软土地基上双排钢板桩结构循环承载特性及计算方法研究

Abstract：Steel double-wall structures, such as offshore cofferdams and seawalls, are constructed on soft grounds to endure cyclic loading like wave etc. The actual bearing characteristics of steel double-wall structures are complicated due to the flexible dynamic amplification of structural displacement and strength cyclic degradation of soft grounds. Currently the specialized design standards are based on assumptions of“integral gravity wall”, “penetrating rigid frame” or “single row pile”, resulting in the design difficulties and application restrictions in conditions of offshore waves and soft grounds. Based on the mesoscopic numerical simulation of structural bearing performance and geotechnical model tests of mechanical properties, a dynamic calculation method is established for the interaction system of the steel double-wall structure and soft grounds. The dynamic calculation method will consider structural characteristics such as the interlock of sheet piles and the spatial effects, contact properties of the sheet piles, anchorage and soil, strength cyclic damage-dependent degradation of soft grounds. On account of the numerical results of the dynamic calculation method, the cyclic bearing characteristics of the steel double-wall structure and soft grounds, such as the dynamic response status, the cooperated mechanism, the instability mode and the ultimate bearing capacity, and influences of relative factors, will be investigated. Through the theoretical analysis of cyclic bearing mechanics, the simplified calculation method of ultimate cyclic bearing capacity will be proposed to provide technical supports for engineering design and application in conditions of offshore waves and soft grounds.

软土地基上双排钢板桩近海围堰、海堤等经受波浪等循环荷载作用，结构位移柔性动力放大以及软土地基强度循环弱化使其实际承载特性复杂。我国现行规范主要基于静力“整体重力”、“嵌固钢架”或“单排板桩”假设，导致近海波浪、软土地基条件下结构设计困难和应用局限。基于双排钢板桩结构受力特点细观数值模拟以及软土力学性能土工模型试验研究，建立考虑钢板桩间锁口连接及空间效应等结构特性、钢板桩—锚杆—土体接触特性、软土强度循环损伤弱化特性的双排钢板桩结构与软土地基相互作用动力计算方法；结合动力计算方法进行数值计算，研究双排钢板桩—软土地基系统动力响应状态、协同作用机理、失稳模式以及极限承载力等循环承载特性及其影响因素；通过循环承载特性理论分析，提出软土地基上双排钢板桩结构极限循环承载力简化计算方法，为近海波浪、软土地基条件下双排钢板桩结构工程设计及应用提供技术支撑。

双排钢板桩适合用作软土地基条件的近海围堰、海堤等型式，具有广泛的应用前景。服役过程中双排钢板桩结构经受波浪等循环荷载作用，造成软土地基上双排钢板桩结构的实际承载特性复杂。本项目基于数值模拟、室内土工试验等手段，建立软土强度循环（损伤）弱化模型，应用到双排钢板桩结构循环承载计算，研究双排钢板桩-软土地基系统受力和位移状态、协同作用机理、失稳模式等循环承载特性及其影响因素，提出基于土体参数的双排钢板桩结构极限承载力简化计算方法。结果表明水平荷载作用下双排直腹钢板桩结构柔性大、自身变形大；双排U型钢板桩结构和曲面直腹钢板桩结构刚度大、自身变形相对较小。通过建立软土不排水强度和孔隙水压力的关系，进一步将软土不排水强度表征为循环荷载作用次数和应力水平的函数进而应用到双排钢板桩结构与软土地基耦合数值计算。考虑软土地基循环弱化时双排直腹钢板桩位移相比未考虑软土地基循环弱化时明显增大，且随着水平力加载幅值和循环荷载作用次数的增加而增大；孔隙水压力主要发生在双排直腹板桩结构附近软土区域，且受回填土体围固压力影响内部软土区域的孔隙水压力随着循环荷载作用次数和循环荷载幅值增加明显增大。提出基于极限平衡的软土地基上双排钢板桩结构承载力简化计算方法与有限元结果吻合较好，可为近海波浪、软土地基条件下双排钢板桩结构工程设计及应用提供技术支撑。