静压群桩挤土效应的关键问题和机理研究

Due to its high efficiency, low noise and low pollution, the jacked pile is being widely used in a large number of construction projects of China. It is also a subject of much research popularity. Although research on the installation process of the single jacked pile, and the squeezing effect on the soil during installation, is fairly thorough, the governing mechanism involved in the pile group installation remain, at least partially, unclear. There is, in particular, a lack of reliable numerical analysis results. In the prior study of applicant, a hybrid Eulerian-Lagrangian finite element method has been preliminarily developed, which can undertake the large-deformation, pore pressure coupled analysis because of its adoption of the effective stress calculation framework. This project intends to refine and utilize this technique to carry out the three dimensional large deformation finite element simulation of pile group installation, which, combined with the transparent soil tests and large-scale model tests, serve to grasp an in-depth understanding of the squeezing effect of pile group installation on the soil. The particular emphasis of this research is placed on addressing three key problems, namely pile heave, pile derivability degradation, and negative skin friction. Research aims to explore the fundamental mechanism of these problems, propose the relevant scientific calculation methods and establish the effective mitigation approaches. The research results will be helpful in improving the design and calculation theories of jacked pile group, providing scientific basis to optimize its design and construction. It is expected to have important practical implication.

在我国大规模开展的城市工程活动中，静力压桩（静压桩）由于其经济高效、噪音小、污染小等诸多优点获得了日益广泛的应用，该领域也是近年来科学研究的热点。虽然静压单桩挤土效应的相关研究已相当深入，但群桩的沉桩挤土机理迄今为止尚未明确，特别是群桩沉桩的数值仿真模拟研究非常匮乏。申请人在早期的研究中已初步开发了采用有效应力计算框架、具有流固耦合计算能力的大变形有限元计算技术-“混合欧拉-拉格朗日”有限元。本项目拟完善和采用该技术开展群桩沉桩的数值仿真模拟，同时结合新型透明土试验和大比尺模型试验，开展群桩挤土效应综合分析。项目将重点分析群桩挤土效应的三个关键问题-桩体上浮，桩塞，桩侧负摩阻力。通过数值仿真模拟和模型试验分析揭示上述关键问题的作用机理，建立相关的科学计算方法，提出有效的防治措施。研究成果将完善静压桩设计与计算理论，为优化静压桩的设计、施工提供科学依据，具有重要的应用价值。

静压桩因施工速度快、工期短、噪音低、单桩承载力高等诸多优点，在我国基础设施建设中得到了广泛应用。静压桩属于挤土桩，其沉桩挤土过程对桩周土应力状态、孔隙水压力、强度、结构等产生显著影响。沉桩后伴随孔压消散、土体固结，桩周土应力、强度、结构将再次改变。本项目针对静压桩沉桩挤土与固结效应引起的桩周土体应力演变、桩截面形状对沉桩挤土与桩基时变承载力特性的影响、静压桩拟入桩对已入桩的桩身水平响应影响、桩间固结效应对静压群桩长期承载力影响等多个相关科学问题开展了深入研究。研究手段主要包括土工离心模型试验和大变形有限元模拟分析。研究结果表明：在静压桩沉桩过程中桩周土体应力演变特征受其水平径向距离和桩埋置深度影响较大；在同截面面积条件下，改变桩截面形状能显著影响桩基承载力；通过优化桩截面形状能有效减少建造成本、提高工程经济性；当已入桩的桩头状态为完全约束时，在拟入桩沉桩安装过程中，已入桩的桩身弯矩峰值始终位于桩头位置。在拟入桩沉桩安装后，受拟入桩的桩身回弹、土体的剪切、卸载影响，已入桩桩身弯矩随固结时间会逐渐降低。受沉桩挤密效应、土体强度增长的影响，在相同贯入深度拟入桩沉桩时的贯入阻力要大于已入桩沉桩时的阻力值；当桩间固结效应不明显时，已入桩和拟入桩沉桩过程中产生的超孔压会出现一定的叠加和累积。桩间固结效应不会改变静压群桩承载力时变曲线特性，但能提高静压群桩整体的长期承载力。基于大量试验和数值分析成果，项目还对静压桩设计施工提出了具体的指导建议。截至项目结题日期，依托本项目已发表SCI论文数目21篇，大幅超过了原定的目标。