Research and Application of a Prefabricated Spatial Reticulated Shell Support System for Large Cross-Section Tunnel in a Complex Urban Environment

Abstract

To solve the technical problems of slow construction progress, low mechanization and high risk of shallow buried large cross-section tunnels in a complex urban environment, a series of spatial reticulated shell (SRS) support structures are developed in this paper. Moreover, the equipment of a multifunctional operation trolley is developed to install the SRS arch, and the construction technology system of the prefabricated SRS structure is proposed for a large cross-section tunnel. Therefore, the deformation characteristics of the end-plate joint component and jointless component are clarified by laboratory experiments. The construction mechanics’ simulation of the SRS arch is performed to obtain the tunnel deformation and structure stress based on the tunnel project of Panyu Square Station of Guangzhou Metro. A field application of the prefabricated SRS arch is carried out to realize the mechanized construction operation. The obtained results reveal that the end-plate joint component has better ductility, and its ultimate bearing capacity is basically consistent with the jointless component. The SRS arch can effectively control the deformation of the surrounding rock, improve the stress state of the structure, and reduce the plastic zone of shotcrete by numerical simulations. The overall stress of the SRS arch by field measurement represents the characteristics of “bigger on the upside and smaller on the downside” and “uneven symmetry”. Additionally, the successful application of the prefabricated SRS arch provides a scientific reference for mechanized construction of large cross-section tunnels.

1. Introduction

In recent years, with the acceleration of urbanization and the rapid increase in the urban population, urban traffic congestion has become increasingly serious, which has become a major bottleneck restricting urban development to a great extent. In consideration of the limited resources of urban aboveground space, scientific and reasonable development of underground space is an important means to alleviate traffic congestion and expand urban functions. For the construction of large cross-section and superlarge cross-section tunnels, most tunnel faces are divided either horizontally or vertically into several drifts, and the drifts are excavated in a predetermined sequence [1], such as the center diaphragm method, cross diaphragm method and double side heading method [2,3,4]. The main advantage of dividing a large cross section into small parts is that the support can be carried out in time after each excavation step, which makes it easier to control the overall settlement and horizontal convergence of the tunnel. However, the total construction sequence will increase significantly, and large mechanized equipment is difficult to operate in limited space, which may reduce the excavation efficiency and eventually increase the construction cost [5,6].

There are many kinds of tunnel primary support methods; among them, grid arch/steel arch and rock-bolt mesh shotcrete are widely used [7,8,9,10]. A large cross-section tunnel with complex soft surrounding rock has poor self-stability after excavation; once the bearing capacity of the supporting structure is insufficient and it cannot be quickly closed into a ring, large deformation easily occurs. If the excavation exposure time is longer, the risk of loosening and collapse accidents will increase significantly [11,12,13]. Furthermore, arch installation is still mainly a manual process, and there are problems, such as low construction efficiency, high labor intensity, and difficulty in ensuring the quality of installation operation [14,15].

Therefore, it is of great significance to determine the appropriate construction method and reasonable support structure to improve the stability of the working face, construction efficiency, and economic benefits. Baumann and Betzle [16] designed a new type of three-leg grid arch with an 8-shaped web reinforcement, and proved the rationality of the structural design by carrying out a series of field application tests. Choi et al. [17] carried out the flexural mechanical performance test of the three-leg grid arch and conducted research on the structural design and parameter optimization of different forms of connecting web bars. Kim et al. [18] designed a lattice type of four-leg grid arch. Through the four-point bending test and numerical simulation, and compared with three-leg grid arch, the differences in ultimate bearing capacity and structural stability were analyzed. Lee et al. [19] carried out research on the mechanical properties of a high-strength grid arch, obtained reasonable structural design parameters, and verified that it has good stability and economy.

The reinforced reticulated shell structure has the advantages of reasonable stress, large overall rigidity, and various forms, which has been widely applied in supporting projects, such as mine roadways and traffic tunnels [20]. The concrete reticulated shell structure gives full play to the performance of steel and concrete materials. After construction, the whole tunnel support system presents a continuous hyperbolic arch structure, which gives the support advantages of being three-dimensional, continuous and light weight, and has the dual characteristics of active support and passive support [21]. Pang et al. [22] designed a new structure of reticulated shell bolting and shotcrete according to the mechanical principle of a large-span reticulated shell structure on the ground and successfully carried out industrial tests in soft rock roadways with high ground stress in the Huainan mining area. Zhang [23] hypothesized that the mesh shell bolt shotcrete support can not only support the roadway alone, but also form a combined support system with anchor cables or surrounding rock grouting, and it can be used in various types of soft rock roadways, dynamic pressure roadways and large section chambers. Li et al. [24] proposed the coupling support technology of anchor cables and reticulated shell linings between the support body and surrounding rock and defined the bearing capacity of the reticulated shell support.

