## 2 Project Background
### Freezing Program for the FSPR Method  
A horizontal zoning and vertical segmentation freezing method is used to avoid an excessive volume of frozen soil, according to the freezing construction organization design papers and pertinent requirements of the Gongbei Tunnel. As indicated in Figure 3, horizontal zoning freezing is mostly used to collaborate with underground excavation construction by separating the pipe curtain section into five zones: A, B1, B2, B3, and C. Vertical zoning freezing entails establishing two freezing stations on the east and west sides of the tunnel near the working shafts at both longitudinal ends, each with two systems (a refrigeration system and a limit system). To manage the opening and shutting of freezing tubes and limiting tubes, the pipe curtain is separated into three zones: freezing zone 1 (64 m), freezing zone 2 (128 m), and freezing zone 3 (64 m) from east to west. \begin{table}
\begin{tabular}{c c c} \hline
**Number** & **Soil Layer Name** & **Soil Layer Characteristics** \\ \hline I & Artificial backfill & \begin{tabular}{c} Composed primarily of granite residual soil layer, crushed stone, and fine sand, it is \\ mildly to moderately compacted and has high permeability. \\ \end{tabular} \\ \hline III-1 & Muck and mucky soil & \begin{tabular}{c} Mostly silt and mucky sand, with consistent development, uniform thickness, flow \\ plastic shape, high water content, high compressibility, and poor strength. \\ \end{tabular} \\ \hline III-2 & Clay and silty clay & \begin{tabular}{c} Mostly silty clay and silt, with iron-manganese nodules and a lot of sand, loose to \\ slightly dense, with a thickness gradually increasing from west to east. \\ \end{tabular} \\ \hline III-3 & Gravelly sand & Primarily made up of gravel sand, loose to slightly dense, well-permeable, and high \\  & & in water content. \\ \hline IV-3 & Mucky soil & \begin{tabular}{c} Mostly mucky soil and humus silt, flow plastic, steady development, high water \\ content, high compressibility, low strength, with soft soil characteristics. \\ \end{tabular} \\ \hline V-1 & Silty clay & Mostly silty clay, unevenly developed, with a maximum thickness of 4.5 m. \\ \hline V-2 & Coarse and gravel sand & \begin{tabular}{c} Mostly medium sand, coarse sand, and gravel sand, widely distributed, mainly in a \\ medium-dense state, slightly sticky. \\ \end{tabular} \\ \hline V-3 & Mucky soil & \begin{tabular}{c} Mostly muddy soil with high water content, high compressibility, flow plastic, \\ under consolidation, and low strength. \\ \end{tabular} \\ \hline VI-2 & Coarse and gravel sand & \begin{tabular}{c} Mostly coarse and gravel sand, with uneven development and significant interface \\ undulation, with medium density. \\ \end{tabular} \\ \hline VII-1 & Residual soil & \
\begin{tabular}{c} Mostly weathered residual soil of granite, unevenly developed, moist, moderately \\ dense. \\ \end{tabular} \\ \hline VIII-1 & Weathered granite &
\begin{tabular}{c} Mostly made up of Biotite porphyry granite that has fully weathered. After \\ absorbing water, the weathered rock mass has become a loose structure that is \\ simple to soften. \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 1: Soil layer characteristics in concealed excavation section. Excessive frost heave can easily result in project risks since the tunnel passes through the customs port complex, near a significant number of subterranean pipelines and building piles, and is primarily surrounded by water-rich and soft soil layers. There are stringent criteria for mitigating the influence of ground frost heave, and the thickness of the frozen soil curtain should not be excessive. To achieve the above two requirements, careful real-time monitoring of the freezing temperature field is required during the building process:  
1. Water sealing between jacked pipes should be safe with a minimum design value of frozen soil curtain thickness.