A study on the behaviour of single piles to adjacent Shield TBM tunnelling by considering face pressures

막장압의 크기를 고려한 Shield TBM 터널 근접시공이 단독말뚝의 거동에 미치는 영향에 대한 연구

Abstract

In the current work, a series of three-dimensional finite element analyses were carried out to understand the behaviour of a pre-existing single pile to the changes of the tunnel face pressures when a shield TBM tunnel passes underneath the pile. The numerical modelling analysed the results by considering various face pressures (25~100% of the in-situ horizontal stress prior to tunnelling at the tunnel springline). In the numerical modelling, several key issues, such as the pile settlements, the axial pile forces, the shear stresses have been thoroughly analysed for different face pressures. The head settlements of the pile with the maximum face pressure decreased by about 44% compared to corresponding settlement with the minimum face pressure. Furthermore, the maximum axial force of the pile developed with the minimum face pressure. The tunnelling-induced axial pile force at the minimum face pressure was found to be about 21% larger than that with the maximum face pressure. It has been found that the ground settlements and the pile settlements are heavily affected by the face pressures. In addition, the influence of the piles and the ground was analysed by considering characteristics of the soil deformations. Also, the apparent safety factor of the piles are substantially reduced for all the analyses conducted in the current simulation, resulting in severe effects on the adjacent piles. Therefore, the behaviour of the piles, according to change the face pressures, has been extensively examined and analysed by considering the key features in great details.

본 연구에서는 Shield TBM 터널굴착이 기 시공된 단독말뚝의 하부를 근접하여 통과할 경우 터널 막장압에 따른 말뚝의 공학적 거동을 파악하기 위해 3차원 유한요소해석을 수행하였다. 이때 터널 막장압의 크기를 터널굴착 이전 springline 위치에서 수평토압의 25~100%로 변화시키면서 그 영향을 고찰하였다. 수치해석에서는 막장압의 변화에 따른 터널굴착으로 유발된 말뚝의 침하, 축력 및 전단응력을 고려하였다. 말뚝의 두부침하는 막장압의 크기를 가장 크게 적용한 조건이 막장압의 크기를 가장 작게 적용한 조건에 비해 약 44% 감소하여 발생하였다. 말뚝의 최대축력은 막장압의 크기를 가장 작게 적용한 조건에서 가장 크게 나타났으며, 이는 막장압의 크기를 가장 크게 고려한 조건에 대비하여 약 21% 큰 것으로 분석되었다. 터널굴착으로 인한 말뚝의 거동은 막장압의 변화에 따른 지반침하의 영향을 크게 받는 것을 알 수 있었으며, 막장압의 크기에 따른 말뚝 및 지반의 거동을 등고선을 이용하여 재분석하였다. 또한 모든 막장압 조건에 대하여 말뚝의 겉보기안전율이 1.0 이하로 산정되어 터널굴착이 인접말뚝에 유해한 영향을 끼치는 것으로 판단된다. 따라서 본 연구를 통해 말뚝의 거동에 영향을 미치는 주요인자를 막장압의 변화에 따라 심도 있게 고찰하였다.

Fig. 1. Sectional view of tunnel crossing bridge foundation (Liu et al., 2014)

Fig. 2. A representative 3D finite element half mesh used in the current study (D: tunnel diameter)

Fig. 3. Sectional view of analysis geometry

Fig. 4. Method used for the tunnel construction using TBM (A = the changes of the tunnel face pressures (0.25~1 in the current work), Z = distance from the surface to the tunnel springline, γ = unit weight of material, K₀ = lateral earth pressure coefficient at rest)

Fig. 5. Relation of axial pile forces and pile head settlements

Fig. 6. Distributions of normalised pile head and soil surface settlements with tunnel advancement (δ_gr,max = 16 mm for face pressure of 262.5 kPa)

Fig. 7. Distributions of normalised tunnelling-induced pile and subsurface soil settlements with depth

Fig. 8. Distributions of normalised axial pile forces with depth

Fig. 9. Distributions of normalised tunnelling-induced axial pile forces with depth

Fig. 10. Distributions of interface shear stresses with depth

Fig. 11. Distributions of tunnelling-induced interface shear stresses with depth

Fig. 12. Distributions of tunnelling-induced relative displacements at the pile-soil interface with depth

Fig. 13. The contour of settlements the pile and subsurface (X-Z Plane)(Y/D = 0)

Fig. 14. The contour of vertical displacements for the pile and subsurface (Y-Z Plane)(Y/D = 0)

Table 1. Summary of numerical analyses

Table 2. Material parameters assumed in the numerical modelling