## 4 Value Analysis of Deformation Allowance of Loess Tunnel
### Suggestions on the Value of Deformation Allowance of Loess Tunnel  
Designing the deformation allowance is a crucial step in tunnel design. Given the unique characteristics of loess, establishing a reasonable deformation allowance range is essential for loess tunnel design and construction. Table 2 in the specifications provides the deformation allowance. Statistical analysis of various loess tunnels reveals that the arch crown settlement is greater than surrounding convergence. Using correlation calculations and considering different factors like soil type, excavation span, surrounding rock levels, moisture content, and cover depth, all three analysis methods consistently identify soil type and excavation span as influential factors with strong correlations. In clay loess and silty loess sections, where moisture content ranges from 14.3% to 22.59%, arch crown settlement typically remains below 100 mm. Hence, a deformation allowance of 10 cm is suitable. In sandy loess sections, moisture content typically falls between 10.3% and 26.5%, with arch crown settlement accumulating within the range of 75 mm to 200 mm. Hence, a deformation allowance of up to 20 cm can be chosen. Considering the cover depth, a deformation allowance of 12 cm is appropriate for depths ranging from 0 to 50 m. For cover depths greater than 50 m, a deformation allowance of 16 cm provides an 84.6% guarantee rate. Based on soil type, surrounding rock levels, cover depth, and moisture content, a suggested range of deformation allowances is provided. Refer to Table 3 for the recommended values:  
Based on the statistical and analytical results presented above, and taking into account the complex and variable geological conditions in loess areas--where tunnels are typically located in both collapsible and non-collapsible loess strata, as well as in areas with heterogeneous fill and newly accumulated loess--the deformation allowance derived from actual monitoring and statistical analysis significantly exceeds the specified value. Based on the summary and analysis, and taking into account factors like cover depth, moisture content, and surrounding rock grade, and referring to the specifications, the recommended deformation allowance values for loess tunnels are provided in Table 4. The appropriate deformation allowance should be determined by considering factors like initial support type and parameters, lining construction timing, and lining parameters in a comprehensive manner. Both safety and cost-effectiveness must be taken into account. Thus, for diverse loess tunnels in different regions, a comprehensive evaluation of various factors is necessary, and the proposed values should be further verified and analyzed through field measurements for validity.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Lane type (Excavation Span)** & **Two Lane (<14 m)** & **Three Lane (>14 m)** \\
**Surrounding Rock Grade** & **Two Lane (<14 m)** & **Three Lane (>14 m)** \\ \hline Grade IV surrounding rock & 5–8 cm & 8–12 cm \\ Grade V surrounding rock & 8–12 cm & 12–15 cm \\ \hline \hline \end{tabular}
\end{table}
Table 2: Specification deformation allowance (JTGD).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Grade IV Surrounding Rock** & **Grade V Surrounding Rock** \\ \cline{2-5}  & **Vault/cm** & **Side Wall/cm** & **Vault/cm** & **Side Wall/cm** \\ \hline Clayey loess & \textless{}10 & \textless{}4 & 15–25 & 10–15 \\ Silty loess & \textless{}10 & \textless{}4 & \textless{}15 & \textless{}10 \\ Sandy loess & 15–20 & 8–12 & 35–47.5 & 15–20 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Suggestions on the value of deformation allowance.