures was discussed. Liu Xinrong et al. [5] Based on the staggered proximity section of the Liangjiang Tunnel in Chongqing, the corresponding numerical model of the staggered proximity tunnel was established, and the influence of the existence of existing tunnels and the change of staggered spacing on the surrounding rock shape variable of the new adjacent tunnel during the excavation of the new tunnel was studied.
In this paper, combined with the construction process of the Yangbajing No. 2 Tunnel Oblique Well Transmission and Exhaust Air Liaison Road in the control project of the Naqu to Lhasa section of National Highway G109, in view of the problems existing in the construction of the upper and lower crossings of the two tunnels, numerical simulation methods are used to analyze the impact of the construction of the upper span liaison air duct on the main tunnel, and the influence of different excavation methods on the main tunnel below is studied, and the excavation method applicable to the project is obtained.
1 Project Overview
1.1 Geographical location and traffic profile
The surface wellhead of Yangbajing No. 2 Tunnel is located between the foot of the hillside slope and National Highway 109, and the tunnel is about 6 275 m long, which is a long tunnel. The tunnel is a double-hole four-lane section, of which the No. 1 inclined shaft turns to the main hole with a burial depth of 390 m and a maximum burial depth of 730 m. Near the cave entrances at both ends of the tunnel area, there are G109, Qinghai-Tibet Railway and other traffic arteries passing through, and the traffic conditions are good; However, the tunnel site area belongs to the alpine erosion engineering geological zone (II.), which is a sub-area of engineering geology of block structure intrusion rocks (II.4), and the terrain is undulating. The route area of the tunnel site area is as high as 4 300 m above sea level, and the climatic boundary is more obvious, which belongs to the cold temperate semi-arid monsoon climate zone of the plateau.
1.2 Upper span liaison air duct construction technology
According to the cross-section size, cross and longitudinal slope of the transportation channel of the inclined shaft fan room, the specific construction technology: first of all, the excavation support construction of the fan room provides power supply and equipment guarantee for the construction of the main tunnel of the No. 2 tunnel into the Yangbajing and the wind turbine room; Then the above passages are constructed in accordance with the construction order of the right line exhaust air connection road, the left line air supply contact road, and the left line exhaust air connection road. The overall layout of Yangbajing Tunnel No. 1 inclined shaft is shown in Figure 1.
In the design of the ventilation inclined well, in order to meet the demand for the supply and exhaust of the left and right holes of the main line, the exhaust duct of one of the holes needs to span the other main hole, of which the contact air duct is the contact channel of the exhaust air outlet and the inclined well, and the upper span of the contact air duct section will produce a large longitudinal slope, and the vertical net distance of the upper span position is 6 to 8 m. In this paper, the contact channel in The inclined shaft No. 1 of Yangbajing Tunnel No. 2 is selected for research and analysis, and the cross-sectional diagram of the inclined shaft is shown in Figure 2.
Figure 1 Schematic layout of Yangbajing Tunnel No. 1 Inclined Shaft Download the original drawing
Figure 2 Cross section of the intersection of Yangbajing No. 2 Tunnel No. 1 Inclined Shaft Download the original diagram
The excavation of the upper span liaison air duct uses glossy blasting, which is divided into two steps: upper and lower. The height of the upper step is 6.85 m, and the height of the lower step is 2 m. The elevation up and down the steps is shown in Figure 3. The overall process flow of the upper contact channel: advanced support → upper step blasting → the upper step initial support→ lower step blasting → lower step initial support (the above stage is circulated) → the upward arch construction → the second lining construction.
Figure 3 Illustration of the height of the steps up and down Download the original image
2 Finite element simulation analysis of upper spanning contact air duct excavation
2.1 Establishment of the basic model
In this paper, the finite element numerical calculation software is used to model the tunnel, and the simulated rock mass and soil mass are all molar-Coulomb failure criterion, and some characteristics and laws of the cross tunnel are summarized by establishing the corresponding three-dimensional tunnel model.
(1) Model size.
Theoretical studies have shown that in tunnel excavation, due to the stress and displacement changes caused by the release of the surrounding rock load, the effect outside the 5 times the hole diameter is less than 1%, the influence outside the 3 times the hole diameter is less than 5%, and the actual left and right boundaries usually take 3 to 5 times the model span, and the upper and lower borders take 3 to 5 times the model height [6]. According to the actual situation at the intersection of the inclined shaft of Yangbajing No. 2 Tunnel, the width of the main tunnel on the right line is 12 m, the height is 9 m, the width of the upper span contact air duct is 5.8 m, the height is 7 m, the x-axis direction of this model is 84 m long, parallel to the upper span liaison air duct, the y-axis direction is 66 m long, parallel to the right line of the main tunnel, and the z-axis direction is 72 m high, according to which the finite element model of the tunnel is shown in Figure 4.
