## 3 Result and Discussion  
### Change of the Temperature Field in Model Test  
The original data of temperature field of model test are shown in Figure 6. According to the temperature data in the box, the temperature at each point decreases rapidly after freezing, and the temperature near the freezing pipe decreases most rapidly. After stopping freezing, the temperature of frozen soil increases rapidly in a short time, and there is an interval near 0 \({}^{\circ}\)C. When the temperature rises to this interval, the temperature rise rate slows down, which is roughly [\(-\)1.4, \(-\)0.6]. This is because in this interval, a large amount of ice changes into water and absorbs latent heat. This interval is the main phase change interval of the muddy clay in Shanghai.  
In order to more intuitively show the distribution of the temperature field, the cloudy map of the freezing process is shown in Figure 7 and that of the melting process is shown in Figure 8.  
According to the temperature cloudy map in the freezing process box, it can be seen that with the passage of freezing time, the expansion of the freezing front above the freezing pipe becomes slower and slower.  
Due to the influence of seepage, the temperature field distribution in the freezing process is not symmetrical about the freezing pipe. At 32 h, the \(-\)10 \({}^{\circ}\)C isotherm above the freezing pipe is 8 cm away from the freezing pipe, while the \(-\)10 \({}^{\circ}\)C isotherm below the freezing pipe is only 5 cm away from the freezing pipe. There is strong convection under the freezing pipe, and the freezing curtain is thinner than that above the freezing pipe.  
In the freezing process of combined formation with seepage, the heat transfer can be divided into two parts, including heat conduction in clay layer and heat convection at the junction of the clay layer and sand layer. During the freezing process, the temperature at the liquid inlet of the freezing pipe is 5 \({}^{\circ}\)C lower than that at the liquid outlet, which causes the temperature of the clay layer near the liquid inlet to be lower than that at the liquid outlet. However, the existence of seepage brings the cold energy of the upstream to the downstream, resulting in the downstream temperature lower than that of the upstream. It can be seen from the test data that under the seepage velocity of 1.2 m/d, the temperature cloudy map in the box shows a \"bell mouth\" opening into the upstream. The upstream temperature is lower than the downstream, which is particularly prominent at the beginning  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **Y1** & **Y2** & **Y3** & **Y4** \\ \hline Height (mm) & 5–200 & 5–200 & 110–310 & 290–490 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Sampling location.  
Figure 6: Temperature distribution during the model test.  
of freezing. This shows that when the seepage velocity is 1.2 m/d, the influence of heat conduction in clay layer on soil temperature is greater than that of heat convection.  
Figure 8: _Cont_.  
Figure 7: Temperature distribution nephogram during the freezing stage (**x** stand for the horizontal position and **y** stand for the distance from the freezing center).  
Within 16 h after stopping freezing, the temperature in the area around the freezing pipe increases, and the temperature in the freezing area is gradually uniform, but the area with a temperature less than 0 \({}^{\circ}\)C does not shrink. During the melting process, the influence of seepage is more obvious. As can be seen from Figure 8, seepage accelerates the melting of the upstream freezing curtain and brings the cold energy to the downstream, delaying the melting of the downstream.