The Study on Deformation and Mechanical Properties of Surrounding Rock to Primary Arch Cover Met

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Combined with the monitoring data of the project site, the stress and deformation law of the tunnel surrounding rock under the influence of time and space effect are analyzed. In addition, the excavation process of the double-layer superimposed primary arch cover method and the traditional arch cover method is compared. Based on the stress and deformation characteristics of the tunnel surrounding rock, the technical advancement of the double-layer primary arch cover method is quantified. The results show that after the tunnel excavation is completed, the vertical displacement of the tunnel surrounding rock is mainly concentrated in the vault and arch bottom of the tunnel. The maximum uplift displacement is 7.263 mm and the maximum subsidence displacement is 14.09 mm. The settlement of the tunnel vault is roughly divided into three stages: slow settlement, rapid settlement and settlement stability; it can be seen from the comprehensive longitudinal span and section monitoring data that the double-layer superimposed primary arch cover method can better reduce the deformation of surrounding rock and increase the stability of the tunnel during excavation. Physical sciences/Engineering/Civil engineering Earth and environmental sciences/Solid earth sciences/Geophysics Earth and environmental sciences/Solid earth sciences/Petrology Tunnel engineering surrounding rock deformation numerical simulation large section tunnel initial support arch cover method Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Introduction In recent years, the construction of underground transportation facilities in China has developed rapidly. The geological conditions faced by tunnel construction are becoming more and more complex, and the tunnel section is also expanding. Large cross-section tunnels are characterized by large height, wide span, poor geological conditions and low stability of surrounding rock. Therefore, in order to ensure the construction safety of large cross-section subway tunnels, engineers proposed a double-layer superimposed initial arch cover method based on the hole pile method. The double-layer superimposed initial arch cover method is favored because of its unique double arch cover structure, which can ensure the stability of surrounding rock to a large extent when excavating large cross-section tunnels. Scholars at home and abroad have done a lot of research work on the mechanical properties of surrounding rock in large section tunnel excavation and the supporting effect of primary arch cover method, and have achieved many useful results. Zhang Fupeng et al. 1 used the numerical simulation method to compare and analyze the structural stress, displacement and plastic zone range of the reverse expansion method and the traditional middle partition wall method during the tunnel excavation process based on the Baowancun tunnel project of Yang 'an second line. You Dongmei et al. 2 studied the influence of different clear distances on the stress distribution of surrounding rock of shallow buried bias tunnel excavation by means of numerical simulation with the shallow buried bias section of Swan Cave Tunnel as an engineering case. Jiang Fengguo et al. 3 took the construction of Harbin urban subway tunnel as the engineering background, based on the regional geological conditions, a finite element numerical model was constructed to analyze the deformation and stress variation of surrounding rock during the construction process of double side heading method and cross middle partition wall method. Combined with on-site monitoring methods, the construction method and stability of large-section tunnels were analyzed. Xu Ping et al. 4 constructed a three-dimensional numerical model of large-section tunnel under the condition of loose accumulation stratum by finite difference method, and analyzed the deformation characteristics and stress of surrounding rock and tunnel structure when excavating large-section tunnel by double-side heading method by numerical simulation. Cao Zhilin et al. 5 studied the influence of unloading on the depth of pressure arch in each pilot tunnel of super-large cross-section tunnel by numerical simulation and theoretical analysis based on the construction of double-side heading method of super-large cross-section underground excavation station of Guiyang Metro Line 2. Liu et al. 6 developed a large-scale assembled geomechanical model test system based on practical engineering, and used the displacement monitoring system to carry out mechanical model test research on the construction process of super-large section tunnel in complex strata. He Peng et al. 7 used the improved DDARF method to simulate the deformation and failure law and crack evolution process of the surrounding rock of the shallow-buried small-distance section of the Daling tunnel, which is based on the Daling super-large section tunnel of the Southeast Second Ring Expressway in Jinan City. Sato et al. 8 measured the whole process of deformation, strain and vibration velocity of surrounding rock by arranging advanced monitoring boreholes in Tono underground test tunnel in Japan, and analyzed the influence of mechanical and blasting excavation methods on the damage range. Su Daozhen et al. 9 selected a number of monitoring sections as test sections for field test research in Xiaopanling Tunnel's import and export section, in view of the fact that large-section carbonized mudstone tunnels are susceptible to landslides, distortion of arches, and frequent arch changes in the course of construction. Based on the actual construction situation of a subway station in Qingdao Metro, Feng Shijie et al. 10 studied and analyzed the combination form of equal thickness initial support and initial support arch cover, and put forward two construction schemes. Based on a station of Qingdao Metro, Gong 11 studied the deformation law and control of the initial support arch cover method by combining numerical simulation with field monitoring data. Kong Chao et al 12 studied the mechanical characteristics of the initial arch cover method in composite strata through numerical simulation, combined with data from model experiments, and analyzed the deformation law of the surrounding rock under different working conditions of the initial arch cover method. Zhao Ying et al. 13 analyzed the applicability of the composite arch cover method to a station of Guiyang Rail Transit Line 2 by numerical simulation, and analyzed the surface settlement, vault settlement and plastic zone distribution of surrounding rock that may be caused by construction. Zhang Guohua et al. 14 analyzed the applicability of double-layer composite arch cover method, double-side heading method and step method by numerical simulation method, combined with the engineering example of a underground excavation station in Chongqing area, and compared and analyzed the actual engineering economic indicators. Zhang et al. 15 monitored the surface subsidence during the excavation of Zhongshan Park Station of Qingdao Metro Line 3 constructed by arch cover method. Shang et al. 16 analyzed the influence of different support measures on the deformation of surrounding rock in the construction process of arch cover method by means of numerical simulation. The results show that the double-layer arch cover method can control the deformation of surrounding rock well and reduce the settlement of foundation. Based on a station of Qingdao Metro Line 3, Wu 17 used the finite element method to simulate the construction process of the arch-cover method in the soil-rock composite stratum, and analyzed the surface and vault settlement, supporting structure and surrounding rock stress during the construction process. The results show that the arch-cover method has good applicability in the soil-rock composite stratum. Based on the above research, it can be seen that the existing results are of great significance to the analysis of the deformation and mechanical properties of surrounding rock during the excavation of large-section tunnels. However, previous studies mainly focused on the changes of mechanical properties of surrounding rock in the process of conventional tunnel excavation methods, and there are few studies on the double-layer superimposed primary arch cover method, which is an excavation method for special strata. Therefore, based