复杂环境下沉井加桩基础水平动力特性及其长期演化规律研究

Cassion-piles foundation is a new type deep water foundation of bridge crossing rivers or seas which combining the advantages of caisson and pile groups. It is usually subjected to horizontal load and moment caused by wind or wave due to the complex marine environment, and the foundation may even suffer scour erosion which threatens the dynamic bearing performance. For the lack of systematic research as well as intensive study, the mechanical mechanism of cassion-piles foundation is still unclear. This research project is directed towards the essential scientific problems pertaining to dynamic properties of cassion-piles foundation, covering the dynamic response of foundation and dynamic properties of foundation under long-term state. Through model tests, theoretical methods as well as numerical simulations, a nonlinear analysis model for lateral dynamic response of cassion-piles foundation will be established, and research efforts will be focused on lateral dynamic properties of cassion-piles foundation and its long-term evolution law considering environmental load and scour effect. An efficient dynamic analysis method for the cassion-piles foundation with large numbers of piles in practical engineering will also be put forward. Key factors influencing the nonlinear dynamic properties and long-term operation state of foundation will also be summarized. The outcome of this proposed research is expected to provide reliable scientific basis and theoretical guidance for design and operation safety of foundation under long-term state.

沉井加桩基础是一种结合沉井及群桩各自优点形成的新型跨海跨江桥梁深水基础型式。在复杂的江河或海洋环境下，基础将承受波浪、强风等耦合水平动荷载作用，且基础周围土体还会受到水流的冲刷侵蚀对基础的承载性能构成潜在威胁。由于缺乏系统性的深入研究，沉井加桩基础受力机理尚不明确。本项目针对沉井加桩基础的动力特性展开研究，主要关注基础的水平动力响应及其长期演化规律。拟采用室内动力模型试验、理论分析和数值模拟相结合方法，建立沉井加桩基础非线性动力分析模型，研究考虑环境荷载及冲刷效应的沉井加桩基础动力性能长期演化规律，并构建适用于实际工程大数目群桩-沉井基础振动简化分析方法，总结各关键因素对复合基础的非线性动力特性和长期性能的影响规律，为复合基础结构设计、长期安全运营提供科学依据和理论指导。

针对新型深水基础型式-沉井加桩复合基础，采用室内模型试验、数值分析及理论计算相结合的方法，获取了新型基础型式在不同激振荷载幅值、激振频率及冲刷条件下的非线性动力响应变化规律，提出了基础的动力响应时域及频域分析方法并探讨了基础在水平动荷载作用下的位移响应及共振频率变化规律，研究了沉井基础底部群桩布置型式对沉井基础水平动力特性的影响。基于平面分析模型，开展了受冲刷前后的沉井加桩基础水平静力和简谐动力响应相关研究，探讨了应力历史及冲刷坑尺寸对基础受力特性的影响规律，并进一步拓展建立基础-土体地震相互作用分析模型，研究了沉井加桩基础在地震及冲刷双重灾害荷载作用下的地震响应。研究表明：随着激振荷载幅值的增大，基础振动幅值呈现明显的非线性增加特征，而基础共振频率逐渐减小，且同等条件下沉井加桩基础与沉井基础相比动力承载性能更强，位移及加速度响应均有所降低，特别是在共振频率附近。冲刷引起的基础侧向土体流失会明显降低基础的动力阻抗，且随着冲刷深度的增大，沉井加桩复合基础动刚度项减小幅度呈非线性增长趋势，阻尼项变化则相对平稳，但冲刷后考虑土体应力历史的动力阻抗计算结果明显小于未考虑应力历史情形，表明土体特性的改变会进一步降低基础的动力承载特性放大结构的动力响应。对承受长期动力荷载作用或动力荷载相对较大时的基础而言，仅考虑土体非线性是不够的，还应考虑土体刚度、强度在长期动力循环荷载作用下土体特性变化及土体脱开效应影响。地震和冲刷双重荷载作用影响下，冲刷效应对基础动力影响效应相比地震作用更加明显，其可能原因为冲刷引起基础周围土体特性改变更加显著。基础底部群桩设计型式对动力特性响应有显著的规律，不同设计型式的群桩基础在低频处的动力位移响应变化相对较大。深水基础中大数目群桩-沉井复合基础动力分析中由于底部群桩数目较大，其振动特性可近似按叠加原理结合桩桩相互作用因子进行简化分析。