Zhang Xudong
Hebei Province Highway Safety Perception and Monitoring Key Laboratory Hebei Institute of Traffic Regulations Ruizhi Traffic Technology Consulting Co., Ltd.
Abstract: Taking the construction of an existing subway intersecting tunnel through an existing subway interval as an example, using MIDAS/GTS finite element software, the impact of the construction period and operation period of the project on the displacement characteristics of the existing subway interval tunnel was simulated and calculated and analyzed, and the safety of the existing subway interval tunnel was studied. The research results show that during construction and operation, the displacement of the tunnel excavation above the tunnel is mainly vertical displacement and meets relevant requirements; compared with the construction stage, deformation in all directions during the operation period has been reduced; from the excavation of the newly built tunnel to the overlapping impact area away from the existing tunnel, the construction has a significant impact on the vertical displacement of the existing tunnel. At this stage, monitoring is required and timely feedback and guidance is provided.
Keywords: Mountain Tunnel; Overlapping through existing tunnels; Construction period; Operation period; Displacement characteristics; Security evaluation;
0 Introduction
Due to geographical conditions and underground space resources, more and more tunnel lines have to cross existing underground structures close by. During the construction and operation of such projects, especially the construction of overlapping through existing tunnels, due to the large area of proximity, it seriously threatens the safety of existing traffic lines and personnel.
For this type of project, many experts and scholars have made a lot of research. He Meide [1] combined with the actual project of tunnels through subway sections through underground crossing passages, studied the deformation laws and stress conditions of existing shield tunnels through shallow buried and hidden tunnels, and proposed corresponding deformation control measures; Zhang Yue et al. [2] conducted numerical simulation of the existing tunnels through parallel to a small clear tunnel, analyzed the deformation and internal forces of the existing tunnels during construction, and studied the feasibility of the upper and lower short step method; Qu Wenbin [3] based on actual projects and using FLAC3D software, he studied the deformation of existing tunnels during orthogonal upstream of the new power tunnel.
Based on the above research and analysis, it can be seen that the research on the construction of existing tunnels through newly built tunnels is mostly limited to the construction period, and there are few research on the construction of non-parallel tunnels through tunnels. Therefore, Midas/GTS software is used to analyze the deformation laws of existing tunnels for the two stages of construction and operation of the project, and evaluate the safety of the project.
1 Project Overview and Model Establishment
1.1 Project Overview
Relying on the project's existing subway tunnel and the proposed tunnel partially overlap and obliquely, the overlap area of the two is about 200 m, and the minimum vertical clear distance is about 13.72 m. Relying on the project's geological conditions are shown in Figure 1.
1.2 Numerical calculation model
relies on actual engineering to establish a refined three-dimensional numerical calculation model as shown in Figure 2. Combined with the terrain, the model plane dimension is 145m×280m, the vertical height is 75.7~145m, and from top to bottom, it is: pure soil fill , strong weathered quartz sandstone , stroked quartz sandstone, and windy quartz sandstone. In the model, the rock and soil bodies use 3D solid units, the soil layers are considered elastoplastics, the material damage criteria are Moore-Coulomb criterion, and the subway interval tunnel and the proposed tunnel lining are simulated by 2D panel units, and the cross-sectional parameters are the same as the actual situation.
1.3 Calculation working condition design
(1) First working condition: initial stress field analysis. This working condition is a site stress analysis before the construction of the subway and the proposed tunnel. It takes into account the soil weight and displacement boundaries to form a coupled stress field and clear the grid's displacement.
(2) Second working condition: construction of existing subway interval tunnels. In this working condition, excavation and backfilling of the existing wires are carried out, and the grid is shifted and zeroed.
(3) Third working condition: Analysis of construction of new tunnels. This step is part-level excavation, and the CRD method is used to excavate. Each step leads two construction steps ahead of the next step. When the construction support of the rear step is completed, the middle partition wall and temporary arch will be removed.
Figure 1 Tunnel geological longitudinal section diagram Download the original image
Figure 2 Numerical calculation model Download the original image
(4) The fourth working condition: tunnel operation analysis. At this time, the personnel load, vehicle load and structural load will be applied in the form of uniform force on the bottom plate of the newly built tunnel to view the impact of the tunnel operation period on existing subway tunnels.
2 Calculation results analysis
2.1 Analysis of the displacement of existing tunnels in the construction stage
The main indicator of the impact of tunnel construction on subway tunnels is structural displacement, which can best reflect the degree of impact of tunnel project construction on subway structure. Model calculations can intuitively read the displacement value of the subway structure, thereby judging the safety of the subway structure from a quantitative perspective. Since the displacement is cleared in operating condition 2, the displacement caused by the subway structure in operating condition 3 affects the displacement. The three-way displacement of the existing subway interval tunnel is shown in Figure 3.