This paper, based on the technical challenges of tunnel construction control in complex urban environments, makes full use of the performance advantages of the SRS structures and underground engineering mechanization construction technology and takes advantage of the characteristics of high-strength seamless steel tubes with good toughness and high stiffness. A spatial steel tube reticulated shell structure is used as the primary support for the first time. Moreover, a construction technology system of the SRS arch for large cross-section tunnel is proposed, and industrial tests are carried out in the tunnel project of Panyu Square Station of Guangzhou Metro, which can provide a reference for mechanized construction technology and prefabricated support technology.

2. Project Overview

Relying on the underground tunnel between Hengli Station and Panyu Square Station of Guangzhou Rail Transit Line 22 (as shown in Figure 1), a field test of the mechanized construction of the prefabricated SRS arch support is carried out at the mileage of ZDK34+370~ZDK34+400, and the tunnel in the test area is mainly located in moderately weathered granite (as shown in Figure 2). The overburden thickness of the vault is 26.5 m, mainly composed of miscellaneous fill, muddy soil, silty medium-coarse sand, and sandy clay. The maximum excavation span of the tunnel is 14.9 m, the height is 11.95 m, and the cross-sectional area is 143 m². The tunnel passes under Luojiayong Bridge, and the minimum horizontal clear distance between the bridge pile and the tunnel is only 2.06 m. In addition, the buildings on the surface are densely built, and the deformation of the surrounding rock of the tunnel should be strictly controlled to minimize the impact on the surrounding environment.

The design parameters of the primary support of the tunnel (as shown in Figure 3) are as follows: the grid arch used in the non-test area adopts a four-leg grid arch with a main reinforcement diameter of 32 mm; the SRS arch used in the test area adopts a nine-leg type with a steel tubes diameter of 50 mm and thickness of 6 mm; π-shaped and U-shaped reinforcement diameter is 14 mm, with a spacing of 1.0 m; and steel mesh diameter is 8 mm, with a spacing of 15 cm × 15 cm. The shotcrete grade is C25, and the thickness is 35 cm; early-strength mortar bolts diameter is 22 mm, the length is 3.0 m, and the row spacing is 1.0 m × 1.0 m. The arch foot is equipped with 42 mm diameter locking anchor tubes, and the single length is 2.0 m.

3. Spatial Reticulated Shell Structure and Experimental Scheme

3.1. Structure Types

The SRS structure can adopt five-leg, seven-leg, nine-leg steel tubes and other types (as shown in Figure 4). This article only conducts mechanical performance test research for the nine-leg steel tube reticulated shell structure. It consists of main limb steel tubes, Π-shaped connecting ribs, U-shaped connecting ribs and end-plates, which are arranged according to certain rules and are connected by high-strength bolts to form a double-layer SRS support structure. The arches are connected to each other to support the tunnel continuously. The surrounding rock pressure is dispersed as a spatial force system through the outer steel tube network, and the inner steel tube network can provide support to the outer steel tube network. High-strength seamless steel tubes are the main force-bearing members, and the external steel tubes and the internal steel tubes form the force mode of the “normal triangle” and “inverted triangle” through the connecting ribs. Consequently, the whole structure not only has the characteristics of high strength and large stiffness, but also has good toughness and three-dimensional stability.

In the construction of underground tunnels, to facilitate transportation and field assembly, the integral ring arch is usually divided into several segments according to the stress characteristics, and each segment is connected by high-strength bolts. The connection mode of the end-plate + bolt is convenient for installation, and the force transmission mechanism is clear, which can ensure the overall stability of the structure. The specific joint construction is shown in Figure 5.

High-strength bolts are used to connect the end-plates of two adjacent arch segments to realize the assembly of the SRS arch into a ring. The bolt holes are evenly arranged between the adjacent steel tubes, and the distance between the adjacent bolt holes is 200 mm. The center distance between the external and internal steel tubes is 220 mm, and the thickness of the end-plate is 20 mm. The joint construction parameters are shown in Figure 6.

