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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39300132
72709
10.1038/s41598-024-72709-0
Article
Design and application of a prefabricated structure for large-span arch open-cut highway tunnels
Lin Zhi 622220970126@mails.cqjtu.edu.cn

1
Liu Lili 3075397035@qq.com

1
Hai Dapeng 1753858518@qq.com

2
Lu Wanqing 2
Yang Chaoce 3
Xu Zhanglong 622160082024@mails.cqjtu.edu.cn

1
1 https://ror.org/01t001k65 grid.440679.8 0000 0000 9601 4335 State Key Laboratory of Bridge and Tunnel Engineering in Mountainous Areas, Chongqing Jiaotong University, Chongqing, 400074 China
2 China Construction Seventh Engineering Bureau Co., Ltd, Zhengzhou, 450003 China
3 Chongqing High-tech Zone Construction Bureau, Chongqing, 401329 China
19 9 2024
19 9 2024
2024
14 2189217 3 2024
10 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
To enhance construction efficiency, improve lining quality, and reduce project costs, this study proposes a prefabricated arch structure scheme for large-span open-cut highway tunnels, specifically targeting the ongoing construction of an eight-lane open-cut tunnel on Chongqing’s Xinsen Avenue. By establishing a finite element “load-structure” integrated calculation model and adopting an iterative analysis method, the impact of joint stiffness variations on internal force distribution was determined. Combined with long-term field monitoring and measurements, a thorough analysis was conducted on the performance of the prefabricated arch structure during different construction stages. The results indicate that after assembly into a ring, the structure undergoes a self-adaptive adjustment phase, during which the joints experience significant stress and deformation. Following top-layer backfilling and water level restoration, the axial force on the joints increases, while stress and deformation decrease, exerting a positive effect on the joint bearing capacity. From the construction period to post-opening operations, the stress variation experienced by the joints remains within a controllable range, demonstrating substantial safety margins. This fully validates the safety and reliability of the prefabricated arch structure scheme.

Subject terms

Civil engineering
Engineering
http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 52078089 52274176 Lin Zhi Chongqing Natural Science Foundation Innovation and Development Joint FundCSTB2022NSCQ-LZX0079 Lin Zhi Research and Innovation Program for Graduate Students in ChongqingCYB23242 Lin Zhi China Construction Seventh Bureau Science and Technology R & D ProjectCSCEC7B-2022-Z-19 2022 Construction Science and Technology Plan Project-Group 1-New Urban Infrastructure Construction(CKZ No. 2022-1-13 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

As China’s population ages and the demographic dividend diminishes, a significant reduction in the number of young and middle-aged laborers at construction sites has resulted in a sharp increase in labor costs, thereby increasing project costs1. Traditional construction methods are plagued by issues such as high labor intensity, extensive material consumption, high carbon emissions, minimal mechanization, inconsistent quality, slow construction speeds, and high vulnerability to natural conditions2. Consequently, an imperative transformation within the construction engineering industry is underway, compelling the comprehensive promotion of new urban construction, advancements in intelligent construction, the utilization of green building materials, and prefabricated construction to achieve the national policy goals ofdual carbon control.

The prefabricated structure demonstrates unique advantages in the construction field due to its notable characteristics of high efficiency, low carbon, and environmental friendliness3,4. Among them, the connection joints of prefabricated components are the core of the prefabricated structure, influencing not only the overall mechanical properties and load-bearing capacity of the structure5, but also being closely related to the manufacturing process of components, construction and assembly efficiency, as well as the waterproof and sealing performance of the structure. For the underground structure of prefabricated open-cut tunnels, this paper adopts the grouting tenon-and-groove joint to promote more efficient and precise rapid assembly of prefabricated structures6,7.

This paper takes Xinsen Avenue in Jinfeng Park of the Chongqing High-tech Zone as the background. The road is the main urban road in the north‒south direction of the park, with a total length of 2.5 km, including a double-arch eight-lane tunnel. The tunnel has a total length of 790 m, a net width of 16.5 m, and a net height of 5 m. The starting section crosses the third-stage tunnel of Gaoteng Avenue and connects with the reconstruction interchange of Gaolong Avenue after leaving the tunnel. The tunnel construction method combines underground excavation and open excavation: K1 + 065 ~ K1 + 255 is the first section of underground excavation, which is 190 m long K1 + 700 ~ K1 + 855 is the second section of underground excavation, which is 155 m long; and the remainder is the open-cut section, which is 445 m long. The section is designed with an assembled lining, and the filling thickness is 8.5–11.5 m, as shown in Fig. 1.

