桩土相互作用的预应力混凝土桩抗拔承载机理及试验研究

In recent years, prestressed concrete hollow piles are frequently utilized as uplift piles to resist buoyant force acted on the bottom of foundation substructure. However, due to differences with traditional uplift piles in characteristics of pile-soil interface and pile compacting effect in pile driving, it is necessary to study the property of pile-soil interaction and influence to friction resistance and bearing capacity caused by pile compacting effect. On the basis of theoretical analysis, numerical stimulation and different scale tests including shear tests, model tests and field tests, the mechanical characteristic of the pile -soil interface will be investigated and the stress-strain constitutive relation will be proposed, and the failure pattern of uplift pile will be also revealed. The stress distribution of pile and the mechanism of bearing capacity will be analyzed by considering three stages, such as under pile driving, after pile driving and normal service condition. The load-displacements, the shaft stresses and fiction stresses of the uplift piles are analyzed by considering the interaction between the pile surface and soil. Comparison with compaction and non-compaction driving piles in model tests, the relationship of pile-soil friction stress and compaction effect, as well as the effect to bearing capacity of the uplift piles, will be discussed. By means of the numerical modeling, in-door model tests and in-situ pile tests, the characteristics of the bearing capacity of compressive piles and uplift piles will be compared considering compaction effect of driving piles, and the reasonable parameters of uplift index will be suggested. The theoritical and experimental research results will reveal the bearing mechanism of uplift piles, and a systemic design theory of uplift piles will be purposed eventually and as a result that will provide a reliable technical guarantee for using the prestressed concrete hollow piles as uplift piles in construction projects.

近年来预应力混凝土桩作为地下结构抗浮基础应用越来越广泛，由于其桩侧表面特性以及沉桩方式与传统的抗拔桩存在较大差异，桩土接触界面特性、沉桩挤土效应对桩侧摩阻力的影响以及抗拔承载特性的发挥机理需要进一步研究。基于理论分析、数值模拟以及不同尺度的室内试验、现场原位试验，研究预应力混凝土桩桩-土界面力学特性并建立接触面本构模型，揭示桩土相互作用的抗拔桩破坏模式；分析抗拔桩挤土条件下沉桩时、沉桩后及工作状态下桩身应力分布规律以及桩土相互作用的承载力发挥机理；研究上拔荷载作用时预应力混凝土桩荷载位移关系以及桩身轴力、侧摩阻力沿桩身逐步发挥的特征；通过挤土条件下抗压桩与抗拔桩的承载力发挥对比，提出预应力混凝土桩抗拔系数的取值建议。在理论及试验研究基础上揭示预应力混凝土桩抗拔承载机理，建立考虑桩土相互作用的抗拔桩承载力设计理论，可为工程应用提供依据，有助于提高预应力混凝土抗拔桩工程应用的可靠性。

本项目采用理论分析、数值模拟以及不同尺度的室内试验、模型试验，对预应力混凝土抗拔桩的桩土接触界面特性、沉桩挤土效应对桩侧摩阻力的影响以及抗拔承载特性的发挥机理开展研究。预制桩在砂土和粉土中的剪切试验结果显示混凝土桩桩-土界面的力学特性可采用指数-双曲线组合模型进行描述，并得到粗糙度对剪切强度的影响规律；通过砂土和粉土中的单-双桩抗拔模型试验，得到抗拔桩挤土条件下沉桩时、沉桩后及工作状态下桩身应力分布规律以及桩土相互作用的承载力发挥机理，获得残余应力的大小、分布规律以及时间效应；依据挤土条件下的混凝土桩抗压和抗拔试验，得到压桩与抗拔桩的承载力大小，并计算出预应力混凝土桩抗拔系数，给出工程中的抗拔系数取值；在理论及试验研究基础上，揭示预应力混凝土桩抗拔承载机理，形成抗拔桩的变形计算方法，并引入变刚度调平设计理念，建立了考虑桩土相互作用的抗拔桩承载力设计理论，其在高压电线杆塔基础抗拔桩设计中的应用显示经济合理，具有较好的经济价值和社会价值。