## 1 Introduction
The field monitoring results indicated that when the fault intersects with the tunnel side wall, the displacement at this section is the largest and significantly greater than that of other sections. [PERSON], [PERSON] and others [15] used three-dimensional finite difference computational software to analyze the seismic reduction technology of large-section tunnels in fault fracture zones. They conducted numerical simulations to analyze the effects of different stiffness lining materials and grouting reinforcement of the surrounding rock on the seismic performance of the lining in fault fracture zones. [PERSON], [PERSON] and others [16], based on a continuous-discrete coupled analysis method, established a multi-scale mechanical model for fault engineering sites. They used an artificial synthetic rock mass model based on the discrete element method to simulate the fault fracture zone and applied the finite difference method to describe the macroscopic dynamic characteristics under the influence of tunnel excavation in the hanging and footwalls of the fault. [PERSON] [17] highlighted the application of shotcrete linings in tunnel construction, demonstrating their effectiveness in controlling surrounding rock deformation. [PERSON], [PERSON] [18] explored the application of the ADECO-RS method in tunnel construction, em phasizing the importance of real-time monitoring and adjustment of surrounding rock deformation for tunnel safety. [PERSON], [PERSON] and others [19] analyzed the causes of large deformation phenomena in carbonaceous shale diversion tunnels and proposed control measures for the large deformation stage. [PERSON] [20] focused on the small-section diversion tunnel of the Liangshan Hydropower Station as a case study and explored the construction technology for ultra-small-section diversion tunnels under limited financial conditions. On the other hand, recent advancements in geotechnical modeling and data-driven approaches have also shown a successful degree of contribution to the subsurface characterization of heterogeneous geological conditions [21; 22; 23; 24; 25]. The integration of 3D geo-modeling and ensemble deep learning approaches further enhances prediction reliability and uncertainty analysis in geological modeling [26]. Moreover, researchers, such as [PERSON] et al. [24] and [PERSON] et al. [27], have emphasized the role of groundwater flow and high-stress zones in excavation stability, suggesting that hydromechanical interactions must be thoroughly considered. Stability assessments of stratified rock masses and reinforcement strategies, such as those presented by [PERSON] et al. [28], provide further insights into securing long-wall operations and excavation fronts. Based on the analysis above, it can be found that the most optimal tunnel excavation methods were mainly determined based on engineers' experience; in other words, the determination process is subjective and not scientific, and the optimal result would not be the optimal one. [PERSON], M.A. [29] proposed a novel multi-objective optimization approach for tunnel support design considering construction cost and safety. While their study provided important insights, it did not incorporate dynamic feedback from staged numerical simulations, nor did it implement a fully automated optimization workflow. However, because of the complex and variable geological conditions during tunnel construction, the construction methods for sections with poor geological conditions still require further research and improvement. In order to address the issues of low construction efficiency and tight project timelines in tunnel engineering, this study, based on the Longshu Tunnel of the Central Yunnan Water Diversion Project, which crosses a slate fracture zone, combines numerical simulation with engineering practice to comparatively study the deformation pattern, stress distribution, and instability mechanism of tunnel surrounding rock when using full-face and step-by-step construction methods in a slate fracture zone.