Fig. 1 Plan layout of the Xinsen Avenue Tunnel in Chongqing.

The standard ring of the prefabricated tunnel in the open-cut tunnel is divided into 2 m rings in the longitudinal direction, including 208 rings on the left and 209 rings on the right, for a total of 417 rings. As shown in Fig. 2, the lining structure adopts a design scheme combining partial prefabrication and cast-in-place methods. The inverted arch adopts the cast-in-place scheme (C block), the arch and the sidewall are composed of prefabricated components, the prefabricated part is divided into 2 blocks (A, B blocks), and the closed-cavity thin-walled prefabricated assembly structure type is adopted8–10 In the circumferential and longitudinal directions, the prefabricated components are connected by grouting mortise joints. The concave and convex tenon and mortise are set at the joint, and the joint is bridged by pouring the slurry into the joint gap to transfer the occlusal shear force and strengthen the bending resistance and anti-deformation ability to ensure the reliability of the structural force transmission.

Fig. 2 Standard ring component diagram.

The open-cut prefabricated structure system is complex, and the structural system changes continuously during construction, significantly affecting the force-bearing, deformation, and stability of the structure. Several factors influence this, including (1) the interaction between joints and structure, strata and structure, and the foundation pit support structure and the prefabricated structure; (2) continuous changes in the structural system during construction, along with changes in backfill, hydrostatic pressure, and other load effects; and (3) changes in load effects during use, such as internal usage loads, earthquake effects, and civil defense load effects. Therefore, calculating and analyzing the overall effect of the structure under the influence of multiple factors is particularly important.

Currently, monitoring efforts for prefabricated structures, such as shield tunnels and underground utility corridors, primarily focus on comprehensive performance monitoring and in-depth analysis of overall structural behavior11–13. Regarding subway stations, Peng et al.14. conducted extensive research on the mechanical characteristics of prefabricated underground station structures during the backfilling stage. They established an incremental method structural calculation model to explore the effects of different conditions on the internal force distribution and stability of these structures. Additionally, Lin et al.15–17, using long-term field monitoring data from prefabricated stations in the Changchun subway, meticulously analyzed the performance and behavior changes of arch joints during various construction stages. Furthermore, Xiao et al.18 combined on-site monitoring with numerical simulation to systematically study the stress characteristics of prefabricated urban utility tunnels under different conditions, identifying potential risk areas and possible failure modes. Hak Joon Kim et al.19 by comparing field-measured data with numerical analysis results, conducted an in-depth analysis of the mechanical behavior of three-hinge prefabricated arch structures at different stages, emphasizing the influence of soil-structure interaction on structural performance. These studies provide valuable references and guidance for the design, construction, and operation of prefabricated underground structures.

Therefore, based on experimental research findings of prefabricated station structures in Changchun subway and integrating results from numerical theoretical analysis20–22, along with long-term on-site monitoring measurements, this study extensively discusses the trends of stress distribution, relative deformation, and relative rotation angle of tunnel joints over the construction process. It provides empirical data on the performance of prefabricated structures under actual loading conditions. This research not only validates the effectiveness and reliability of prefabricated assembly techniques in large-span underground structures but also offers essential technical references for similar project designs and constructions.

Calculation method based on prototype joint tests

The calculation of prefabricated structure joints is based on extensive theoretical analysis, supplemented by experimental research results from various types of 1:1 scale prototype joints23–29. The bending test loading method is shown in Fig. 3, where the axial force is provided by the lateral force N, and the bending moment is provided by the vertical force FM. Shear tests with separately controlled shear force and axial force can be conducted by adjusting the movable pads of the control device, with the shear force provided by the vertical force FM. The experiment explored the mechanical behavior, stiffness, load-bearing capacity, reasonable construction, grouting mechanism, and waterproof characteristics of joint structures, mastering the key mechanical behavior patterns of joint structures, the constitutive relationships of the joints, and forming the design and verification methods for the load-bearing capacity of joint structures.

Fig. 3 The schematic diagram of the bending loading mode.

The grouting mortise joint exhibits variable stiffness, and its characteristics are significantly influenced by the axial force and bending moment. The stiffness values for the joint are derived from test curves, as depicted in Fig. 4a29. These test curves illustrate the performance variations of the joints under diverse axial forces and bending moments, providing a scientific foundation for their design and verification.

