## 2 Deformation Allowance of Loess Tunnel and Statistical Monitoring Samples
### Basic Information of Monitoring Samples  
This paper's statistical samples consist of 148 monitoring sections from 29 tunnels, as indicated Table 1. Relevant instructions are as follows:  
1. The data in this paper are sourced from published academic papers. Some information cannot be found, and it is replaced by \"/\" in the table. 2. In the table, the surrounding rock grade and cover depth pertain to the conditions at the monitoring section's location. Figure 3: Deformation composition of tunnel surrounding rock. 3. According to the classification of surrounding rock according to the specifications [34], the higher the level of surrounding rock, the looser the soil, and the worse the properties of the surrounding rock. From the rock mass level, it can be seen that the rock mass levels of each tunnel monitoring section are classified into grade IV and grade V. The grade IV surrounding rock consists of a crushed stone-like structure, characterized by a loose arrangement of angular gravel and soil that is relatively soft. Due to the soft texture of the loess layer, which contains small and loose particles, most of the surrounding rock is classified as grade V, with grade IV surrounding rock being relatively scarce. 4. In terms of excavation methods, the bench method comprises the majority of the monitoring sections, while the CRD (Cross Diaphragm) and CD (Center Diaphragm) methods represent smaller proportions. So, even though the loess tunnel falls into the category of soil tunnel, the bench method does not perform as well as the CRD method and CD method in managing stratum deformation. However, due to its notable benefits in construction efficiency and cost-effectiveness, it remains the primary construction approach for loess tunnels. 5. Regarding the spatial distribution of tunnels, among the 29, 10 are in Shaarxi Province, 9 in Gansu Province, 4 in Henan Province, 3 in Shanxi Province, 1 in Ningxia Hui Autonomous Region, 1 in Qinghai Province, and 1 in Inner Mongolia Autonomous Region. 6. Due to the complexity of loess's geological and hydrological conditions, tunnels vary significantly. Typically, a loess ground profile consists of Q1, Q2, Q3, and Q4 loess layers, each with distinct physical and mechanical properties in ascending order. There are 10 tunnels in the clayey loess, 13 in the sandy loess, and 7 in the silty loess sections. 7. Typically, tunnel deformation allowances in loess areas consider factors such as cover depth, tunnel width, surrounding rock conditions, construction method, and moisture content variations. In the research, qualitative descriptions prevail, and quantitative analysis is infrequent. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Geographical** & **Tunnel** & **Soil Type** & **Excavation** & **Surrounding** & **Moisture** & **Excavation** & **Reference** \\  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Geographical** & **Tunnel** & **Soil Type** & **Excavation** & **Surrounding** & **Moisture** & **Excavation** & **Reference** \\
Generally, typical loess in China consists of aeolian sediments [62; 63; 64], supported by the following characteristics. Loess soil particles in China transition from coarse to fine and from northwest to southeast. The mineral and chemical composition of loess exhibits relative consistency across a wide area, with more significant variations observed in the northwest region compared to the southeast. In Liu [65], China's loess is classified into three regions from northwest to southeast: sandy loess, silty loess, and clayey soil (Figure 4). Moving from sandy loess to clayey loess, there is a decrease in sand content and porosity, along with an increase in clay content and cohesion. Tunnel mechanical properties and deformation behavior vary among different loess soil types. Hence, deformation allowance design for loess tunnels of varying soil types should also differ.