往复荷载下考虑桩土界面损伤特性的桩基累积沉降变形研究

Firstly, based on the pile-soil interface indoor model test, random fractal theory and random fractal model and fractal crushing of computer technology structure are used, to describes the evolution of the microstructure of the pile soil interface during loading and establishes the relationship between the random fractal evolution and its physical and chemical characteristics. Secondly, beginning the theory of elastic-plastic statistical damage, based on degree of softening damage which fractal dimension of random fractal model respond the structural soil around the pile under load. A useful random damage constitutive model of pile-earth coupling system for pile-soil interface which is established. Then, the model test of pile foundation under long-term cyclic loading is carried out, and the important factors which affect the settlement of pile foundation are analyzed. And the development rule of pile foundation settlement is further discussed. Finally, from the microcosmic angle of normal stress, shear stress and tangential displacement of pile-soil interface, fully considering the pile foundation settlement law of groups-soil-pile cap composite deformation system, this paper established the calculation model of pile foundation settlement considering the fatigue damage characteristics of pile-soil interface under cyclic loading, discussed the influence of the fatigue degradation of soil around the pile on the settlement and deformation of the pile structure system, proposed the calculation method of pile foundation settlement under long term load, and reasonably predicted and controlled pile foundation settlement and deformation.

本项目首先依托室内桩土界面模型试验，采用随机分形理论和计算机技术构造随机分形模型和破碎分形生成，描述桩-土界面在受荷过程中微（细）观结构演变规律，建立起随机分形演化与其物理、化学等特征联系；其次，从弹塑性统计损伤理论出发，基于随机分形模型的分维数反映桩周结构性土体在荷载条件下的软化损伤程度，建立起桩-土耦合体系可供应用的桩土界面随机损伤本构模型；然后，开展长期往复荷载作用下的桩基室内模型试验，分析影响桩基沉降的重要因素，进一步探讨桩基累积沉降的发展规律；最后，由桩-土界面的法向应力、剪切应力、切向位移等微观角度入手，充分考虑桩-承台-土综合协调变形系统下的桩基沉降规律，建立往复荷载作用下考虑桩土界面疲劳损伤特性的桩基沉降计算模型，探讨考虑桩周土体疲劳退化对桩基结构系统沉降变形的影响，提出长期荷载作用下桩基结构的累积沉降计算方法，合理预测和控制桩基础的累积沉降变形量。

本研究紧密围绕长期往复荷载作用下软粘土地区桩基累积沉降变形这一工程问题，以桩土界面剪切破坏为切入点，采用室内试验、理论分析和数值仿真等研究手段对桩土接触面剪切特性及其对桩基承载力的影响开展了深入而系统的研究，首先，开展了粉质黏土-混凝土结构接触面的室内直剪试验，探究并验证了粗糙度和法向应力这两大主要因素对接触面剪切特性的影响，通过对接触面不同剪切破坏形式转变的内在机理分析，指出界限法向应力的存在是接触面剪切特性形式转变的原因；其次，利用MTS电液伺服加载系统进行了往复荷载作用下带台单桩模型试验，获得了往复列车荷载条件下，不同桩长、不同动静荷载比值和不同荷载频率对单桩动力特性的影响规律；然后，根据试验分析结论，并结合土-结构接触面剪切过中的“三阶段”模式，充分考虑剪切全过程中的土-结构相对剪切变形和土体自身附加变形，对无厚度单元接触面统计损伤本构模型进行改进，提出了土-结构接触面剪切全过程的统计损伤改进模型；最后，基于室内直剪试验数据和修正后的无厚度单元接触面本构模型，给出了无厚度单元接触面本构模型的仿真实现流程及具体实施步骤，采用FLAC3D建立了直剪试验力学模型验证了其适用性，并通过抗拔桩仿真模型分析了桩体力学参数和土体力学参数等因素对桩基承载力的影响规律。本项目提出的全过程统计损伤本构模型可以反映接触面“三阶段”的剪切全过程，且参数取值更加便捷、准确，计算结果更加符合工程实际，为剪切过程中土-结构界面的一些重要力学问题提供了新的解决途径，也为地下工程结构的安全性和稳定性计算提供了基础理论支撑。