on the line section project of Dengying Depot of Qingdao Metro Line 4, this paper analyzes the variation law of surrounding rock stress during the excavation of double-layer primary arch cover method, and studies the stress deformation and stability of surrounding rock during the excavation of large section tunnel. The research results are expected to provide useful reference for the design and construction of similar engineering cases. Numerical Modeling project profile Relying on the engineering tunnel, it belongs to the single-hole four-line tunnel of the vehicle entry and exit section line. The actual situation on site is shown in Figure 1. The total length of the tunnel is 44.5 m, the maximum diameter of the section is 30.059 m, the maximum buried depth is 26.82 m, the section area of the tunnel is more than 50 m 2 and the buried depth is less than 2 times the span, which belongs to the shallow buried large section tunnel. The stratum conditions of the tunnel are plain fill, moderately weathered granite, and slightly weathered granite from top to bottom. The stratum stratification is shown in Figure 2. Almost all of the tunnels are located in slightly weathered granite. The surrounding rock grades are mainly IV and V. The excavation section line of the main tunnel is the underground excavation interval, and the interval is constructed by mining method. Establishment of Numerical Calculation Model The model is established by hypermesh software and imported into FLAC3D software after meshing. The Mohr-Coulomb plastic constitutive model is used for both soil and rock, which can obtain more realistic simulation results. In order to eliminate the boundary effect, the model size is set to length * width * height = 140m * 45m * 80m. In order to facilitate the establishment of the model and the subsequent calculation, the hypermesh software is used to divide the region and group in the model establishment stage. The specific situation of the model is shown in Figure 3. physical and mechanical parameters of rock and soil According to the relevant survey and design data of the surrounding rock of the tunnel, the physical and mechanical parameters of the soil layer within the excavation range are weighted average, and the weight is the thickness of each soil layer. The selected physical and mechanical parameters of rock and soil are shown in Table 1. Table 1 Physical and mechanical parameters of rock and soil stratum density(kg · m -3 ) bulk modulus (MPa) shear modulus (MPa) c(kPa) φ (°) plain fill 1800 22 7 19.2 18 moderately weathered rock 2500 8621 6198 500 40 slightly weathered rock 2600 18000 10300 2000 45 imitating construction status Based on the actual project of Qingdao Metro, the specific excavation sequence of the double-layer primary arch cover method is shown in Figure 4. According to the on-site construction process, the single excavation length of the numerical model is 1.6m, and the whole excavation process is divided into 89 steps. Firstly, the pilot tunnel 1 and pilot tunnel 2 are excavated, and the lining and temporary side wall support are applied. Excavate the rock mass on both sides of the annular guide pit of the upper half section of the arch, and complete the first layer of the upper half section. The length of the first support of the second floor is 20 ~ 25m. After the initial support of the upper half section of the arch is completed, the core rock mass of the lower half section of the arch is excavated. It is divided into several construction sections along the longitudinal direction of the interval, and the intermediate rock mass at the lower part of the interval is excavated in sequence according to the diagram, and the initial support of the side wall is constructed in time. Numerical simulation results Displacement analysis of tunnel surrounding rock After the tunnel is excavated according to the established steps of the project, the displacement cloud diagram of the surrounding rock of the tunnel is shown in Figs.5 and 6. It can be seen from the diagram that after the normal excavation of the tunnel is completed, the vertical displacement of the surrounding rock of the tunnel is mainly concentrated at the arch bottom and vault of the tunnel. The vertical displacement of the tunnel arch bottom shows an upward uplift trend, and the maximum uplift displacement is 7.263 mm. The displacement of the tunnel vault is mainly manifested as subsidence, with the maximum subsidence displacement of 14.09 mm and the maximum horizontal displacement of 5.657 mm. The displacement of surrounding rock is approximately symmetrically distributed along the central line of the tunnel. This is mainly because the tunnel excavation destroys the original rock stress of the surrounding rock, resulting in stress redistribution of the surrounding rock. This value is far less than the standard value of subway excavation vault settlement in the specification. Analysis of maximum shear strain of tunnel surrounding rock The maximum shear strain cloud diagram of surrounding rock after tunnel excavation is shown in Fig.7.It can be seen from the figure that after the completion of tunnel excavation, the maximum shear strain of surrounding rock is mainly concentrated in the surrounding rock on both sides of the side wall and the initial support of the first layer. This shows that in the process of tunnel excavation, the shear stress of the tunnel arch foot and the side walls on both sides is the largest, and the surrounding rock is the weakest. In the process of tunnel excavation, it is necessary to strengthen the monitoring and support, which indirectly shows that the arch cover of the double-layer primary arch cover method can effectively transfer the shear stress to the surrounding rock on both sides of the tunnel arch foot and the side wall. Stress Analysis of Surrounding Rock of Tunnel The vertical stress cloud diagram of surrounding rock after tunnel excavation is shown in Fig.8.The stress cloud diagram is symmetrically distributed along the middle line of the tunnel, and the maximum vertical stress is 23.33 MPa. The vertical stress at the arch feet on both sides of the tunnel is opposite to the vertical stress of most surrounding rock. This is because the principle of the initial arch cover method is to transfer the stress of the surrounding rock at the upper part of the tunnel to the arch feet on both sides through the arch cover, so as to ensure the stability of the surrounding rock during the construction of the tunnel. Therefore, the monitoring and protection of the arch foot on both sides of the tunnel should be strengthened when using the primary arch cover method for tunnel construction, and measures should be taken to ensure its stability during excavation. Comparative Analysis of Numerical Simulation and Field Monitoring Data Monitoring and measuring the data in the process of tunnel construction is an indispensable part of tunnel construction, and it is the key to the information design and construction of the tunnel. The monitoring data can be used as the basis for judging the stability of surrounding rock and the reliability of supporting structure. According to the relevant specifications, the deformation of the surrounding rock of the tunnel is monitored. It can be seen from Figs.5 and 6 that the displacement of the tunnel vault and the arch bottom is the largest area of the vertical displacement of the surrounding rock during the entire tunnel excavation process and after the excavation is completed, and the stress is more concentrated. Therefore, the vault settlement and the arch bottom uplift value of the tunnel are mainly monitored. The on-site monitoring data of the vault and arch bottom settlement at the monitoring section are compared with the numerical simulation vault settlement, as shown in Fig.9. Figure 9 (a) is the vault settlement curve. As shown in the figure, in the numerical simulation process, when the number of excavation steps is 0-14 steps, the tunnel vault displacement value increases slowly. This is because when the tunnel is excavated to the 14th step, the first part of the rock mass of the upper half section of the tunnel is excavated to 22.4m, which does not reach the location of the monitoring point of 22.5 m. When the number of excavation steps is 14-38 steps, the vault settlement increases rapidly to 11.3mm. As the excavation process continues, the vault displacement continues to remain in a relatively stable state. According to the