考虑非线性剪切蠕变的冻土与混凝土接触面力学特性研究

Bearing capability, deformation and long-term stability of pile foundation in permafrost region are directly influenced by the creep properties of the interface between frozen soil and pile. However, the existing designs of pile foundation lack the consideration of time effect in permafrost region. In this project, the creep characteristics of interface between frozen soil and concrete are chosen as the stay point of this kind of problem. Based on the current research of contact interface test, systematic research of creep mechanical properties and influencing factors of frozen soil-concrete interface are conducted with the self-developed large-scale flowing deformation shear apparatus, which aims to study the creep shear principle of contact interface and non-linear flowing deformation parameters. The deformation condition in the process of contact interface creep could be judged on account of creep curve，and the relationship during creep displacement, time and shear stress is established, which contributes to propose the computing method of stable value in different states and judge the stability of interface. The creep phenomenological equation of the frozen soil-concrete interface is established by introducing the phenomenological theory. Combined with the creep deformation characteristics of contact interfaces, the state failure eigenvalues are defined and the long-term strength criterion is determined of contact interfaces, which aims to establish long-term strength prediction models for contact interfaces. Research results of this project can provide theoretical basis and important reference for design and long-term life prediction of infrastructure in permafrost region.

桩-冻土接触面蠕变特性直接影响多年冻土区桩基础的承载、变形及长期稳定性，而现有的多年冻土桩基设计缺乏对时间效应的考虑。本项目以冻土-混凝土接触面蠕变特性为研究该类问题的落脚点。在现有接触面试验研究成果的基础上，以自主研制的大型流变剪切仪为试验平台，开展冻土-混凝土接触面的蠕变力学特性及影响因素的系统研究，探讨接触面的剪切蠕变规律，研究接触面非线性蠕变参数；依据蠕变曲线对接触面蠕变过程进行变形状态判定，建立蠕变位移量、时间与剪应力的函数关系，得到各变形状态阈值的确定方法；引入现象学理论，建立冻土-混凝土接触面蠕变现象学方程；结合接触面蠕变变形特征，定义接触面状态破坏特征值，确定接触面的长期强度准则，建立接触面长期强度预测模型。该研究成果将为多年冻土区桩基础的设计、长期承载力预测等提供理论依据。

在青藏地区的铁路建设中，常采用“以桥代路”的形式来避免冻土地区的稳定状态受到破坏。冻土区桩基础的承载力主要由桩端阻力、桩侧冻结力组成，由于水和冰的存在，冻土表现出强烈的流变特性，因此，多年冻土区桩基在服役期荷载作用下也会产生流变效应，桩基承载性能会发生劣化，表现为长期荷载作用下桩-土相对位移逐渐增大，桩基础沉降加大。冻结力是导致冻土区桩基承载力远大于融土区桩基础的主要原因，在全球变暖及人类工程活动的双重作用下，为实现多年冻土区桩基础沉降变形的科学预测，保证桥梁桩基在服役期的安全运营，需重点研究桩-冻土接触面的蠕变特性。本项目采用自制的大型应力控制式剪切仪开展了冻结粉土-混凝土、冻结砂土-混凝土、冻结粉质黏土-混凝土接触面剪切蠕变试验，研究了接触面蠕变特性及影响因素，探究了接触面的剪切蠕变机理，建立了冻土-混凝土接触面现象学方程和长期强度预测模型，并结合弹塑性理论提出了考虑损伤变量的接触面蠕变函数形式。研究结果表明：冻土-混凝土接触面蠕变变形特性与温度、含冰量、混凝土粗糙度、土样类型、荷载水平及加载方式等有关。冻土-混凝土接触面蠕变变形特征与冻土蠕变变形特征相似，包括低应力条件下的衰减蠕变、稳定蠕变及高应力下的加速蠕变阶段。温度越高，接触面蠕变效应越显著。稳定蠕变速率受到干密度及含冰量的综合影响，存在最优含冰量时的最小稳定蠕变速率。随含冰量的增大试样黏性变形占比增大，蠕变效应越显著。Burgers黏弹性模型能较好地模拟高含冰量冻结砂土-混凝土接触面蠕变曲线。随着粗糙度的增大，剪切破坏类型由接触面滑移破坏向土体内部破坏发展，粗糙度越大试样衰减蠕变时间越长、稳定蠕变阶段越明显、蠕变稳定性越强，且稳定蠕变速率随粗糙度的增大而减小。提出的改进Nishihara模型能够较好的描述冻土-混凝土接触面蠕变全过程。本项目的研究成果对多年冻土区既有桩基的沉降控制和新建桩基的承载设计具有一定的指导意义。