深厚软土地区摩擦型桩沉降的时间效应研究

A new approach is presented to investigate the time effect for the settlement of frictional piles embedded in deep soft soil. First, the pile is divided into finite segments, and the viscoelastic models composed of the nonlinear springs and dampers are used to simulate the surrounding soil and pile end soil. Then, the displacement equations of pile can be established. By means of iterative method and together with the boundary-initial conditions of the pile-soil system, the displacement, axial force and shaft resistance of each pile segment can be obtained. Meanwhile, the fictitious soil pile model and the additional mass model are proposed to simulate the pile end soil and the soil plug of the pipe pile, respectively. On this basis, the influence of the properties of the pile end soil (i.e. the thickness of the pile end soil, the sediment and the soft substratum) and the soil plug (i.e. the height and mass density of the soil plug) on the settlement properties of the pile is investigated. Detailed model tests and in-situ tests are also put forward to evaluate the reliability of the proposed theoretical models and the calculation methods. Meanwhile, a more simple and accurate method is put forward to test the axial force and shaft resistance of the pile. Based on all the above works, a more effective method and the corresponding computer program are made available to better analyze the long-term settlement of frictional piles embedded in deep soft soil considering the complicated pile-soil conditions and thus provides valuable guidance for the design of the pile foundations.

本申请提出一种新的深厚软土地区摩擦型桩沉降时间效应计算方法。将桩身划分为若干段，采用非线性弹簧和阻尼器组合得到的元件模型模拟桩周土及桩底土，建立桩身位移求解方程组，通过方程组的联立迭代并结合桩-土体系的边界条件和初始条件，求得桩身任意位置的沉降、桩身轴力及侧摩阻力随时间的变化规律；在此基础上，提出采用虚土桩模型模拟桩底土、附加质量模型模拟桩芯土塞，研究桩底土性质（厚度、桩底沉渣、软弱下卧层等）及管桩桩芯土塞（土塞高度、密度等）对桩基沉降时间效应的影响；在理论研究的基础上，进行详细的模型试验及现场试验，以验证所提出的理论模型和计算方法的可靠性，并提出一种更为简便精确的桩身轴力及侧摩阻力测试方法。以期通过本项目的研究，建立复杂工况下深厚软土地区摩擦型桩长期沉降性状的预测分析方法，并编制相应的计算程序，为桩基础的设计提供理论指导。

摩擦桩在软土地区应用广泛，其沉降表现出明显的随时间变化的特性。为此，本研究提出了一种新的桩基沉降时间效应计算方法，求得了桩身任意位置处的沉降、侧摩阻力及桩身轴力。在此基础上，分析了土体非线性、桩底土性质、侧阻软化、端阻硬化及桩芯土塞等因素的影响规律。主要成果如下：（1）随着时间的推移，桩顶沉降逐渐增大，并最终趋于稳定，表现出明显的随时间逐渐发展的规律；（2）荷载水平较高时，侧阻的软化现象会导致桩基沉降量的增大，其影响随着侧阻软化程度的增大而增强，端阻的硬化现象则会导致桩基沉降量的减小；（3）桩底土性质对桩的沉降特性影响显著，随着桩底土厚度的增大，桩顶沉降量逐渐增大，且其增大速度逐渐减小；（4）桩芯土塞的存在增大了管桩的侧摩阻力，并进而减小了桩顶沉降，且其影响程度受到桩身参数及挤土效应的影响。本研究能够为复杂工况下深厚软土地区摩擦桩长期沉降特性预测提供参考。