Through analysis of Figure 3, it can be seen that due to the excavation of the newly built tunnel, Line 2 has floated 4.05mm and the maximum horizontal displacement is 1.84mm, which meets the requirements of the Metro Group on the engineering deformation control indicators in the security zone of the 10mm, meets the requirements of the Guangdong Provincial Standard "Technical Specifications for the Protection of Existing Structures of Urban Rail Transit" for the horizontal and vertical displacement value of 15mm, and meets the requirements of the industry standard "Technical Specifications for the Protection of Urban Rail Transit" for the Safety Protection of Urban Rail Transit" [5] (CJJ/T The specified requirements for the horizontal and vertical displacement control values of station and tunnel structures with safety control indicator values in 202-2013 are less than 20mm; compared with horizontal displacement, the vertical displacement is more obvious, which is the main displacement of the subway structure. Therefore, during the construction of such projects, monitoring and measurement of the vertical displacement of existing tunnels is necessary to ensure the safety of construction.
Figure 3 Cloud diagram of existing tunnel structure displacement in the construction stage Download the original image
2.2 Analysis on the displacement impact of existing tunnels in the operation stage
operating stage displacement changes in existing subway tunnels are shown in Figure 4.
Figure 4 Cloud map of existing tunnel structure displacement in the operation stage Download the original image
Analysis of Figure 4 It can be seen that since the new tunnel will float 3.74mm after operation, the horizontal maximum displacement will still meet the relevant requirements; compared with the horizontal direction displacement, the main displacement of the existing subway inter-segment tunnel structure is still vertical displacement. And it has been reduced compared with the construction stage, which is mainly due to the existence of vehicle loads in the upper personnel and vehicles that can offset the structural impact caused by partial excavation of unloading .
2.3 Further analysis of the displacement of existing tunnels
According to the above analysis, vertical displacement is the key analysis indicator, and the impact is mainly concentrated in the construction stage. Now, the vertical displacement value of the existing tunnel at each calculation step is extracted, and the vertical displacement time curve of the existing tunnel is drawn as shown in Figure 5.
Figure 5 Vertical displacement time curve Download the original image
Through the analysis of Figure 5, it can be seen that from the fifth excavation step, that is, when the new tunnel is excavated, it will affect the existing tunnel. After step 36, when the proposed project turns out above the existing tunnel, the impact will be basically eliminated. Therefore, when the new tunnel is overlaid through the existing subway tunnel, the construction must be strictly controlled from the downstep to the newly built tunnel being away from the overlapping area with the existing tunnel, the blasting method must be strictly controlled, the vertical displacement of the existing tunnel is strengthened, and the construction is feedback and guidance is promptly provided. After being away from the overlapping area, the construction control can be appropriately relaxed to increase the construction speed within a safe range.
3 Conclusion
(1) The impact of excavation of a new tunnel on the displacement of an existing tunnel is mainly vertical displacement, with a floating up by 4.05mm and a maximum horizontal displacement value of 185mm, all of which meet the relevant control standards.
(2) The impact of the new tunnel on the displacement of the existing tunnel during the operation period is still mainly vertical displacement, with a floating up by 3.74mm and a maximum horizontal displacement of 0.64mm, all of which meet the relevant requirements and are reduced compared with the construction stage. This is mainly due to the existence of vehicle loads of the upper personnel and vehicles that can offset the structural impact caused by partial excavation and unloading.
(3) When the construction of a new tunnel is overlaid through the existing subway section tunnel, the construction from the downstep to the newly built tunnel being away from the overlapping influence area with the existing tunnel, all of which affect the displacement of the existing tunnel structure is affected. It is necessary to strengthen control of the construction of the new tunnel, and the vertical displacement of the existing tunnel is strengthened, and timely feedback and guidance are provided to ensure construction safety.
References
[1] He Meide. Research on deformation control of existing shield tunnels through shallow buried and hidden excavation tunnels [D]. Beijing: Beijing Jiaotong University, 2015.
[2] Zhang Yue, Cao Wei. Construction monitoring and numerical simulation analysis of existing tunnels parallel to the small clear tunnel [J]. Northern Transportation, 2016(10):56-59.
[3] Qu Wenbin. Research on the impact of construction of newly built power tunnels on surface settlement and deformation of existing tunnels [J]. Municipal Technology, 2020(5):192-196.
[4] Shenzhen Metro Group Co., Ltd. Project management measures for subway operation safety protection zones and construction planning control zones [Z]. Shenzhen: 2016.
[5] Ministry of Housing and Urban-Rural Development of the People's Republic of China. Technical specifications for safety protection of urban rail transit structures: CJJ/T 202-2013[S]. Beijing: China Construction Industry Press, 2013.
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