Figure 4 Finite element model Download the original image
(2) Model boundary conditions.
The intersection of the Yangbajing No. 2 tunnel in this study belongs to the deep burial tunnel, and the burial depth of this model has been considered, without considering the self-weight of the upper covered rock and soil. The upper boundary is a free surface, and the left and right boundaries and the lower boundary constrain the displacement of their respective normals, that is, the displacement of the corresponding nodes is set to zero [7].
(3) Unit type.
In this model, two tunnels intersect up and down, the structure is complex, and the mesh hybrid simulation using finite element software is used. Solid element simulation is used for the surrounding rock, and shell element simulation is used for initial support, and it is precipitated on the core soil of the excavation.
Due to the complexity of the stress near the intersection segment, the mesh near the intersecting segment in the model was encrypted, and the final total number of mesh nodes was 185 024.
(4) Material parameters.
Combined with the on-site geological survey data, the surrounding rock at the intersection of the cross-contact air duct on the Yangbajing Tunnel is Grade IV., and the surrounding rock parameters mainly refer to the physical and mechanical parameters given by the geological survey data; The parameters of the tunnel support structure are combined with the tunnel design drawing and selected according to the values given by the relevant specifications.
2.2 Simulation of excavation and support processes
In the construction sequence, the order consistent with the actual project is adopted, that is, the excavation of the main tunnel of the right line is carried out first, then the support of the main tunnel is carried out, and then the excavation of the upper contact air duct is carried out, and finally the support of the upper contact air duct is carried out. In ANSYS, it is possible to simulate the elimination and addition of materials using killing and activating elements, i.e. using this life-and-death function of the unit to simply and effectively simulate the excavation and support process of the tunnel. When the tunnel is excavated, it can be directly selected to kill the excavated unit, which can realize the simulation of excavation; When applying support, the corresponding support part of the unit that was killed during excavation is first activated, and then its material properties are changed. Therefore, using the life-and-death function of the unit to simulate the excavation and support of the tunnel is more accurate and the results are more reliable than simply using the "empty unit" to simulate excavation and changing the material number to simulate the support [8].
2.3 Calculate cross-section selection
According to the excavation process of the upper span liaison air duct, 4 sections were selected: the upper right span of the contact air duct was excavated 4 m section, the upper right span of the contact air duct was excavated to 1/2 section, the upper right span was excavated to complete the section, and the upper span of the contact air duct was all excavated to complete the section. The sections are shown in Figure 5.
2.4 Analysis of calculation results of upper spanning contact air duct construction
2.4.1 Analysis of initial support deformation of main tunnel
Figure 6 shows the initial support deformation of the main tunnel of the upper span when the upper span contact air duct is constructed using the full section method, and Figure 7 calculates the initial support deformation of the main tunnel of the section when the upper span contact air duct is constructed using the upper and lower step method.
(1) The disturbance of the surrounding rock by the excavation of the upper span contact air duct will also cause the initial support deformation of the main tunnel below, and in the process of excavation of the upper span contact air duct, the deformation of the main tunnel will also change with the excavation site to the intersection of the two tunnels. Therefore, during the excavation of the upper span contact air duct, it is necessary to strengthen the deformation monitoring of the intersection of the main tunnel below to prevent large deformation.
Figure 5 Tunnel section selection Download the original image
Figure 6 Initial support displacement of the main tunnel of the full-section method Download the original diagram
Figure 7 Initial support displacement of the main tunnel of the upper and lower steps Download the original diagram
(2) From the construction method, although the settlement value caused by the excavation of the full section and the upper and lower step method is concentrated in several excavation intervals, the maximum deformation value of the initial support when the full section method is excavated is 0.565 3 mm, the maximum deformation value of the initial support when the upper and lower step method is excavated is 0.382 51 mm, and the deformation of the upper and lower step method can be reduced by 32%, which is due to the fact that the step-by-step excavation during the construction of the upper and lower step method has a better inhibitory effect on the initial support deformation of the main tunnel below.
(3) When excavating to the upper right span of 1/2 section, the deformation generated by the upper and lower step method is much smaller than the deformation caused by the excavation of the full-section method, and the impact of the upper and lower step method excavation on the initial support deformation of the main tunnel is smaller. Therefore, it is more appropriate to use the upper and lower step method for the construction of the upper span contact air duct, and the deformation is easy to control.
2.4.2 Analysis of initial support force of the main tunnel
Figure 8 shows the cloud map of the initial support stress of the main tunnel of the upper span when the upper span liaison air duct is constructed using the full-section method. Figure 9 shows the cloud map of the initial support stress of the main tunnel of each section calculated during the construction of the upper span liaison air duct using the upper and lower step method.
