# Excavation Method Comparison and Optimization for a Super Large Cross-Section Tunnel
## 2 Project Overview
### On-Site Monitoring  
The Xiaoyuan Tunnel represents the first instance of a single-tunnel, four-lane, large cross-section highway tunnel in Jiangxi Province. An illustration of the tunnel construction limits and internal outlines is provided in Figure 2. The tunnel, characterized by its linear design, is situated in an area with complex geological conditions that present numerous potential and unpredictable geological hazards. Given these circumstances, it is crucial to meticulously monitor the relaxation zones and displacements of the surrounding rock, as well as to accurately assess the development of surrounding rock deformation. To this end, the deformation of the tunnel is measured by a multipoint displacement meter, which has an uncertainty of 0.5, an accuracy of \(\pm 1\%\)FS (i.e., full scale), and a measurement range of 0-100 mm. The horizontal convergence of the surrounding rock is gauged by a laser range finder, which was positioned 4.5 m above the ground and close to the side wall of the tunnel. Moreover, each tunnel section is equipped with a laser rangefinder, and the distance between two adjacent laser rangefinders (arranged along the tunnel traffic direction or the tunnel's longitudinal direction) is 25 m. For the material of actual tunnel construction, C25 shotcrete and C35 reinforced concrete are selected to construct the primary lining and secondary lining, respectively. The specifics of the above measurements are depicted in Figure 3.  
Figure 1: Longitudinal section of the engineering geology. The monitoring results for the top of the arch, along with the left and right arch shoulders, are presented in Figure 4. The data clearly demonstrate consistent settlement trends across these points. Initially, the settlement values increase rapidly but gradually slow down and eventually stabilize. Notably, the monitoring data exhibit significant variability during the initial 15 days, with settlement values beginning to stabilize from the 16 th day onward. This stabilization corresponds with the excavation of the tunnel's left guide hole on the 16 th day, which exposed the tunnel's critical surface and induced the most substantial deformation of the surrounding rock. Following the implementation of initial support measures, vault settlement reached total deformation. By the 15 th day, the excavation of the right guide hole was essentially complete, and the vault displacement largely stabilized. Subsequently, the tunnel then underwent a second lining construction, which resulted in a marked decrease in displacements at all monitoring points. The maximum settlement at the top of the arch vault was recorded at 15.8 mm and eventually stabilized. The settlement at the left arch shoulder consistently exceeded that at the right,  
Figure 3: Monitoring instrument and measuring point layout. (**a**) The measurement points distribution; (**b**) displacement meter layout. Figure 2: Schematic diagram of tunnel construction limits and internal outlines (unit: centimeter). with the maximum settlement reaching 12.4 mm. Additionally, displacement monitoring was conducted at two waist monitoring points in tunnel section ZK2985+420, with the results depicted in Figure 5.  
As depicted in Figure 5, the convergence change curve of the two waists exhibits a pronounced steep trend in the initial 15 days, attributed to the appearance of the critical section during the early stages of tunnel construction. As excavation progressed to the 20 th day, with the near completion of the right guide hole, the convergence values at the two waist points began to stabilize. At this stage, the maximum convergence value recorded for both waist points was 8.6 mm. Hence, the convergence curve of the two waists can be comprehensively divided into three distinct stages: an initial rapid growth phase, subsequently transitioning to a deceleration phase, and finally stabilizing at a slower rate. Figure 4: Monitoring curves for the top of arch, left arch shoulder, and right arch shoulder. Figure 5: Monitoring curves of two waist convergence values.