基于微结构失稳机制的非饱和原状黄土本构模型研究

Under loading, the bonds between the particles or aggregates in the unsaturated natural loess break gradually, the particles or aggregates slide into the pores, and the macro-deformation appears. The researchers are working on the methods which can correctly calculate the macro-deformation of the sample caused by that the bonded structure changes into unbonded structure. This project will establish the calculation method of this deformation from micro-view. The latest technologies are used to measure the pore distribute change and the change of particles or aggregates supporting the pore during loading, and establish the relations between the stress and the diameter of pore that begins to collapse, the stress and microstructure that becomes instability, the stress and deformation caused by collapse of pore. In order to analyze the deformation mechanism caused by microstructure collapse deeply, the discrete element method (DEM) is used to produce the DEM sample in which the pore structure and bonded structure are similar to unsaturated natural loess, analyze the macro response and the micro structure change of the sample during loading, obtain the change of generalized effective stress (GES) which considers the effect of bonds and capillary water, and establish the relations between GES and the diameter of the pore that begins to collapse, GES and microstructure that becomes instability, GES and plastic strain, plastic work and the diameter of the pore that begins to collapse, plastic work and microstructure that becomes instability, plastic work and plastic strain. Finally, the constitutive model of natural loess will be built based on these relations. The completion of the research project will help to improve the calculation method of the deformation of loess under loading and the knowledge on deformation characteristics of unsaturated natural loess.

非饱和原状黄土受荷过程中其内部颗粒/团粒间胶结逐渐破坏，颗粒/团粒滑移入孔隙中，呈现出宏观变形，准确计算逐渐破损过程中试样的宏观变形是科研人员致力解决的问题。本项目拟从微观角度建立该变形的计算方法。采用最新的微结构测试技术，获得非饱和原状黄土受荷过程中孔隙分布的定量变化和支撑孔隙的颗粒/团粒结构的变化，建立应力和开始塌陷孔隙直径、失稳结构、失稳变形的关系；为了更深入地分析由微结构失稳产生的变形机制，基于三维离散元法，生成类似非饱和原状黄土孔隙结构和胶结结构的离散元试样，分析加载过程中试样的宏观响应和微结构变化，获得考虑胶结作用和毛细水作用的广义有效应力的变化规律，建立广义有效应力、塑性功分别与开始塌陷孔隙直径、失稳结构、塑性变形的关系。最后基于这些关系构建非饱和原状黄土的本构模型。本项目的完成将有助于完善受荷下非饱和原状黄土变形的计算方法，提高对非饱和原状黄土变形特性的认识。

本项目采用室内宏微观试验、数值模拟和理论分析相结合的方法开展了研究。在黄土微结构定量描述方面，通过电镜照片图像分析获得了不同区域黄土粉粒和孔隙尺寸、形状和空间排布的定量数据以及黄土颗粒/团粒和孔隙结构构成，确定了代表整体试样微结构的电镜扫描土样面积S，通过压汞试验获得了不同区域黄土孔隙分布差异。在原状黄土受荷下微观变形机制方面，试验结果表明相同固结应力下饱和原状黄土比饱和重塑黄土粒间孔隙变化小，粒内孔隙两者相近基本保持不变; 相近孔隙比下，饱和重塑黄土相对初始状态粒间孔隙变化较大，说明饱和原状黄土团粒更加稳定; 对比粒间孔隙体积换算的孔隙比变化和宏观孔隙比变化发现两者接近相等，说明原状黄土试样的宏观变形为粒间孔隙变化，这为原状黄土宏观变形的微观机制提供了依据。进行了等吸力三轴试验，获得了加载前后的孔隙分布变化，与前面结论类似，加载过程中主要为粒间孔隙变化，粒内孔隙基本不变。建立了考虑粉粒和黏粒颗粒形态、孔隙特性、胶结特性（毛细水胶结和胶结物胶结）的结构性黄土试样，并进行了加载试验，鉴于加载过程中微结构结构变化扑捉的困难，尚未获得系统成果，针对理想黄土试样（由圆形颗粒和胶结物胶结组成）离散元数值试验获取的破损参数B，建立了将土体视为胶结元和摩擦元两种颗粒结构的本构模型。鉴于上述本构模型在确定破损参数B时存在一维压缩和三轴压缩过程中表达式不统一的问题，考虑变形参数、屈服条件和强度参数是含水率的函数，建立了以含水率为状态变量的本构模型，该模型比上述模型在参数确定方面更容易且模型的力学响应更加合理。课题研究着眼于服务 “西部大开发”和“一带一路”重大战略下深厚堆积黄土区重大工程建设，研究成果为黄土宏观变形下的微观机制积累了新认识，构建的本构模型丰富了受荷下非饱和原状黄土变形的计算方法。