monitoring data, when the monitoring days are 0 ~ 15 days, the vault settlement remains in a relatively stable state, and the displacement value increases slowly. When the monitoring days are 15 ~ 38 days, the vault settlement increases rapidly. At 39 ~ 90 days, the settlement displacement value remained basically unchanged. Figure 9 (b) is the displacement curve of tunnel arch bottom. As shown in the figure, in the process of numerical simulation, when the number of excavation steps is 0-14 steps, the uplift value of tunnel arch bottom increases slowly. When the number of excavation steps is 14-38 steps, the uplift value of the tunnel arch bottom increases rapidly to 5.47mm. As the excavation process continues, the uplift value of the arch bottom continues to remain in a relatively stable state. According to the monitoring data, when the monitoring days are 0-15 days, the uplift value of the tunnel arch bottom remains in a relatively stable state, and the displacement value basically increases slowly. When the monitoring days are 15 ~ 38 days, the uplift value of the tunnel arch bottom increases rapidly. During 39 ~ 90 days, the uplift displacement value remained basically unchanged. Through comparative analysis, it is found that the two curves have almost the same change trend, which can be roughly divided into three stages: slow deformation, rapid deformation and basic stability. The maximum settlement value of the vault is also not much different. The field monitoring data and the numerical simulation data maintain correspondence in magnitude and time-space effect. However, there are still some differences in the shape of the two curves. This is because the tunnel adopts the mine method for blasting construction. The blasting will cause certain disturbance to the monitoring points and cause certain measurement errors. Moreover, the monitoring points of the tunnel are welded on the arch frame, and the tunnel excavation is easy to cause the deformation of the arch frame. The deformation of the arch frame is elastic deformation, so the grid steel frame also has a certain influence on the monitoring data of the monitoring points. Comparative study of double-layer primary arch cover method and traditional arch cover method The traditional arch cover method is widely used because it can achieve better results for special strata. The double-layer primary support arch cover method is optimized based on its construction experience, that is, it has one more layer of primary support than the traditional arch cover method. Therefore, the unsupported excavation, the traditional arch cover method and the double-layer superimposed primary support arch cover method are compared to analyze the effect of different numbers of primary supports on the stability of surrounding rock during tunnel excavation. Monitoring was carried out at 0,5,10,15,20 and 22.5 m in the longitudinal direction of the tunnel vault. Through numerical simulation of the excavation process, the settlement of each monitoring point under no support, single-layer initial support and double-layer initial support is shown in Figure 10. The displacement settlement at the vault of monitoring point 1 is the largest among all monitoring points. The arch bottom of monitoring point 2 is the position where the surrounding rock of the tunnel has the largest upward extrusion uplift displacement. After the tunnel excavation is completed, the maximum displacement of the surrounding rock appears on the connection between the vault and the arch bottom. In the settlement displacement diagram of monitoring points 3,4 and 5,6, it can be seen that the vertical displacement of the monitoring point is symmetrical about the center line of the tunnel section, indicating that the vertical displacement of the surrounding rock is symmetrically distributed, and there is no bias state in the excavation process of the tunnel. The size of the settlement at monitoring points 5 and 6 is: double-layer initial support, one-layer initial support and no support construction. The reason for this situation is that the core concept of the initial support arch cover method is to excavate the tunnel arch in the weak surrounding rock, and set up the arch cover structure with large bearing capacity to bear the load of the upper weak soil. The monitoring points 5 and 6 are located at the arch foot of the tunnel. The upper soil load is transmitted to the arch feet on both sides through the arch cover, resulting in an increase in the displacement of the surrounding rock. Through Fig.10, the following conclusions can be drawn. In terms of the effect of controlling the settlement of the tunnel vault, the double-layer initial support and the first-layer initial support have obvious advantages over the unsupported construction. With the increase of the longitudinal distance, the superiority of the double-layer initial support over the first-layer initial support gradually appears. The settlement trend of the tunnel vault longitudinal 0,5,10,15,20,22.5 m is roughly the same during the excavation process. With the increase of the longitudinal distance, the advantage of the double-layer primary support is gradually emerging compared with the one-layer primary support. The section at the middle section of the tunnel is taken as the monitoring surface to monitor the deformation of the surrounding rock during the excavation of the tunnel, that is, the section at 22.5 m. The distribution of each monitoring point on the monitoring section is shown in Fig.11. Fig.12 shows the vertical displacement curve of each monitoring point during excavation. The displacement settlement at the vault of monitoring point 1 is the largest among all monitoring points. The arch bottom of monitoring point 2 is the position where the surrounding rock of the tunnel has the largest upward extrusion uplift displacement. After the tunnel excavation is completed, the maximum displacement of the surrounding rock appears on the connection between the vault and the arch bottom. In the settlement displacement diagram of monitoring points 3,4 and 5,6, it can be seen that the vertical displacement of the monitoring point is symmetrical about the center line of the tunnel section, indicating that the vertical displacement of the surrounding rock is symmetrically distributed, and there is no bias state in the excavation process of the tunnel. The size of the settlement at monitoring points 5 and 6 is: double-layer initial support, one-layer initial support and no support construction. The reason for this situation is that the core concept of the initial support arch cover method is to excavate the tunnel arch in the weak surrounding rock, and set up the arch cover structure with large bearing capacity to bear the load of the upper weak soil. The monitoring points 5 and 6 are located at the arch foot of the tunnel. The upper soil load is transmitted to the arch feet on both sides through the arch cover, resulting in an increase in the displacement of the surrounding rock. Fig.13 shows the horizontal displacement of the monitoring points of the section. It can be seen from the figure that the horizontal displacements of the monitoring points 3,4 and 5,6 are symmetrical about the center line of the tunnel. The displacement at the arch feet on both sides is 1.6mm, which is much larger than 0.308mm at the haunch. Because of the particularity of the position of the arch feet on both sides in the double-layer composite arch cover method, the arch feet should be monitored in time during the construction process to ensure the safety and stability of the arch feet in the construction process. Conclusion ⑴ After the completion of tunnel excavation, the vertical displacement of tunnel surrounding rock is mainly concentrated in the arch bottom and vault of the tunnel. The vertical displacement of the tunnel arch bottom shows an upward uplift trend, and the maximum uplift displacement is 7.263mm. The displacement of the tunnel vault is mainly manifested as subsidence, with the maximum subsidence displacement of 14.09 mm and the maximum horizontal displacement of 5.657mm. ⑵ The actual monitoring data can be well matched with the numerical simulation data. The settlement of the tunnel vault is roughly divided into three stages: slow settlement stage, rapid settlement stage and stable settlement stage. Comparing the numerical simulation results with the tunnel monitoring data, it can be seen that the field monitoring data are in good agreement with the numerical simulation data, indicating that the reliability of the numerical simulation results is high, and the construction of the tunnel using the double-layer primary arch cover method is reasonable. ⑶ It can be seen from the comprehensive longitudinal span and section monitoring data that, in addition to the special arch foot position, the displacement of the monitoring point in both the X direction and the Z direction always satisfies : no support > single-layer initial support > double-layer initial support. This shows that support and lining can greatly limit the displacement of the surrounding rock of the tunnel. The double-layer superimposed initial support arch cover method can better reduce the deformation of the surrounding rock and increase the stability of the tunnel during excavation. 