既有建筑增扩地下空间是解决城市停车空间紧张的有效手段。桩基础作为高层建筑的承力构件，地下降水开挖过程中其承载力会有所损失。若不采取有效处理措施必然会引起既有桩基附加变形，甚至造成既有建筑倾斜、整体倒塌破坏。如何分析降水开挖条件下既有桩基承载特性和变形控制机理，优化控制既有建筑差异沉降的补桩模式，是既有建筑增扩地下空间工程中亟待解决的关键科学问题。针对该难题，本项目拟采用已有试验数据分析、模型试验、数值模拟和理论分析等综合手段展开研究，建立桩侧和桩端荷载传递时效性模型，提出既有桩基承载力时效性分析方法，建立降水卸荷条件下桩侧和桩端荷载传递模型，提出考虑降水开挖影响的既有桩基承载特性分析方法，形成开挖卸荷后既有旧桩和补打新桩组合桩基协同变形控制分析方法。预期成果有望揭示既有建筑增扩地下空间全过程中既有桩基承载特性和变形控制机理，为既有建筑增扩地下空间成熟实用技术的研发和推广应用提供理论依据。

Excavation under existing building is an effective mean to expand underground space that caters for the need of parking space. As piles are served as load-transfer structures of a high-rise building, bearing capacity of the existing piles will be eliminated due to excavation and dewatering beneath an existing building. Additional displacement of existing pile foundation will then be caused, and is likely to lead to incline, even collapse of the whole building if some effective measures on improvement of the pile bearing capacity are not conducted. Therefore, there is a need to analyze bearing behavior and mechanism of deformation control of piles under the condition of excavation and dewatering beneath existing building, and to optimize distribution of the supplementary piles to reduce differential settlement of an existing building. A variety of measures will be involved such as analysis of reported pile-test results, model test, numerical simulation and theoretical analysis. This research project aims to establish load transfer models of skin friction and end resistance considering time effect, and propose an analysis method to clarify time effect of pile capacity. Load transfer models of skin friction and end resistance of the piles under the condition of excavation and dewatering are also the prospective research focuses. Moreover, this study intends to propose an analysis method of existing pile response considering the effect of excavation and dewatering, and is expected to establish a novel method to analyze cooperative deformation of the pile groups composed of old existing piles and new supplementary piles. The results are expected to reveal behavior and mechanism of deformation control of piles during the whole process of excavation and dewatering stages, and the research is an attempt to provide theoretical support of the underground space development technique.