## 6 Conclusions  
For the open excavation and story-adding project at the Pingguo Yuan Station of Beijing metro line 6, the existing station and retaining structure of the foundation pit created by open excavation were analyzed using finite element simulation. Subsequently, the effects of three stabilizing measures were verified based on field measurements. Accordingly, the following conclusions are drawn:  
1. After unloading the soil above the station, the vertical load at the top disappeared. Under lateral and bottom loads acting on the station, the overall station uplift in the foundation pit area changed from subsidence to uplift, and the uplift value of the roof reached 2.59 mm. The sidewall exhibited an internal crowding trend, and the stress state of the station top plate changed to upper tension and lower compression. The stress state of the upper sidewall is changed to outside compression. In addition, the inside tension state and the stresses on the station bottom plate and lower sidewall are relieved.
2. After the construction of the station, the surrounding rock is partially broken on both sides of the station. Accordingly, the plastic zone is further expanded owing to the foundation pit excavation, which is simultaneously connected to the underground excavation fracture surface. This enlarges the plastic zone of the surrounding rock, leading to an increase in the horizontal load acting on the station, which then leads to an internal extrusion trend within the structure.
3. Under the existing structure, the horizontal displacement of the embedded pile can be well controlled; however, the bottom of the end-suspended pile exhibits an obvious slip. The effects of connecting the pile bottom and primary support of the existing station, establishing grouting reinforcement after pile construction, and reserving earth Derm on the horizontal displacement of the end-suspended pile are discussed through numerical simulation. Subsequently, we concluded that after establishing the connection between the pile bottom and primary support of the existing station, the slip of the pile bottom was significantly reduced, and the piles bulged in the middle with stability. After grouting reinforcement, the horizontal displacement of the pile decreased by 60%, and the slip of the pile bottom persisted. Reserving the earth Derm has a certain controlling effect on the pile bottom slip of the end-suspended pile at the pit corner.  
Figure 20: Comparison diagram of uplift value of station top plate.  
* Based on finite element simulation, combined with the measured analysis, after establishing three safety reinforcement measures, the maximum horizontal displacement of the end-suspended pile was controlled to within 1.5 mm. Moreover, the pile bottom hardly slipped, and the stability of the retaining structure was significantly improved, all of which can provide guidelines for similar constructions.  
Conceptualization, J.W. and S.W.; methodology, H.L.; software, H.L.; validation, E.L., Z.W., and S.W.; formal analysis, S.W. and H.L.; investigation, S.W.; resources, E.L.; data curation, H.L.; writing--original draft preparation, H.L.; writing--review and editing, J.W. and S.W.; visualization, S.W.; supervision, S.W.; project administration, E.L.; funding acquisition, J.W. and Z.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Shandong Provincial Key R&D Program (Major Scientific and Technological Innovation Project) Project, grant number 2019 JZZY010428, the Postdoctoral Innovation Project of Shandong Province, grant number 202003080, the Natural Science Foundation of Shandong Province, grant number ZR2020 QE265 and China Railway Construction Corporation Limited's 2020 annual scientific research plan, grant number 2020-B05.  
Not applicable.  
The data are contained within the article.  
We are grateful to the China Railway Fourteenth Bureau Group Co., Ltd., for providing us with the experimental platform.  
The authors declare no conflict of interest.