## 5 Discussion  
As a result of the measurement of the temperature change of the ground due to ground heating, it was confirmed that it reduced to the power function according to the horizontal distance from the heat source. The underground temperature distribution of the energy pipe for utilizing geothermal energy uses the linear heat source model by [PERSON] (1882). In this model, the temperature decreases from the heat source in the form of an exponential function and the tendency is similar to the decreasing function determined from the experimental results of this study.  
The phase change of the pore water due to ground heating occurred through the generation of the pressure gauge and the bubble. This phenomenon means that the heat transfer characteristic due to the ground heating is very complicated. This is because the phase change of the pore water and the heat transfer due to the water vapor occurred simultaneously. On the other hand, the leakage of the water vapor to the surface also means the loss of heat energy. [PERSON] (1960) [PERSON] (1960) even knew that heat would be lost if the water vapor was evacuated. A sealing device was applied to solve this problem. The heat transfer characteristics by ground heating needed to be further investigated through laboratory experiments and numerical analysis.  
The increase in the tip resistance of the heated ground by the CPT was up to 1.2 m from the heat source and the improvement of the soil strength was reduced to the first order linear equation. It has been experimentally verified that the temperature and the strength of the soil increase together through the heat transfer from the electric heating pipe. However, it was difficult to clarify the relationship between the heat transfer and the strength improvement of the soft soil due to the inhomogeneity of the ground.  
Additionally, the discharge of the water vapor in the indoor ground heating experiment performed by [PERSON] (2008) means a loss of heat energy. At deep depths, the loss of heat energy may not occur because the earth pressure suppresses the discharge of water vapor. However, as a result of the experiment, the ground was not homogeneous and it was disturbed by the boring for the installation of the heating pipe. The water vapor was discharged through these gaps and, consequently, a loss of heat energy occurred.  
Figure 16: The variation of the pore pressure according to the horizontal distance from the electric heat pipe (Cases 5, 6 and 8), (**a**) a point 28 cm in the horizontal direction; (**b**) 33 cm; (**c**) 45 cm.  
As a result of applying the ground heating method to the deep soft ground, it was easy to install the electric heating pipe after boring. However, although the borehole was filled with bentonite, this gap served as a passage through which the water vapor was discharged. Therefore, it was necessary to increase the strength so that an electric heating pipe can be inserted into the soft ground of the site.  
In the secondary heating, the electric heating tube of Case 1 was lost and could not be heated. The electric heating tube was weak on the part of the connection with the lead wire supplying electric power. Therefore, it is necessary to improve the connection part of the electric heating pipe for practical use.