托换桩-土钉墙组合支护体系的破坏模型和变形控制机理

Before excavating foundation pit adjacent to buildings, it is the most important to protect the safety of existing buildings. Retaining and protecting of foundation excavation is a common solution to this problem. The combination of underpining pile and soil nailing wall is a new retaining and protecting system. It has the combined characteristics of vertical underpinning and horizontal supporting in structure. It can both limit the lateral deformation of slope of foundation pit and prevent vertical settlement of adjacent building foundation. Through deformation compatibility among lateral displacement of foundation pit,settlement of the building at the pit top and settlement of underpinning pile , it can distribute the upper structure load and soil pressure between underpinning piles and soil nails, forming an interactive system of underpinning piles, soil nailing walls and upper structure. Failure model of the combination, and deformation compatibility and their impact to load distribution transfer, are two of key scientific questions of underpinning pile-soil nail-upper structure. This subject will deal with three aspects of researches about above-mentioned questions: 1. deformation compatibility and the path of distribution transfer; 2.suppoting structure failure model; 3. bearing capacity and deformation performance of combination of underpinning pile-soil nailing. This subject will take a way of theory analysis, numerical analysis, model test and field test to achieve the research object of load distribution transfer path and its failure model based on deformation compatibility in the combined supporting system of underpinning pile- soil nailing wall. Research findings have theoretical and practical application values.

紧邻建筑物开挖基坑时最重要是保护已有建筑物安全，基坑支护是最常用的解决办法。托换桩-土钉墙作为一种新型组合支护体系，构造上具有竖向托换和水平支护相结合的特点，支护原理上具有限制基坑边坡侧向变形和邻近建筑物基础沉降相结合的特征。通过基坑侧向变形、坡顶建筑物沉降、托换桩桩顶沉降之间的变形协调完成上部结构荷载和土压力在托换桩和土钉之间的分配，形成托换桩-土钉墙-上部结构的相互作用体系。托换桩-土钉-上部结构共同作用下的破坏模式、变形协调过程及其对荷载的分配传递过程的影响是其中的两个关键科学问题。本项目拟围绕上述问题开展三个方面研究：1.变形协调及荷载分配传递路径；2.支护结构破坏模式；3.支护结构的承载能力和变形性能。本项目拟采用理论分析、数值分析、模型试验和现场测试的研究路线，达到阐述托换桩-土钉墙组合支护体系中基于变形协调的荷载分配传递路径及其破坏模式的研究目标，研究成果具有理论和应用价值。

提出虚拟开挖应力的概念模拟土体开挖状态，通过等于2倍静止土压力的虚拟释放应力还原非弹性半无限体的开挖状态至弹性半无限体状态，进而建立了基于Mindlin解的素土边坡变形计算方法。根据工程实际中遇到的土钉墙塌方的情况，把土钉墙的破坏方式分为三种，即局部破坏、整体破坏和土体破坏，土钉的抗拔承载力是由土钉与面层的节点强度决定的，而不是由土钉的长度决定的。针对新型托换桩-土钉墙组合支护结构，假定沉降大于托换桩沉降范围内的坡顶建筑物荷载全部转移至托换桩，阐述了托换桩-土钉墙组合支护的工作机理，对支护结构的变形协调机理和荷载传递路径进行了探讨，提出了支护结构侧向变形、坡顶沉降以及托换桩沉降及内力的计算方法。根据托换支护法的研究需要，研制了一套托换支护模型试验平台，该平台较好的完成了土钉抗拔试验、单桩承载力试验、复合土钉墙模型试验和托换支护法模型试验。从室内模型试验和有限元数值模拟对托换桩-土钉墙支护体系进行研究，通过试验数据对比分析得出托换桩-土钉墙支护体系在加载、开挖等过程中的变形规律和承载机理。最后利用FLAC3D软件对周边既有建筑物基坑进行数值模型，对比分析了托换支护法数值模拟和室内模型试验土体位移场的变化情况、周边建筑物的位移、微型桩内力和土钉的轴力等。