To match the axial force and bending moment and achieve stable joint stiffness, a multi-iteration calculation method is adopted. The iterative process is as follows: first, assume that the joint is rigid, i.e., the existence of the joint is ignored, and the initial internal forces of the continuous structure are calculated to determine the initial stiffness of the joint; then, the initial stiffness is used for the overall structural analysis of the joint, and the stiffness is adjusted based on the internal forces of each joint. Through this cyclic iteration, stable and matched internal force and stiffness data are finally obtained, and the final internal force values of the structure are determined. The iteration of the joint stiffness can be found in the principle shown in Fig. 4b30. After approximately three iterations, a stable joint stiffness value with an error of less than 5% relative to the actual value can be obtained.

Fig. 4 Variation diagram of joint stiffness.

When analyzing the load-bearing capacity of the joints, the experiment revealed their load-bearing characteristic curves31 (Fig. 5a), presenting four stages: the linear phase (I), quasilinear phase (II), nonlinear phase (III), and ultimate load-bearing phase (IV), with the proportion of each stage shown in Fig. 5b. The four inflection points correspond to the appearance of cracks (inflection point 1), crack development (inflection point 2), tenon crack penetration (inflection point 3), and structural crack penetration (inflection point 4). In phases I and II, the resistance to bending mainly comes from the resistance moment; when the bending moment exceeds the resistance moment, the tenon begins to bear the bending load, cracks gradually connect, deformation accelerates, and the joint softens. Eventually, the tenon cracks penetrate, and the structural cracks connect, leading to structural failure.

Fig. 5 Schematic diagram of joint test characteristics.

Load-structure method model

Principle of the load-structure method

The load-structure method, commonly used for two-dimensional analysis of underground structures, is widely applied due to its simple model, clear force concept, and ease of directly obtaining structural deformation and internal force parameters (such as axial force, shear force, and bending moment) for section design. This model has a long history of application and rich practical experience in the field of tunnel engineering. Numerous engineering cases have validated its accuracy in predicting tunnel lining loads under various conditions, providing us with a reliable theoretical foundation and reference basis. In the load-structure model of prefabricated open-cut tunnel structures, the structure acts as the main load-bearing entity, while the strata serve both as the source of loads and as the source of elastic support, constraining the deformation of the structure; the interaction between the structure and the strata is manifested through the constraints imposed on the structure by the elastic supports. According to the calculation method of the surrounding rock pressure in Appendix G of the “Code for Design of Road Tunnel” (JTG D70-1-2018), Fig. 6 shows a schematic diagram of the load-structure model.

Fig. 6 Schematic of the load-structure model.

1 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:k=K\bullet\:L\bullet\:d$$\end{document}

where k represents the stiffness of the foundation springs in compression and shear (kN/m), K is the coefficient of subgrade reaction (kN/m3), L denotes the spacing between concentrated foundation springs (m), and d is the calculation length of the soil layer along the longitudinal direction of the underground structure (m).

Beam-spring model

In the load-structure model, the joints of the assembled tunnel structure are simulated by beam-spring elements. The prefabricated component is simulated by a variable cross-section beam, and the joint is simulated by a rotating spring, an axial spring, and a shear spring. The model accurately calculates the deformation and internal force of the assembled open-cut tunnel structure by introducing the mechanical parameters of the joints, such as bending stiffness (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{K}_{\theta\:}$$\end{document}), axial stiffness (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{K}_{n}$$\end{document}) and shear stiffness (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{K}_{s}$$\end{document}) (see Fig. 7).

Fig. 7 Joint beam-spring model.

Analysis model formulation

When using GTS NX simulation software for modeling and analysis, as shown in Fig. 8a, the two-dimensional load-structure model of the prefabricated structure is based on a single ring width of 2 m as the unit structural parameter. The main contact parameters in the model include the following: (1) Backfill plain concrete, plain concrete backfill is used in the gap between the two holes and the side of the tunnel near the slope. (2) Other contact parameters, according to the specific situation of the surrounding rock of the tunnel, the elastic resistance coefficient of the filling part (vault) is k = 10 MPa/m, the elastic resistance coefficient of the undisturbed soil part is k = 30 MPa/m, and the elastic resistance coefficient of the inverted arch part (rock layer part) is 100 MPa/m.

The structural elements are modeled according to the closed-cavity thin-walled components used in actual engineering, which correspond to both solid and hollow sections, as shown in Fig. 8b.

