## 4 Engineering Case Study  
### Engineering Background  
The Y25-Y26 pipe-jacking project across the Chu River channel is located in Shuangdun Town, Changfeng County, Hefei City, Anhui Province. The project site belongs to the Jianghuai undulating plain landform, with a micro-landform of hills and valleys. The geotectonic location of the region belongs to the southern edge of the North China Plateau, and the secondary tectonic unit belongs to the Hefei Basin. The stratigraphic distribution from top to bottom is as follows: filling soil, 0.5 to 1.5 m thick; silty chalky clay, 1.2 to 2.7 m thick; clay, 5.9 to 7.5 m thick; silty clay, 4.8 to 6.0 m thick; strongly weathered mudstone, 1.7 to 2.6 m thick. The physical and mechanical parameters of each stratigraphic layer are shown in Table 2.  
The Y25-Y26 project crossing the Chu River channel adopted mud-water-balanced mechanical pipe jacking. The pipe jacking crossed the stratum of silty clay, and the thickness of the pipe-jacking cover layer was 3.0 m. The width of the river valley in the section where the pipeline crosses is 55 m, the width of the river bed is 25 m, the height of the river bank is 9.46 m, and the average depth of the river water in the dry season is about 3.0 m. The pipe jacking was composed of three circular reinforced concrete pipes with an internal diameter of 3.0 m and an external diameter of 3.6 m. The sandwich width of adjacent pipes was 2.8 m. The project had remarkable features such as high environmental requirements, a small overburden thickness, and a large pipe diameter. Controlling the stability of the pipe-jacking tunnel surface was the priority of engineering control. Section and plan views of the place where the pipe jacking crosses the river are shown in Figure 10.  
Figure 9: Relationships between different support ratios and the displacement of the tunnel face: (a) H\({}_{s}\) = 3 m; (b) H\({}_{s}\) = 2 m; (c) H\({}_{s}\) = 1 m.  
In Figure 11, according to the principle of optimal value calculation, the jacking area was divided into a non-river impact area (0-20.24 m and 54.76-75 m) and a river impact area (20.24-54.76 m). The underground water level was \(-\)3 m from the surface of the riverbank. The construction stratum was located in a silty clay layer, and K\({}_{0}\) was the static soil pressure coefficient, taken as 0.7. The heights described by H\({}_{\text{a}}\), H\({}_{\text{b}}\), H\({}_{\text{c}}\), H\({}_{\text{r}}\), H\({}_{\text{s}}\), H\({}_{\text{u}}\), and H\({}_{\text{w}}\) in the above equation are also given in the upper part of Figure 11. The dimension of the simple model of the project in the upper half of Figure 11 was kept the same as the jacking length in the bottom half of the figure. The bottom half of Figure 11 describes the upper limit value P\({}_{\text{max}}\), the lower limit value P\({}_{\text{min}}\), the optimal value P\({}_{\text{o}}\), and the measured value P\({}^{\prime}\) of the support pressure on the tunnel face for different jacking distances. A partial enlargement of the optimum value for jacking distances from 20.24 m to 25 m is also given. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Jacking** & **Jacking** & **Measured** & **Jacking** & **Jacking** & **Measured** \\
**Distances (m)** & **Force (kN)** & **Value P\({}^{\prime}\) (kN)** & **Distances (m)** & **Force (kN)** & **Value P\({}^{\prime}\) (kN)** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 6: Background project measurements. Figure 11: Support pressure values on tunnel face at different jacking distances. The change rules for the upper limit value P\({}_{\rm max}\), lower limit value P\({}_{\rm min}\), and optimal value P\({}_{\rm o}\) of the tunnel-face support pressure were basically the same: At jacking distances from 0 m to 10 m (non-river impact area), the support pressure values were constant at 4868.249 kN, 2230.598 kN, and 3098.977 kN, respectively.