Pile–raft foundations are widely used in soft soil engineering due to their good integrity and high stiffness. However, traditional design methods independently design pile–raft foundations and superstructures, ignoring their interaction. This leads to significant deviations from actual conditions when the superstructure height increases, resulting in excessive costs and adverse effects on building stability. This study experimentally investigates the interaction characteristics of pile–raft foundations and superstructures in soft soil under different working conditions using a 1:10 geometric similarity model. The superstructure is a cast-in-place frame structure (beams, columns, and slabs) with bamboo skeletons with the same cross-sectional area as the piles and rafts, cast with concrete. The piles in the foundation use rectangular bamboo strips (side length ~0.2 cm) instead of steel bars, with M1.5 mortar replacing C30 concrete. The raft is also made of similar materials. The results show that the soil settlement significantly increases under the combined action of the pile–raft and superstructure with increasing load. The superstructure stiffness constrains foundation deformation, enhances bearing capacity, and controls differential settlement. The pile top reaction force exhibits a logarithmic relationship with the number of floors, coordinating the pile bearing performance. Designers should consider the superstructure’s constraint of the foundation deformation and strengthen the flexural capacity of inner pile tops and bottom columns for safety and economy.

In this study, aiming at the practical application of additional layer excavation technology in the renewal and reconstruction of old urban communities, the influence of different excavation methods on the settlement behavior of existing structures is deeply analyzed through numerical simulation experiments. It is found that the frame columns show basin-type settlement in the block layered excavation, basin excavation and central island excavation. The cumulative settlement of the middle column is the largest, and the corner column is the smallest. The settlement phenomenon is mainly concentrated around the frame columns Z6, Z7, Z10 and Z11, and the change law of the cumulative settlement value conforms to the linear function distribution. By defining the cumulative settlement rate (a) as an index to measure the influence degree of the existing structure, it is found that the a value of the middle column is the largest, the side column is the second and the corner column is the smallest, and the a value of block layered excavation is the largest, and the central island excavation is the smallest. Considering the construction period, block layered excavation may be a more appropriate choice. The comparative analysis of the research results and the model experiment results confirms the feasibility of the improved model experiment, provides an important theoretical basis and practical guidance for the renewal and transformation of old urban communities and is of great significance for improving the utilization efficiency and safety of existing buildings.