## 4 Results of Ground Heating Method  
### Heat Transfer Characteristic  
Figure 7 shows the variation of the ground temperature due to ground heating. Figure 7a shows the whole sensor. The temperature of the ground was increased by heating twice for 48 h. The pattern in which the temperature was decreased after the heating was finished was also similar. Figure 7b compares the variation in the ground temperature due to the installation of the steam discharge hole. As in the study by [PERSON] [2], the heat energy was lost due to the discharge of the water vapor. However, in the secondary heating, the electric heating pipe of Case 1 was lost and it was not heated. The electric heating pipe has a weak connection with the lead wire supplying the electric power. Therefore, for practical use, it is necessary to improve the connection part. The temperature of the ground in Case 4, which is 3 degrees in the 120-degree direction, was increased up to 86.0 degrees.  
Figure 6: The conceptual diagram of the field experiments performed in this study.  
Figure 7: The variation of the ground temperature due to ground heating. (**a**) Overall sensor; (**b**) comparison of the temperature variation due to the installation of the steam discharge hole.  
The test results first examined the temperature change by the ground heating. Table 2 and Figure 8 show the temperature measured in the Case 1 experiment. Table 2 shows the initial temperature of each sensor, the maximum temperature due to heating, and the temperature gradient (\(dT/dt\)) at the first heating (48 h). The initial temperature was increased from 18.0 \({}^{\circ}\)C to 48.43 \({}^{\circ}\)C at a point 0.3 m away from the electric heating pipe with an exothermic temperature of 450.0 \({}^{\circ}\)C. The temperature gradient was 0.413 \({}^{\circ}\)C/h at 30 cm and decreased with the increase in the distance from the heat source.  
The temperature changes shown in Table 2 and Figure 8a were considered as the temperature change curves according to the horizontal distance from the heat source. As shown in Figure 8b, it was expressed as a power function of \(y=405.6x^{-0.63}\). In addition, the temperature gradient along the horizontal distance was in the form of an exponential function: \(y=1.1559e^{-0.035x}\) (Figure 8c).  
Figure 9a shows the change of the underground vapor pressure due to ground heating. The vapor pressure sensor was installed at the same point as thermometer No. 2 and No. 3-3. The vapor pressure increased by about 20 kPa due to the ground heating and the pore water evaporated into water vapor. As shown in Figure 9b, it was confirmed that the air bubbles leaked to the surface of the ground and the water vapor generated by the heating was discharged to the surface because the sensor was not shielded.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Sensor** & \(D\) (**m**) & \(T_{i}\) (\({}^{\circ}\)C) & \(T_{max}\) (\({}^{\circ}\)C) & \(dT/dt\) (\({}^{\circ}\)C/h) \\ \hline
3-1 & 0.3 & 18.00 & 48.43 & 0.413 \\
3-2 & 0.6 & 17.18 & 29.56 & 0.141 \\
3-3 & 1.2 & 16.01 & 20.21 & 0.018 \\ \hline \end{tabular} \(D\): Horizontal distance from the heat source; \(T_{i}\): Temperature before heating; \(T_{max}\): Maximum temperature after heating; \(dT/dt\): Temperature gradient.  
\end{table}
Table 2: The temperature variation due to ground heating (Case 3).