软硬互层地基中桩基础的地震动弯曲性能及其评价研究

As the commonly used foundations, piles are designed to carry the loadings of the superstructures. Evidence from the recent earthquakes indicates that plastic hinge may generate in the pile shaft at the interface between the soft and firm soils due to the excessive flexural bending of the shaft. The damage of pile foundations may induce severe collapse of the superstructures. The bending moment at the interface of the soft-firm soil layer during earthquakes is lack of design and calculation methods. In this project, the determination of the flexural bending behavior of the shaft at the soil layer interfaces is investigated for seismic analysis. Based on the model test, damage mechanics and wave propagation theory, the load transfer mechanism and deflection of the shaft are examined by considering the kinematic pile-soil interaction. A subgrade reaction model for the soils surrounding the shaft will be presented to take the continuum of the foundations. Then, a novel theoretical nonlinear dynamic p-y curve based on statistic damage mechanics is proposed to simulate the nonlinear and softening property of reaction to the shaft by the soil during earthquakes. Moreover, the kinematic pile-soil-pile interaction factor will be presented to investigate the flexural bending of the pile groups. Furthermore, the calculation for the seismic bending moment at the soil layer interfaces will be determined according to the boundary conditions of the shaft. Lastly, the flexural kinematic bending mechanism of the shaft at the soil layer interfaces will be studied. The design methodology in this study will propose effective measures for seismic pile bending at the soft-firm soil layer interfaces on account of technical feasibilities and economic benefits. The outcome of the project will improve the shortage of the seismic analysis of piles in the current code and enrich the seismic design theory in China.

地震过程中，桩基础常在软硬互层地基界面处因桩身弯矩过大而出现塑性铰，导致上部结构坍塌破坏，这引起了学术界广泛关注，但已有计算方法尚远落后于实际应用。项目拟模拟桩土系统横向地震动挠曲响应过程，揭示软硬相互地基中桩基础的弯曲破坏机理，对桩基础的地震安全评价具有明显工程价值和理论研究意义。项目拟通过振动台模型试验、损伤力学以及波动理论等手段，考虑桩土运动相互作用，研究地震荷载下软硬地基中桩土荷载传递性状，确定单桩地震动弯曲曲率计算方法，采用损伤力学建立桩侧土抗力的动力p-y理论曲线，构建层状地基中群桩基础横向地震动相互作用计算理论，得到不同工况和边界条件下软硬土层界面处桩身挠曲曲率和变形的计算方法，揭示桩基础地震动弯曲破坏机理，最终提出地震荷载下软硬地基界面处桩身弯曲性能计算理论。本项目预期成果可补充现行规范关于软硬互层地基中桩基础地震动弯曲性能计算内容，丰富桩基础抗震设计理论。

我国交通基础设施中公路桥梁数量庞大，层状地基中桥梁桩基础地震动弯曲破坏及其性能评价是近年来桩基抗震研究热点之一。本项目由基本理论出发，以动力温克尔弹性地基梁分析为基础，结合基桩-土-基桩位移衰减函数，构建相应力学概化模型，阐明了地震作用下桥梁柱墩基础荷载传递机理，揭示了软硬交互地层中桩基础的弯曲破坏机理，提出了一套基于时程分析法的公路桥梁桩基础的抗震性能评价理论框架与技术方法。最后，结合振动台试验和三维数值模拟手段，确定了场地和桩基础的基本动力特性，探讨了不同强度地震波以及地震波激振方向下桩基础的地震响应。本项目所述计算方法，改进了传统反应谱法中缺乏桩土相互作用分析基础、计算时间慢、精度低等诸多不足之处，从而能更精确细致地计算关键薄弱部位内力响应。项目研究成果对补充现行规范和丰富桩基础抗震设计理论，具有重要的理论指导意义和工程实践价值。