# Reasonable Support Parameters for Loess Tunnel Under Different Water Contents: A Case Study
## 5 Conclusions  
Based on Fengshouling Tunnel, the support parameters with grade V rock mass suitable for different water contents were investigated by the rock strength test, on-site monitoring, and numerical simulation. The results can be summarized as follows:  
1. The cohesion \(c\) of the loess decreased exponentially with the water content, whereas the internal friction angle \(\varphi\) showed a linear correlation with the water content. Additionally, it was observed that cohesion \(c\) was more significantly influenced by changes in water content compared to the internal friction angle \(\varphi\). 2. The crown settlement was greater than the horizontal convergence. The maximum deformation value was 64 mm, which is much less than the design reserved deformation. The primary support structure mainly bore the compressive stress, and the force on the crown was greater than at other sites, where it was smaller and distributed symmetrically. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Support Parameters**} & **Shotcrete** & \multicolumn{2}{c}{**Steel Rib**} & \multicolumn{2}{c}{**Safety Factor**} \\ \cline{2-7}  & **Axial** & **Axial** & **Bending** & **Stocrete** & **Steel Rib** & **Steel Rib** \\  & **Force/kN** & **Force/kN** & **Moment/kN\_m** & **Stocrete** & **Compressive** & **Tensile** \\ \hline condition 1 & 1718.7 & 238.9 & 61.8 & 2.6 & 1.8 & 2.9 \\ condition 2 & 1493.4 & 220.9 & 45.2 & 3.1 & 2.1 & 3.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Calculation results under saturated water content. Figure 19: Distribution of safety factors. 3. The deformation of the surrounding rock is much smaller than the reserved deformation, and the stress value of the support structure is much smaller than the design value. The support structure has a large safety reserve and does not meet the needs of economic construction. The optimization of the support parameters can be achieved by reducing the support strength. 4. The deformation behavior under different water contents can be characterized by three stages: rapid growth, continuous growth, and slow growth. It was observed that the final settlement value at the crown increases linearly with the water content under same support strength. The surrounding rock deformation in loess tunnels predominantly occurs during the initial phase of excavation. This suggests that strict control over the surrounding rock deformation is essential during the early construction stage of loess tunnels. 5. For general water content levels ranging from 10% to 17%, the recommended support parameters consist of I18 steel ribs with a longitudinal spacing of 60 cm and C25 shotcrete with a thickness of 24 cm. For high water content levels between 17% and 25%, the suggested parameters are I20a steel ribs with the same 60 cm spacing and C25 shotcrete increased to a thickness of 26 cm. In cases of extremely high water content exceeding 25%, the surrounding rock is reinforced with curtain grouting, and the support parameters recommend I22a steel ribs with a 60 cm longitudinal spacing and C25 shotcrete with a thickness of 26 cm. Conceptualization, H.C. ([PERSON]) and Y.L. ([PERSON]); Methodology, Y.L. ([PERSON]) and J.C.; Validation, H.C. ([PERSON]); Formal analysis H.C. ([PERSON]); Resources, Y.L. ([PERSON]); Investigation, H.C. ([PERSON]); Data curation, L.G. and P.W. ; Supervision, H.C. ([PERSON]) and Y.L. ([PERSON]). All authors have read and agreed to the published version of the manuscript. The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 52278394), by \" Key Research and Development Program of Shaanxi Province\" (Grant No. 2024 SF-YBXM-628), and by \"Shaanxi Province Innovation Capability Support Plan\" (Grant No. 2024 RS-CXTD-42). Not applicable. The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy. The authors are grateful for the comments provided by the anonymous reviewers.