黏性-砂性土中沉箱（井）加桩基础冲刷机理及安全评估方法研究

Scour is a problem generally faced by water structures which may result in weakening the bearing capacity of bridge foundation and lead to damage of bridge. Therefore, it is necessary to evaluate the damage potential of bridge foundation induced by scour. To better understand the scour mechanics of caisson-pile foundation in clay-sand sediment and propose a useful method to evaluate the damage potential of the caisson-pile foundation induced by scour, this project firstly carries out large contact surface model test and multi-directional loading test of caisson-pile foundation to evaluate the effect of clay content on coupled effect of vertical load and horizontal load on caisson-pile foundation considering the horizontal load induced by water and wind. Then, model test of scour around caisson-pile foundation in water sink is carried out to study the scour mechanism around caisson-pile foundation in clay-sand sediment. Meanwhile, centrifuge test is conducted to evaluate the effect of stress history induced by scour on bearing properties of the caisson-pile foundation. Based on the results from those tests, we are trying to propose a progressive failure limit analysis method to calculate the bearing properties of caisson-pile foundation under scour in clay-sand sediment. Finally, parametric study is carried out using the proposed method and the FEM to establish a safety assessment method for caisson-pile foundation subjected to scour in clay-sand sediment. Case studies are also carried out to verify the safety assessment method. The aim of the project is to provide theoretical tools for assessing the safety of caisson-pile foundation which is used as the foundation of bridge or the foundation of offshore wind turbines subjected to scour.

冲刷是水上构筑物(如桥梁、海上风机)普遍面临的问题，可能导致桥梁基础的承载能力削弱而破坏，因而合理预测冲刷带来的危害及对冲刷作用下桥梁基础进行安全评估是非常必要的。本研究针对当前黏性-砂性土中沉箱（井）加桩基础冲刷机理和危害评估方法研究所存在的不足，考虑上部结构及水流冲刷带来的水平力，开展大型接触面模型试验和沉箱（井）加桩多向加载模型试验，分析土层黏粒含量对竖向与水平向荷载耦合效应的影响；开展室内水槽冲刷试验，研究黏性-砂性土中沉箱（井）加桩基础的冲刷机理及冲刷承载性能；进行离心模型试验，探讨冲刷带来的土体应力历史对沉箱（井）加桩基础的承载性能的影响；进而提出黏性-砂性土中可真正考虑土层、荷载耦合效应和冲刷相互耦合的沉箱（井）加桩基础的渐进破坏极限分析方法；采用理论方法和数值模拟进行参数分析，最终建立沉箱（井）加桩基础的冲刷安全评估方法，应用于实际工程，为桥梁冲刷安全评估提供理论依据。

桥梁基础和上海风电基础往往处于砂性和黏性土中，波流冲刷会对V-H-M组合荷载作用下的新型基础承载性能造成不利影响，如何解决复杂情况的基础的承载性能演变的科学问题及评估承载性能的演变变得尤为重要。本项目通过模型试验、数值分析、理论推导等多种手段，对冲刷条件下砂性-黏性土中沉井加桩基础在V-H组合荷载作用下的承载性能演化进行研究。多向荷载耦合荷载下的风电基础承载性能模型试验揭示了多向荷载在风电基础上的耦合机理。水槽冲刷模型试验揭示了黏性-砂性土土性对冲刷深度的影响特性及冲刷对沉井加桩基础的水平承载能力的影响机制。在此基础上通过数值模拟对砂土-黏土土性、多向荷载耦合作用、冲刷作用对风电基础的承载性能的影响规律进行了分析，结果表明砂土中竖向荷载对单桩基础的水平承载性能具有提高作用，而黏土中竖向荷载的存在会降低单桩基础的承载性能。由于沉井底部承载作用，竖向荷载对沉井加桩基础的水平承载性能具有显著提高作用，当竖向荷载为设计承载力时，水平承载性能可提高50%以上。同时，参数分析揭示了各影响因素对冲刷作用下土性图V-H荷载耦合作用的影响规律，并在此基础上建立了黏性-砂性土中多向荷载作用下单桩及沉井加桩基础统一分析理论为解决多向荷载作用下基础工程设计提供了相对准确的分析方法。同时基于基础振动特性，建立了冲刷坑深度对基础振动特性影响的分析理论，结合反演分析技术，形成了基于振动测试技术测定冲刷深度的方法。从理论上解决了冲刷深度测定困难的问题。