Fig. 8 FEM finite element analysis (Image generated using MIDAS GTS NX, version 2019).

Calculation results andanalysis

Calculation conditions

This study mainly considers three unfavorable working conditions in the long-term use of tunnels to calculate the load effect. Condition 1: both sides of the tunnel sidewall are rocks, and the maximum height of the upper filling is approximately 10.0 m; Condition 2: the two sides of the tunnel sidewall are in situ soil, and the maximum height of the upper filling is approximately 8.5 m; Condition 3: the two sides of the tunnel sidewall are in situ soil, and the maximum height of the upper filling is approximately 11.5 m; then, considering the unfavorable conditions of the three working conditions, the envelope design is carried out.

For the load-structure model (see Fig. 6), the surrounding rock pressure values of the lining structure under various working conditions are listed in Table 1.

Table 1 Calculation values of the surrounding rock pressure.

Calculation conditions	Vertical distribution pressure
q/(kN/m)	The vertical distribution pressure on both sides of the middle partition wall
qz/(kN/m)	Horizontal distribution pressure on both sides of the lining
e/(kN/m)	
External	Internal	
E1	E2	e1	e2	
Condition 1	220	120	57.2	124.4	57.2	88.4	
Condition 2	190	120	62.7	148.5	62.7	102.3	
Condition 3	250	120	82.5	168.3	82.5	122.1	

Joint parameters

Through multiple iterations of the joint stiffness under each working condition and its stress environment, the final stiffness value (2 m ring width) is shown in Table 2.

Table 2 Final joint stiffness values.

Calculation conditions	Bending stiffness \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${K}_{\theta\:}$$\end{document} (kN m/rad)	Tangential stiffness \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${K}_{n}$$\end{document} (kN/m)	Rotating angle θ (rad)	
Condition 1	AB joint	1.85E + 06	1.17E + 07	1.26E−03	
AC joint	1.95E + 06	1.05E + 07	1.21E−03	
BC joint	2.04E + 06	1.05E + 07	1.81E−03	
Condition 2	AB joint	1.82E + 06	1.17E + 07	1.12E−03	
AC joint	1.77E + 06	1.05E + 07	1.17E−03	
BC joint	1.87E + 06	1.05E + 07	1.78E−03	
Condition 3	AB joint	2.02E + 06	1.17E + 07	1.39E−03	
AC joint	1.91E + 06	1.05E + 07	1.53E−03	
BC joint	2.02E + 06	1.05E + 07	2.19E−03	

Calculation results

Structural internal force calculations and verification.

Three distinct surrounding rock loads are applied to the open cut assembled tunnel to assess the lining’s response under varied load conditions. Due to space constraints, only the internal force diagram for working condition 1 is presented. Figure 9 shows the bending moment, axial force, and shear force.

Fig. 9 Prefabricated tunnel internal force diagram (Image generated using MIDAS GTS NX, version 2019).

The analysis indicates that according to Fig. 9a, bending moments are primarily concentrated at the sidewalls, arch feet, and arch crown, with the BC joint sustaining the maximum bending moment. This phenomenon could stem from disparities in the stiffness of the backfill materials on both sides of the tunnel, which may originate from variations in the type of backfill materials, degree of compaction, or unevenness during the construction process. Figure 9b reveals that the axial force distribution is relatively uniform, primarily manifested as a compressive state, indicating that the tunnel’s main structure exhibits excellent integrity and load-bearing capacity when subjected to pressure from the surrounding rock and overlying strata. Additionally, Fig. 9c demonstrates that shear forces are predominantly concentrated at the arch haunches and arch feet. The higher shear force values, attributed to the stiffness differences in the backfill materials on both sides of the tunnel, reflect the discontinuity in internal force distribution and stress concentration phenomena. These findings underscore the significant impact of the uniformity of backfill materials on the internal force distribution of tunnel structures, emphasizing the need for attention during design and construction to ensure structural stability and safety.

To facilitate the acquisition of analysis results at key positions of the structural system, a total of 32 sets of key sections were defined at corresponding locations on the structural model, as shown in Fig. 10.

Fig. 10 Checking node location diagram (Image generated using MIDAS GTS NX, version 2019).

