## 5 Conclusions  
This study investigated the application of coir fiber in different soil types by analyzing its effects on soil physical properties, mechanical properties, and crack resistance. The key conclusions are as follows:  
1. The incorporation of coir fiber significantly increased soil porosity and aeration, with the most pronounced improvement observed at fiber lengths in the range of 3-4 cm and content in the range of 0.4-0.6%, leading to enhanced water permeability and aeration performance.
2. Fiber reinforcement notably improved soil cohesion and shear strength, with the optimal combinations being 3 cm fiber length and 0.4% content for Soil-type A, and 4 cm fiber length and 0.4% content for Soil-type B.
3. Fiber length and content had a significant effect on crack resistance, with 3-4 cm fibers at 0.4% content exhibiting the best performance.
4. Correlation analysis revealed a strong positive relationship between soil porosity and shear strength, confirming that fiber reinforcement not only improves pore structure but also enhances overall mechanical performance. Additionally, the crack ratio showed a negative correlation with shear strength and reinforcement coefficient, indicating that fiber incorporation effectively mitigates the adverse impact of cracks on soil stability.
5. Response surface analysis identified the optimal fiber combinations for Soil-type A as a 3.051 cm fiber length and 4.07% content, and for Soil-type B as a 3.376 cm fiber length and 0.456% content.  
Overall, coir fiber significantly improved both the physical and mechanical properties of the soil, particularly in enhancing shear strength and crack resistance. The findings provide a theoretical basis for selecting appropriate fiber length and content combinations in engineering applications and clarify the interrelationships between key parameters, offering valuable insights for future research and practical implementation. This research offers a promising reference for the practical application of natural fiber-reinforced ecological restoration materials in highway slopes for protection.  
Conceptualization, W.Z.; Methodology, Y.Q. and W.Z.; Software, Y.Q.; Validation, Y.Q. and Y.L.; Formal analysis, Y.Q.; Investigation, P.W., W.Z., L.W. and G.Y.; Resources, G.S.; Data curation, Y.Q., W.Z. and Y.L.; Writing--original draft, Y.Q. and W.Z.; Writing--review & editing, Y.L. and G.S.; Supervision, G.S.; Project administration, Y.Q. and G.S.; Funding acquisition, P.W., L.W. and G.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Shanxi Luqiao Construction Group, Horticulture Greening Engineering Co., Ltd., under the Major Scientific Research Project Cooperation Agreement (grant number 200-662504058). The APC was also funded by this project.  
Not applicable.  
No new data were created or analyzed in this study.  
Authors [PERSON], [PERSON] and [PERSON] were employed by the company Shanxi Luqiao Construction Group, Horticulture Greening Engineering 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. The sponsors had no role in the design, execution, interpretation, or writing of the study.