# Ground Stability Analysis in Non-Open-Cut Tunneling Method Using Small-Diameter Steel Pipe Piles
## 4 Analysis of Pile Behavior Based on Model Test  
### Overview of Model Test  
Two processes affect pipe piles during construction cycles: Stage 1, pulling back the pressure panel; Stage 2, propelling the precast structure forward. During Stage 1, if the ground is weak, the pile might be bent downward because of the collapse of the tunnel face. During Stage 2, the pipe might be lifted and bent upwards. Therefore, 36 model tests were performed to confirm the behavior of the piles during these two processes; their details are summarized in Table 3. To consider the tunnel size and their excavation sequence effect, three types of tests were conducted, as illustrated in Figure 3. The basic height of a one-level tunnel model is 125 mm, which is minimized to 1/20 th of the actual tunnel height of 2.5 m. The height of the two-level tunnel model is 250 mm. Figure 4 depicts the construction of a two-level tunnel. In this case, the large tunnel face is divided into two levels to prevent large-scale accidents of ground settlement that occur during Stage 1. As the non-open-cut method is generally used for the construction of underground crossings at existing roads at low cover depths, the cover depths in the experiments in this study were determined to be 1.5, 2, and 3.5 m, and the behavior of the pile was analyzed at each cover depth. Figure 2: The step of tunnel construction in the non-open-cut method using small steel pipe piles: (**a**) installation of the forepoling using small steel pipe piles. (**b**) Placing the pressure panel at the tunnel face. (**c**) Propulsion of the precast structure (Stage 2). (**d**) Pulling back the pressure panel (Stage 1). (**e**) Excavation of the tunnel face using excavator. Figure 4: Two levels under construction tunnel. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Shape of** & & & & \\
**Tunnel** & & & & \\
**Structure** & & & & \\ \hline Type 1 & one level & propelling precast & with or without & 175/125/75 \\  & one level & structure & shoe structure & 175/125/75 \\  & & pulling back & with or without & 175/125/75 \\  & & pressure panel & shoe structure & 175/125/75 \\ Type 2 & one level; same & propelling precast & with or without & 175/125/75 \\  & height as two levels & structure & shoe structure & 175/125/75 \\  & & pressure panel & shoe structure & 175/125/75 \\  & & propelling precast & with or without & 175/125/75 \\ Type 3 & two levels & structure & shoe structure & 175/125/75 \\  & & pulling back & with or without & 175/125/75 \\  & & pressure panel & shoe structure & 175/125/75 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test scenarios of model tests. Figure 3: Test cases for shape of tunnel structure: (**a**) Type 1—one level. (**b**) Type 2—one level but with same height as two levels. (**c**) Type 3—two levels. The experimental equipment for the model tests was designed, as displayed in Figure 5, to analyze the behavior of the pile. The size of the soil box was 1000 mm \(\times\) 900 mm \(\times\) 700 mm, and it was designed to be large enough to prevent any effects on the results of the model test. To analyze the behavior of the pile during the three construction types, the experimental equipment was designed such that it can propel and pull back the precast structure, upper pressure panel, and lower pressure panel using three motors. As displayed in Figure 5c, the shoe structure is connected to the precast structure using bolts so that it remains removable. It was also designed to enable the behavior of the pile and ground to be checked based on the existence of a shoe structure. To determine the specifications of the pile, the properties of the ground for construction were determined first. The non-open-cut method is generally used for construction under the condition of weak ground stiffness, such as on weathered soil. In this study, the properties of the ground were determined, as presented in Table 4, by averaging the properties of the ground acquired from other studies that have applied the non-open-cut method [9; 10; 11; 12].