敞口混凝土管桩土塞形成机理与荷载传递

Soil plug has a marked impact on the installation of open-ended concrete pipe pile and its bearing capacity. This project proposes a systematic study on the soil plug formation mechanisms and load transfer characteristics related to open-ended concrete pipe pile through combining laboratory model tests, numerical calculation and theoretical analysis. Main research content includes: (1) Based on visualized double-skinned model pile tests and constant normal stiffness direct shear tests, study is carried out on the microscopic developments and macro mechanical responses of soil plug during pile sinking and loading with a view to revealing the formation and working mechanisms of soil plug under different influence factors and establishing the correlation between the microscopic parameters and macro mechanical index; (2) Considering soil particle crushing and the wear of inner wall of pile, the particle flow is simulated numerically based on the laboratory test results to reveal the influences of installation method, section dimension and soil distribution on the soil plug effects; and (3) Taking the residual stress into account, the load transfer model of open-ended concrete pipe pile is established and resolved on the basis of test and numerical analysis results, and a regression equation will be established to express the influence of soil plug on the characteristics of pile installation and bearing capacity. The research results shall provide theoretical references for the design and construction of open-ended concrete pipe pile.

土塞效应显著影响敞口混凝土管桩的贯入和承载特性。本项目拟结合室内模型试验、数值仿真模拟及理论分析，对敞口混凝土管桩土塞形成机理与荷载传递特征开展系统研究。具体内容包括：（1）通过可视化双壁模型桩试验和恒刚度界面剪切试验，研究沉桩及加载过程中土塞的细观演变和宏观力学响应，揭示多工况多因素下土塞的形成机理和工作机制，建立细观参数与宏观力学指标的关联；（2）考虑土颗粒的破碎及内壁的磨损，基于室内试验结果建立颗粒流仿真模型，揭示沉桩方式、截面尺寸及土层分布对土塞效应的影响规律；（3）考虑残余应力的影响，基于试验及数值分析成果，构建并解答敞口混凝土管桩荷载传递理论模型，建立土塞对敞口混凝土管桩贯入及承载性能影响机制的函数表达。研究成果将为敞口混凝土管桩的设计和施工提供理论依据。

土塞效应显著影响敞口混凝土管桩的贯入和承载特性。结合室内模型试验、离散元数值模拟及理论分析，对敞口混凝土管桩土塞形成机理与荷载传递特征开展系统研究。具体成果包括：（1）研制了大比例沉桩试验系统和双壁模型试验桩，并开展了砂土和黏性土开口和闭口管桩的贯入试验，得到了不同工况下桩壁内外侧和桩端阻力分布，揭示了土塞的形成和作用机制。基于试验结果，并纳入应力折减系数、挤土敏感系数、侧阻退化系数三个参数，提出了适用于开口混凝土管桩的基于静力触探试验的承载力设计方法。同时，量化敞口管桩内、外侧摩阻力不同的发挥机理，建立了开口管桩“桩中桩”荷载传递解析计算模型，解析了开口管桩特有的荷载传递过程。（2）结合数字图像相关测量技术（DIC），研制了可视化桩-土界面剪切仪类比桩-土体系界面体系，并建立了纳入破碎机制的离散元模型，揭示了不同初始法向应力、剪切位移幅值、界面粗糙度下，混凝土/钢桩与砂土/黏土界面的循环弱化效应宏细观机制，量化了界面剪切弱化因子与剪切带分布特征。（3）研制现场试验系统，提出了创新性的安装多种测试元件的现场试验方法，将光纤测试技术和硅压阻式传感器用于管桩试验，实现了桩贯入过程贯入阻力分布的实时采集，系统研究了黏性土中静力压桩过程桩的受力特性，着重分析了有效径向应力降低的“侧压力退化”现象。（4）开展开口桩贯入砂土地基的颗粒流数值模拟，揭示了桩土-体系的实时响应，揭示了紧邻桩体的剪切带分布以及约5~9倍桩径的桩周土影响范围，建立了桩径与闭塞效应和土塞高度的相关性。沉桩过程中桩外侧摩阻力随着沉桩深度的增加出现“侧阻退化效应”，并计算出桩外侧摩阻力与沉桩深度和桩径比值的幂函数方程。