3.2. Component Preparation

Studying the flexural mechanical properties of end-plate joint components and jointless components is an important step in the design of the SRS arch, and it is crucial to accurately evaluate the overall mechanical properties of the structure. The total length of the processed joint components and jointless components is 3200 mm. The main limb steel tubes adopt seamless steel tubes for structural purposes with the same diameter and thickness, and the grade is Q420B. Π-shaped and U-shaped bars adopt HRB400 threaded bars with a diameter of 14 mm. The joint components are connected by 11 M24 high-strength bolts of 10.9 grade, and there are two bolts for fastening at the connection between each steel tube and the end-plate. The end-plate is made of Q235B carbon structural steel with a thickness of 20 mm; in order to improve the convenience and speed of bolt connection, the back of the end-plate can be welded without reinforcing ribs. The component types are shown in Figure 7.

The design parameters of the processed components are shown in

Table 1. Main structural parameters of the processed components.

Component Type	Steel Tube (mm)		End-Plate Thickness (mm)	Length (mm)	Π-Shaped Rebar Diameter (mm)	U-Shaped Rebar Diameter (mm)	Bolt Size
	Size	Limbs Number
Jointless component	50 × 6	9		3200	14	14
End-plate joint component	50 × 6	9	20	3200	14	14	M24

, and the properties of the steel materials are shown in

Table 2. Mechanical properties of the steel materials.

Material	Elastic Modulus (GPa)	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation Ratio (%)
14 mm reinforcement	198	430.34	576.81	23.60
20 mm thickness steel plate	205	254.62	427.25	24.58
50 × 6 mm steel tube	205	457.78	604.61	25.47

.

3.3. Loading Scheme

Through the four-point pure bending test, the failure characteristics and bearing mechanism of the end-plate joint component and jointless component are further analyzed, which provide a certain evaluation of the safety and adaptability of this support structure.

The static load test is carried out by a Y32-500A four column hydraulic servo loading system with a range of 0~5000 kN. Simple support at both ends and the four-point bending loading mode are adopted to ensure that the joint is in a pure bending state, and the vertical load is collected by a pressure sensor. Before the test, geometric and physical alignment is conducted. The loading process is divided into three steps: first, the component is preloaded to ensure good contact between the component and the loading device; second, the load increment of each stage is 10 kN, and the pressure is maintained for 3 min and monitored in real time; third, when the load pressure does not continue to increase, the displacement loading mode is changed, and the loading rate is 1 mm/min until the component is unstable. The layouts of the loading device and measuring instrument are shown in Figure 8. Schematic diagrams of the loading test model of SRS components are shown in Figure 9.

3.4. Layout of Measuring Points

For the end-plate joint component, 18 metal strain gauges are pasted on the back of one side of the end-plate along the vertical and horizontal directions, 9 strain gauges are pasted on a monitoring section of the main limb steel tubes along the longitudinal direction, and 6 bolt axial force measuring points are set. For the jointless component, 2 monitoring sections are arranged on the surface of the main limb steel tubes, including 18 longitudinal strain gauges and 9 circumferential strain gauges. Digital displacement meters are installed at the tripartite point and mid-span of the component to record the deformation. The specific arrangement of the strain gauges is shown in Figure 10.

4. Experiment Results and Discussion

4.1. Experiment Phenomenon and Deflection Displacement

Figure 11 shows the failure mode of the end-plate joint component, and Figure 12 shows the failure mode of the jointless component. At the initial stage of loading, the components do not change significantly. When the external load increases to 60% of the ultimate strength, the steel tubes both represent obvious bending deformation. At the same time, the π-shaped ribs are subjected to the load, and obvious deformation occurs in the arc section. When loaded to the ultimate bearing state, the components deform significantly, the vertical load tends to be stable, and the bending springback is large after unloading.

The end-plate joint component is welded to realize a fixed connection between the main limb steel tubes and the end-plate. The arch segments are fastened with high-strength bolts, and no brittle failure occurs during the loading process. The deformation of the end-plate in the tension zone is not obvious, and the axial force of the bolt on the tension side is large, but it does not break. Due to the absence of weak links, such as weld seams and bolted connections, the jointless component has good integrity, a simple force transmission mechanism during loading.

Figure 13 shows the moment–mid-span deflection displacement curves of the end-plate joint component and jointless component. At the initial stage of loading, the mid-span deflection of the two kinds of components changes linearly with the bending moment. The growth rate of the deflection of the end-plate joint component is slow, showing higher deformation resistance and bending stiffness. When the load continues to be applied, the growth rate of the deflection gradually accelerates, and the components begin to enter the stage of plastic deformation. The plastic development period of the jointless component is longer, and the end-plate joint component first reaches the ultimate bearing state. At the later stage of loading, the deflection displacement continues to increase, while the external load remains relatively stable.

