## 5 Field Application and Monitoring Analysis
### Analysis of Engineering Application Effect  
In order to test the application effect of the SRGB support structure, the stress conditions of the SRGB support structure during the construction process were monitored on site. The main test items include the settlement and convergence of surrounding rock, surrounding rock pressure, steel tube stress and concrete spray layer stress. The layout of measuring points on the test section is shown in Figure 27.  
Figure 25: Mechanized installation of the arches. (**a**) Assembled arches near the ground and (**b**) the installation of upper bench arches.  
Figure 26: Mechanized installation of arch waist and inverted arch. (**a**) Arch waist and (**b**) inverted arch.  
Figure 28 shows the vault settlement and peripheral convergence duration curves of the SRG arch support section and the grid arch support section. The black line represents the grid arch support, and the red line represents the SRG arch support.  
After nearly 30 days of real-time monitoring, the maximum settlement of the vault with the SRG arch support is 12.8 mm, which is 5.9 mm less than that with the 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 SRG 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.  
Figure 29 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). During the whole construction process, the spatial reticulated grid structure was in the elastic deformation stage, and the stress state was good. The stress on the spandrel and the vault is generally larger, followed by the arch waist, and the arch foot is the least stressed. The concrete sprayed layer is dominated by  
Figure 27: Schematic diagram of the layout of monitoring components.  
Figure 28: Duration curves of the displacement of the surrounding rock.  
tensile strain, and the local position exceeds the ultimate tensile strength of concrete, but the overall structure still has high safety redundancy.  
In general, by relying on the field test of the spatial reticulated grid structure carried out by the underground tunnel project of Guangzhou Metro, the first application of this structure in an urban subway tunnel has been realized. The mechanized construction method can effectively shorten the arch erection time. The high-strength, high-rigidity, and high-toughness mechanical properties of the spatial reticulated grid restrain the deformation of the surrounding rock to a great extent, and achieved good application results.