Abstract: In order to study the influence of eccentric unloading induced by excavation of foundation pit on the existing shield tunnel, a refined threering shield segment model is established by using software MIDAS/GTS NX. The initial confining pressure of shield lining ring is calculated based on the modified routine method, and the additional confining pressure generated by excavation and unloading of the foundation pit is calculated based on the Mindlin formula. By applying the total confining pressure on the shield segment model, the transverse deformation and internal force of the shield lining are simulated and calculated. Additionally, the ellipticity is adopted as an index to evaluate the safety of tunnels, and the influence of the eccentric unloading of the foundation pit on the shield tunnel is studied. The results show that: (1) During the eccentric unloading process, the oblique "flattening" effect would be occurred to lining structure and showing "oblique ellipse" deformation. (2) Due to the stress concentration, the stress is large at the joint between two segments, and the maximum stress of concrete is between the adjacent block and the standard block. (3) The maximum stress of concrete and bolts increases with increasing ellipticity. (4) As the displacement of the retaining structure of the foundation pit increases, the stress release coefficient β of the sidewall will increase gradually, and the horizontal displacement of the tunnel will also increase. Therefore, during excavation of foundation pits, the displacement of retaining structure should be strictly controlled to ensure the safety of the shield tunnel.

Key words: foundation pit excavation, shield tunnel, eccentric unloading, confining pressure, segment, lining structure, crosssectional deformation