The mid-span deflection of the end-plate joint component is approximately 46.0 mm when it reaches the ultimate bearing state, and the ultimate bearing capacity is 212.6 kN·m. The maximum allowable deflection of jointless component is approximately 54.2 mm, and the ultimate bearing capacity is 209.5 kN·m, which accounts for 98.5% of end-plate joint component, indicating that the end-plate joint component has good ductility and high bearing capacity.

4.2. Moment–Strain Curves of the End-Plate Joint Component

4.2.1. Steel Tube Strain

To determine the stress distribution law of the main limb steel tubes of the end-plate joint component, the bending moment–steel tube strain relationship curves are obtained (as shown in Figure 14). The label “STS” in the figure represents the monitoring item steel tube strain, “1” represents the longitudinal strain measuring point 1, and the rest are the same.

It can be seen from Figure 14 that all measuring points in the outer main limb steel tubes show compressive strain, and the strain values of measuring points STS-1 and STS-4 are larger than others. When reaching the ultimate bearing state, the strain values of measuring points STS-1 and STS-4 are −2435 με and −2347 με, respectively. The converted stress values are 501.6 MPa and 483.5 MPa, which exceed the yield strength of the steel tube. The internal main limb steel tubes measuring points STS-5~STS-9 all show tensile strain, and the change trend is basically the same. The maximum strain value of all measuring points ranges from 1357 με to 1634 με, which does not exceed the yield strength of the steel tube.

4.2.2. Axial Force of Bolts

The type selection and force characteristics of bolts directly affect the reliability and safety of joint connections. Therefore, it is necessary to study the mechanical properties of bolts. Figure 15 shows the relationship curves between the bending moment and axial force of the bolts, in which HSB-1~HSB-3 are the upper bolts and HSB-4~HSB-6 are the lower bolts.

By analyzing the force of the bolts at different positions, it can be seen that the lower bolts bear a greater tensile load than the upper bolts. Among them, the axial force of the HSB-5 bolt at the limit state is 48.0% higher than the final pretightening force, which exceeds the yield strength by 3.3%. The axial force of the HSB-4 bolt and HSB-6 bolt are increased by 9.3% and 14.0%, compared to the final pretightening force, respectively. However, the axial force of the upper bolts is decreased slightly.

In conclusion, the lower bolts are generally stressed, and the force of each bolt is in good condition without failure occurrence, which indicates that the bolt selection is reasonable.

4.3. Moment–Strain Curves of the Jointless Component

Figure 16 and Figure 17 show the moment–steel tube strain curves of the 1/2 section and 1/8 section of the jointless component, respectively.

For the 1/2 section, the external steel tube measuring points STS-1, STS-3, STS-5, and STS-7 are all compressive strains, and the maximum strain values of the measuring points range from −319 με to −867 με; the steel tube measuring points STS-2, STS-4, STS-6, and STS-8 are all tensile strains, and the maximum strain values of the measuring points range from 56 με to 320 με. The internal steel tubes measuring points STS-9, STS-11, STS-13, STS-15, and STS-17 are all tensile strains, and the strain at ultimate bearing state is 1991 με, 2346 με, 2614 με, 2204 με and 1992 με. The measuring points STS-11 (483.3 MPa) and STS-13 (538.5 MPa) exceed the yield strength of the material. The steel tube measuring points STS-10, STS-12, STS-14, STS-16, and STS-18 are all compressive strains, and the maximum strain values range from −535 με to −655 με.

For the 1/8 section, the measuring points STS-1, STS-2, STS-3, and STS-4 are compressive strains, and the maximum strains are −2097 με, −1266 με, −2398 με, and −1206 με, respectively. Among them, the stress value of measuring point STS-3 is approximately 494.0 MPa, which exceeds the yield strength of the material. The measuring points STS-5, STS-6, STS-7, STS-8, and STS-9 are all tensile strains, and the maximum strains are 2386 με, 1927 με, 1826 με, 1790 με and 1358 με, respectively. The stress value of STS-5 is approximately 491.5 MPa, which exceeds the yield strength of the material.

5. Numerical Analysis of the Supporting Performance of the Spatial Reticulated Shell

5.1. Numerical Calculation Model

The plastic damage model of ABAQUS can be used to analyze the plastic damage range of concrete and the stress state of the supporting arch in the primary supporting structure of the tunnel [25], and then obtain the numerical solution of the supporting performance of the SRS arch and grid arch. The calculation model simulates the surrounding rock range of 80 m × 60 m × 60 m. According to the site geology and actual construction conditions, the stress and strain state of the primary support under the surrounding rock load is simulated.

