## 4 Behavior of Coupled Holder-Pile-Ground System during Earthquake
### Settlement Suppression Mechanism by Pile (Pile Member Force Evaluation)  
Figure 18 shows the changes in the axial force distribution acting on a typical pile at point a in Figure 7. The axial force acting on the pile increased from during the earthquake to afterwards. The changes in the axial force of the pile due to seismic motion can be interpreted as follows:  
(1) Prior to earthquake occurrence  
The axial force acting on the pile head was approximately 120 kN and was almost constant in the depth direction. As described in Section 2.2, in addition to this axial force, the holder load is shared by the bottom foundation and the frictional force between the peripheral surface of the pile. (2) From soon after to 12 s after the start of the earthquake  
As shown in the one-dimensional analysis (Figure 12), the B layer began to undergo consolidation settlement immediately after the earthquake. Because the pile was present, the force that supports the bottom foundation (ground reaction force) was lost owing to the settlement of the B layer, and the axial force of the pile increased to approximately 220 kN at the pile head. Additionally, the axial force of the pile did not increase significantly at this stage in the deep clay layer. Figure 17: Status of pile damage progress: seismic motion (A): (**a**) input seismic motion and damage time; (**b**) pile damage. Figure 18: Changes in axial force distribution of pile over time: seismic motion (A). (3) From 12 s after the start of the earthquake to the end  
The repeated loads caused by the seismic motion resulted in a decreased mean effective stress (i.e., effective confining pressure) in the sandy soil layer. This decrease in the effective confining pressure results in a positive excess pore water pressure and an increased axial force on the pile. It is also generally inferred that a clay layer is less susceptible to seismic damage. However, the Ylc layer was not only in an extremely soft state, but the duration of the seismic motion (A) was long, and a large shear deformation occurred in the ground, as shown in Figure 12. Therefore, the clay layer experienced further decreases in the effective confining pressure and alternatively gradual increases in the axial force of the pile, even in the deep part. (4) During consolidation settlement (after the earthquake)  
After the end of the earthquake, consolidation settlement occurred first in the liquefied sand layer and then in the clay layer after a delay. This consolidation settlement caused negative friction, and the axial force greatly increased in locations where the piles were undamaged. In this way, the consolidation settlement in the B layer during the earthquake in (2) and the decreased effective confining pressure of the sandy Yus and clay Ylc layers during the earthquake in (3) caused stress redistribution, increasing the pore water pressure and axial force of the pile. From the M-N failure diagram in Figure 8, it can be observed that the increased axial force of the pile increased the allowable bending moment (i.e., increased the yield strength of the pile). Figure 19 shows the shifts in the M-N failure diagram of the member force acting on the pile during the earthquake as well as its bending moment diagram; indeed, the bending moment acting on the pile during the earthquake increased, causing gradual damage to the pile and maintaining its support functions for some period of time. Therefore, although the pile was ultimately damaged in almost all sections, the deformation of the holder ground was suppressed to some extent. Figure 20 shows the shear strain distribution in the without-piles and with-piles models. The shear strain was the maximum value until the end of the earthquake. The without-piles model shows a concentration in the region where the shear strain is predominant near the surface layer, but the with-piles model shows that the shear strain was predominant around the pile as well as near the surface layer, and it can be observed that the pile resisted the shear deformation by the seismic motion.