坡顶沉桩挤土效应及坡体变形细观机理研究

It is frequently encountered that the problem of pile sinking adjacent slope crest in engineering practice,because of more complex boundary and constraint conditions than pile sinking in the case of axial symmetric condition,it often causes more engineering accidents,and has become an key scientific problems in dire need of slove. In this project, the theoretical analysis and visualization model test will be conducted, the particle flow will be simulated numericaly by mesoscopic measurement and processing technology of the digital image, and the macro mechanism of soil squeezing adjacent to the slope crest due to the pile sinking and mesomechanism of slope deformation will be demonstrated. The main contents included in this research work are as follows: (1) Based on the Source-Sink theory and Lagrange interpolation method,the boundary stresses at the top and the inclined slope will be corrected, and the solutions to the slope deformation due to the installation of a group of piles will be obtained on the basis of the existing solution of single pile; (2) By the observation of the sandy model test for simulating the whole process of pile sinking, the development of soil displacement caused by the pile sinking adjacent to the slope and the changes in soil particles in meso scale will be illustrated; (3) By using the non-circular/spherical particle, the numerical simulation of soil performance in meso scope will be conducted through the comparison of the test results. The macro and meso characteristics of slope soil under different working conditions will be investigated. The outcome of this research work can be used for the estimation of the soil displacement due to the pile sinking adjacent to the slope crest, as a reference for the engineering design, the determination of construction parameters, and the control of soil deformation. Furthermore, the stability of the slope subjected to the installation of piles can be analysed by mesoscopic mechanism.

坡顶沉桩问题在工程实践中经常遇到，因其具有比轴对称沉桩更复杂的边界条件和约束条件，引发的工程事故更多，已成为当前亟待解决的关键科学问题。本项目研究旨在理论分析与可视化室内模型试验基础上，利用数字图像细观量测与处理技术，开展颗粒流数值仿真模拟，揭示坡顶条件下沉桩宏观挤土效应与细观变形机理。主要研究内容包括：(1) 基于源-汇理论和拉格朗日插值法，修正坡顶边界以及斜坡倾斜边界应力，在已有单桩理论解答的基础上建立坡顶条件下群桩沉桩挤土位移解答；(2) 通过观察砂土斜坡模型试验，揭示沉桩全过程中坡体挤土位移模式的演化与土体颗粒细观变化；(3) 采用非圆(球)形颗粒，通过与试验结果对比进行细观数值模拟，研究多工况多因素下斜坡土体宏细观变化规律。所得成果不仅可对邻近斜坡条件下沉桩挤土位移进行定量评估，为工程设计、施工参数设置以及挤土位移控制提供参考，而且还能为斜坡挤土变形与稳定分析提供细观力学解释。

近年来，静压沉桩凭借其工期短、无污染及质量稳定等优势广泛用于实际工程中，然而，沉桩施工产生的挤土效应会对周围环境造成不利影响。目前岩土工程界对水平地面场地条件下的沉桩研究较为深入。相比之下，对邻近边坡等非轴对称边界和约束条件下沉桩机理和规律的认识相对不够，坡顶沉桩引起的工程事故时有发生。因此，充分认识坡顶沉桩过程中的力学机理和土体位移规律已成为当前工程领域迫切需要解决的课题。本项目针对这一问题进行了理论分析、试验研究以及数值模拟，同时运用颗粒流软件结合模型试验从细观层面上对坡顶沉桩挤土规律进行了分析。建立了位移非轴对称和应力非轴对称情况下圆孔扩张的理论解答，研究了侧压力系数K0≠1时应力非轴对称及位移非轴对称条件下沉桩扩孔挤土解答。结果表明：传统的圆孔扩张理论假定K0=1高估了临界扩孔压力和极限扩孔压力，也低估了塑性区半径的扩展范围；倾斜位移边界条件的存在对圆孔两侧的挤土位移有明显影响，孔距自由边界的距离越大，自由边界对挤土位移的影响也越小。利用边坡稳定性理论计算方法，研究了邻近边坡沉桩过程中沉桩力、桩端塑性区以及桩体抗滑作用对滑动面安全系数的影响，结果显示沉桩力及桩端塑性区对滑动面稳定性产生不利影响，使安全系数降低；当桩体穿过滑动面之后，桩体起到抗滑作用，滑动面安全系数显著提高。通过可视化模型试验，利用实时动态摄录技术和数字图像细观结构分析技术，对邻近斜坡沉桩过程中的挤土效应进行研究，通过土压力及位移量测设备揭示沉桩全过程中斜坡力学响应及挤土位移模式的演化规律，结果显示：随着桩体的不断沉入，斜坡挤土位移影响范围越来越大，桩周土体位移模式也不断发生变化，紧邻桩身以及靠近桩端范围内的土体，在桩体径向挤压和桩侧摩擦的共同作用下，产生了向下的位移，而桩身一定距离以外的土体，受地表及斜坡自由边界的影响，表现为竖向隆起位移；当沉桩至斜坡底面以下深度时，后续沉桩对斜坡的挤土位移影响不大。运用PFC数值软件，针对室内试验工况进行数值模拟，结果表明：随着桩体的不断沉入，斜坡挤土位移影响范围越来越大，斜坡土体的位移模式也不断发生变化；紧邻桩身以及靠近桩端范围内的土体，在桩体径向挤压以及桩侧摩擦的共同作用下，产生了向下的位移，而桩身一定距离以外的土体，受地表及倾斜自由边界的影响，表现为竖向隆起位移；当沉桩至斜坡底面以下深度时，后续沉桩对斜坡的挤土位移影响不大。