软土地区地下室增层开挖条件下既有桩基承载性状研究

Excavating beneath existing piled raft to add or rebuild a new basement is one of the underground space use technologies that cater for the need of conservation-orientated society. As piles are served as the final load-transfer structures, excavation under the existing foundations is likely to lead to cataclysm of the whole buildings, particularly in soft ground condition. This research project aims to depict the comprehensive behavior of piles subjected to excavation beneath the piled raft. A variety of measures will be involved such as analytical and numerical modeling, field pile-test simulation, laboratory test, regression analysis of reported pile-test results and construction monitoring of real cases. The prospective research focuses include: (1) previous set-up effect and current appraisal of the capacity of piles; (2) how the overburden pressure, soil properties and excavation extent affect or control the pile behavior in relation to soil-cutting under the raft; (3) the load-transfer pattern and the interactive working performance of a pile group composed of existing piles and nearby newly-installed ones. The results are expected to reveal the behavior of piles during the excavation and post-rehabilitation stages, thereby to establish appropriate evaluation method for pile capacity. The research is an attempt to provide theoretical support for the design and construction of such underground space development technique.

在既有桩筏基础下开挖土方、增层改建地下室是一种顺应节约型社会建设的地下空间开发技术。桩基础作为结构传力的最终构件，在"挖墙脚"式的增层开挖过程中，一旦灾变必牵动建筑物全局，在软土地区该问题更为突出。本项目力图通过解析解答和数值模拟、原位仿真桩基试验、室内试验、已有试桩数据的回归分析、实际工程案例施工全程监测等多种手段，描绘增层改造条件下桩基承载行为的立体剪影。拟开展研究内容包括：既有工程桩在服役前期的承载力时效增长规律和现状承载力评价；桩上段挖土卸荷过程中上覆压力、土性、开挖面等参数对既有桩基承载性状的影响和控制条件；既有桩邻近位置补入新桩后的基桩荷载转移模式以及组合群桩体系的协调工作性能。预期成果有望揭示既有建筑物地下室增层开挖施工过程中和增层施工后的桩基础承载行为及评价方法，为这一地下空间开发新技术迈向成熟应用奠定理论基石。

在既有建筑物下开挖增建地下室，可在不拆除重建上部结构的前提下增加地下空间。在大规模工程应用之前，须首先澄清既有建筑桩基、托换桩在开挖卸荷条件下的承载性状。本项目通过现场试验、室内模型试验、理论推导和数值模拟等手段，分析了以下问题：（1）桩-土界面剪切时效增长规律及在役桩基现状承载力评价与测试方法；（2）地下增层开挖诱发的土体响应，比如坑底回弹量、开挖被动区土体侧移等；（3）开挖卸荷条件下既有受荷基桩的荷载-变形响应，包括上覆土压力的减小、坑底土回弹、桩周土侧移三种主要因素的影响；（4）开挖卸荷条件下既有受荷基桩的屈曲稳定性评价；（5）地下增层托换过程中的桩基协同承载性能与桩-柱荷载转移关键风险分析。研究结果表明：（1）基于对数时间增长的桩-土界面剪切模型包含三阶段增长特征；（2）超固结比法可更好反映开挖卸荷土体的应力状态；（3）开挖卸荷诱发基桩侧摩阻力分布可采用五段式概化，开挖深度是诸多因素中对既有基桩承载及变形影响最大的；（4）支挡式开挖条件下，开挖深度、支护桩-基桩水平距离、桩基承台刚度、桩身轴向荷载都对既有基桩附加内力有较大影响；（5）开挖深度和桩顶、桩端约束条件对基桩屈曲稳定临界荷载影响明显；（6）地下增层桩基托换过程存在明显的次生应力和变形，截桩顺序是控制风险的关键所在。本项目的科学发现与工程实施并行，形成了既有建筑地下增层桩基技术体系，已应用于多项工程，获得多项科技奖励。