# Numerical Analysis for Ground Subsidence Caused by Extraction Holes of Removed Piles
## 3 Results and Discussion
### Case Where Three Extraction Holes of Removed Piles Are Left in a Hollow State  
Figure 11 shows the contour diagram for the amount of subsidence when three extraction holes of removed piles are hollow. Figure 12 presents the amount of subsidence of the ground surface, the clay layer, and the gravel layer during the pile removal process. Figure 13 presents the amount of subsidence of each layer after the pile removal process. Figure 14 shows the compressive stress acting on the extraction hole of removed pile. As can be seen from Figure 11, as in the case of one or two extraction holes of removed piles, the location where the maximum subsidence occurs immediately after the completion of the extraction and after a certain amount of time has elapsed after the extraction is near the lower part of the clay layer around the extraction hole of removed pile. There is no difference. However, since pile (1) is pulled out first and then pile (2) and pile (3), the maximum subsidence occurs in the clay layer sandwiched between pile (2) and the extraction hole of the removed pile (1). Figure 10: Compressive stress concentrated on two extraction holes of removed piles. Figure 11: Deformation of the surrounding ground when three extraction holes of removed piles are hollow. Figure 12: Subsidence of the surrounding ground in the process of forming three extraction holes of removed piles. Figure 13: Subsidence of the surrounding ground after the formation of three extraction holes of removed piles. From Figure 12, the amount of subsidence is seen to decrease on the ground's surface from 8 to 20 m with the withdrawal of piles (1) and (2). This is thought to be due to the fact that suction force is generated due to the pulling out of the piles and the stress that is acting upward, as in the case of one or two extraction holes of removed piles. In addition, as shown in Figure 13, the amount of subsidence in the surrounding ground is larger than that in the case of one or two extraction holes of removed piles. As a result, it is considered that the phenomenon of depression in the ground increases as the number of extraction holes of removed piles increases. Therefore, in the case of three extraction holes of removed piles, it is necessary to inject the filler simultaneously with the extraction of each pile in order to prevent the cavity from expanding sooner than in the case of one or two extraction holes of removed piles [35; 36; 37]  
Figure 14 shows that the compressive stress concentrated in the extraction hole of a removed pile is greater immediately after pile (2) is extracted than immediately after pile (1) is extracted. However, immediately after pile (3) is extracted, the clay layer expands from 2 to 10 m, and pile (2) is extracted. The compressive stress is smaller than immediately after the removal. If the extraction holes of removed piles do not form in a straight line, the compressive stress acting on the third and subsequent holes will become smaller. In addition, in the case of three extraction holes of removed piles, the compressive stress acting on the ground surface is larger than that in the case of one or two extraction holes of removed piles. As a result, when multiple extraction holes of removed piles are formed, the compressive stress acting on the ground surface increases, and the difference from the compressive stress acting on the extraction hole of removed pile under the clay layer is reduced, so that the strength of the filler is more uniform. In addition, since the amount of subsidence shows the maximum value in the clay layer with \(Z=10\) m, it is necessary to consider that point as a standard. Moreover, since the age of cement management is generally in units of weeks, it is necessary to pay attention to the strength one week after its injection. Therefore, it is desirable to develop a compressive strength of 32 kN/m\({}^{2}\) within one week after pulling out the piles.