According to the “Code for Design of Road Tunnel”, when e0 ≤ 0.2 h, the bearing capacity is controlled by the compressive strength, and the lining structure is in a state of minor eccentric compression. When e0 ≥ 0.2 h, the bearing capacity is controlled by the tensile strength, and the lining is in a state of significant eccentric tension. The strength safety factor of the lining structure represents the safety state of the lining. When in a minor eccentric compression state, it is calculated using Eq. (2), and in a significant eccentric tension state, it is calculated using Eq. (3).2 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$KN=\phi\:\alpha\:{R}_{a}bh$$\end{document}

3 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$KN=\phi\:\frac{1.75{R}_{1}bh}{\frac{6{e}_{0}}{h}-1}$$\end{document}

where K is the safety factor; N is the axial force; Ra is the ultimate compressive strength of the concrete; R1 is the ultimate tensile strength of the concrete; α is the eccentricity influence coefficient; b and h are the width and thickness of the section, respectively; and ϕ is the longitudinal bending coefficient.

During the tunnel support structure design process, the safety factor serves as a crucial metric for evaluating the tunnel’s safety condition and devising suitable measures. An analysis of the forces and eccentricities across 32 key sections of the lining structure reveals the safety factors, as shown in Fig. 11. The findings show that the safety factors at the arch feet and crown are relatively low. At the arch crown (Sect. 1), a joint section, there is a concentration of internal force distributions. Furthermore, the arch foot (Sect. 15), acting as a convergence zone for vertical and lateral forces, exhibits greater internal forces than adjacent locations, aligning with phenomena depicted in previously discussed stress contour maps.

Fig. 11 Safety factor radar.

2. Joint load-bearing verification.

As shown in Fig. 12, the calculation results indicate that the design values are all below the limit of the linear phase, meaning that the internal forces and deformations of the joints are within the linear stage of the load-bearing characteristic curve. The design values for the bending load-bearing capacity have a considerable safety margin from the load-bearing limit. This analysis demonstrates that under various conditions, the assembled structure exhibits reliable load-bearing performance, providing solid technical support and assurance for the design and construction of large-span open-cut arch tunnels.

Fig. 12 Schematic diagram of joint bending load-bearing verification.

Insitu monitoring of assembled structural joints

Construction stage

The monitoring period starts from the formation of the arch ring with the prefabricated components A and B and lasts until the tunnel is officially open to traffic, totaling 7 months. The key backfilling steps during the monitoring period are as follows: backfilling the arch foot support material in Area I, backfilling the evenly distributed load material in Area II, backfilling the manually compacted top arch material in Area III, and finally, backfilling with topsoil for landscaping and the long-term usage stage. A diagram of the backfilling construction steps is shown in Fig. 13.

Fig. 13 Schematic diagram of construction stage(Illustration created by Lili Liu using CAD, version 2024).

Monitoring scheme

The Xin Sen Avenue prefabricated open-cut tunnel underwent comprehensive monitoring and measurement for the first time. Monitoring commenced with the assembly of prefabricated components A and B at the crown to form a ring, continuing throughout the construction period. It spanned from the backfilling of concrete at the sidewalls to soil over the crown, extending three months post official opening for traffic, totaling 6 months. The monitored tunnel length was 790 m, with 445 m constituting the prefabricated assembly section. The arrangement consisted of 2-m-wide prefabricated components, totaling 417 rings, as shown in Fig. 14. Test Sect. 2 was situated between rings 206 and 207 in the left line assembly section of the tunnel.

Fig. 14 Test ring distribution diagram. (Illustration created by Zhanglong Xu using CAD, version 2024)

At the arch crown AB joint, a variety of instruments—concrete strain gauges, rebar stress meters, crack meters, inclinometers, and anchor dynamometers—are deployed for joint performance monitoring. The configuration of these monitoring instruments is illustrated in Fig. 15. Strain gauges, embedded within the joint, monitor concrete strain; rebar stress meters, similarly embedded, assess rebar stress; and crack meters, positioned on the water-facing side, track joint movement. Inclinometers, mounted on both sides of the joint on prefabricated components, measure the joint’s relative rotation angle by tracking the tilt of the connected components. The specific sensor parameters are detailed in Table 3, with the on-site sensor arrangement shown in Fig. 16. The comprehensive monitoring system includes sensors, data loggers, data transfer units (DTUs), and a cloud platform, enabling data sharing with users through web and mobile applications.

Fig. 15 Schematic diagram of the test element layout.

Table 3 Joint sensor parameters.

