地震作用下多年冻土区桥梁桩基础-冻土体系相互作用机理及抗震计算模型研究

The Qinghai-Tibet plateau, mainly distributed by permafrost in China, is also an area with a high potential occurrence of earthquakes. The pile foundation is widely used for bridges in this region. The bridge pile foundation is prone to be destroyed by the earthquakes, so it is essential to evaluate its seismic performance. The dynamic properties of the frozen soil are influenced significantly by the temperature and water content (hydro-thermal characteristics). However, the hydro-thermal effect of frozen soil on the seismic analysis of the bridge pile foundation in permafrost region was neglected in previous researches. These results were insufficient to reveal the interaction mechanism of the bridge pile-frozen soil system subjected to earthquakes. Therefore, there was no appropriate pile-frozen soil interaction model for the seismic analysis of the bridge pile foundation in permafrost region. In this project, the widespread railway bridge pile foundation with gravity pier in permafrost region will be selected as research object. Through model tests, theoretical analysis and numerical simulations, the failure characteristics, hysteretic behavior and energy dissipation of the bridge pile-frozen soil system under cyclic lateral loading will be analyzed. On the basis of these results, we aim to reveal the interaction mechanism of the bridge pile-frozen soil system subjected to earthquakes. The bridge pile-frozen soil interaction model with consideration of hydro-thermal characteristics of the frozen soil will be provided to analyze the seismic responses of the bridge pile foundation-frozen soil system. Combined with the quasi-static testing results, our goal is to provide seismic performance evaluation index and seismic fortification criteria of bridge pile foundation in permafrost region.

我国青藏高原多年冻土区处于地质构造活跃地带，该区域桥梁工程普遍采用桩基础，其遭遇地震的危险性和可能性较大，抗震问题突出。冻土的动力特性受其温度和含水量（水热效应）影响显著，然而以往在多年冻土区桥梁桩基础抗震研究中未充分考虑冻土水热效应，其成果无法有效揭示地震作用下桥梁桩基础-冻土体系的相互作用机理，因而缺乏合理的桩-冻土相互作用抗震计算模型。为此，本项目选择我国多年冻土区铁路桥梁中普遍采用的重力式桥墩桩基础为研究对象，采用模型试验、理论分析和数值模拟等研究手段，探究多年冻土区桥梁桩基础-冻土体系在水平反复荷载作用下的破坏形态、滞回耗能等特性，揭示地震作用下桥梁桩基础-冻土体系的相互作用机理，建立考虑冻土水热效应的桥梁桩基础-冻土体系相互作用计算模型，对多年冻土区桥梁桩基础进行非线性地震反应分析，并结合拟静力试验结果，提出多年冻土区桥梁桩基础的抗震性能评估指标及抗震设防标准。

我国青藏高原地区多年冻土分布广泛，同时该区域地质构造活跃，地震活动频繁，多年冻土区公路和铁路等基础设施遭遇地震的危险性和可能性较大。考虑到多年冻土的敏感和脆弱性，为尽量降低工程修建对多年冻土的热影响，公路和铁路修建在高温高含冰量多年冻土区通常采用“以桥代路”的方式，而桩基础由于其承载力高、热扰动小等优势，成为了多年冻土区桥梁工程基础形式的首选。为系统研究多年冻土区桥梁桩基础地震破坏机理及抗震性能，确保其合理的抗震设计，本项目以我国多年冻土区铁路桥梁中普遍采用的重力式桥墩桩基础为研究对象，采用模型试验、理论分析和数值模拟等研究手段，探究了地震作用下多年冻土区桥梁桩基础-冻土体系相互作用机理，并建立了考虑冻土效应的桥梁桩基础-冻土体系相互作用计算模型，分析了多年冻土区桥梁桩基础的地震响应及抗震性能。.研究结果表明：首先，季节冻土层或多年冻土层均会显著改变地震作用下桥梁桩基础的破坏形态，包括桩基础混凝土开裂位置、塑性铰的出现等，冻土层的存在使桩基础的塑性区域扩大，破坏程度显著增加；其次，季节冻土层或多年冻土层的存在会显著提高桩土体系的水平承载能力，拟静力模型试验结果显示季节冻土层和多年冻土层存在时桩-土体系的水平承载能力分别提高了约50%和60%，而桩-土体系变形能力的变化并不显著；另外，季节冻土层或多年冻土层的存在及其厚度变化会显著影响桩土体系的滞回特性和耗能能力，尤其是加载后期的耗能能力明显高于无冻土情况。最后，数值分析结果显示，冻土温度变化对桩-冻土体系承载力、刚度退化和桩身位移的影响最明显，初始含水率和土体干密度的影响则相对较小；多年冻土退化导致地震作用下桩身最大曲率明显减小，使桥梁桩基础的破坏概率大幅度下降，而墩顶最大位移呈增大趋势。以上研究成果充分揭示了冻土-桩基础的相互作用机理，对多年冻土区既有桥梁桩基础的抗震性能评估和新建桥梁桩基础的抗震设防具有重要的指导意义。