# Ground Stability Analysis in Non-Open-Cut Tunneling Method Using Small-Diameter Steel Pipe Piles
## 5 Numerical Analysis of Pile Behavior under Conditions of Actual Tunnel Size
### Results of Numerical Analysis for Stage 1 of Each Construction Cycle  
The behavior of the ground during Stage 1 of each construction cycle according to the existence or nonexistence of a pile, each cover depth, and each cycle is depicted in Figure 15 through contouring. Whereas the displacement of the ground occurred up to the surface of the ground when there was no pile, the settlement of the ground was restrained during any cycle when the pile was installed.  
Figure 14: Behavior of tunnel face with respect to time: (**a**) 1.2 s, (**b**) 3 s, (**c**) 4.8 s, (**d**) 6 s.  
Figure 15: Ground displacement during Stage 1 of each construction cycle.  
When there was no pile, a larger amount of ground settlement was observed because the arching effect of the ground was as low as the cover depth (Figure 16). As the cover depth decreased, the ground settlement increased because of the lack of the arching effect of the ground. A ground settlement of 194 mm was measured when the pile was not installed; however, the ground settlement decreased to below 38.5 mm when the piles were installed at a cover depth of 1.5 m. It was observed that the amount of ground settlement increased during the progress of the construction step (Figure 17). Because the length of the overlapping section of the installed piles is 1.373 m, as listed in Table 5, a new pile would further be installed at the tunnel face between the fourth and fifth cycles. Thus, the amount of ground settlement was estimated to be between 31.7 and 38.5 mm.  
The stresses acting on the piles during Stage 1 based on each cover depth and cycle are depicted in Figure 18. The stress contours in two or three nearest piles from the tunnel face were represented with blue color because these piles were bent downward, which helped to restrain the ground settlement.  
Figure 16: Displacement of ground surface during Stage 1 of each construction cycle based on cover depth.  
Figure 17: Surface settlement trough according to progress of construction step at cover depth of 1.5 m.  
As the cover depth increased, the stresses acting on the piles increased as the ground load increased (Figure 19). By analyzing the stresses acting on the piles as the construction progressed, it was seen that they generally increased until the installation of the next pile. During the second cycle, Pile-1 could not help prevent the ground settlement well because it was located in front of the shoe structure. In addition, because the relative locations of the pile and shoe structure were varied based on the progress of the construction, it was observed that the point of maximum stress moved to the latter part of the pile during the third to fifth cycles (Figure 20). Considering the overlapping length of 1.373 m of the pile, as listed in Table 5, the stress acting on Pile-1 was estimated to be between 152.83 (fourth cycle) and 168.19 MPa (fifth cycle).  
Figure 19: Maximum stresses acting on piles during Stage 1 of each construction cycle based on cover depth.  
Figure 18: Stress acting on piles during Stage 1 of each construction cycle.