Sensor model	Standard range	Sensitivity	Resolution	
Intelligent string steel bar stress meter

JMZX-4XXHAT

	− 200–350 MPa	0.1 MPa	0.2%FS	
Embedded intelligent string strain gauge

JMZX-215HAT

	± 1500 µε	0.1 µε	0.03%FS	
Intelligent string anchor cable meter

JMZX-3108HAT

	800 kN	0.1 kN	0.5%FS	
Surface-type intelligent joint gauge

JMDL-2210AT

	100 mm	0.01 mm	0.5%FS	
Surface inclinometer probe

JMQJ-7315ADS

	± 15~°±30°	0.001°	± 0.5%FS	

Fig. 16 Test element layout site. (Image captured by Zhanglong Xu). (a) Monitoring Ring for Tenon-Groove Joint; (b) Inclinometer, Anchor Gauge and Joint gauge; (c) Stress gauge; (d) Strain gauge.

Analysis of the joint stress monitoring results

The test results of the concrete strain gauge are microstrain. To analyze more intuitively, the strain is converted into stress. The elastic modulus of concrete is 3.45 × 104 MPa of C50 concrete in the specification. Figure 17 shows the stress variation curve of a test ring measuring point with time, and the test rings are all located in the middle of the assembly section of the right line of the tunnel. As shown in Fig. 17:

At the start of the assembly and arch closure between blocks A and B (origin of the coordinate axis), stress concentrations are observed near the water-facing surface at measurement points C1 and C2. Compressive stresses increase at these points, while tensile stresses increase at points on the opposite, non-water-facing surface. This phenomenon is attributed to the formation of a new structure after the large components A and B are assembled and the arch is closed. The structure undergoes self-adaptive adjustment under the combined effects of self-weight and construction loads, resulting in a redistribution of internal forces within the structure, characterized by compression in the upper part and tension in the lower part of the joints.

From the completion of sidewall rubble concrete backfilling in region I to the start of layered backfilling of the topsoil in region III, all measurement points exhibit a slow increase in stress with oscillatory characteristics. This behavior is attributed to the restraint exerted on the arch base due to the backfilling with stone concrete of the sidewalls. Upon completion of the layered backfilling of the topsoil, the stress at all measurement points increases due to the increased vertical load. After the completion of the protective backfilling, approximately 2.0–3.0 m thick above the arch crown structure, the maximum tensile stress is observed at position C4, while the maximum compressive stress appears at position C1, with a maximum stress value of 0.33 MPa.

Throughout the entire construction process, the overall variation in joint stresses is relatively small. The greatest stress variation occurs at position C4 on the non-water-facing tensile side, followed by position C1 on the water-facing compressive side. After the completion of protective backfilling over the arch, the joints exhibit characteristics of compression in the upper part and tension in the lower part. Upon backfilling with planting soil, particularly during long-term use, due to the relatively shallow thickness of the overlying soil and smaller vertical loads, all measurement points in the monitoring ring transition to a state of compressive stress under the lateral earth pressure exerted by the filling soil, exerting a beneficial effect on the mortise and tenon joints.

Fig. 17 The curve of concrete stress varying with time. S1 represents backfilling in Region I; S2 represents backfilling in Region II; S3 represents backfilling in Region III; S4 represents backfilling with Planting soil; S5 represents long-term use.

The intelligent string steel bar stress meter yields stress as its test result. Figure 18 illustrates the time-based stress variation curve at a test ringmeasuring point. As observed in Fig. 18:

After assembly into a ring, under the influence of structural self-weight and construction loads, longitudinal reinforcing steel bars at main water-facing measurement points R1, R2, and R4 experience increased compressive stresses. In contrast, measurement points R3, R5, and R6, located near the non-water-facing side, exhibit tensile stresses. This indicates that after assembly into a ring, the upper steel bars at lap joints mainly bear compression, while the lower bars experience tension, with compression generally dominating.

From the completion of sidewall rubble concrete backfilling in region I to the start of layered backfilling of the topsoil in region III, all measurement points show an increase in steel stress, with a greater increase observed in compressive stress. During the phase of layered backfilling with topsoil, the stress in steel at all joint measurement points initially increases and then stabilizes, maintaining an overall state of compression in the upper part and tension in the lower part. At this stage, the position R4 on the compressive side exhibits the highest compressive stress, reaching 8.80 MPa, while position R6 on the tensile side shows the highest tensile stress, reaching 5.39 MPa.

