组合荷载下大直径基桩耦合作用机理及承载力计算方法研究

Large-diameter pile foundations as an effective supporting system have been widely used in practice. However, the study of bearing behavior and calculation approach of large-diameter pile foundation under combined loads is not enough. In response to the present situation that investigations on coupling effect with consideration of combined vertical and lateral loading are obviously insufficient, the following studies will be systematically carried out. Firstly, a series of experimental research on shear mechanical properties of pile-soil interface under constant and varying normal stress is performed to investigate the formation mechanism of strengthen effect of friction and to establish corresponding theoretical system. Moreover, based on the results of finite element simulation, this work aims to reveal the separation effect of pile-soil interface and to propose a simplified model to analyze the contact and separation status for pile-soil interface. And then, considering the influence of strengthen effect of friction and separation effect of pile-soil interface, numerical and simplified theoretical models of shaft resisting moment, base resisting moment, base lateral resistance and additional moment due to p-Δ effect are presented, respectively. Furtherly, model tests for the bearing mechanism of pile foundation with consideration of the influence of loading sequence are conducted as well. The influence of coupling mechanism of combined loading on the internal force and deformation behavior of large-diameter pile foundation are therefore comprehensively investigated. On this basis, the engineering practical simplified calculation model and design method for the vertically and laterally loaded pile with large-diameter are further proposed, and thus provides the theoretical basis for the design and optimization of large-diameter pile foundations.

大直径桩基础作为有效的支撑体系已在工程实践中得到了广泛应用，然而，组合荷载作用下的大直径基桩承载性状以及计算方法研究还不够深入。针对竖向-水平联合荷载作用下耦合效应研究存在明显不足的现状，系统地开展以下几个方面的研究。开展常/变法向应力下桩-土界面剪切力学性状试验，探讨摩阻力增强效应的形成机理并建立相应的理论体系；基于有限元模拟深入揭示桩-土界面脱开效应机制，进而构建桩-土界面接触与脱开状态的简化计算模型；在考虑摩阻力增强效应及桩-土界面脱开效应影响的基础上分别建立桩身抗力矩、基底抗力矩、基底水平阻力以及p-Δ效应影响的附加弯矩数值模型与简化理论模型；结合不同加载顺序影响下的组合受荷基桩承载特性模型试验结果，全面揭示组合荷载的耦合作用机理。并在此基础上进一步提出适合于工程应用的组合荷载耦合作用下基桩承载力简化模型和计算方法，为大直径桩基础的承载力设计与优化提供充实的理论基础。

大直径桩基础因承载力大、可靠性高、稳定性强等优点被广泛应用于跨江跨海桥梁、海上风电等基础工程中，已成为我国基础建设发展战略中一个重要方向。然而，目前对大直径基桩水平尺寸效应形成机制、组合荷载作用下桩基承载内力耦合机理的研究还不够充分，缺乏系统的承载机理与设计理论研究，严重制约了大直径桩基础的发展。为此，本项目首先利用自主研制的大型界面剪切设备开展了系列动态法向压力作用下桩-土界面剪切力学特性试验研究，结合试验研究成果提出了摩阻力增强效应理论，并结合所构建的桩-土界面脱开效应模型及计算方法，实现了组合荷载作用下桩周侧摩阻力分布模式的精准确定；在此基础上，创新提出了单位长度桩身抗力矩概念并推导了解析解与简化解，结合有限元模拟分析，从量化角度充分揭示了大直径桩基水平尺寸效应形成机理，破解了传统小直径桩基设计理论无法应用于超大直径桩基的力学根源；随后，基于自主研制的伺服加载系统开展了基桩承载特性的室内模型系列试验研究，探究了不同维度荷载下的桩土相互作用机理及其承载特性，进而建立了以桩周竖向侧摩阻力为核心的桩身截面竖向内力与水平内力相互耦合的作用机制，并提出了考虑耦合效应的桩基承载力迭代计算流程及创建了桩身响应传递矩阵解，攻克了组合荷载作用下桩身内力与变形精准计算难题，实现了大直径桩基水平尺寸效应理论与组合承载设计关键技术的重要突破。最后，基于Akima非线性插值理论，提出了扣除侧阻抗力矩影响的大直径桩基净值p-y曲线反推新技术，为大直径桩基水平尺寸效应原位测试研究提供了数据分析新手段，奠定了工程实用性的基础。本项目研究成果引领和支撑了大直径桩基组合承载设计计算理论的工程应用，推广潜力极大，对我国桩基工程理论发展具有重要意义。