Figure 8 Full-sectional method main tunnel support stress cloud diagram Download the original diagram
(1) The equivalent force of the main tunnel is continuously increasing during the excavation of the upper contact air duct, and it is concentrated in the vault position of the part that intersects with the upper span contact air duct. For the force of the initial support of the main tunnel after the completion of the excavation of the upper span liaison air duct, different from the situation after the main tunnel is excavated alone, due to the existence of the upper span contact air duct, the vault of the main tunnel is subjected to upward pulling stress, and concentrated on the intersection of the upper span liaison channel and the main tunnel. In the process of tunnel construction, special attention should be paid to the changes in stress and deformation at the intersection of the main tunnel and the upper span liaison air duct to ensure the safety of construction.
(2) From the perspective of construction methods, the maximum tensile stress generated on the arch of the main tunnel when the full section method is completed is 0.463 37 MPa, and the maximum compressive stress generated at the arch foot of the main tunnel is -0.103 8 MPa; When the full excavation is completed by the up and down step method, the maximum tensile stress is generated at the vault of the main tunnel of 0.269 05 MPa, and the maximum compressive stress is -0.028 261 MPa at the arch foot of the main tunnel. It can be seen that the upper span contact air duct has a significant effect when using the upper and lower step method of construction, the tensile stress can be reduced by 35%, and the compressive stress can be reduced by 72%, which is due to the increase in the vertical net distance between the two tunnels when excavating on the upper step, and the use of the upper step excavation surface after the lower step excavation reduces the disturbance of the main hole, so it is more appropriate to use the upper and lower step method for the excavation of the upper span contact air duct.
Figure 9 Up and down step method main tunnel support stress cloud diagram Download the original diagram
3 Conclusion
In view of the problems existing in the construction of the upper and lower crossings of the above two tunnels and the upper span of the liaison air duct, as well as the stress concentration at the intersection and the large slope of the upper span contact air duct, in order to ensure the safety state of the construction process, the following conclusions are drawn from the comparison of the construction of the upper span contact air duct full section method and the upper and lower step method on the initial support of the main tunnel.
(1) The simulation results of the excavation process of the upper span contact air duct show that the force and deformation of the lower main tunnel will be affected by the construction of the upper contact air duct, especially the part where the upper contact air duct intersects with the lower right main tunnel, the deformation and force are more obvious, and should be considered during the construction process.
(2) Comparative analysis of the displacement and force of the initial support of the main tunnel on the right line under the two construction methods of full-section excavation and upper and lower step excavation, the relevant results show that the deformation can be reduced by 32% when the upper and lower step method is adopted, the tensile stress at the vault of the main tunnel can be reduced by 35%, and the compressive stress at the arch foot of the main tunnel can be reduced by 72%. Therefore, it is more appropriate to excavate the upper span liaison air duct by using the upper and lower step method, which is convenient for timely construction
bibliography
Li Yufeng,Peng Limin,Lei Mingfeng. Research Progress on Design and Construction Technology of Cross Tunnel Engineering[J].Journal of Railway Science and Engineering,2014,11(1):67-73.
Gong Jianwu,Qi Peilin. Study on spatio-temporal mechanical characteristics of excavation of three-lane ultra-proximity tunnel[J].Highway,2016,(8):217-221.
Meng Dong,Liu Qiang,Peng Limin,et al. Research on blasting vibration response of railway overpass tunnel construction[J].Journal of Hefei University of Technology: Natural Science Edition,2015,(3):363-368.
Luo Zhixiang,Zhang Minchao,Zhong Zuliang. Study on the Influence of Space Cross Tunnel Blasting Construction on Adjacent Tunnel Structure[J].Chinese Journal of Underground Space and Engineering,2018,(S1):205-213.
Liu Xinrong, Wang Jiming, Li Dongliang, et al. Experimental study on the three-dimensional model of surrounding rock deformation of interlaced new tunnel[J].Modern Tunnel Technology,2017,54(5):171-179.
CHEN Peihuang. Monitoring measurement and numerical simulation analysis of mechanical characteristics of upper and lower overlapping tunnels[J].Railway Construction Technology,2019,(2):97-101.
Ye Jianzhong,Tu Meiji,Qiu Fan. Numerical simulation analysis of the impact of close construction of subway small net distance superimposed interchange tunnel[J].Urban Rail Transit Research,2016,19(12):27-31,35.
Cheng Bangfu,Cheng Lin. Numerical simulation and comparative analysis of monitoring data of excavation method of a weak surrounding rock highway tunnel[J].Journal of Anhui Jianzhu University,2020,28(1):7-13.