Progressive failure mechanism of existing-supplementary double-layer piles retaining excavation beneath existing underground space

The increasing degree of urbanization has resulted in traffic and space congestion. When there is no longer any aboveground space for development, the existing space underground is excavated, and the construction of underground buildings is expected to solve the above problems. In the excavation of an underground layer, it is necessary to drive supplementary and existing piles into the soil to form a double-layer pile-supported structure. Aiming at an existing-supplementary double-layer pile-supported structure and considering the pile spacing of the supplementary piles, use of crown beams, and other factors, this study performed a series of indoor model tests and an simulation analysis in combination with finite element analysis software to reveal the bearing characteristics of this structure. The results showed that the installation of supplementary piles could effectively inhibit soil slippage after pile driving, redistribute the soil pressure load of the existing piles, and reduce the number of soil cracks between the piles. With the increase in the supplementary pile spacing, the deformation of the existing-supplementary double-stacked piles gradually intensified, and the bending moment of the pile body and the horizontal displacement at the top of the piles increased. The installation of crown beams in the existing piles could strengthen the connection between the existing and supplementary piles, reduce the number of soil cracks between these piles, and change the force mode of the pile body. Considering the geological conditions and the influence of existing buildings, the deformation laws of the supporting structure and the soil around the foundation piles during the excavation of an underground layer of an L-shaped foundation pit were revealed. The research results are expected to provide a scientific and theoretical basis for the design and construction of downward additional support structures for existing buildings, along with socio-economic significance.