软土能源桩温度蠕变的微观机理与热力耦合宏观模型研究

Energy piles possess wide engineering opportunities in China’s developed coastal and riparian urban agglomerations. The ground condition of these areas are usually soft clays, in which the energy piles, under cyclic thermal loads, may undergo long-term accumulated settlements and stability problems, which involves complex multi-field geotechnical mechanisms. Developing a theoretical model that is able to accurately describe and predict these mechanisms has long been a major task to the engineering and academic researchers. In recent years, research on the macro thermodynamic models with clear physical mechanisms has got fundamental progress. The project aims to start from the study of the micro mechanisms of thermal creeps of soft clays under cyclic thermal loads. Then, a unified thermodynamic theoretical framework and model is going to be developed which is able to quantitatively and qualitatively describe the visco-plastic dissipative mechanism of soft clays, from micro to macro. The key question of the project is to study the Thermo-Electro-Osmosis micro mechanism between clay particles and the pore bound water and how to implement it into the thermodynamic constitutive model in order to quantitatively describe the macro thermal-mechanical-coupled properties of soft clays. The project is going to develop multi-field axisymmetric finite-element codes for analyzing and predicting problems of energy piles in soft clays. Centrifuge model tests and thick-walled hollow cylinder tests will be used for validating the proposed theories. The project is expected to provide theoretical and experimental support for the application of energy piles in coastal and riparian areas.

能源桩在我国经济发达的沿海沿江城市群有着广泛的工程应用空间。沿海沿江地区多为软土地质，而软土地基中能源桩在循环温度荷载作用下的长期沉降累积和稳定性涉及诸多复杂的多场耦合岩土力学机制。建立能够准确描述和预测这些力学机制的理论模型，一直是工程界和学术界为之努力的重要目标。近年来，对于具有清晰物理机制的宏观热力学模型的研究取得了根本性进展。本项目拟从研究软土在循环温度荷载作用下蠕变的微观机制入手，发展统一热力学理论框架和模型，由微观至宏观、定量定性描述软土的黏塑性耗散机制。其中核心问题涉及土颗粒-孔隙结合水的Thermo-Electro-Osmosis微观机制，以及如何将其嵌入热力学本构中从而宏观定量描述软土的热-力耦合性质。项目将发展多场耦合的轴对称有限元程序对软土中能源桩问题进行分析和预测，并运用离心机试验、中空厚壁圆柱实验等手段进行理论验证，为能源桩在沿海沿江地区应用提供理论和实验基础。

TTS模型是以非平衡态热力学为基础的新一代岩土本构模型，能够为岩土材料力学响应提供更本质的物理基础。较弹塑性本构模型而言，TTS模型摒弃了经典塑性力学中的若干概念（屈服函数、流动法则、硬化规律等），以颗粒固体流体动力学（GSH）为理论框架，能够统一描述岩土材料温度效应、临界状态、率相关性、各向异性、循环滞回、多场耦合、小应变刚度等重要性质。.本课题主要是针对不同固结程度的黏土在应力-（循环）温度变化条件下的微观 Thermo-Electro-Osmosis 机制进行理论研究和实验研究，并将机制嵌入清华岩土热力学模型，开展从微观到宏观层次上的理论模型研究、有限元数值模拟和实验验证，形成以热力学概念为基础的具有清晰物理机制的宏观力学表达，建立软土中能源桩长期变形积累和（短期、长期）稳定性的预测方法。.首先，通过测量土样比重变化的实验手段，对温度变化条件下结合水与自由水的转换进行直接观测和定量描述。结果表明，固体颗粒比重会随着温度的升高而降低；考虑到土壤成分的热膨胀，当温度升高时，结合水转化为自由水。 .其次，从单个土颗粒受力平衡的角度出发，开展考虑温度影响的有效应力微观力学模型研究。.然后，以非平衡态热力学为基础，以颗粒固体流体动力学为理论框架，对考虑自锁弹性能的统一能量耗散体系进行定量描述。在此基础上，完善了考虑微观Thermo-Electro-Osmosis物理机制的温度场-应力场耦合 TTS 本构模型，应用考虑温度变化的常规三轴实验对模型参数进行标定。发展 TTS 模型的轴对称有限元程序，对地基和能源桩模型试验及多种单元试验进行数值模拟。.最后，根据《桩基地热能利用技术标准》指导完成了雄安城市计算（超算云）中心能源桩项目。对能源桩进行TRT（恒定热流密度）和TPT（恒定进口水温）测试，用于评估能源桩的换热能力，并向桩内埋设传感器计算温度变化引起的桩结构力学响应。