Abstract: To explore the influence of large-area unbalanced surface surcharge on the deformation of retaining structures, a case study is conducted on a metro deep foundation pit project in Wuhan with large-area soil piles distributed outside the construction red lines on both the eastern and western sides. The eastern soil piles are divided into Zones 1, 2, and 3 with heights of 4－5 m, 13－14 m and 8.0－9.5 m respectively, and the height of western soil piles ranges from 9－13 m. The coverage range of eastern soil piles is obviously larger than that of the western side, forming a typical form of surface unbalanced surcharge. A three-dimensional numerical model based on PLAXIS 3D is established, and the constitutive hardening soil model with small-strain stiffness  is adopted to simulate the whole excavation process of the metro deep foundation pit. The reliability of the numerical model is verified by comparing with field monitoring data. Furthermore, sensitivity analyses are carried out to discuss the influences of surcharge distance and unbalanced surcharge condition on retaining structure deformation. The research conclusions are drawn as follows: (1) Large-area surcharges on both sides of the pit cause asymmetric deformation of retaining walls, namely larger deformation occurs at positions with greater surcharge and smaller deformation at positions with less surcharge. (2) The deformation of retaining walls adjacent to end shafts is dominated by spatial effect, which exerts a more prominent influence than surface surcharge. (3) The surcharge distance is negatively correlated with the lateral displacement of retaining walls, and the influence becomes more significant with the decrease of distance. (4) Removing the surcharge on one side of the pit can reduce the lateral displacement of retaining walls on the same side, but will lead to a remarkable increase in the displacement of the opposite side, which fails to guarantee all pit deformation values within the early warning range.

Key words: deep foundation pit, unbalanced surcharge, retaining structures, deformation characteristics, finite element, hardening soil model with small-strain stiffness