The mesh generation of the overall and detailed models is shown in Figure 18. The upper boundary of the model is established to the surface, the bottom constrains the displacement in the X, Y, and Z directions, the two sides constrain the displacement in the X direction, and the front and rear constrain the displacement in the Z direction. Different colors indicate different formation conditions, which are divided into five types of formations, from top to bottom are miscellaneous fill, mucky soil, silty medium-coarse sand, sandy clay, and moderately weathered granite. The three-dimensional models of the SRS arch and grid arch are made with the help of relevant software, and then imported into ABAQUS for parameter assignment and calculation. In the model, the three-node quadratic space beam element (B32) is used for the steel tube and steel bar, the eight-node linear hexahedron nonconforming element (C3D8I) is used for the concrete spray layer, and the eight-node reduced integration element (C3D8R) is used for the surrounding rock. The steel skeleton and shotcrete are modeled separately, and the embedded function is used to establish the connection between them to achieve the effect of common deformation.

The geotechnical parameters of the formation are shown in

Table 3. Geotechnical parameters of the surrounding rock.

Material	Stratum Thickness (m)	Bulk Density (kN/m3)	Elastic Modulus (MPa)	Poisson’s Ratio	Cohesive Force (kPa)	Internal Friction Angle (°)
Miscellaneous fill	3.2	18.5	4	0.3	12	7
Mucky soil	2.4	17.1	10	0.4	5	6
Silty medium-coarse sand	3.7	18.5	15	0.3	1	25
Sandy clay	2.5	18.5	20	0.3	18	15
Moderately weathered granite	48.2	25.0	5 × 103	0.2	400	35

, which are from the engineering geological survey report. The Mohr–Coulomb elastic–plastic model is adopted for the stratum, and the bilinear follow-up strengthening model is selected for the constitutive relationship of steel tubes and steel bars, which conforms to the Von Mises criterion [26].

The specific types of steel material in numerical simulation are shown in

Table 4. The specific types of steel materials in numerical simulation (Unit: mm).

Steel Tube		Steel Rebar		Π-Shaped Rebar		U-Shaped Rebar
Material	Size	Material	Diameter	Material	Diameter	Material	Diameter
Q420	50 × 6	HRB400	32	HRB400	14	HRB400	14

, and the mechanical properties of the steel materials are shown in

Table 5. Mechanical properties of the steel materials.

Material	Elastic Modulus (GPa)	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation Ratio (%)
14 mm steel rebar	198	430.34	576.81	23.60
32 mm steel rebar	201	445.64	592.50	19.28
50 × 6 mm steel tube	205	457.78	604.61	25.47

. In addition, the elastic modulus of the mortar bolt is 200 GPa, the bulk density is 78.5 kN/m³, and the Poisson’s ratio is 0.3. The elastic modulus of C25 shotcrete is 23 GPa, the bulk density is 24.0 kN/m³, and the Poisson’s ratio is 0.3.

The uniaxial tension and compression stress–strain constitutive relationships of the concrete are defined by the following equations [27]:

(1)

Tensile stress–strain relationship of the concrete

$$y=\left\{\begin{array}{cc}x\left(1.2-0.2x^5\right),\hfill & \hfill x\le 1\\ \frac{x}{{\alpha }_{\mathrm{t}}{\left(x-1\right)}^{1.7}+x},\hfill & \hfill x>1\end{array}\right\}$$

(1)

$$x=\epsilon /{\epsilon }_{\mathrm{t}}$$

(2)

$$y=\sigma /f_{\mathrm{t}}$$

(3)

where ${\epsilon }_{\mathrm{t}}$ denotes the peak strain of the uniaxial tensile stress–strain curve; ${\alpha }_{\mathrm{t}}$ is the parameter value of the falling section of the uniaxial tensile stress–strain curve; and $f_{\mathrm{t}}$ is the uniaxial tensile strength of the concrete. The concrete tensile stress–strain curve is shown in Figure 19a.

