## 5 Conclusions  
In response to settlement control requirements in complex and sensitive areas, this study takes the large-section tunnels beneath the Beijing Capital Airport runway and the Taiyuan Railway Station as the background to develop a theoretical approach and key techniques for micro-settlement control. The following conclusions are drawn:  
1. A theoretical method for surface micro-settlement control in large-section tunnels is proposed. A calculation formula is derived to reveal the relationships between surface settlement and factors such as pipe roof stiffness, support system stiffness, pipe roof construction procedures, and groundwater level changes. Surface micro-settlement control techniques, including increasing the pipe roof stiffness, reinforcing the support system, reducing the group pipe effects, maintaining pressure and reducing resistance around the pipe, and controlling the groundwater level, are proposed.
2. The stiffness of the pipe roof and the overall stiffness of the support system are critical factors in controlling surface micro-settlement in large-section tunnels. This study proposes a method for determining the appropriate stiffness for the pipe roof and support system. The stiffness should be selected from the transition segment between the steep decline and the gentle slope on the stiffness-settlement curves of the pipe roof and the support system. If the stiffness of the pipe roof combined with the primary and temporary supports fails to meet the requirements for micro-settlement control, an integrated support system with higher stiffness can be adopted.
3. The primary causes of surface settlement during pipe roof construction are voids between the pipes and the surrounding soil, as well as the settlement superimposition effect of group pipe construction. This study proposes a pressure-maintaining and resistance-reducing technique around the pipe by appropriately regulating grouting pressure to minimize surface settlement during single-pipe jacking. It is recommended that the spacing between simultaneously installed jacked pipes be greater than half the width of the settlement trough (\(\lambda i\)) to mitigate the settlement superimposition effect.  
Tunnel construction causes groundwater loss, resulting in a drop in the groundwater level, which subsequently leads to over-consolidation settlement in the soil. For over-consolidation-sensitive strata, such as medium sand and coarse sand, water-blocking methods like freezing, grouting, or a combination of both are required. In cases where a circumferentially enclosed pipe roof is used, the internal pipe freezing method is prioritized for its superior effectiveness. For over-consolidation-insensitive strata, such as gravel and cobble with high permeability, water-blocking treatments are not required.  
Conceptualization, Z.T. and Z.Z.; methodology, Z.T.; investigation, Z.D.; resources, Z.L.; writing--original draft preparation, Z.T., and Z.D.; writing--review and editing, Z.Z. and Z.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Key R&D Program of China (2023 YFC3806701).  
The data that support the findings of this study are available upon request from the corresponding author, [[PERSON]], upon reasonable request.  
Authors [PERSON] and [PERSON] were employed by the company China Railway Construction Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.