Model Tests and Interpretation of Earth Pressure Behind Existing and Newly Added Double-Row Piles Retaining Underground Supplementary Excavation

In urban redevelopment, adding basements beneath existing buildings often requires specialized retaining structures, such as existing and newly added double-row piles, yet their complex load-sharing mechanism is not yet fully understood. This study addresses this gap through a series of physical model tests, systematically investigating the influence of two key variables: the row spacing and the newly added/existing pile length ratio. The results reveal that row spacing is a critical factor governing the system’s stability and cooperative behavior. The newly added piles bear the majority of the earth pressure, effectively shielding the existing piles. A distinct, layered pressure distribution was observed in the inter-row soil, a phenomenon that classical earth pressure theories cannot adequately predict. Based on a comprehensive evaluation of structural performance, deformation control, and stability, this study proposes an optimized configuration with a row spacing of 4D and a newly added/existing pile length ratio of 9/6. This configuration achieves an effective balance between structural performance and economic efficiency, offering valuable practical guidance for the design of supplementary retaining systems in basement addition projects.