# Numerical Analysis for Ground Subsidence Caused by Extraction Holes of Removed Piles
## 3 Results and Discussion
### Case Where Two Extraction Holes of Removed Piles Are Left in a Hollow State  
Figure 7 shows the contour diagram for amount of subsidence when two extraction holes of removed piles are left hollow. Figure 8 presents the amount of subsidence of the ground surface, the clay layer, and gravel layer during the pile removal process. Figure 9 presents the amount of subsidence of each layer after the pile removal process. Furthermore, Figure 10 shows the compressive stress acting on the extraction holes of removed piles. From Figure 7, it is noted that there is no difference in the location where the maximum amount of subsidence occurs between the time immediately after pulling out the piles and several days after pulling out the piles near the bottom of the clay layer around the extraction hole of the removed pile, as in the case of one extraction hole of a removed pile. However, since pile (2) is pulled out after pile (1) is pulled out, the maximum amount of subsidence occurs in the clay layer sandwiched between pile (2) and the extraction hole of the removed pile (1). From Figure 8, the amount of subsidence of the ground's surface decreases from 8 to 20 m due to the pulling-out of piles (1) and (2). This is thought to be due to the fact that a suction force is generated by pulling out the two piles, as in the case of pulling out one pile, and stress is acting upward. In addition, the amount of subsidence immediately after the  
Figure 7: Deformation of surrounding ground when two extraction holes of removed piles are hollow. completion of the extraction of pile (1) is larger than in the case of one extraction hole of a removed pile. This is because pile (2) still exists in the ground immediately after pile (1) is pulled out, but pile (2) is deformed and starts to fall down toward the extraction hole of the removed pile, and the compressive stress concentrated on the extraction hole of removed pile increases. This is very conceivable. In addition, as shown in Figure 9, the amount of subsidence in the surrounding ground is larger than that in the case of one extraction hole of removed pile. As a result, the number of extraction holes of removed piles increases and the depression phenomenon is promoted by an increase in the number of cavities in the ground. Therefore, it is considered necessary to prevent the cavity from expanding earlier when there are two extraction holes of removed piles as compared to the case of one extraction hole of a removed pile. It is important to inject the filler into the extraction hole of a removed pile simultaneously with the formation of the extraction hole of a removed pile [26; 27]. Figure 10 shows that the direction of the compressive stress concentrated on the extraction hole of removed pile is greater immediately after pile (2) is withdrawn than immediately after pile (1) is withdrawn. As a result, when the number of extraction holes of removed piles increases, it affects the periphery of the previously formed extraction holes of removed piles, and the compressive stress increases in the lower part of the soft ground. In addition, in the case of one extraction hole of a removed pile, the compressive stress is about 10 \(\mathrm{kN/m^{2}}\) on the ground surface, which is smaller than the compressive  
Figure 8: Amount of subsidence of the surrounding ground in the process of forming two extraction holes of removed piles. Figure 9: Subsidence of the surrounding ground after the formation of two extraction holes of removed piles. stress in the extraction hole of the removed pile under the clay layer. However, when there are two extraction holes of removed piles, a compressive stress of approximately 20 kN/m\({}^{2}\) acts on the ground surface extraction hole of removed pile immediately after the extraction of pile (2). It acts on the extraction hole of the removed pile at the bottom of the clay layer one week after extraction. The difference in compressive stress is small.