(2)

Compressive stress–strain relationship of the concrete

$$y=\left\{\begin{array}{l}\frac{nx}{n-1+{\left(x\right)}^n},x\le 1\\ \frac{x}{{\alpha }_{\mathrm{c}}{\left(x-1\right)}^2+x},x>1\end{array}\right\}$$

(4)

$$n=\frac{E_{\mathrm{c}}{\epsilon }_{\mathrm{c}}}{E_{\mathrm{c}}{\epsilon }_{\mathrm{c}}-f_{\mathrm{c}}}$$

(5)

$$x=\epsilon /{\epsilon }_{\mathrm{c}}$$

(6)

$$y=\sigma /f_{\mathrm{c}}$$

(7)

where ${\epsilon }_{\mathrm{c}}$ denotes the peak strain of the uniaxial compressive stress–strain curve; ${\alpha }_{\mathrm{c}}$ is the parameter value of the falling section of the uniaxial compressive stress–strain curve; and $f_{\mathrm{c}}$ is the uniaxial compressive strength of the concrete. The concrete compressive stress–strain curve is shown in Figure 19b.

5.2. Numerical Results and Analysis

The cloud diagrams of the supporting arches are shown in Figure 20. The numerical results show that the maximum stress at the arch foot of the SRS arch is 187.7 MPa, which does not exceed the yield strength of the steel tube. To further improve the bending capacity of the arch in the joint area, the back of the end-plate can be strengthened by adding reinforcement. In addition, the maximum stress of the main reinforcement at the arch foot of the grid arch is 240.7 MPa, which does not reach the yield strength of 400 MPa.

According to Figure 21, the plastic failure zone of the concrete structure is mainly concentrated in the arch foot and vault, where the compressive stress is relatively focused and the plastic strain value is larger. The arch vault and arch bottom are tensile cracking areas, the plastic value and distribution range of the arch shoulder are small, and no obvious damage has occurred. When supported by the SRS arch, the plastic damage area of the concrete can reach 45%, while the grid arch is more than 60%. Due to the high bearing capacity of the SRS arch, it can provide high support resistance when the concrete is damaged and reduce the plastic zone.

Beyond that, the numerical results also show that the vault settlement is 18.0 mm and the peripheral convergence is 11.2 mm in the case of the SRS arch support, and the vault settlement is 24.6 mm and the peripheral convergence is 14.3 mm in the case of grid arch support. The comparative analysis indicates that the SRS arch has a positive impact on the control of the surrounding rock deformation and the improvement of structure stress and has higher supporting stiffness and bearing capacity, which can meet the safety and quality requirements of tunnel engineering.

6. Prefabricated Spatial Reticulated Shell Support System and Field Test

To solve the technical problems of precise hoisting and assembly of the heavy SRS arch, a three-arm high-degree-of-freedom multifunctional operation trolley, and longitudinal connection device are developed, which effectively improve the construction efficiency and ensure the quality of the project. The SRS arch is prefabricated in the factory according to the design requirements of the drawings, stacked in an orderly manner, and then transported to the tunnel working face by horizontal and vertical transport machinery. The left, middle and right mechanical arms of the multifunctional operation trolley are manipulated to grab three arches to the appropriate height. Then, the bolt connection is carried out, and the arches are lifted to the specified position through the middle mechanical arm; finally, the left and right mechanical arms are manipulated to complete the installation of the rest of the arches. After the installation of the arch, auxiliary procedures, such as laying the metal mesh, setting the anchor rod and shrunk-down anchor pipe, and spraying the concrete, are carried out. During construction, the cross operation is orderly, and the process connection is reasonable, forming the construction system of a large cross-section tunnel prefabricated SRS arch support (as shown in Figure 22).

The multifunctional operation trolley adopts the structure of three arms and three baskets. The manipulator and the hanging basket are integrated into one, with the functions of automatic grasping and fine-tuning at the end of the manipulator, which can realize the mechanized installation of the arch. Replacing the manipulator fixture can meet the installation requirements of different types of arches. The boom system can realize three-stage expansion. The middle boom can load 1.5 t heavy objects, the left and right booms can load 0.7 t heavy objects, the maximum lifting height of the boom can reach 13.0 m, the maximum operating width can reach 16.0 m, and the direction of the manipulator can be controlled by adjusting the rotating platform. The circumferential joint of the SRS arch is connected by the end-plates and bolts. Holes are reserved on the outside of the end-plates, and U-shaped steel bars are placed in the reserved holes to realize the longitudinal connection of two adjacent arches.

To further master the working state of the prefabricated SRS arch and ensure the safety of mechanized construction, based on the results of laboratory tests and numerical analysis, the field application of the prefabricated SRS arch in a large cross-section tunnel is carried out. The back of the end-plate at the arch foot is welded with reinforcing ribs to improve the deformation resistance. The specifications and types of various steels are consistent with the laboratory test.

