# Numerical Analysis for Ground Subsidence Caused by Extraction Holes of Removed Piles
## 3 Results and Discussion
### Case Where One Extraction Hole of Removed Pile Is Left in a Hollow State  
Figure 3 shows the contour diagram for amount of subsidence when one extraction hole of a removed pile is left hollow. Figure 4 presents the amount of subsidence of the ground surface, the clay layer, and the gravel layer during the pile removal process. Figure 5 presents the amount of subsidence of each layer after the pile removal process. The clay layer was \(Z=10\) m, which was selected because this value shows the maximum subsidence in the clay layer. Figure 6 shows the compressive stress acting on the extraction hole of removed pile. From Figure 3, 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 pile and several days after pulling out the pile. In addition, as shown in Figure 4, the amount of subsidence on the ground surface decreases from 8 to 20 m. For this reason, stress is seen to be acting upward as the pile is being pulled out. This is thought to be due to the suction force that acts as the pile is being pulled out. This phenomenon also occurs at the point of 2 m in the gravel layer during the withdrawal. Figure 3: Deformation of the surrounding ground when one extraction hole of a removed pile is hollow. However, as shown in Figure 5, the amount of subsidence increased with time in the clay layer and the ground surface after the formation of the extraction hole of removed pile. This is thought to be due to the occurrence of depression. By pulling out the pile, a hole is formed and a cavity appears. When this cavity forms, a loosened area is created around the extraction hole of the removed pile. It gradually collapses and the hollow part expands. In addition, the loose area around the extraction hole of removed pile as well as the cavity expands. Sinking is the growth of this cavity reaching the surface, which can occur almost instantaneously. In addition, the subsidence phenomenon caused by mining activities, etc., continues for several months to several years after the completion of these activities [29]. Figure 4: Subsidence of the surrounding ground during the process of forming one extraction hole of a removed pile. Figure 5: Subsidence of the surrounding ground after forming one extraction hole of a removed pile. Figure 6: Compressive stress concentrated on one extraction hole of a removed pile. For this reason, it is important to prevent the ground cavity from initially expanding due to the formation of the extraction hole of removed pile. In addition, the clay layer has more subsidence than the ground surface, even after the hole has been formed. This is because the stress is concentrated in the extraction hole of the removed pile due to the formation of the hole, and the earth pressure increases as the lower clay layer is formed. However, the amount of subsidence does not become maximum near the deepest part of the clay layer. This is thought to be because the lower gravel layer supports the soft clay layer. From Figure 6, the compressive stress immediately after pulling out the pile can be seen to be almost the same as one week after pulling out the pile. This is thought to be due to the convergence of the consolidation subsidence over time. The compressive stress is small at 18-20 m from the ground surface. This is because this part is a gravel layer, and the vertical displacement over time is considered to be very small compared to the subsidence in the clay layer. 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, the filler needs to develop a level of strength of 28 kN/m\({}^{2}\) within 7 days.