挤土效应对砂土中静压桩水平承载特性影响的机理研究

Jacked piles belong to displacement piles. The soil around the pile will be forced to deform and its stress state is subject to change when the piles are jacked into the ground. Previous study on the squeezing effect of jacked piles focuses mainly on the penetration resistance, state change of soil around the pile and the effect of soil squeezing on the vertical bearing capacity of pile. Although some of previous experimental study on laterally loaded piles involved the influence of pile installation method, systematic study on the squeezing effect of jacked piles in sand and its effect on the lateral bearing characteristics is still scant. In the current engineering practice, the effect of non-uniform distribution of sand along the horizontal direction, resulting from the squeezing effect, on the lateral soil-pile interaction is not taken into account. This project will study the squeezing effect of open and closed-ended piles in the sand ground with different relative densities by use of indoor model tests and μXCT tests. Its influence on the lateral bearing characteristics of both stiff and flexible piles will be studied as well. Theoretical analysis will be conducted by use of the cavity expansion method incorporating the state-dependent dilatancy model to reveal the cavity expansion mechanism in sand. Numerical analysis will be performed as well by use of the material point method to simulate the process of pile jacking and lateral loading. Finally, based on the results of experimental study, theoretical and numerical analyses, a method will be proposed to determine the parameters of p-y models that can take into account of squeezing effect.

静压桩属于挤土型桩，在施工过程中，桩体压入会使桩周土体变形，应力状态改变。以往关于沉桩挤土效应的研究主要集中在沉桩过程中桩体所受的贯入阻力、桩周土体的状态变化、以及挤土效应对桩基竖向承载力的影响等方面；虽然部分水平受荷桩的试验研究涉及沉桩方式的影响，但就砂土场地中的沉桩挤土效应及其对桩基水平承载特性的影响方面仍缺少系统性的研究，在分析与设计中也基本不考虑挤土后砂土密度沿水平向非均质分布对桩土水平相互作用的影响。本项目将通过室内模型试验结合μXCT技术，研究开、闭口桩在不同密度砂土场地中的挤土规律，以及挤土效应对刚性桩和柔性桩水平承载特性的影响机理；同时结合与状态相关剪胀性砂土模型，应用圆孔扩张法进行砂土中扩孔的理论分析，应用数值分析物质点法，实现对砂土中沉桩及水平加载的全过程模拟与分析；最后综合试验、理论和数值研究的成果，建立能考虑挤土效应影响的砂土场地中水平受荷桩p-y模型参数确定方法。

静压桩属于挤土型桩，在施工过程中，桩体压入会使得桩周土体变形，应力状态改变。以往关于沉桩挤土效应的研究主要集中在沉桩过程中桩体所受的贯入阻力、桩周土体的状态变化、以及挤土效应对桩基竖向承载力的影响等方面；虽然部分水平受荷桩的试验研究涉及沉桩方式的影响，但就砂土场地中的沉桩挤土效应及其对桩基水平承载特性的影响方面仍缺少系统性的研究，而在分析与设计中也基本不考虑挤土后砂土密度沿水平向非均质分布对桩土水平相互作用的影响。本项目结合室内模型试验、基于圆柱孔扩张的理论分析和大变形数值仿真，开展了如下研究：1）挤密效应对砂土中水平受荷桩承载特性影响的模型试验；2）砂土颗粒形态表征及基于二维颗粒的压桩过程细观变形机制试验；3）基于圆柱孔扩张理论的沉桩过程挤密效应分析；4）桩贯入过程的大变形数值模拟与分析；5）挤密效应对桩基水平承载特性及p-y曲线的影响；6）挤密效应对桩基竖向承载特性的影响。项目成果揭示了桩贯入过程的桩土作用机理、挤密效应对桩基水平承载特性及p-y曲线的影响规律，还揭示了挤密效应对桩基竖向承载特性的影响规律。结果表明，采用挤密桩相对于无挤密桩的承载力变化率在-20%~60%之间，并与砂土相对密度呈负相关关系，即对松砂中的水平承载力有提高作用，相反却会降低密砂中的水平承载力。挤密桩与无挤密桩的抗压承载力比值约为1.1~7，而抗拔承载力比值则在0.7~5.5之间，比值随着桩的长径比和砂土相对密度的变化大致呈线性关系。研究成果可为砂土地基挤土桩的水平和竖向承载力的计算和桩基设计提供重要依据。