Physical Model Study on Staged Underexcavation Rectification of an Existing Tilted High-Rise Building Using Cement-Based Materials

To investigate the settlement response and rectification performance of existing tilted high-rise buildings during staged underexcavation in complex collapsible-loess areas, a severely tilted high-rise building was selected as the case study. An integrated approach combining field investigation, geotechnical testing, a 1:100 physical model test, and three-dimensional finite-element analysis was employed to investigate the causes of building inclination and the deformation response during staged underexcavation rectification. The results indicate that insufficient compaction of the inter-pile soil, together with wetting-induced collapse and mechanical softening, jointly contributed to the development of differential foundation settlement. The stage-wise rectification displacement increments during staged underexcavation decreased successively from 0.85 mm to 0.48 mm and 0.43 mm. The prototype-scale rectification displacement derived from the physical model was 176 mm, corresponding to a rectification rate of 83.17%. As underexcavation proceeded, the additional rectification response induced by each stage generally decreased, exhibiting a progressively attenuating trend. Numerical simulations yielded average rectification rates of 95.73% and 98.44% for the one-time and staged underexcavation schemes, respectively, with a final rectification displacement of 202.10 mm for the staged scheme. Under the investigated conditions, staged underexcavation exhibited better rectification performance than one-time underexcavation. The progressively reduced excavation volumes of 120, 100, and 80 m3, combined with inter-stage stabilization, promoted gradual stress redistribution and deformation adjustment of the structure–foundation–soil system, supporting the feasibility of staged underexcavation under the investigated collapsible-loess conditions. These findings provide a useful reference for the controlled and precise implementation of rectification engineering for tilted high-rise buildings in complex collapsible-loess areas.