The typical section is selected to monitor the arch internal force, shotcrete strain, contact pressure between the surrounding rock and primary support, vault settlement, and convergence deformation. The field installation of monitoring components is shown in Figure 23. The specific layout of the monitoring points on the monitoring section is shown in Figure 24, in which GJ is the arch strain gauge, YB is the concrete strain gauge, YL is the pressure cell, GD is the vault settlement, and SL is the horizontal convergence.

Based on the analysis of the monitoring data of the test section, the SRS arch can fully meet the safety construction requirements of an urban shallow-buried large cross-section tunnel with the bench method in a surrounding rock of grade IV. The deformation of the surrounding rock and the stress of the structure are within the scope of the design index.

6.1. Displacement of the Surrounding Rock

Figure 25 shows the vault settlement and peripheral convergence duration curves of the SRS arch support section and the grid arch support section. In Figure 25, the black line represents the grid arch support, and the red line represents the SRS arch support.

Figure 25 shows that the maximum settlement of the vault with the SRS arch support is 12.8 mm, which is 5.9 mm less than that with grid arch support; in addition, the convergence deformation of the upper and lower benches is 8.1 mm and 4.0 mm, accounting for 68.6% and 75.5% of the convergence deformation of the grid arch support respectively. The main reason is that the SRS arch has high support stiffness and bearing capacity. Under the condition of mechanized construction, it can quickly complete the accurate assembly of a heavy arch frame, close the surrounding rock in time, and effectively control the deformation after tunnel excavation.

Combined with the numerical simulation data introduced above, in terms of vault settlement and peripheral convergence results, the SRS structure shows a higher bearing capacity, which to a certain extent shows that the numerical results and the measured results are in good agreement. It is verified that the SRS support structure has a good application effect on the deformation control of large section tunnel in the urban environment and can further reduce the impact of tunnel engineering on surrounding buildings.

6.2. Surrounding Rock Pressure

Figure 26 shows the pressure duration curves of the surrounding rock in the supporting section of the SRS arch. Due to site construction, the YL-3 pressure cell on the right shoulder of the upper bench was damaged, and the measured data were missing.

Figure 26 shows that the distribution of the surrounding rock pressure in different positions is not the same. The contact pressure in the arch waist and arch shoulder increases more obviously, and the maximum pressure value is significantly higher than that in other positions. The pressure tends to be stable approximately six days after the excavation of the lower bench. The contact pressure of the right arch waist is the largest (244.9 kPa), followed by the left arch shoulder (124.0 kPa), left arch waist (103.5 kPa), right arch foot (70.5 kPa), vault (65.4 kPa) and left arch foot (55.8 kPa). There is bias pressure in the tunnel, and the contact pressure on the right side is obviously greater than that on the left side.

6.3. Arch Stress

Figure 27 shows the stress distribution of the steel tubes of the SRS arch. The black solid line in the figure represents the stress of the external steel tubes, and the red dashed line represents the stress of the internal steel tubes. The positive sign denotes tensile stress, and the negative sign denotes compressive stress.

The data show that the stress value of the steel tubes at each measurement point of the SRS arch is less than the yield strength of the material itself, and the structure is in the stage of elastic deformation. The stress distribution of the arch is characterized as “bigger on the upside and smaller on the downside” and “uneven symmetry”. The stress on the arch shoulder and the vault is generally larger, followed by the stress on arch waist, and that on the arch foot is the smallest. The external steel tubes are mainly subjected to compressive stress, while the internal steel tube is mainly subjected to tensile stress.

The maximum compressive stress of the SRS arch occurs on the external steel tube of the right arch shoulder, reaching 40.4 MPa, and the minimum compressive stress is −13.1 MPa, which is located on the internal steel tube of the right arch foot. The maximum tensile stress occurs on the internal steel tube of the right arch foot, reaching 47.0 MPa, and the minimum tensile stress is 18.3 MPa, which appears on the external steel tube of the right arch waist. The whole SRS arch is in a good stress state and can meet the demand of on-site support strength.

6.4. Shotcrete Stress

Figure 28 shows the stress distribution of shotcrete at various positions of the monitoring section. The blue dotted line in the figure represents the shotcrete stress, the positive sign represents tensile stress, and the negative sign represents compressive stress.

The stress distribution of shotcrete also presents the characteristics of “bigger on the upside and smaller on the downside “, and “uneven symmetry”. The stress on the vault, the right arch shoulder and the right arch waist is generally larger, while the stress on the left is smaller. Shotcrete is mainly manifested as tensile stress. The maximum tensile stress occurs at the right arch waist and reaches 20.9 MPa, which exceeds the ultimate tensile strength. The maximum compressive stress appears in the vault, which is −9.3 MPa and within a reasonable range. Although the local shotcrete has undergone plastic failure, the SRS arch itself has high strength, and good ductility, which can ensure the safety and stability of the overall structure.

