开挖条件下饱和软粘土地基桩基础竖向承载特性研究

Abstract：Considering the lack of research on the pile bearing characteristics in saturated clay under excavation, firstly, the coefficient of K0 of different stress path under K0-consoildation and the undrained shear stress-strain under K0-conlidation are analyzed by stress path triaxial tests, variation of the stress and strength characteristics of the soil around pile are studied through stress path triaxial tests. Secondly, a model of pile vertical loading system can effectively control vertical effective stress of the soil around pile is developed out. The laboratory model tests of a single pile in saturated clay under excavation are carried out. Based on the model tests and the stress path triaxial tests, an analytical method is developed to analyze the ultimate frictional resistance in saturated clay under excavation. Then, a simplified nonlinear approach for analysis of a single pile in saturated clay under excavation is proposed. Finally, the laboratory model tests of the interaction between pile and pile and the rigid pile group subjected to vertical loading in saturated clay under excavation are carried out. Based on the model tests and the theoretical analysis, an analytical method is developed to analyze interaction between pile and pile subjected to vertical loading in saturated clay under excavation. Then, a simplified nonlinear method for rigid pile group subjected to vertical loading in saturated clay under excavation is given. The study will provide experimental and theoretical basis for the design of pile foundation in saturated clay under excavation.

摘要：针对开挖条件下饱和软粘土地基中桩基础竖向承载特性研究存在的不足，本课题首先通过三轴试验对K0超固结土体的K0系数及不排水抗剪强度进行研究，以揭示开挖卸载条件下饱和软粘土地基中桩周土体应力状态和强度特性的变化规律；其次研发一套可以有效控制桩周土体竖向有效应力的竖向受荷桩模型试验系统，对开挖条件下饱和粘土地基中竖向受荷单桩进行试验研究，基于模型试验和土体三轴试验结果，提出开挖条件下饱和软粘土地基中桩侧极限摩阻力及竖向受荷单桩简化计算方法，并与模型试验结果进行对比验证。最后对开挖条件下饱和软粘土地基中竖向荷载下的桩-桩相互作用及刚性筏板群桩基础的承载特性进行试验研究和理论分析，建立开挖条件下饱和软粘土地基中竖向桩-桩相互作用计算模型，进而提出开挖条件下饱和软粘土地基中竖向受荷刚性筏板群桩基础非线性简化计算方法。研究成果将为开挖条件下桩基础的设计提供试验和理论依据。

针对开挖条件下饱和土地基中桩基础竖向承载特性研究存在的不足。项目通过室内模型试验和理论分析相结合的方法对开挖条件下非均质地基中桩基础竖向承载特性展开研究。针对目前开挖条件下竖向受荷桩基础模型试验研究中存在的问题，本课题首先研发了一套高精度开挖条件下桩基础竖向承载特性模型试验系统，对开挖条件下饱和土地基中桩基础竖向承载特性进行了室内模型试验；针对试验所用土体进行了应力路径三轴试验，得出了相应室内模型试验试验过程中桩周土体的应力状态和强度参数。综合应力路径三轴试验结果和模型试验结果对开挖条件下桩基础的承载特性进行分析研究。建立了开挖条件下非均质地基中竖向受荷单桩非线性计算方法，得出了开挖条件下非均质地基中基桩顶面柔度系数。采用有限元法对筏板进行了分析。在考虑桩周土体开挖卸荷影响的基础上，基于层状弹性半空间理论，对开挖条件下非均质地基中桩土体系相互作用进行了分析。建立了开挖条件下桩土体系的柔度矩阵，采用有限元和有限差分相结合的方法得到了开挖条件下非均质地基中柔性桩筏基础非线性分析方法。通过与已有试验和理论分析结果进行对比，验证了简化方法的正确性。计算结果表明：常规试桩法和套管试桩法高估了基桩竖向极限承载力和桩顶刚度，采用常规试桩和套管试桩法得到的承载力作为设计值，偏于不安全；开挖卸荷引起桩周土体回弹使得桩身出现拉力，开挖深度较大时桩身中下部的配筋应充分考虑由于开挖引起的沿桩身的拉力，防止开挖过程中基桩被拉断。开挖卸荷将会引起群桩基础和桩筏基础承载力和刚度的降低；忽略开挖卸荷的影响，将低估开挖条件下桩筏基础中土体承担荷载的比例，并使得分析结果中筏板底部基桩桩顶荷载分布的不均匀性偏大。本项目的研究成果，可为开挖条件下群桩基础及桩筏基础变刚度调平设计提供理论分析工具。