## 4 Engineering Case Study
### Engineering Background
The support pressure decreased for jacking distances from 10 m to 25 m. At jacking distances from 25 m to 50 m (river impact area), the support pressure values were also constant, at 2669.977 kN, 309.910 kN, and 1044.870 kN, respectively. However, there were some differences between the three: when the jacking distance exceeded the intersection of the underground water level line and the dike line, the decrease rates for the upper limit value P\({}_{\rm max}\) and lower limit value P\({}_{\rm min}\) increased, while the optimal value P\({}_{\rm o}\) did not change significantly. When the jacking distance was greater than 20.24 m (moving from the non-river impact area to the river impact area), the optimal value of the tunnel-face support pressure, P\({}_{\rm o}\), changed abruptly, decreasing from 2351.527 kN to 1133.573 kN, but there was no significant change in the upper limit value P\({}_{\rm max}\) or the lower limit value P\({}_{\rm min}\). In the process of pipe jacking at distances from 20.24 m to 25 m, the rate of decrease in the optimal value P\({}_{\rm o}\) showed a linearly decreasing law. Each decrement was approximately 22.5 kN, which is reflected in the partial enlargement in the upper left corner of the bottom half of the graph in Figure 11. The overburden thickness was 3 m at jacking distances from 25 m to 50 m. In Section 3.3, the numerical simulation allowed us to calculate the ultimate support pressure as 881.786 kN, while the optimal value of support pressure at the tunnel face was 1044.87 kN. These results are relatively similar. It can be seen in Figure 11 that the measured value of the tunnel-face support pressure, P\({}^{\prime}\), fluctuated between 1700 kN and 3600 kN. The measured value P\({}^{\prime}\) basically lay between the upper limit value P\({}_{\rm max}\) and the lower limit value P\({}_{\rm min}\), and it was most similar to the optimal value P\({}_{\rm o}\) in the non-river impact area and to the upper limit value P\({}_{\rm max}\) in the river impact area. In an actual pipe-jacking process, the parameters that need to be considered include the jacking force, peripheral friction, and jacking face resistance, as shown in Figure 12. Firstly, the jacking force exerted by the jacking machine should not be varied frequently over short distances and needs to be kept constant. It is difficult to achieve a continuous change in the applied jacking force over a short period of time. Secondly, if the jacking force is kept constant, the peripheral friction becomes larger and the jacking face resistance becomes smaller as the pipe is jacked (a pair of interacting forces, the jacking face resistance and the support force, which is equal in magnitude and opposite in direction to the jacking face resistance). Finally, as the jacking distance increases, the peripheral friction increases and the jacking speed decreases. When the pipe is jacked a certain distance, the jacking force needs to be increased to restore the jacking speed. In this background project, when the jacking distance is from 4 to 12 m, the jacking force is kept constant at 4000 kN. In this process, the peripheral friction gradually becomes larger, and the jacking face resistance becomes smaller (the support force becomes smaller), its value decreases from 3278 kN to 1834 kN. When the jacking distance is from 24 m to 43 m, the jacking force remains unchanged at 6000 kN, but the peripheral friction becomes larger and the jacking face resistance decreases from 2675 kN to 1819 kN. As a result, the measured value P\({}^{\prime}\) fluctuates up and down. The focus of this paper is on the study of support pressure in the tunnel face. There is not a lot of research work carried out on the construction of three lines of pipe jacking. Clearance \(D\) is a very important parameter in pipe-jacking construction projects.