Building Rectification by Underexcavation and Practical Design Method

Abstract

An 11-story high-rise building with a shear-wall structure utilized a raft foundation based on a deep alluvial deposit. After structure completion, the building tilted beyond acceptable limits. This paper presents the investigation, settlement analysis, building rectification, and foundation remediation. The purpose of this study is to provide a common method of addressing similar problems and propose two formulas to guide design. First, through site investigation and settlement reanalysis, it was determined that the large settlement and building eccentricity accounted for the building tilt. Second, underexcavation was used for the side with less subsidence to address the building differential settlement, and precast micropiles were driven into the foundation to improve foundation safety. Finally, two formulas for underexcavation were proposed to apply to general rectification projects. A simplified analysis method was adopted for formula derivation. In accordance with the force balance in the foundation soil before and after underexcavation, the preferable hole spacing could be expressed as a multiple of the hole diameter, i.e., λ=K/(K−1), which mainly depends on the safety factor of the bearing capacity of the foundation soil. Assuming that the extraction volume of soil equals the forced settlement volume for inclination correction, the diameter of the hole is obtained, i.e., d=4λs/π, which mainly depends on the proposed forced settlement. The key parameters were verified by the case history. The case history and practical design method provide a framework for projects experiencing differential settlement. Buildings sometimes incline. Reasons for the inclination are various, such as severe foundation settlement, surrounding underground construction, neighboring foundation pit excavation, and underground water drainage. Underexcavation is a cost-effective and “soft” method of rectifying buildings. It is suitable for buildings with shallow foundations. This paper presents a case history, and theoretical formulas for underexcavation design are proposed. In the case study, the methods of investigating and analyzing building inclination and the technology of inclination correction can provide valuable references for similar projects. The theoretical formulas can provide the key parameters of limit hole spacing and hole diameter for a correction design and enable a designer to quickly give a correction scheme for general conditions. For specific projects, appropriate adjustments should be made according to geotechnical conditions, the surrounding environment, and the suitability of construction. Notably, even with the formulas to guide design and construction, in view of the complexity of geological issues, close monitoring is still essential.