## 5 Conclusions  
This paper explores the application of smoothed particle hydrodynamics (SPH) in tunnel and underground engineering, highlighting its capability to address complex issues related to large deformations and dynamic loading conditions. By deriving the theoretical framework of SPH and presenting two practical examples, this study validates the method's efficacy in simulating intricate tunnel scenarios. SPH has emerged as a significant advancement in numerical simulation, offering a robust framework for analyzing and optimizing tunnel design and construction processes, as well as effectively managing large-scale deformations and dynamic loads. The findings from this research not only affirm SPH's practical utility, but also establish a foundation for future advancements and refinements in numerical simulation techniques for tunnel and underground engineering. The key conclusions are summarized as follows:  
1. This study provides a comprehensive examination of the application and future prospects of smoothed particle hydrodynamics (SPH) in tunnel and underground engineering. It thoroughly reviews the defining characteristics and theoretical foundations of the SPH method, supplemented with examples derived from the Navier-Stokes (N-S) equations. The discussion highlights SPH's distinctive advantages in addressing complex flow dynamics and large deformation challenges. As a meshless technique, SPH demonstrates significant efficiency in managing dynamic boundaries, fluid-solid interactions, and multi-field coupling conditions. 2. As a meshless technique, smoothed particle hydrodynamics (SPH) offers distinct advantages for simulating fluid-solid interactions and multi-field coupling conditions, particularly in the dynamic environments of tunnel construction involving water and mud flows. SPH has demonstrated its versatility across a range of fields, including astrophysics, fluid dynamics, and solid mechanics. Its effectiveness in addressing various physical phenomena and incorporating material properties underscores its ability to enhance the accuracy of simulation results. The applicability of SPH is further validated through simulations of water and mud behavior during tunnel construction, the interaction between rock and soil structures, and the failure mechanisms of brittle materials. 3. This paper provides an in-depth examination of the theoretical foundations of smoothed particle hydrodynamics (SPH) and its potential applications in tunnel excavation and underground engineering. SPH has proven effective in simulating fluid dynamics and soil deformation during tunnel blasting, with simulation outcomes aligning closely with actual monitoring data. This validates the method's feasibility and effectiveness for practical engineering applications. Despite its notable performance in managing large deformations in soft rocks and the mechanical cutting of hard rocks, SPH faces significant challenges. These include the complexity of constitutive models for surrounding rock masses, the rheological characteristics of tunnel structures, and the inherent spatiotemporal effects during excavation. Additionally, the fundamental theory of SPH requires further development, particularly in multi-field coupling scenarios. As computational resources and SPH theory continue to advance, the method's applicability in tunnel and underground engineering is expected to expand. Future research should focus on addressing current technological limitations and exploring synergistic approaches with other numerical simulation techniques to enhance SPH's utility in complex engineering environments. In summary, SPH, as an emerging numerical simulation tool, holds significant promise for tunnel and underground engineering and warrants further investigation and practical application. Conceptualization, R.F. and T.C. ; methodology, R.F. ; software, T.C. ; validation, M.L. and S.W. ; formal analysis, R.F. ; investigation, R.F. ; resources, T.C. ; data curation, M.L. ; writing--original draft preparation, R.F. ; writing--review and editing, T.C.