Research on the Deformation Laws of Deep Foundation Pit Construction of Metro Station in Soft Upper-Hard Lower Strata

Deep excavations in composite “soft upper-hard lower” strata present significant deformation control challenges due to strong stiffness contrasts. This study investigates the deformation characteristics of a deep metro foundation pit in Jinan under zoned excavation conditions. A three-dimensional finite element model was developed to simulate the staged excavation process, and the spatiotemporal evolution of diaphragm wall deflection and ground settlement was analyzed, with particular focus on the influence of soft soil thickness. The results show clear spatial variation, with maximum lateral wall displacements exhibiting a typical “bulging” profile along the longer sides of the pit. The normalized maximum wall displacement (δhm/He) ranges from 0.051% to 0.090%, while the ratio of maximum ground settlement to wall displacement is 0.35–0.57, indicating lower deformation levels compared to homogeneous soft soils. As the soft soil thickness increases, the wall displacement increases and the location of maximum displacement migrates downward and outward. Under the investigated conditions, a transition toward a “kick-out” deformation mode occurs when the soft soil thickness reaches the transition range identified in the parametric analysis. These findings provide a quantitative basis for deformation prediction and support design in composite strata.