6.5. Process Time Statistics

The installation of the SRS arch is mainly divided into four processes: construction preparation, arch assembly, longitudinal connection, and metal mesh binding. There are obvious differences from the traditional arch in terms of clamping mode, arch erection method, component weight, etc. The field test has carried out a statistical analysis on the operation time of arch erection on the upper step. With the increase in the number of arch erection, the cooperation between the equipment and the operator becomes more tacit, the performance of the equipment becomes more stable, and the efficiency of the vertical arch is significantly improved.

Two rings of the SRS arch are installed in each cycle, with a footage of about 2.0 m, and a total of 8 cycles of operation time are counted. After several rounds of equipment debugging by the operators and the continuous improvement of coordination between various processes, the total time from the initial 487 min is gradually shortened to 239 min (as shown in Figure 29). The construction efficiency is significantly improved, and the construction progress is also greatly accelerated.

7. Construction System Superiority and Optimization Suggestions

7.1. Construction System Superiority

Mechanized construction has greatly reduced the input of operators, from the original more than 10 people per team to only 4 people to operate the trolley equipment and complete the auxiliary process; it takes approximately 2.0 h to install the grid arch by the manual method. However, the multifunctional trolley can complete the erection of a single ring arch in 1.2 h, indicating that erection efficiency is significantly improved.

The SRS arch adopts a standardized design, factory prefabrication and assembly construction and has the characteristics of convenient and reliable joint connection, good integrity, and even force. Moreover, the SRS arch emphasizes the timeliness and effectiveness of the support; that is, it is closed into a ring in time, bears the load of the surrounding rock in real time, and provides effective support resistance to restrain the deformation of the surrounding rock. The mechanization of arch installation can greatly reduce the time that workers are exposed to the surrounding rock under the vault of the tunnel, reduce the probability of engineering accidents, and ensure the safety of construction operations.

7.2. Optimization Suggestions

The overall size of the multifunctional operation trolley can be optimized to make it lighter and smaller and to better exert its performance advantages in tunnels of different cross-sections. The original two-wheel steering can be optimized to four-wheel steering, reducing the turning radius of the trolley.

The manipulator is equipped with a quick-change device to ensure that the manipulator can be replaced in a short time, to realize the grasping and assembly of the SRS arch, steel arch and other type arches, and to further improve the flexibility and applicability of the multifunctional operation trolley.

8. Conclusions

The new prefabricated SRS arch adopts high-strength seamless steel tubes as the main load-bearing parts to improve the supporting rigidity and bearing capacity, forming a supporting structure suitable for tunnels under complex surrounding rock conditions. Relying on the undercut tunnel project of Guangzhou Metro Panyu Square Station, the bearing characteristics and supporting performance of the SRS arch structure are studied by laboratory experiments, numerical analysis, and field measurement. The main conclusions are as follows:

(1)

A construction system of the prefabricated SRS arch for a large cross-section tunnel in a complex urban environment is developed, which mainly includes a three-arm and three-basket multifunctional operation trolley, a high-strength SRS arch and a longitudinal connection device.

(2)

Through the indoor model test, the deformation characteristics and bearing mechanism of end-plate joint components and jointless components are clarified. It is verified that the SRS arch structure has good ductility and high bearing capacity and meets the joint design requirements. The research results provide a theoretical basis for the parameter design and engineering application of this structure.

(3)

With the help of the numerical simulation method, the stress distribution characteristics of the SRS arch and grid arch and the plastic damage of the concrete are mastered. Moreover, it is concluded that the SRS arch has a positive impact on controlling the deformation of the surrounding rock and ameliorating the stress of the support structure.

(4)

Through the field test of the prefabricated SRS arch, mechanized construction is realized. The measured data show that the SRS arch can be closed and formed into a ring in time, and the structural force is reasonable, which can meet the demand of support strength and provide new technical support for the development of urban underground large spaces in the future.

Based on the stratum conditions of the underground tunnel at Panyu Square Station of Guangzhou Metro, the key technology research on the mechanized construction of large-section tunnels by the step method was carried out, but the adaptability of other construction methods and stratum conditions was not studied. Therefore, it is necessary to expand the construction technology to underground engineering construction with different construction methods, stratum conditions, and section sizes in the future.