深水桥梁垫层隔震基础冲刷演化机理与设计建议

Scour Effects on Local Scour Around Isolated Cushion Foundation of Deepwater Bridges and Its Dynamic Evolution

群桩和沉井作为跨江海桥梁的基础支撑体系已得到广泛应用,但近年来实际工程的水深不断增加,现有技术已难以适用于50 m以上的超深水环境条件。为研究垫层隔震基础这一新型深水桥梁基础的冲刷演化机理,针对周期为1.2 s,波高分别为4、6、8 cm的3种波浪条件,以及海流流速为25 cm·s^(-1),波浪波高为6 cm,周期为1.2 s的波流共同作用条件,开展砂土中垫层隔震基础冲刷演化机理的波流水槽模型试验研究,结合仅单向流作用下的基础体系冲刷演化机制,重点考察并梳理了隔震垫层的5种典型破坏形式及其特点。结果表明:仅存在单向流作用时,垫层的破坏形式主要为边缘破坏、剪切破坏和完全破坏,且严重程度随流速增大而增大;存在波浪作用时,海床泥沙及垫层材料起动后将发生振荡,导致隔震垫层还会发生掩埋破坏和掏底破坏;波流共同作用时,产生的影响比二者单独作用时更严重,对基础体系的正常工作产生严重影响。此外,通过分析交界面位置附近材料在冲刷过程中的滑动与滚动过程,认为颗粒材料的滑动和滚动是造成隔震垫层材料流失的主要原因。最后,对比分析了国内外抛石设计规范中的粒径计算方法,并在此基础上对垫层材料的设计提出建议。

Pile group foundations and caisson foundations have been widely used as support systems for sea bridges. However, the existing technology has been unable to meet the requirements for severe environmental conditions in the design and construction because of the increasing water depth in the actual project, e.g., depths greater than 50 m. To investigate the dynamic mechanism of scour around the isolated cushion foundation embedded in the sand, flume tests were carried out under wave heights of 4, 6, and 8 cm, with a period of 1.2 s, as well as the condition of the combined wave(H=6 cm, T= 1.2 s) and current(V=25 cm·s^(-1)). Compared with the results of current-only experiments, five typical failure types and their characteristics were investigated and summarized. It can be concluded that scour under current conditions mainly includes edge failure, shear failure, and entire failure, and the failure is more serious when the flow velocity increases. When waves act, sand and cushion materials are eroded and vibrated, so coverage failure and hollow-out failure occur. For the combined condition, scour is more severe than when considering current and wave conditions separately and has a significant influence on the safety of the foundation system. In addition, microscopic analysis was carried out in this study to investigate the flow-soil interface, where rolling and sliding are deemed to be the microscopic reasons for the loss of cushion materials. Recommendations based on the design codes of riprap used domestically and internationally were compared and analyzed for their potential for the design of cushion materials. Recommendations are provided based on the behavior of these codes.