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12:32:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4055936/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4055936/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":52581119,"identity":"116feb64-7b27-41b5-84bd-9bf6a56e6406","added_by":"auto","created_at":"2024-03-13 08:27:37","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":455625,"visible":true,"origin":"","legend":"\u003cp\u003eIn-out section line\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/124ecb4292bb01355b07deb6.png"},{"id":52581118,"identity":"4d77c185-d6d7-4ce9-a592-e3b18802d2c9","added_by":"auto","created_at":"2024-03-13 08:27:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":43815,"visible":true,"origin":"","legend":"\u003cp\u003eStrata stratification\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/c7670ac43513098548887382.png"},{"id":52580939,"identity":"9f5961f7-9b12-4be3-860f-2b71a327858a","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":47923,"visible":true,"origin":"","legend":"\u003cp\u003eNumerical calculation model\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/43d9d38071626c017f578bb4.png"},{"id":52580938,"identity":"54e324db-68e6-4fdc-84c6-57ec27c66fd7","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":99511,"visible":true,"origin":"","legend":"\u003cp\u003eExcavation sequence of double-layer primary arch cover tunnel\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/b0b4dfbe6f7950126dcb9336.png"},{"id":52580948,"identity":"fe4e60c3-01f2-4598-8835-288c02582d27","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":173599,"visible":true,"origin":"","legend":"\u003cp\u003eHorizontal displacement nephogram of surrounding rock\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/24107a8c4b138cbee6c57d6a.png"},{"id":52581120,"identity":"2ec46fd9-2788-4ff9-9245-27987a799284","added_by":"auto","created_at":"2024-03-13 08:27:37","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":190696,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement cloud of surrounding rock\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/52f116b0e2165ccdd67a1ff3.png"},{"id":52580941,"identity":"aca3e657-adec-44a5-8aea-aeb959434209","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":76788,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement cloud of surrounding rock\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/b31eb43be12c5e70f9f67784.png"},{"id":52580951,"identity":"bc5de364-76b1-49a6-ae0f-c3acabdd474b","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":228246,"visible":true,"origin":"","legend":"\u003cp\u003eStress cloud diagram of tunnel surrounding rock\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/65bb61f34700654300bfa1d7.png"},{"id":52580945,"identity":"337545f6-5700-4b1c-b8ea-931ad92ad72f","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":38373,"visible":true,"origin":"","legend":"\u003cp\u003eTunnel vault and arch bottom displacement numerical simulation and monitoring data comparison\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/0fd34b329de821f1354031cc.png"},{"id":52580943,"identity":"565376de-d364-4a5d-8cc2-529b38d67a54","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":55504,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement of vault monitoring point\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/a4cf9fa5cccd4a9b7995f6e5.png"},{"id":52580950,"identity":"51086e43-91c3-43f7-b6cf-e6f979f277ed","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":80868,"visible":true,"origin":"","legend":"\u003cp\u003eMonitoring point layout of monitoring section\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/6f22c3a5ae48a5bb49bdd861.png"},{"id":52581121,"identity":"3e4bf9d6-09ca-438d-9f8e-f64786f57ab1","added_by":"auto","created_at":"2024-03-13 08:27:37","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":59808,"visible":true,"origin":"","legend":"\u003cp\u003eVertical displacement of monitoring points of cross section\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/772ab9d2e99fcdbcfd092417.png"},{"id":52580949,"identity":"05151b1b-9df5-4ce4-b6ea-cb8a75d8d73d","added_by":"auto","created_at":"2024-03-13 08:19:37","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":46338,"visible":true,"origin":"","legend":"\u003cp\u003eHorizontal displacement of monitoring points of cross section\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/3a95bc3a69d4ac86e62ca5c8.png"},{"id":61290703,"identity":"8009a685-0da3-4639-bb38-87ac85d818e1","added_by":"auto","created_at":"2024-07-29 07:16:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2194846,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4055936/v1/7b42c7f2-536c-4901-8a73-1eca14e8e1d8.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The Study on Deformation and Mechanical Properties of Surrounding Rock to Primary Arch Cover Met","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIn recent years, the construction of underground transportation facilities in China has developed rapidly. The geological conditions faced by tunnel construction are becoming more and more complex, and the tunnel section is also expanding. Large cross-section tunnels are characterized by large height, wide span, poor geological conditions and low stability of surrounding rock. Therefore, in order to ensure the construction safety of large cross-section subway tunnels, engineers proposed a double-layer superimposed initial arch cover method based on the hole pile method. The double-layer superimposed initial arch cover method is favored because of its unique double arch cover structure, which can ensure the stability of surrounding rock to a large extent when excavating large cross-section tunnels.\u003c/p\u003e\n\u003cp\u003eScholars at home and abroad have done a lot of research work on the mechanical properties of surrounding rock in large section tunnel excavation and the supporting effect of primary arch cover method, and have achieved many useful results. Zhang Fupeng et al.\u0026nbsp;\u003ca href=\"#w1\"\u003e\u003csup\u003e1\u003c/sup\u003e\u003c/a\u003e used the numerical simulation method to compare and analyze the structural stress, displacement and plastic zone range of the reverse expansion method and the traditional middle partition wall method during the tunnel excavation process based on the Baowancun tunnel project of Yang 'an second line. You Dongmei et al.\u0026nbsp;\u003ca href=\"#w2\"\u003e\u003csup\u003e2\u0026nbsp;\u003c/sup\u003e\u003c/a\u003estudied the influence of different clear distances on the stress distribution of surrounding rock of shallow buried bias tunnel excavation by means of numerical simulation with the shallow buried bias section of Swan Cave Tunnel as an engineering case. Jiang Fengguo et al.\u0026nbsp;\u003ca href=\"#w3\"\u003e\u003csup\u003e3\u003c/sup\u003e\u003c/a\u003e took the construction of Harbin urban subway tunnel as the engineering background, based on the regional geological conditions, a finite element numerical model was constructed to analyze the deformation and stress variation of surrounding rock during the construction process of double side heading method and cross middle partition wall method. Combined with on-site monitoring methods, the construction method and stability of large-section tunnels were analyzed. Xu Ping et al.\u0026nbsp;\u003ca href=\"#w4\"\u003e\u003csup\u003e4\u003c/sup\u003e\u003c/a\u003e constructed a three-dimensional numerical model of large-section tunnel under the condition of loose accumulation stratum by finite difference method, and analyzed the deformation characteristics and stress of surrounding rock and tunnel structure when excavating large-section tunnel by double-side heading method by numerical simulation. Cao Zhilin et al.