地震作用下软土-桩基-桥梁之间相互作用机理及损伤评估研究

Observations from past earthquakes have shown that many bridges were collapsed not because of the severe damage in superstructure, but rather, due to the earthquake-induced failure or excessive deformation in both the piles and piers. Two major problems associated with earthquakes in soft ground are the amplification of seismic-induced ground motion by soft soil layer(s), and the stiffness degradation of soft clay during the seismic loading, respectively. As a result, the seismic clay-pile-bridge interaction and the damage mechanism of the system are highly complex and not fully understood. Some further studies are required to provide valuable insights into the fundamental mechanisms underlying seismic soil-pile-bridge interaction. This study consists of shaking table model tests, three-dimensional finite element simulations and theoretical analyses. The shaking model table model tests will be performed to examine the influences of several key factors on the seismic response of the clay-pile-bridge systems. Subsequently, a series of three-dimensional finite element validation analyses are carried out to back-analyze the shaking table model tests. The validated numerical procedure is then extended to perform a series of parametric studies to extend the work into other scenarios which cannot be easily realized through physical experiments. Based on the results of both the numerical parametric and experimental studies, theoretical analyses are then performed to systematically investigate the seismic clay-pile-bridge interaction; seismic damage evaluations of both the pile and pier under different conditions are systematically analyzed. Besides, dimensionless correlations are derived using multivariate regression analyses to predict the maximum bending moments/curvatures of both the pile and pier. Furthermore, seismic fragility analyses will be performed on a typical soft clay-pile-bridge system via the validated three-dimensional finite element method. The findings drawn from this study likely contribute to the improvement of the existing seismic analytical method for pile foundation and provide a useful reference and quick tool for the seismic design of pile-bridge system constructed in soft clay deposits.

地震中桥梁的破坏屡见不鲜，桥梁发生落梁倒塌的主要原因在于其基础或桥墩在地震作用下产生了破坏或较大的侧向变形。由于软土对地震波的显著放大效应及其刚度弱化特性，软土-桩基-桥梁之间相互动力作用关系非常复杂，它们之间相互动力作用机理及其地震损伤特性尚未研究清楚，软土中桩基-桥梁系统的抗震安全性有待进一步探究。本项目基于此而提出，拟采用振动台模型试验、三维有限元数值分析和理论分析相结合的方法，揭示地震类别及烈度、软土-桩基-桥墩系统的几何-力学参数对整个系统地震响应的影响规律，探明软土-桩基-桥墩之间相互动力作用关系及地震损伤特性，推导出可预测地震作用下软土-桩基-桥墩系统中桩和桥墩最大弯矩（曲率）响应的数学预测公式，确立软土地区典型桩承多跨桥梁系统的地震易损性曲线。项目研究成果有助于完善既有的桩基抗震分析理论，同时可为软土地区桩基-桥梁系统的抗震设计提供参考。

众多震后调查表明，软弱土地基中桩承桥梁的倒塌破坏时常发生，且发生破坏的位置主要聚集于桩基础和桥墩。由于软黏土（软土）对地震波的显著放大效应及其刚度弱化特性，软土-桩基-桥梁之间相互动力作用关系及其地震损伤特性尤为复杂，软土中桩基-桥梁系统的抗震安全性尤为值得关注。本项目采用振动台模型试验、三维有限元数值分析和理论分析相结合的方法，系统分析地震动强度、软土-桩基-桥墩系统的几何-力学参数对整个系统地震响应的影响规律，推导出可预测地震作用下软土中桩基及桥墩最大加速度和弯矩响应的预测公式，探究了软土中桩基和桥墩的地震破坏概率。研究发现：当基岩峰值加速度小于0.15 g时，软土对地震波的放大效应较为强烈，而当基岩峰值加速度大于0.2 g时，软土的阻尼耗能减弱地震波的震动强度；由于软土动力特性的非线性和软土-桩基之间相互作用关系的复杂性，桥墩和桩基最大弯矩与基岩峰值速度有着强烈的非线性特征，当基岩峰值加速度大于0.15 g或基岩峰值速度大于0.2 m/s时，桩基-桥墩系统基本进入塑性变形阶段；桩身抗弯刚度和梁跨结构质量分别对桩基和桥墩的地震响应影响更大，表明软土中桩基和桥墩地震响应分别取决于地震过程中软土的动力作用和上部结构的惯性力作用。此外，地震易损性分析表明，软土中桩承桥墩系统的地震破坏主要聚集于桩基，其破坏概率随着桩身抗弯刚度的增大而显著降低，而对上部结构质量的变化较为不敏感。当承台峰值速度达到0.4 m/s时，桩的抗弯刚度分别为4.909×10^5 kNm2、1.473×10^6 kNm2和3.436×10^6 kNm2时，桩基超越中等破坏程度的概率分别为70%、17%和1%，表明增加桩的抗弯刚度可以显著提高桩的抗震能力。本项目研究成果有助于完善既有的桩基抗震分析方法，同时可为软土地区桩基-桥梁系统的抗震设计提供参考。