## 3 Results and Analysis
### Changing Law of Internal Force of Support Structure  
The stress of the supporting pile is measured by the earth pressure box buried at the top of the pile. The stress release of the surrounding rock can be measured before the installation of the middle column, and the stress variation trend can be measured after the installation of the middle column. Figure 7 illustrates the trend of the overburden stress on the gypsum arch. It is evident that the overburden depths in the central column force correspond to 6.3 m, 12.6 m, and 18.9 m, respectively, according to the similarity ratio relationship. During the construction of the arch cover, the existence of the gypsum arch cover results in a small stress release of the overburden on the arch. The peak of stress reduction occurs at the position where the thickness of the overburden is 36 cm, with a stress value of 1.2 kPa. This corresponds to the working condition of the demolition of the central diaphragm wall. Prior to the removal of the diaphragm wall, there is a slow release of overburden pressure, but not much reduction. Following the removal of the diaphragm wall, the soil above compressed downwards but remained confined by the arch cover support structure. This led to an increase in internal pressure, which eventually stabilized. The fluctuations in soil pressure within the overburden were insignificant, and the overall condition has remained stable. Figure 8 illustrates the force changes above the gypsum arch cap after excavation. Negative values indicate downward compression of the arch, resulting in compressive stresses, while positive values indicate upward bulging of the arch cap, resulting in tensile stresses. Tensile stresses were observed at the location of the right arch gridle during the construction of the arch cap while excavating side drift A. The excavation of the left drift altered the state of the perimeter rock of the arch, resulting in slight uneven stress at the  
Figure 6: Ground settlement monitoring arrangement and monitoring results. (**a**): Settlement monitoring point layout. The blue block in the figure (**a**) is the main station. The red points is the arrangement of the eight monitoring points. (**b**): Patterns of change at different settlement monitoring sites. Figure 7: Trend of overburden stress on gypsum arch. left and right ends of the arch cap. However, the excavation of side drift B caused the surrounding rock below the left arch haunch to hollow out, and the surrounding rock above squeezed the arch cover, returning the left arch crown to a state of compressive stress. The maximum compressive stress in the arch is located at the top, with a maximum value of 21.5 kPa. The deformation stress at 20 cm of the monitored section is greater than that at 45 cm. The addition of the central column resulted in a reduction of compressive stress in the arch cap. Analysis showed that solidification of the gypsum at the top of the pile caused expansion, which in turn lifted the arch cap, counteracting some of the extruded stresses from the downward deformation of the arch cap. During the removal of the initial support of the central drift, the arch cover experienced rapid stress release. Subsequently, after the removal of the initial support of the central drift, the stress release rate of the arch cover gradually slowed down and eventually stabilized. Figure 9 illustrates the stress trends in the arch feet under various operating conditions. As shown in the figure, the stresses in the left and right arch feet exhibit only minor fluctuations during the mid-guideway stage of the excavation. The excavation of the side drifts led to the release of compressive stresses at the foot of the arch, resulting in a significant reduction in compressive stresses on the same side. During excavation of the lower surrounding rock, the arch foot bears part of the upper earth pressure transmitted by the arch cover, leading to increased compressive stress.