\u0026nbsp;\u003ca href=\"#w5\"\u003e\u003csup\u003e5\u003c/sup\u003e\u003c/a\u003e studied the influence of unloading on the depth of pressure arch in each pilot tunnel of super-large cross-section tunnel by numerical simulation and theoretical analysis based on the construction of double-side heading method of super-large cross-section underground excavation station of Guiyang Metro Line 2. Liu et al.\u0026nbsp;\u003ca href=\"#w6\"\u003e\u003csup\u003e6\u003c/sup\u003e\u003c/a\u003e developed a large-scale assembled geomechanical model test system based on practical engineering, and used the displacement monitoring system to carry out mechanical model test research on the construction process of super-large section tunnel in complex strata. He Peng et al.\u0026nbsp;\u003ca href=\"#w7\"\u003e\u003csup\u003e7\u003c/sup\u003e\u003c/a\u003e used the improved DDARF method to simulate the deformation and failure law and crack evolution process of the surrounding rock of the shallow-buried small-distance section of the Daling tunnel, which is based on the Daling super-large section tunnel of the Southeast Second Ring Expressway in Jinan City. Sato et al.\u0026nbsp;\u003ca href=\"#w8\"\u003e\u003csup\u003e8\u003c/sup\u003e\u003c/a\u003e measured the whole process of deformation, strain and vibration velocity of surrounding rock by arranging advanced monitoring boreholes in Tono underground test tunnel in Japan, and analyzed the influence of mechanical and blasting excavation methods on the damage range. Su Daozhen et al.\u0026nbsp;\u003ca href=\"#w9\"\u003e\u003csup\u003e9\u003c/sup\u003e\u003c/a\u003e selected a number of monitoring sections as test sections for field test research in Xiaopanling Tunnel's import and export section, in view of the fact that large-section carbonized mudstone tunnels are susceptible to landslides, distortion of arches, and frequent arch changes in the course of construction. Based on the actual construction situation of a subway station in Qingdao Metro, Feng Shijie et al.\u0026nbsp;\u003ca href=\"#w10\"\u003e\u003csup\u003e10\u003c/sup\u003e\u003c/a\u003e studied and analyzed the combination form of equal thickness initial support and initial support arch cover, and put forward two construction schemes. Based on a station of Qingdao Metro, Gong\u0026nbsp;\u003ca href=\"#w11\"\u003e\u003csup\u003e11\u003c/sup\u003e\u003c/a\u003e studied the deformation law and control of the initial support arch cover method by combining numerical simulation with field monitoring data. Kong Chao et al\u0026nbsp;\u003ca href=\"#w12\"\u003e\u003csup\u003e12\u003c/sup\u003e\u003c/a\u003e studied the mechanical characteristics of the initial arch cover method in composite strata through numerical simulation, combined with data from model experiments, and analyzed the deformation law of the surrounding rock under different working conditions of the initial arch cover method. Zhao Ying et al.\u0026nbsp;\u003ca href=\"#w13\"\u003e\u003csup\u003e13\u003c/sup\u003e\u003c/a\u003e analyzed the applicability of the composite arch cover method to a station of Guiyang Rail Transit Line 2 by numerical simulation, and analyzed the surface settlement, vault settlement and plastic zone distribution of surrounding rock that may be caused by construction. Zhang Guohua et al.\u0026nbsp;\u003ca href=\"#w14\"\u003e\u003csup\u003e14\u003c/sup\u003e\u003c/a\u003e analyzed the applicability of double-layer composite arch cover method, double-side heading method and step method by numerical simulation method, combined with the engineering example of a underground excavation station in Chongqing area, and compared and analyzed the actual engineering economic indicators. Zhang et al.\u0026nbsp;\u003ca href=\"#w15\"\u003e\u003csup\u003e15\u003c/sup\u003e\u003c/a\u003e monitored the surface subsidence during the excavation of Zhongshan Park Station of Qingdao Metro Line 3 constructed by arch cover method. Shang et al.\u0026nbsp;\u003ca href=\"#w16\"\u003e\u003csup\u003e16\u003c/sup\u003e\u003c/a\u003e analyzed the influence of different support measures on the deformation of surrounding rock in the construction process of arch cover method by means of numerical simulation. The results show that the double-layer arch cover method can control the deformation of surrounding rock well and reduce the settlement of foundation. Based on a station of Qingdao Metro Line 3, Wu\u0026nbsp;\u003ca href=\"#w17\"\u003e\u003csup\u003e17\u003c/sup\u003e\u003c/a\u003e used the finite element method to simulate the construction process of the arch-cover method in the soil-rock composite stratum, and analyzed the surface and vault settlement, supporting structure and surrounding rock stress during the construction process. The results show that the arch-cover method has good applicability in the soil-rock composite stratum.\u003c/p\u003e\n\u003cp\u003eBased on the above research, it can be seen that the existing results are of great significance to the analysis of the deformation and mechanical properties of surrounding rock during the excavation of large-section tunnels. However, previous studies mainly focused on the changes of mechanical properties of surrounding rock in the process of conventional tunnel excavation methods, and there are few studies on the double-layer superimposed primary arch cover method, which is an excavation method for special strata. Therefore, based on the line section project of Dengying Depot of Qingdao Metro Line 4, this paper analyzes the variation law of surrounding rock stress during the excavation of double-layer primary arch cover method, and studies the stress deformation and stability of surrounding rock during the excavation of large section tunnel. The research results are expected to provide useful reference for the design and construction of similar engineering cases.\u003c/p\u003e"},{"header":"Numerical Modeling","content":"\u003cp\u003e\u003cstrong\u003eproject profile\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eRelying on the engineering tunnel, it belongs to the single-hole four-line tunnel of the vehicle entry and exit section line. The actual situation on site is shown in Figure 1. The total length of the tunnel is 44.5 m, the maximum diameter of the section is 30.059 m, the maximum buried depth is 26.82 m, the section area of the tunnel is more than 50 m\u003csup\u003e2\u003c/sup\u003e and the buried depth is less than 2 times the span, which belongs to the shallow buried large section tunnel. The stratum conditions of the tunnel are plain fill, moderately weathered granite, and slightly weathered granite from top to bottom. The stratum stratification is shown in Figure 2. Almost all of the tunnels are located in slightly weathered granite. The surrounding rock grades are mainly IV and V. The excavation section line of the main tunnel is the underground excavation interval, and the interval is constructed by mining method.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEstablishment of Numerical Calculation Model\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe model is established by hypermesh software and imported into FLAC3D software after meshing. The Mohr-Coulomb plastic constitutive model is used for both soil and rock, which can obtain more realistic simulation results. In order to eliminate the boundary effect, the model size is set to length * width * height = 140m * 45m * 80m. In order to facilitate the establishment of the model and the subsequent calculation, the hypermesh software is used to divide the region and group in the model establishment stage. The specific situation of the model is shown in Figure 3.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ephysical and mechanical parameters of rock and soil\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAccording to the relevant survey and design data of the surrounding rock of the tunnel, the physical and mechanical parameters of the soil layer within the excavation range are weighted average, and the weight is the thickness of each soil layer. The selected physical and mechanical parameters of rock and soil are shown in Table 1.