考虑黏土热弹塑性行为的能量桩工作机理与分析方法研究

Energy piles are piles equipped with heat exchange pipes through which a heat-carrying fluid circulates and exchanges heat with the ground. This technology couples the structural role of classical pile foundations to that of heat exchangers with the energy supply. As a result of good energy efficiency and economy, energy piles have been have been used increasingly in recent years. The mechanical behavior of energy pile foundations under the combined thermo-mechanical loading has attracted the attention of researchers. However, the thermo-elasto-plastic behavior of soils under the cyclic temperature loading and the effects on the pile-soil interaction are not clear yet. In this study, the thermo-elasto-plastic behavior of saturated clay will be studied by laboratory tests, and then a thermo-elasto-plastic constitutive model will be proposed. Based on the results of the numerical analysis, the model test and the field study, the load-settlement characteristics of the single pile and the pile group will be studied systematically. The influences of cyclic load-unloading and variations of mechanical properties on load transfer characteristics will be discussed in detail. In addition, the interactions among the energy pile, the structural pile, the soil and the raft will also be studied. By incorporating a load-transfer model which can reflect the effects of cyclic temperature changing, the load - settlement analysis method of single energy pile will be established. In this method, the temperature-induced soil deformation will be considered. The finite element method will be used to determine the pile-pile interaction factor under the thermo-mechanical coupling load. Then the simplified analysis method for energy pile group foundations will be established. Based on the results of model tests and numerical analyses, the method for bearing capacity of energy pile foundations will be proposed. The results of this study can improve the design methodology and speed up the application of energy piles.

能量桩是将地源热泵系统中的传热管埋置在桩体内部，同时起到换热和承载作用的桩。由于具有良好的节能性和经济性，能量桩的应用日益增多，热力耦合作用下能量桩的力学性状也已成为研究的热点，但循环温度荷载下土体的热弹塑性行为及其对能量桩工作机理的影响尚不明确。本课题拟通过室内试验明确饱和黏土力学特性与温度之间的关系，建立考虑循环温度效应的热弹塑性本构模型。在此基础上发展能量桩热力耦合数值分析方法，联合模型槽和现场试验，系统研究能量桩单桩和群桩的承载及沉降特性，阐明循环温度变化引起的土体周期受载和力学特性演变对荷载传递特性的影响规律，揭示群桩基础中能量桩-土-筏板-非能量桩之间的相互作用机理。进而提出能反映循环温度效应的荷载传递模型，建立考虑土体温度变形的能量桩单桩荷载-沉降分析方法；基于桩-桩相互作用系数的概念，建立能量桩群桩基础简化分析方法。研究成果可为完善能量桩的设计理论和推广应用提供参考依据。

能量桩是指将地源热泵系统中的地下换热管设置在桩体内部，桩同时起到承载和换热的作用。能量桩技术不仅可以节省换热管钻孔埋管的施工费用，又具有稳定性和耐久性好、不占用额外地下空间等优点。. 为保障能量桩承载安全，分析其在热-力荷载下的工作特性，项目采用试验分析、理论推导和数值模拟的方法对土体力学性能的温度效应、温度对桩-土位移模式及设计方法进行了研究。采用温控三轴仪研究了温度变化对黏土压缩和剪切特性的影响。结果表明，温度循环后黏土有残余体积应变，其大小随温度循环周数的增加而增加，并逐渐趋于稳定值。基于广义塑性理论给出了热-力耦合作用下土体体积应变的计算模型。若黏土在排水条件下升温，稳定后的不排水和排水剪切的峰值强度有明显增加，临界摩擦角基本保持不变。不同温度下黏土不排水剪切过程中均产生正的超孔压，排水剪切中土体体积均持续减小，表明高温下偏应力偏应变曲线出现软化的原因与强超固结土的剪胀机理有区别。温度循环一周后，黏土的不排水峰值强度有明显提高，温度变化幅值越大，应力应变关系的软化现象越明显。基于试验结果，提出了考虑温度影响的剪胀比公式。. 采用有限元分析了能量桩单桩和群桩在热-力荷载下的工作特性，结合实验分析了不同桩顶约束条件、不同荷载水平下温度循环对桩顶沉降、桩侧摩阻力和桩身轴力的影响。若群桩基础中只有部分桩温度变化，桩-筏-桩的相互作用使桩顶荷载重新分配，基础可能发生倾斜。基于荷载传递法，采用曼辛法则模拟温度循环过程中桩土界面的卸载和再加载特性，通过再加载过程中刚度的折减近似考虑塑性变形的积累，建立了热力耦合作用下能量桩单桩工作特性的简化分析方法。进一步考虑桩侧剪应力在土层中的传递，反映群桩之间的相互作用，建立了能量桩群桩基础工作特性的简化分析方法。该方法能反映桩土界面上的非线性、桩顶的约束条件和能量桩位置的影响，能直接计算所有桩的位移和轴力，可用于大规模能量桩群桩基础的设计计算。