长期热循环下桩土力学特性及作用机理研究

Heat exchanger pile technology is one of the new building energy saving techniques in which the heat exchanger is constructed together with pile foundation. Under the long term cycling thermal load, mechanism and changes of mechanical properties of pile and soil, and of behavior of pile-soil interface are the key are key scientific problems required to be solved in the design of heat exchanger pile. Therefore, a method of combining laboratory test, model test, theoretical modeling with numerical analysis is used in this subject. Firstly, the thermo-drainage consolidated triaxial shear test and the thermal consolidated test are carried out on the soil around the pile. The effects of temperature on the strength, consolidation and strain accumulation of soil are analyzed quantitatively, and the change rules of heat hardening or softening of soil under different temperature are revealed. Secondly, model test method is used, and static loading tests are carried out under different temperature and under the cyclic heating/cooling case for a certain temperature increment, then the long-term cycling thermal effect of the heat exchanger piles are revealed. Finally, a shear model of pile-soil interface and a numerical calculation model for the heat exchanger pile-soil system are developed，then mutual verification are executed on numerical calculation results and model test results. Furthermore, the calculated result are used to assess the influence of the long-term thermal cycling effect on the bearing capacity of pile foundation, and to optimize the design of heat exchanger pile, and to put forward the corresponding technical measures. The results of this subject, on the one hand, can reveal the mechanical properties of heat exchanger pile and soil under complex environmental conditions, develop multi-physical field soil mechanics and promote inter-disciplinary, on the other hand can provide an important scientific and technical support for the design and application of heat exchanger pile.

热交换桩技术是与桩基工程协同施工的、新的建筑节能技术。在长期热循环作用下，桩土力学特性、界面荷载传递及作用机理等是亟待解决的关键科学问题。为此，本项目采用室内试验、模型试验、理论建模和数值模拟相结合的方法，首先对桩周土开展热排水三轴剪切试验、热固结试验，定量地揭示温度对土的强度、固结和应变累积特性的影响，判定土的热硬化或软化规律；其次采用模型试验方法，开展不同温度增量及循环加热/卸热作用下的桩基静载荷试验及测试，揭示热交换桩的长期热循环作用机理；在此基础上，建立桩土界面剪应力-位移模型及桩土系统数值模型，将计算结果与模型试验结果相互验证，评价长期热循环效应对桩基承载力的影响，进而优化热交换桩的设计计算，提出相应的技术措施。本项目一方面旨在揭示复杂环境条件对热交换桩基础工程特性的影响，促进学科交叉，另一方面为热交换桩基础的优化设计提供必要的科学依据和技术支撑。

本项目围绕热交换桩，采用室内试验、模型试验、理论和数值方法，开展了长期热循环下桩土力学特性及作用机理研究。首先采用温控静动三轴仪，开展了不同温度（25℃、45℃和65℃）、不同围压（100kPa、150kPa、200kPa）及先加温后剪切、先剪切后加温条件下宁波软黏土的热力学特性，分析了温度对土的应力~应变及抗剪强度等指标的影响。其次，研制了大型温控固结仪，开展了循环温度下土的固结试验，获得土的e~p、e~logp曲线及压缩系数等指标；在广义三元件模型、西原模型基础上，引入考虑温度影响的元件参数，建立了耦合温度的三元件流变模型和西原模型，探讨了的热流变固结性状。第三，以宁波软黏土为对象，开展预制热交换桩桩-土模型试验，研究了温度对桩身轴力、桩侧摩阻力、桩顶位移、桩身应力应变、土体孔压、地表沉降等的影响规律，揭示了长期热循环作用对桩基承载特性的影响。最后，考虑能源桩桩土界面法向温度应力增量对初始剪切刚度的影响，对传统桩土界面荷载传递双曲线模型进行修正，建立了考虑温度影响的桩土界面荷载传递模型，分析了温度对能源桩侧摩阻力的影响；并基于上负荷面、tij和温度等价应力概念，提出了可考虑超固结比、结构性、温度效应和中间主应力的黏土热弹塑性本构模型，并进行二次开发，在Abaqus程序中开展热交换桩基承载特性数值模拟，计算获得不加热(室温30℃)、加热至45℃(60℃)、先加热至45℃(60℃)再自然降温至30℃、先加热至45℃再自然降温至30℃(进行两次循环加热)六种工况及30℃→45℃→30℃(两次循环)工况下桩土温度、应力、孔压时程曲线，荷载沉降、桩侧阻力、桩身轴力，进一步阐明长期热循环作用对桩基承载特性的影响机理；利用有限元模型，计算分析了长期热循环作用下热交换桩有承载力特性，评价了长期热循环作用对热交换桩基础承载力设计的影响，并提出了长期热循环作用下桩端和桩侧后注浆对热交换桩附加沉降的控制措施。