\u003c/p\u003e\n\u003cp\u003eTable 1 Physical and mechanical parameters of rock and soil\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"579\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"19.23743500866551%\" valign=\"top\"\u003e\n \u003cp\u003estratum\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003edensity(kg\u003cstrong\u003e\u0026middot;\u003c/strong\u003em\u003csup\u003e-3\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.291161178509533%\" valign=\"top\"\u003e\n \u003cp\u003ebulk modulus (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003eshear modulus\u003c/p\u003e\n \u003cp\u003e(MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.145580589254767%\" valign=\"top\"\u003e\n \u003cp\u003ec(kPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.157712305025996%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026phi; (\u0026deg;)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"19.23743500866551%\" valign=\"top\"\u003e\n \u003cp\u003eplain fill\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e1800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.291161178509533%\" valign=\"top\"\u003e\n \u003cp\u003e22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.145580589254767%\" valign=\"top\"\u003e\n \u003cp\u003e19.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.157712305025996%\" valign=\"top\"\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"19.23743500866551%\" valign=\"top\"\u003e\n \u003cp\u003emoderately weathered rock\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e2500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.291161178509533%\" valign=\"top\"\u003e\n \u003cp\u003e8621\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e6198\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.145580589254767%\" valign=\"top\"\u003e\n \u003cp\u003e500\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.157712305025996%\" valign=\"top\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"19.23743500866551%\" valign=\"top\"\u003e\n \u003cp\u003eslightly weathered rock\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e2600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.291161178509533%\" valign=\"top\"\u003e\n \u003cp\u003e18000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.584055459272097%\" valign=\"top\"\u003e\n \u003cp\u003e10300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.145580589254767%\" valign=\"top\"\u003e\n \u003cp\u003e2000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.157712305025996%\" valign=\"top\"\u003e\n \u003cp\u003e45\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;imitating construction status\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on the actual project of Qingdao Metro, the specific excavation sequence of the double-layer primary arch cover method is shown in Figure 4. According to the on-site construction process, the single excavation length of the numerical model is 1.6m, and the whole excavation process is divided into 89 steps. Firstly, the pilot tunnel 1 and pilot tunnel 2 are excavated, and the lining and temporary side wall support are applied. Excavate the rock mass on both sides of the annular guide pit of the upper half section of the arch, and complete the first layer of the upper half section. The length of the first support of the second floor is 20 ~ 25m. After the initial support of the upper half section of the arch is completed, the core rock mass of the lower half section of the arch is excavated. It is divided into several construction sections along the longitudinal direction of the interval, and the intermediate rock mass at the lower part of the interval is excavated in sequence according to the diagram, and the initial support of the side wall is constructed in time.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNumerical simulation results\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDisplacement analysis of tunnel surrounding rock\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAfter the tunnel is excavated according to the established steps of the project, the displacement cloud diagram of the surrounding rock of the tunnel is shown in Figs.5 and 6. It can be seen from the diagram that after the normal excavation of the tunnel is completed, the vertical displacement of the surrounding rock of the tunnel is mainly concentrated at the arch bottom and vault of the tunnel. The vertical displacement of the tunnel arch bottom shows an upward uplift trend, and the maximum uplift displacement is 7.263 mm. The displacement of the tunnel vault is mainly manifested as subsidence, with the maximum subsidence displacement of 14.09 mm and the maximum horizontal displacement of 5.657 mm. The displacement of surrounding rock is approximately symmetrically distributed along the central line of the tunnel. This is mainly because the tunnel excavation destroys the original rock stress of the surrounding rock, resulting in stress redistribution of the surrounding rock. This value is far less than the standard value of subway excavation vault settlement in the specification.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAnalysis of maximum shear strain of tunnel surrounding rock\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe maximum shear strain cloud diagram of surrounding rock after tunnel excavation is shown in Fig.7.It can be seen from the figure that after the completion of tunnel excavation, the maximum shear strain of surrounding rock is mainly concentrated in the surrounding rock on both sides of the side wall and the initial support of the first layer. This shows that in the process of tunnel excavation, the shear stress of the tunnel arch foot and the side walls on both sides is the largest, and the surrounding rock is the weakest. In the process of tunnel excavation, it is necessary to strengthen the monitoring and support, which indirectly shows that the arch cover of the double-layer primary arch cover method can effectively transfer the shear stress to the surrounding rock on both sides of the tunnel arch foot and the side wall.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStress Analysis of Surrounding Rock of Tunnel\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe vertical stress cloud diagram of surrounding rock after tunnel excavation is shown in Fig.8.The stress cloud diagram is symmetrically distributed along the middle line of the tunnel, and the maximum vertical stress is 23.33 MPa. The vertical stress at the arch feet on both sides of the tunnel is opposite to the vertical stress of most surrounding rock. This is because the principle of the initial arch cover method is to transfer the stress of the surrounding rock at the upper part of the tunnel to the arch feet on both sides through the arch cover, so as to ensure the stability of the surrounding rock during the construction of the tunnel. Therefore, the monitoring and protection of the arch foot on both sides of the tunnel should be strengthened when using the primary arch cover method for tunnel construction, and measures should be taken to ensure its stability during excavation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eComparative Analysis of Numerical Simulation and Field Monitoring Data\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMonitoring and measuring the data in the process of tunnel construction is an indispensable part of tunnel construction, and it is the key to the information design and construction of the tunnel. The monitoring data can be used as the basis for judging the stability of surrounding rock and the reliability of supporting structure. According to the relevant specifications, the deformation of the surrounding rock of the tunnel is monitored. It can be seen from Figs.5 and 6 that the displacement of the tunnel vault and the arch bottom is the largest area of the vertical displacement of the surrounding rock during the entire tunnel excavation process and after the excavation is completed, and the stress is more concentrated. Therefore, the vault settlement and the arch bottom uplift value of the tunnel are mainly monitored. The on-site monitoring data of the vault and arch bottom settlement at the monitoring section are compared with the numerical simulation vault settlement, as shown in Fig.9.