Throughout the entire construction process, the steel stresses are generally low, indicating a significant safety margin. However, the phase of topsoil backfilling has a somewhat adverse effect on the mortise and tenon joints. During this phase, there is a significant increase in compressive stress in the upper steel bars of the joints, accompanied by a corresponding increase in tensile stress in the lower bars. After completion of overlying soil backfilling, especially during long-term use, the stress state of the structure undergoes noticeable changes: the upper steel bars bear higher compressive stresses, while the steel at position R3 in the lower part transitions from initial tensile stress to compression.

Fig. 18 The curve of steel reinforcement stress varying with time. S1 represents backfilling in Region I; S2 represents backfilling in Region II; S3 represents backfilling in Region III; S4 represents backfilling with Planting soil; S5 represents long-term use.

The anchor force measurement results are axial forces. Figure 19 depicts the variation of forces at the monitoring points over time. From Fig. 20, it can be observed that:

At the beginning of the arch formation and assembly into rings for components A and B, tightening bolts resulted in tensile stresses with an initial value of 133.11 kN. As the newly assembled structure adapted and stabilized over time, the anchorage forces showed a decreasing trend and gradually stabilized.

From the completion of sidewall rubble concrete backfilling in region I to the start of layered backfilling of the topsoil in region III, the anchorage forces exhibited a decreasing trend. This decrease can be attributed to the compression exerted by the stone concrete of the sidewalls on the arch ring, leading to reduced stresses. After the completion of sidewall backfilling, all measurement points showed oscillatory tensile stress fluctuations. With the commencement of topsoil layered backfilling, there were significant fluctuations in anchorage forces, increasing to 133.28 kN. During long-term usage, due to reduced construction disturbances and decreased loads, the overall trend of anchorage forces at the joints was downward.

Throughout the entire construction process, the anchorage forces remained generally low, indicating a significant safety margin. However, the phase of topsoil backfilling had a somewhat adverse effect on the joints. During this phase, bolt stresses increased significantly. After completion of overlying soil backfilling, especially during long-term usage, the structural stress state exhibited a noticeable downward trend.

Fig. 19 The curve of bolt stress varying with time. S1 represents backfilling in Region I; S2 represents backfilling in Region II; S3 represents backfilling in Region III; S4 represents backfilling with Planting soil; S5 represents long-term use.

Analysis of the joint deformation results

During the whole construction stage of the vault joint, the monitoring surface (backwater surface) shows deformation toward the tensile side, and the deformation behavior is shown in Fig. 20 The width of the grouting section of the joint is 5 mm; that is, the initial width of the non-grouting section at both ends of the joint is 5 mm. During the whole construction process, the joint is not opened, and the grouting section still maintains good bonding. The back surface of the joint produces relative deformation in the opening direction, and the front surface of the joint produces relative deformation in the compression direction.

Fig. 20 Schematic diagram of relative rotation angle change of vault joint.

Figure 21 shows the curve of the joint’s relative rotation angle over time. Due to instrument malfunction, some data is missing in the blank section. The following can be seen:

At the start of arch formation and ring closure with components A and B, the initial relative rotation angle of 0.025° was due to significant self-adaptive deformation and the high sensitivity of the instruments to disturbances during initial data collection. As the newly formed structure adapted and stabilized, the relative rotation angle decreased over time.

Following region I stone concrete backfill, the relative rotation angle stabilized and fluctuated around 0.022°. Upon the commencement of the top layer of backfill, the relative rotation angle gradually increased, peaking at 0.044°. During water level restoration, the relative rotation angle gradually decreased. At the onset of road surface restoration, increased vertical pressure led to an increase in the relative rotation angle, which eventually stabilized.

Throughout the Construction and Current Operational Phases: The water-facing side of the arch crown joint deformed toward tension. The deformation diagram in Fig. 20 indicates minimal joint opening during the construction loading process, with the grouted section maintaining strong adhesion.

Fig. 21 The curve of relative rotation angle of arch joint varying with time. S1 represents backfilling in Region I; S2 represents backfilling in Region II; S3 represents backfilling in Region III; S4 represents backfilling with Planting soil; S5 represents long-term use.

Figure 22 shows the curve of the relative deformation of the backwater surface of the vault joint with time, and the data have been collected thus far. The following can be seen:

Due to the initial high sensitivity of the large-span structure’s adaptive deformation and the disturbance from early instrument acquisition, the relative deformation initially starts with a larger value. As the new spliced structure stabilizes adaptively, the relative deformation shows a stable trend over time.

