# Ground Stability Analysis in Non-Open-Cut Tunneling Method Using Small-Diameter Steel Pipe Piles
## 2 Problems with Existing Non-Open-Cut Methods  
During tunnel construction under existing operational roads using the non-open-cut method, the allowable value for ground settlement and heave to ensure the safety of road or railway traffic is 13 mm [3]. If the value of ground settlement or heave exceeds the allowable value, construction should be stopped and recovery plans and measures should be considered to prevent recurrence. Pavement damage, such as cracks, ground settlement, and heave could occur at each construction step in the non-open-cut method, as detailed in Table 1[3]. The cases of blocking off traffic owing to ground settlement or heave during tunnel construction using the non-open-cut method along various expressways in the Republic of Korea are presented in Table 2[3,4].  
\begin{table}
\begin{tabular}{c c c} \hline
**Construction Step** & **Cause of Occurrence** & **Damage Type** \\ \hline installing temporary support & relaxation of face of slope & crack and settlement of shoulder \\ installing steel pipe & relaxation and disturbance around & heave or settlement of pavement \\  & steel pipe & heave of pavement \\ grouting & excessive grouting injection & heave of pavement \\  & pressure & crack and settlement of pavement \\ excavation of lower part & deformation of pipe roof and & crack and settlement of pavement \\  & settlement of lower part of support & crack and settlement of pavement \\ removing temporary support & relaxation of face of slope & crack and settlement of pavement \\ \hline \end{tabular}
\end{table}
Table 1: Type of pavement damage occurrence possible at each step of construction of the underground multilevel crossing road under the highway, using the non-open-cut method.  
\begin{table}
\begin{tabular}{c c c} \hline
**Name of Expressway** & **Construction Method** & **Displacement (Damage Type)** \\ \hline Gyeongbu & STS (steel tube slab) & 71 mm (settlement) \\ Youngdong & PRS (pipe roof structure) & 90 mm (settlement) \\ Youngdong & UPRS (upgrade pipe roof structure) & 76 mm (heave) \\ Gyeongbu & UPRS & 100 mm (heave) \\ Gyeongbu & NTR (new tubular roof) & 80 mm (settlement) \\ \hline \end{tabular}
\end{table}
Table 2: Case of blocking off traffic urgently during construction in the non-open-cut method.  
[PERSON] et al. [5], analyzed the behavior of the ground during the construction of a railway underground crossing, in which the front jacking and PRS methods were applied. According to the study, 50-60% of the total displacement occurred during the insertion of medium- and large-diameter steel pipe piles and excavation inside the piles. [PERSON] et al. [6], simulated RPS using three-dimensional numerical analysis, which indicated that one-third of the total displacement occurred during installation of the steel pipe piles during the construction of the pipe roof. [PERSON] et al. [7], analyzed the pattern of settlement at each construction stage at three construction sites using the front jacking method, and, as per their findings, 27-56% and 44.3% of the total displacement occurred during the pipe pile insertion step.