\u003c/p\u003e\n\u003cp\u003eFigure 9 (a) is the vault settlement curve. As shown in the figure, in the numerical simulation process, when the number of excavation steps is 0-14 steps, the tunnel vault displacement value increases slowly. This is because when the tunnel is excavated to the 14th step, the first part of the rock mass of the upper half section of the tunnel is excavated to 22.4m, which does not reach the location of the monitoring point of 22.5 m. When the number of excavation steps is 14-38 steps, the vault settlement increases rapidly to 11.3mm. As the excavation process continues, the vault displacement continues to remain in a relatively stable state. According to the monitoring data, when the monitoring days are 0 ~ 15 days, the vault settlement remains in a relatively stable state, and the displacement value increases slowly. When the monitoring days are 15 ~ 38 days, the vault settlement increases rapidly. At 39 ~ 90 days, the settlement displacement value remained basically unchanged. Figure 9 (b) is the displacement curve of tunnel arch bottom. As shown in the figure, in the process of numerical simulation, when the number of excavation steps is 0-14 steps, the uplift value of tunnel arch bottom increases slowly. When the number of excavation steps is 14-38 steps, the uplift value of the tunnel arch bottom increases rapidly to 5.47mm. As the excavation process continues, the uplift value of the arch bottom continues to remain in a relatively stable state. According to the monitoring data, when the monitoring days are 0-15 days, the uplift value of the tunnel arch bottom remains in a relatively stable state, and the displacement value basically increases slowly. When the monitoring days are 15 ~ 38 days, the uplift value of the tunnel arch bottom increases rapidly. During 39 ~ 90 days, the uplift displacement value remained basically unchanged.\u003c/p\u003e\n\u003cp\u003eThrough comparative analysis, it is found that the two curves have almost the same change trend, which can be roughly divided into three stages: slow deformation, rapid deformation and basic stability. The maximum settlement value of the vault is also not much different. The field monitoring data and the numerical simulation data maintain correspondence in magnitude and time-space effect. However, there are still some differences in the shape of the two curves. This is because the tunnel adopts the mine method for blasting construction. The blasting will cause certain disturbance to the monitoring points and cause certain measurement errors. Moreover, the monitoring points of the tunnel are welded on the arch frame, and the tunnel excavation is easy to cause the deformation of the arch frame. The deformation of the arch frame is elastic deformation, so the grid steel frame also has a certain influence on the monitoring data of the monitoring points.\u003c/p\u003e\n\u003cp\u003eComparative study of double-layer primary arch cover method and traditional arch cover method\u003c/p\u003e\n\u003cp\u003eThe traditional arch cover method is widely used because it can achieve better results for special strata. The double-layer primary support arch cover method is optimized based on its construction experience, that is, it has one more layer of primary support than the traditional arch cover method. Therefore, the unsupported excavation, the traditional arch cover method and the double-layer superimposed primary support arch cover method are compared to analyze the effect of different numbers of primary supports on the stability of surrounding rock during tunnel excavation.\u003c/p\u003e\n\u003cp\u003eMonitoring was carried out at 0,5,10,15,20 and 22.5 m in the longitudinal direction of the tunnel vault. Through numerical simulation of the excavation process, the settlement of each monitoring point under no support, single-layer initial support and double-layer initial support is shown in Figure 10.\u003c/p\u003e\n\u003cp\u003eThe displacement settlement at the vault of monitoring point 1 is the largest among all monitoring points. The arch bottom of monitoring point 2 is the position where the surrounding rock of the tunnel has the largest upward extrusion uplift displacement. After the tunnel excavation is completed, the maximum displacement of the surrounding rock appears on the connection between the vault and the arch bottom. In the settlement displacement diagram of monitoring points 3,4 and 5,6, it can be seen that the vertical displacement of the monitoring point is symmetrical about the center line of the tunnel section, indicating that the vertical displacement of the surrounding rock is symmetrically distributed, and there is no bias state in the excavation process of the tunnel.\u003c/p\u003e\n\u003cp\u003eThe size of the settlement at monitoring points 5 and 6 is: double-layer initial support, one-layer initial support and no support construction. The reason for this situation is that the core concept of the initial support arch cover method is to excavate the tunnel arch in the weak surrounding rock, and set up the arch cover structure with large bearing capacity to bear the load of the upper weak soil. The monitoring points 5 and 6 are located at the arch foot of the tunnel. The upper soil load is transmitted to the arch feet on both sides through the arch cover, resulting in an increase in the displacement of the surrounding rock.\u003c/p\u003e\n\u003cp\u003eThrough Fig.10, the following conclusions can be drawn. In terms of the effect of controlling the settlement of the tunnel vault, the double-layer initial support and the first-layer initial support have obvious advantages over the unsupported construction. With the increase of the longitudinal distance, the superiority of the double-layer initial support over the first-layer initial support gradually appears. The settlement trend of the tunnel vault longitudinal 0,5,10,15,20,22.5 m is roughly the same during the excavation process. With the increase of the longitudinal distance, the advantage of the double-layer primary support is gradually emerging compared with the one-layer primary support.\u003c/p\u003e\n\u003cp\u003eThe section at the middle section of the tunnel is taken as the monitoring surface to monitor the deformation of the surrounding rock during the excavation of the tunnel, that is, the section at 22.5 m. The distribution of each monitoring point on the monitoring section is shown in Fig.11. Fig.12 shows the vertical displacement curve of each monitoring point during excavation.\u003c/p\u003e\n\u003cp\u003eThe displacement settlement at the vault of monitoring point 1 is the largest among all monitoring points. The arch bottom of monitoring point 2 is the position where the surrounding rock of the tunnel has the largest upward extrusion uplift displacement. After the tunnel excavation is completed, the maximum displacement of the surrounding rock appears on the connection between the vault and the arch bottom. In the settlement displacement diagram of monitoring points 3,4 and 5,6, it can be seen that the vertical displacement of the monitoring point is symmetrical about the center line of the tunnel section, indicating that the vertical displacement of the surrounding rock is symmetrically distributed, and there is no bias state in the excavation process of the tunnel.\u003c/p\u003e\n\u003cp\u003eThe size of the settlement at monitoring points 5 and 6 is: double-layer initial support, one-layer initial support and no support construction. The reason for this situation is that the core concept of the initial support arch cover method is to excavate the tunnel arch in the weak surrounding rock, and set up the arch cover structure with large bearing capacity to bear the load of the upper weak soil. The monitoring points 5 and 6 are located at the arch foot of the tunnel. The upper soil load is transmitted to the arch feet on both sides through the arch cover, resulting in an increase in the displacement of the surrounding rock.\u003c/p\u003e\n\u003cp\u003eFig.13 shows the horizontal displacement of the monitoring points of the section. It can be seen from the figure that the horizontal displacements of the monitoring points 3,4 and 5,6 are symmetrical about the center line of the tunnel. The displacement at the arch feet on both sides is 1.6mm, which is much larger than 0.308mm at the haunch. Because of the particularity of the position of the arch feet on both sides in the double-layer composite arch cover method, the arch feet should be monitored in time during the construction process to ensure the safety and stability of the arch feet in the construction process.