弯曲河段涌潮与桥墩基础耦合作用机理研究

There are more than ten built or planned to build bridges crossing the Qiantang River and more than a third of them located in curved channels. There is short of full understanding of the flow structure near piers of bridge and the failure modes of its foundation subjected to tidal bores in a curved channel, which results in restricting for safety construction and maintain of bridge engineering. Aimed at investigating the analytical theory and computing method for the 3D nonlinear coupling dynamic responses of the pier structure and its foundation subjected to tidal bore loadings in curved channels, an united application of theoretical analysis, field test, model experiment and numerical simulation are studied comprehensively by use of the interdisciplinary theories and methods in soil mechanics, hydraulics and estuarine dynamics. Based on OpenFOAM platform, a pile-soil-fluid coupling mode will be established to predict nonlinear dynamic behavior of tidal bore-induced motion of pier foundation and surrounding sediments in curved channels. This research will probe the mechanism of riverbed deformation and failure, reveal the characteristics of flow structure around pier and the failure mode of foundation, and explore the reasonable structural type and the protective countermeasure of catastrophe for scouring around bridge piers. This preoject is a new exploration of design theory and analytical method for river-crossing bridges in a macro-tidal estuary and the achievements can provide technical support for similar projects in the Qiantang River estuary as well as the domestic similar estuaries.

钱塘江涌潮河段已建和拟建跨江桥梁已经达到10余座，其中位于弯曲河段的超过三分之一。目前对涌潮作用下弯曲河段桥墩附近水流结构和基础稳定性破坏模式还缺乏充分地认识，使得桥梁工程的建设与维护受到诸多限制。本项目运用理论分析、现场测试、数值模拟和模型试验相结合的手段，采用土力学、水力学与河口动力学等跨学科的理论和方法，对涌潮作用下桥墩基础和水流的三维非线性耦合动力响应分析理论与计算方法进行综合研究。基于OpenFOAM平台，采用桩-土-水动力三维耦合作用模型，结合现场测试和模型试验的结果，深入探讨涌潮引起的涉水建筑物附近河床变形和破坏机理，揭示弯曲河段桥墩附近水流结构特性和基础稳定性破坏模式，探索适合于强潮河口与深厚粉沙土地基的桥墩基础合理形式和冲刷防护对策。本项目是我国强潮河口桥梁工程基础设计理论和分析方法的新探索，可为钱塘江河口及国内同类工程建设提供合理实用的技术支撑。

弯曲河段涌潮的传播特性和水动力结构与顺直河段存在显著的差异，影响桥墩局部冲刷和床面形态的演变。目前对强涌潮水动力作用下桥墩基础及河床响应的研究较少，对桥墩附近水流特性与基础稳定性破坏模式缺乏充分地认识，制约强涌潮区桥梁工程的建设与维护。本项目采用理论研究、现场观测、资料分析、模型试验和数值模拟相结合的研究方法，根据涌潮及弯道水流的动力特性，考虑河床和桥墩基础土体的受力特点，建立了涌潮与桥墩基础土体三维耦合作用模型，结合现场测试和模型试验的结果，分析了涌潮引起的河床变形和破坏机理，揭示了弯曲河段桥墩附近水流结构特性和基础土体稳定性破坏模式。研究结果表明，游荡性河口弯曲河段滩槽演变剧烈，桥墩冲刷受滩槽形态和水流结构的双重制约；弯曲河段的滩槽演变受径流和潮汐的影响，在年内和年纪间存在周期性调整；涌潮在弯曲河段由于离心力的存在导致自由水面的重新分布，呈现复杂的传播与反射特征，加速了河床凹岸冲刷、凸岸淤积的过程；涌潮锋面及随后的急流会在表面形成交叉菱形波，使之与缓流弯道的冲淤特性差异较大，致使底沙的运动轨迹与底流存在较大分歧，促进泥沙横向的异岸转移；由于桥墩的阻水作用，引起桥墩附近水流结构的变化，形成马蹄形涡旋和不同尺度的环流，加大局部水流的紊动强度，增大河床土体内部的压力梯度，引起桥墩附近泥沙输移强度的剧增，形成局部冲刷坑，进而降低桥墩基础的稳定性；通过耦合OpenFOAM中的两相流模型、土体和结构响应模型构建三维耦合模型，模拟了涌潮作用下河床动力响应及考虑结构物的相互作用问题；桥墩局部冲刷坑的形成是一个渐变的过程，提高涌潮作用下桥墩基础土体的抗瞬时液化能力，能显著地减少局部冲刷坑的深度。项目的开展完善了强涌潮河口桥梁基础的稳定性评价技术体系，为河口区桥梁工程的建设和运营提供了必要的科学依据和技术储备。