## 6 Conclusions  
This paper proposes a new support structure of steel corrugated plate-mold bag concrete based on a circular shaft. The mechanical effect of the support structure under engineering conditions is verified using theoretical analysis, numerical simulation, and field testing, and the following conclusions are drawn:  
1. Mold bag concrete can fill the structural gap between the excavation surface and the steel corrugated panel, reduce the loss of soil, and help control the deformation and settlement caused by the excavation. Part of the gas and water in the concrete inside the mold bag is squeezed out with the grouting process, reducing the void space and the water-cement ratio, which is conducive to the rapid setting of the concrete and achieving rapid support. The combination of steel corrugated plate and mold bag concrete can form a rapid support technology for underground engineering.
2. The interaction force between the steel corrugated plate and the mold bag concrete contact surface is influenced by four main factors: the excavation depth, the thickness of the steel corrugated plate, the thickness of the mold bag concrete, and the excavation radius of the circular shaft. As the stiffness of the concrete inside the mold bag increases, most of the external loads are borne by the mold bag concrete, and the load borne by the steel corrugated plate is gradually reduced to 30%.
3. Numerical simulation results show that the thinner the steel corrugated plate and the mold bag concrete, and the larger the excavation depth and radius of the circular shaft, the stress of the structure will increase and the radial deformation of the structure will increase. If the design depth of the shaft is small, the grouting pressure generated during construction will be the most significant factor leading to the deformation of the structure.
4. The experiment shows that grouting pressure is the strongest safety factor that affects the stress state of corrugated steel plates. A reasonable grouting pressure allows the concrete to fill the mold bag and achieve self-compaction so that the steel corrugated plate is evenly stressed. The woven geotextiles and non-woven geotextiles selected for the test are both breathable, permeable, and impervious to the slurry, achieving a wrapping and shaping effect on the concrete.
5. In the field process test research, this paper only carried out the first phase of the test, initially verifying the feasibility of the new support structure construction process and structural mechanical effect. In the next step of the experimental research, an in-depth study should be carried out on how to optimize the design and installation of the mold bag, strengthen the control of construction errors, and improve the construction and installation accuracy and speed. For the corrugated steel panels exposed on the outside to do anti-corrosion treatment, such as taking measures to avoid or reduce the corrosion of steel by spraying plating on the surface or spraying anti-corrosion spray paint.
6. The combination of steel corrugated plate and mold bag concrete for underground engineering support structures is feasible and provides new ideas for underground engineering support technologies such as tunnels and pits, which are of great significance.  
Conceptualization, P.L. and M.Z.; Methodology, P.L. and Z.H.; Software, S.W. and Z.H.; Investigation, P.L.; Resources, P.L., M.Z. and Z.H.; Data curation, S.W. and Z.H.; Writing--review & editing, S.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant No. 51978018 and 52278383), and the Beijing Natural Science Foundation (Grant No. 8222005).  
Data Availability Statement: The data are available from the author upon request. The data are not publicly available due to the data was collected individually by laboratory personnel.  
The authors declare no conflict of interest.