## 4 Conclusions  
The soft and fractured strata can cause significant deformation of surrounding rock during tunnel excavation. This study uses Dali Section I of the central Yunnan Water Diversion Project, Construction Section 3, as its research background. Researchers adopted the field monitoring test method for the monitoring section. To further verify the accuracy of the field monitoring test, numerical simulation simulated the stress-strain changes in the surrounding rock during the excavation process of the monitoring section. The results of the field monitoring experiment were compared and verified with the results of numerical simulation, and the original support scheme was optimized to address the issue of significant deformation in fractured and soft rock layers during tunnel excavation. The primary conclusions are as follows:  
1. The stress variations in the monitoring test exhibited a pattern of a steep increase, followed by a gradual decrease with continuous excavation. In the first 8 m of excavation, there was a pronounced increase in stress, succeeded by a decelerated growth rate in the subsequent 12 m, ultimately stabilizing. This pattern suggests that the initial stress changes rapidly due to the stress release from tunnel excavation. As time progresses, the stress redistributes and gradually stabilizes.
2. The deformation of the surrounding rock post-tunnel excavation was suboptimal. However, through the analysis of optimized support conditions, the simulated crown displacement was decreased to 0.16 cm, achieving a reduction of 96% relative to the original scheme. In softer and more intricate geological layers, the displacement was minimized to 0.52 cm, reflecting a reduction of 87%. This optimization scheme can enhance construction safety. Regarding the existing support system, the peak stress on the monitored steel arch was \(-\)18.92 MPa, suggesting compression. The optimization thus involved reinforcing the support with H150 steel arches in a closer arrangement. Since shotcrete support was not utilized during construction, monitoring and numerical simulation indicated that the axial force at the crown could attain 69.02 kN. Hence, in actual construction scenarios, anchor rods should be integrated promptly post-initial support, followed by shotcrete application to establish a comprehensive shotcrete support system.  
The deformation of the surrounding rock is pivotal for tunnel construction safety. Field monitoring tests remain the most direct method to assess the stability of the surrounding rock. When paired with numerical simulation for comparative analysis, effective solutions to the problem of extensive deformation in the surrounding rock can be proposed. This dual-method approach offers protection and guidance for the safe construction of this tunnel and other analogous tunnels.  
Conceptualization, T.W. and H.L.; methodology, M.K.; software, B.Z.; validation, T.W., H.L. and J.S.; formal analysis, Y.L.; investigation, T.W.; resources, H.L.; data curation, T.W. and Y.Y.; writing--original draft preparation, T.W.; writing--review and editing, H.L.; visualization, T.W.; supervision, H.L.; project administration, M.K.; funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Postgraduate Education Reform and Quality Improvement Project of Henan Province, grant numbers YJS2022 JD02 and YJS2022 AL006.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
Authors [PERSON] and [PERSON] were employed by the company Henan Water Conservancy Investment Group 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.