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Assessment of Twin Metro Tunnels Settlement

Assessment of twin metro tunnels settlement due to adjacent excavation with strengthening measures


A. INTRODUCTION

The effect of an excavation on adjacent infrastructure is a sensitive issue in urban environment, especially when excavating next to existing subway tunnels.

In such scenarios, a numerical analysis with detailed consideration of the staging sequence is a strong tool for the prediction and subsequently mitigation of any detrimental effect on nearby structures, utility and/or transportation networks.

In this case study, our DeepEX software is used to evaluate the deformations of the existing Shanghai Metro twin tunnels due to a deep excavation for the construction of the new Commercial City Complex.

The excavation wall and bracing system along with the location of the twin tunnels are illustrated in figure 1a according to [1].

Sections indicating the soil profile and depth of the tunnels are illustrated in figures 1b and 1c. Description and thickness of the soil properties of the site are illustrated in figure 2.

Figure 1: a)Top view of the excavation site b)section I-I and c) II-II as presented in [1]


Figure 2: Table of soil properties as presented in [1]


Due to the weak soil properties on site and since the twin Shanghai Metro tunnels had to be in full operation during the excavation, additional strengthening measures where adopted.

Specifically pumping consolidation or precompression by pumping water out of ground was used to reduce the amount of water in the soft soil layers and to improve the soil shear strength and deformation modulus before excavation.

Additionally, the soft soil adjacent to the diaphragm walls and surrounding the two metro tunnels was further strengthened using piles comprising of a cement-soil mix.

The areas affected by the strengthening measures are illustrated in figure 3.

Figure 3: a) Section with strengthening affected areas according to [1] b) layout of soil-cement mix piles [1]


B. MODEL OF EXCAVATION IN DEEPEX

The numerical model of the excavation is created in DeepEX according to the description of the project, published in [1].

The finite element method is used to assess the interaction between the excavations and the preexisting twin tunnels. The model constructed in DeepEX is illustrated in figure 4:


Figure 4: a) Stage 1- Site prior to the excavation b) Stage 2 – Strengthening measures


The DeepEX soil zone option is used for the simulation of the strengthening measures. In Stage 2, prior to the initiation of the excavation, two equivalent soil zones of the soil mix piles and pumping strengthening zones are defined with the deepex soil zone tool, and equivalent Mohr coulomb properties are assigned to the soil zones. The remaining stages follow the construction sequence described in [1].


C. ANALYSIS RESULTS

The stress results of the diaphragm retaining walls and twin metro tunnels are illustrated in figure 5.

Figure 5: Moment, axial force and displacement results for the tunnels and retaining walls


The displacement results of the soil medium on the final stage of the excavation are illustrated in the following figures (figure 6-7).

Figure 6: Horizontal displacement contour ux


Figure 7: Vertical displacement contour uz


Finally, in-site measurements of the deflection of the diaphragm wall for the final stage of the excavation process (stage 9) are compared with the deflection approximated by the DeepEX FEM model in figure 8.

Figure 8: Analysis result comparison with wall deflection measurements


D. REFERENCES

[1] Z. F. Hu, Z. Q. Yue, J. Zhou, and L. G. Tham, “Design and construction of a deep excavation in soft soils adjacent to the Shanghai Metro tunnels,” Can. Geotech. J., vol. 40, no. 5, pp. 933–948, 2003.

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