During the backfilling process of the region I stone concrete, the relative deformation at the joints fluctuates within the range of 3.0 to 3.5 mm. After layer-by-layer backfilling of topsoil, the relative deformation at the joints shows an increasing trend. Particularly during the start of planting soil backfilling, the maximum values recorded are 3.8 mm for monitoring ring 1 and 4.4 mm for monitoring ring 2. During long-term usage, relative deformation gradually decreases, consistent with the trend of relative rotation angles shown in Fig. 21.

Fig. 22 Relative deformation of the arch crown joint over time. S1 represents backfilling in Region I; S2 represents backfilling in Region II; S3 represents backfilling in Region III; S4 represents backfilling with Planting soil; S5 represents long-term use.

The results indicate that at the initial stage of assembly and the formation of a ring by the top components, the structure went through an adaptive phase where the joints experienced significant stress and deformation. After backfilling, the joints were subjected to increased axial forces, leading to an increase in stress and deformation, although the overall range of change in joint stress and deformation was minimal. During the construction process, the double dowel joints at the arch crown endured two types of stress through the interlocking of the two dowels, namely, tensile and compressive stresses. From the end of construction disturbances to the current operational phase, there is a sufficient safety margin. These findings have significant practical implications for the design and construction of highway open-cut prefabricated tunnels and their structural design.

Conclusion

This study proposes a prefabricated arch structure scheme for large-span open-cut highway tunnels and conducts in-depth theoretical analysis and on-site practical verification based on the eight-lane open-cut tunnel currently under construction in the Xinsen Avenue project in Chongqing. By establishing a finite element “load-structure” integrated calculation model and adopting an iterative analysis method, the study investigates the influence of joint stiffness variations on the internal force distribution of the structure. Combined with long-term on-site monitoring and measurement data, a comprehensive analysis of the performance of the assembled tunnel joint structure during the construction phase is conducted, yielding the following conclusions:

Through prototype joint tests and theoretical analysis, the mechanical properties, stiffness, and bearing capacity of prefabricated structural joints have been comprehended. Integrating experimental research results and numerical theoretical analysis from prefabricated station structures in the Changchun Metro, the use of a multi-iteration analysis method has proven effective and efficient in completing the overall analysis of the assembled tunnel structure with variable stiffness joints. Furthermore, it verifies that the prefabricated structure meets design requirements in terms of strength and durability.

Long-term monitoring results indicate that the ultra-large-span arch-shaped open-cut assembled tunnel with mortise-and-tenon joints undergoes five key excavation and backfilling steps. The concrete exhibits compressive stress both above and below, while the relative rotation angle of the arch crown joint initially increases and then decreases, mirroring a similar trend in the relative deformation on the water-resistant side of the arch crown joint. The S5 long-term service stage emerges as a crucial control node for joint closure, where the joint initially opens and then closes tightly. By the end of the entire monitoring period, the joint performance is optimized. Overall, during construction, the two tenons of the grouted mortise-and-tenon joint rely on their mutual engagement to bear external forces, with overall modest variations in joint stress and deformation.

This study proposes a prefabricated arch structure scheme for large-span open-cut highway tunnels, but there are still some limitations that need improvement. First, the load-structure model is primarily suited for predicting short-term and medium-term loads, and its ability to assess long-term effects is limited. Second, the model includes simplifications and assumptions that may impact the accuracy of the results. Lastly, although long-term on-site monitoring has been conducted, the monitoring period may be insufficient to fully evaluate the durability and degradation of the structure, necessitating an extended observation period. In summary, while this research represents significant progress in the development of prefabricated arch structures, further studies are needed to optimize the design and construction.

Author contributions

Z.L.: conceptualization, methodology, investigation, and data curation. L.L.: Writing-original draft, writing-review and editing. D.H.: Conceptualization, supervision, validation, funding acquisition. W.L.: Methodology, investigation, funding acquisition, data curation. C.Y.: Visualization, resources, supervision. Z.X.: Writing-review and editing, validation.

Funding

The authors would like to acknowledge the financial support received from the National Natural Science Foundation of China (Grant numbers 52078089 and 52274176), the Chongqing Natural Science Foundation Innovation and Development Joint Fund (CSTB2022NSCQ-LZX0079), and the Research and Innovation Program for Graduate Students in Chongqing (CYB23242), China Construction Seventh Bureau Science and Technology R & D Project(CSCEC7B-2022-Z-19), 2022 Construction Science and Technology Plan Project-Group 1-New Urban Infrastructure Construction (CKZ No. 2022-1-13).

Data availability

The data will be made available upon request. Corresponding author should be contacted if someone wants to request the data from this study.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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