季节性冻土区桥梁桩基水平振动破坏机理与减灾技术研究

Bridge pile foundations in cold regions experienced severe damage in past winter earthquakes that warrants further investigation for understanding the damage mechanism and developing mitigation technologies for the hazards caused by seasonally frozen soils. Based on a representative pile foundation in seasonally frozen soils in China, the thermal and mechanical properties of selected frozen soils and replacement materials under cyclic loading will be obtained through laboratory experiments, and used to construct constitutive models that account for the effects of water/ice content, temperature, loading cycles, strain rate, density, etc. Quasi-static cyclic loading tests of large-scale model piles embedded in frozen soils and replacement materials will be carried out to gather data for comparison of pile performance and understanding of plastic hinging mechanism, and assessment of the effectiveness of the local replacement method in hazard reduction. Meanwhile constitutive or material models for frozen soils, local replacement materials, and the soil- or replacement material-pile interface will be developed, calibrated, and implemented into the OpenSees platform. Impact of major factors such as frozen soil or replacement material stiffness and strength, geometry parameters such as thickness and radius, and the sensitivity of pile performance to these parameters will be analyzed by conducing three-dimensional finite element analyses. Cyclic p-y curves for frozen soils and replacement materials will be derived from the model test data, and key parameters and testing methods for obtaining these parameters will be proposed for constructing p-y curves to be used in beam-on-nonlinear-Winkler-foundation type of analysis. A simplified fixity depth model will be provide for design of piles foundations as a cantilever beam in seasonally frozen soils by using the local replacement method EPS and XPS, which has important theoretical significance and engineering value.

大量震害资料表明，地震易导致季节性冻土区桥梁桩基严重受损，有必要深入研究其水平振动破坏机理及减灾技术。首先，对我国震区典型季节性冻土开展冻融土水-热-力性质试验，建立水-热-力耦合的土动力本构模型，并统计其计算参数。其次，开展拟静力循环荷载作用下冻融土桩基大比例模型试验，揭示桩基动力响应机理及破坏模式。然后，提出季节性冻土区桥梁桩基动力p-y曲线与关键参数，导出桩基动力分析的理论解答。同时，对OpenSees数值计算平台进行二次开发，引入冻土、置换材料动力本构模型，及桩-置换材料和桩-冻土接触本构模型，并基于三维数值模型对关键参数进行敏感性分析。在此基础上，综合模型试验、理论与数值分析等手段，探索采用EPS、XPS等土工合成材料置换桩周表层冻土时的减震效果，提出基于表层置换技术的季节性冻土区桥梁桩基简化设计方法，具有重要的理论意义及工程价值。

对于季节性冻土区的桥梁桩基，桩周土体冻结后强度将急剧增加，其对桩基的水平约束显著增强，使得桥梁桩基侧向位移性能大大降低。在地震等水平动载作用下，极易导致桩基发生开裂、折断等脆性破坏。为提高寒区桥梁桩基的抗冻及抗震性能，本项目研制了一种对温度不敏感的聚氨酯胶-橡胶颗粒-砂混合材料(PolyBRUS)来置换桩周部分土体，从而改善寒区桩基的抗震性能。本项目的主要工作及研究成果如下：. (1)通过室内土工试验，测试了试验材料的物理力学参数，确定了废旧轮胎颗粒材料的改良质量配比为橡胶颗粒：砾砂：聚氨酯胶结料=3:2:1，利用改良废旧轮胎混合材料制作了若干三轴试样。. (2)通过室内静三轴剪切试验，获得了不同试验条件下改良废旧轮胎混合材料试样的应力-应变关系曲线，破坏强度、切线模量及切线泊松比等静力特性参数。通过非线性拟合和回归分析，获得了考虑温度及冻融循环影响的改良废旧轮胎混合材料静力本构模型及模型参数确定方法。. (3)基于室内动三轴试验，获得了改良废旧轮胎混合材料试样的动应力-动应变关系曲线，动弹性模量及阻尼比等动力特性参数。据此建立了考虑温度、冻融循环和围压影响的改良废旧轮胎混合材料动力本构模型及模型参数确定方法。. (4)预制了三个1/12比例的钢筋混凝土桩基，开展了不同温度条件下置换桩和非置换桩的室内循环加载模型试验，获得了桩基的横向荷载-位移滞回曲线、弯矩分布等，分析了保桩-土系统在常温和冻结条件下横向力学特性。. (5)通过二次开发，引入改良废旧轮胎混合材料的本构模型，建立相应的数值计算模型，分析了置换范围和置换深度对桩-土体系地震响应的影响。结果表明：PolyBRuS材料能有效改善桩基的抗震性能，较优的置换范围为2倍基桩半径，置换深度为冻土层厚度。