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003e⑴ After the completion of tunnel excavation, the vertical displacement of tunnel surrounding rock is mainly concentrated in the arch bottom and vault of the tunnel. The vertical displacement of the tunnel arch bottom shows an upward uplift trend, and the maximum uplift displacement is 7.263mm. The displacement of the tunnel vault is mainly manifested as subsidence, with the maximum subsidence displacement of 14.09 mm and the maximum horizontal displacement of 5.657mm.\u003c/p\u003e \u003cp\u003e⑵ The actual monitoring data can be well matched with the numerical simulation data. The settlement of the tunnel vault is roughly divided into three stages: slow settlement stage, rapid settlement stage and stable settlement stage. Comparing the numerical simulation results with the tunnel monitoring data, it can be seen that the field monitoring data are in good agreement with the numerical simulation data, indicating that the reliability of the numerical simulation results is high, and the construction of the tunnel using the double-layer primary arch cover method is reasonable.\u003c/p\u003e \u003cp\u003e⑶ It can be seen from the comprehensive longitudinal span and section monitoring data that, in addition to the special arch foot position, the displacement of the monitoring point in both the X direction and the Z direction always satisfies : no support\u0026thinsp;\u0026gt;\u0026thinsp;single-layer initial support\u0026thinsp;\u0026gt;\u0026thinsp;double-layer initial support. This shows that support and lining can greatly limit the displacement of the surrounding rock of the tunnel. The double-layer superimposed initial support arch cover method can better reduce the deformation of the surrounding rock and increase the stability of the tunnel during excavation.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eHao Ziyi, the sole author, completed all the content independently.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eZHANG F P, LEI S Y, YANG R et al. Numerical simulation of the construction of forked tunnels with super small spacing[J]. China Science paper, \u003cstrong\u003e14 (02)\u003c/strong\u003e:157-163 + 209(2019).\u003c/li\u003e\n \u003cli\u003eYOU D M, XU L S, CHEN H. Numerical simulation of shallow buried biasing double-hole tunnel[J]. China Science paper, \u003cstrong\u003e15(10)\u003c/strong\u003e:1132-113(2020).\u003c/li\u003e\n \u003cli\u003eJIANG F G, BAI L L, SONG M et al. Analysis on construction stability of large section tunnel of Harbin City Metro[J]. Journal of Jilin University\u0026nbsp;（Engineering and Technology Edition）,\u0026nbsp;\u003cstrong\u003e50(04)\u003c/strong\u003e:1419-1427(2020).\u003c/li\u003e\n \u003cli\u003eXU P, YANG X Z, LI Y S, et al. Study on Adaptability of Double Side Drift Method in Construction of Large⁃section Tunnels in Accumulation Strata[J].Modern Tunnelling Technology,\u003cstrong\u003e\u0026nbsp;59(S1)\u003c/strong\u003e:803-810(2022).\u003c/li\u003e\n \u003cli\u003eCAO Z L, XIE Q, SONG Z P et al. Effect of unloading on surrounding rock pressure of super large-section tunnel[J]. Chinese Journal of Rock Mechanics and Engineering, \u003cstrong\u003e39(S1)\u003c/strong\u003e:2882-2891(2020).\u003c/li\u003e\n \u003cli\u003eLIU C, LI S C, ZHOU Z Q et al. Model test on mechanical characteristics of surrounding rock during construction process of super-large section tunnel in complex strata[J]. Rock and Soil Mechanics, \u003cstrong\u003e39(09)\u003c/strong\u003e:3495-3504(2018).\u003c/li\u003e\n \u003cli\u003eHE P, LI S C, LI L P et al. Discontinuous deformation of surrounding rock for small-space tunnel with super-large section in jointed rock mass[J]. Chinese Journal of Geotechnical Engineering, \u003cstrong\u003e40(10)\u003c/strong\u003e:1889-1896(2018).\u003c/li\u003e\n \u003cli\u003eSATO T, KIKUCHI T, SUGIHARA K. In-situ experiments on an excavation disturbed zone induced by mechanical excavation in Neogene sedimentary rock at Tono mine, central Japan[J]. Engineering Geology, \u003cstrong\u003e56(1/2)\u003c/strong\u003e: 97\u0026ndash;108(2000).\u003c/li\u003e\n \u003cli\u003eSU D Z, LUO J J. Experimental study and prediction on large section tunnel construction deformation of surrounding rock in soft ground[J]. Chinese Journal of Rock Mechanics and Engineering,\u003cstrong\u003e\u0026nbsp;35(S2)\u003c/strong\u003e:4029-4039(2016).\u003c/li\u003e\n \u003cli\u003eFENG S J, GAO X, LI Q F et al. Research on Double-layer Primary Support Composite Arch-Cover Structure Combination Form[J]. Urban Mass Transit, \u003cstrong\u003e25(07)\u003c/strong\u003e:152-156(2022).\u003c/li\u003e\n \u003cli\u003eGONG X D. Deformation Pattern and Control of Qingdao Metro Station Adopting Primary Arch Cover Construction Method in Upper Soft and Lower Hard Stratum[J]. Urban Mass Transit, \u003cstrong\u003e24(06)\u003c/strong\u003e:109-114(2021).\u003c/li\u003e\n \u003cli\u003eKONG C, GAO X Q, YAO Y, et al. The deformation and mechanical properties of surrounding rock of primary support arch-cover construction method in upper-soft and lower-hard stratum[J]. 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Monitoring and analysis of excavation induced subsidence of subway station constructed and control measures[J] Chinese Journal of Rock Mechanics and Engineering, \u003cstrong\u003e31(S1)\u003c/strong\u003e:3413-3420(2012).\u003c/li\u003e\n \u003cli\u003eSHANG Y L，DU S J，HAN T，Y et al.Case study on deformation contro ol of upper-soft and lower-hard large span tunnel station using combined control technology and monitoring demonstration[J]Sains Malaysiana, \u003cstrong\u003e46(11)\u003c/strong\u003e:2091-2099(2017).\u003c/li\u003e\n \u003cli\u003eWU X F, LV W J, ZHANG L M et al. Stability Analysis and Support Optimization of Surrounding Rock for Shallow Embedded Subway Station[J]. Chinese Journal of Underground Space and Engineering, \u003cstrong\u003e8(05):\u003c/strong\u003e1059-1064(2012).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Tunnel engineering, surrounding rock deformation, numerical simulation, large section tunnel, initial support arch cover method","lastPublishedDoi":"10.21203/rs.3.rs-4055936/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4055936/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn order to explore the stress and deformation mechanism of surrounding rock in the construction process of the primary arch cover method, the finite difference method is used to simulate the construction process of the primary arch cover method of shallow buried large section hard rock tunnel. Combined with the monitoring data of the project site, the stress and deformation law of the tunnel surrounding rock under the influence of time and space effect are analyzed. In addition, the excavation process of the double-layer superimposed primary arch cover method and the traditional arch cover method is compared. Based on the stress and deformation characteristics of the tunnel surrounding rock, the technical advancement of the double-layer primary arch cover method is quantified. The results show that after the tunnel excavation is completed, the vertical displacement of the tunnel surrounding rock is mainly concentrated in the vault and arch bottom of the tunnel. The maximum uplift displacement is 7.263 mm and the maximum subsidence displacement is 14.09 mm. The settlement of the tunnel vault is roughly divided into three stages: slow settlement, rapid settlement and settlement stability; it can be seen from the comprehensive longitudinal span and section monitoring data that the double-layer superimposed primary arch cover method can better reduce the deformation of surrounding rock and increase the stability of the tunnel during excavation.\u003c/p\u003e","manuscriptTitle":"The Study on Deformation and Mechanical Properties of Surrounding Rock to Primary Arch Cover Met","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-13 08:19:32","doi":"10.21203/rs.3.rs-4055936/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6e94871f-42ce-4e6a-b1e2-277914392e9d","owner":[],"postedDate":"March 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":29313467,"name":"Physical sciences/Engineering/Civil engineering"},{"id":29313468,"name":"Earth and environmental sciences/Solid earth sciences/Geophysics"},{"id":29313469,"name":"Earth and environmental sciences/Solid earth sciences/Petrology"}],"tags":[],"updatedAt":"2024-07-29T07:08:06+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-13 08:19:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4055936","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4055936","identity":"rs-4055936","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}