## 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  
Figure 21: Cloud diagrams of concrete plastic damage. (**a**) SRS arch and (**b**) grid arch.  
Figure 20: Stress nephogram of the SRS arch and grid arch (Unit: MPa). (**a**) SRS arch and (**b**) grid arch.  
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.  
Figure 22: Construction system of the prefabricated SRS arch support.  
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.