## 4 Results of Ground Heating Method
### Soil Improvement Effect  
A static electronic piezo-cone penetration test (CPT) was carried out to verify the effect of soil improvement (ASTM D 5778, 2000). The cone had a measuring range of 100 MPa, a tip area of 10 cm\({}^{2}\), a sleeve area of 150 cm\({}^{2}\), and the electric resistance sensor of the strain gauge type. It is a cone type cone and its aspect ratio is 0.8. The tip resistance of the CPT depends on the water content of the soil [36; 37; 38]. In this study, the strength of the sintered ground due to ground heating was verified by the CPT test. The soil classification index (\(I_{c}\)) profile of the clay was estimated as described in References [38; 39] based on the CPT tests data. Figure 10 shows the comparison between the soil classification index (\(I_{c}\)) profile obtained before (reference CPT test) and after heating. The soil classification index had slightly decreased after ground heating because the soil resistance increased and the pore water pressure decreased due to the ground heating. Figure 10: The Soil Classification Index obtained by the CPT tests data (before and after ground heating). Figure 9: The variation of the vapor pressure by ground heating using an electric heat pipe. (**a**) The variation of the vapor pressure; (**b**) the bubbles on the ground surface. Because the CPT was only possible belowground at a temperature of 50 \({}^{\circ}\)C, the test was performed three days after ground heating was terminated. The strength of the ground before ground heating was measured by performing the CPT in advance. After ground heating, the test was performed for a total of four times at 0.35 m, 0.97 m, and 1.31 m from the heat source in the horizontal direction. Figures 11 and 12 show the result of the measurement of the tip resistance from the CPT. Since the purpose of this study was to understand the change of the soil strength due to electric heating, only the results of 3.5-6.5 m, where the electric heating pipe was installed, are shown in the graph. In Figures 11 and 12, the tip resistances after heating were increased from before heating. Figure 11: The variation of the tip resistance due to ground heating using an electric heat pipe (Case 4), (**a**) a point 35 cm in the horizontal direction; (**b**) 97 cm; (**c**) 131 cm. Figure 12: The variation of the tip resistance due to ground heating using an electric heat pipe (Cases 5, 6, and 8), (**a**) a point 28 cm in the horizontal direction; (**b**) 33 cm; (**c**) 45 cm. It is difficult to quantify the increase in the strength due to ground heating only by the tip resistance graphs shown in Figures 11 and 12, so the average tip resistance of the heating zone was calculated. In this case, the section where the tip resistance is irregularly increased due to the sand seam was excluded. Table 3 shows the change of the average tip resistance due to the ground heating for a total of 96 h. The average tip resistance change at the point 1.31 m away from the heat source was increased by two percent at 540.0 kPa before heating and at 555.0 kPa after heating. Figure 13 shows the change of the tip resistance according to the horizontal distance as a result of the CPT of Case 4 shown in Table 3. The change of the tip resistance according to the horizontal distance from the heat source was estimated by the linear function of \(y=-3.2942x+983.79\). It was heated up to 120 cm in the horizontal direction from the electric heating pipe which was the heat source, and the tip resistance had also slightly increased. Figure 14 shows the change of the tip resistance according to the horizontal distance as a result of the CPTs in Cases 5, 6 and 8, shown in Table 3. Unlike Case 4 in Figure 13, the increase in strength concerning the horizontal distance from the electric heating pipe was not calculated as a linear equation. This is because Figure 14 shows the result of Cases 5, 6 and 8 according to the horizontal distance from the electric heating pipe.