考虑微观机理的膨胀土体变模型及地基变形计算

Volume change model is the basis of the relationships of yield stress-suction and shear strength-suction, the volume change rules must be determined for building constitutive relationship between saturated soil and unsaturated soil.But the current volume change constituve models are insufficient,and the calcuation methods for expansive soil ground deformation are too simplified. Based on the microstructure of compacted expansive soils, this research project will search the function relationship between the content of water phase in different position of soil pores,which control the deformation of the soil, and void ratio as well as degree of saturation. A conceptual model will be constructed to explain the evolution law of water phase in pore structure during wetting or drying under different confining stress. Based on the conceptual model,combining with the traxial test results under different stress paths,the parameters in BExM,which are hard to be obtained from the traditional tests, will be simplified to develop a volume change constitutive model that could describe the volume change behavior in the progress of wetting/drying under the full range of confining stress by modifying the liear NCL to nonlinear NCL. A set of special SWCC tests will be conducted to ascertain the nature influence factor of SWCC by maintaing constant of stress state or void ratio of compacted expansive soil specimens, respectively. Moreover,a 3D spacital curved surface,which can reflect the change relationships among void ratio,matric suction and degree of saturation,will be established, and its correspoding equation will be derived. Based on the developed volume change constitutive model,and combining with the seepage theory of unsaturated soils, a modified method will be presented to calculate the deformation of expansive soil ground more accurately by considering the influnence of initial state and stress path. A large-scale ground model test will be conducted to validate and modify the methods.

体变模型是研究非饱和土屈服应力-吸力、剪切强度-吸力关系的基础，要建立饱和土与非饱和土之间的本构联系就必须确定土体积变化规律。而现有体变模型还存在不足，膨胀土地基变形计算方法也过于简化。 本项目从压实膨胀土微观结构入手，探求孔隙内不同形态水相含量与孔隙比和饱和度之间的函数关系，建立压实膨胀土在不同应力水平下脱/吸湿微观结构水相演化概念模型；在此基础上，简化BExM模型中难以获取的模型参数，结合不同应力路径的体变试验，将正常压缩曲线由半对数坐标系中的直线修正为曲线，建立能够描述全应力范围内压实膨胀土脱湿/吸湿体变特性的数学本构模型；通过分别控制应力和孔隙比的SWCC试验，探明SWCC的本质影响因素，建立能够考虑体变的三维土水特征曲面及其函数方程；以体变模型为基础，结合非饱和土渗流理论，建立考虑初始状态和应力路径影响的压实膨胀土地基变形计算理论与方法，并通过地基模型试验加以验证和修正。

本项目基于膨胀土地基灾害频发，而现有膨胀土体变理论尚有缺陷、膨胀土地基变形计算方法过于简化的背景，开展了以下四个内容的研究：.（1）膨胀土体变的微观机理研究；.（2）膨胀土体积变化本构模型研究；.（3）孔隙比和应力状态对膨胀土SWCC的影响研究；.（4）膨胀土地基变形计算理论与方法研究。. 取得的成果如下：.（1）从压实膨胀土的孔隙结构入手，通过室内试验，研究了非饱和土状态变量净平均应力和基质吸力（含水率）对压实膨胀土宏、微观孔隙结构演化的影响，探明了压实膨胀土体变的微观机理，从而为建立膨胀土体变模型打下了基础；.（2）建立了考虑压实膨胀土微观孔隙结构影响的体变本构模型。该模型能够描述等含水率和等基质吸力状态下，压实膨胀土的非线性NCL特性以及压缩过程中饱和度的变化，并能够预测压缩量随竖向应力的增大先增大后减小的非单调变化规律。.（3）试验结果表明，孔隙比才是影响SWCC的直接因素，而应力状态是通过影响孔隙比而影响SWCC的间接因素。因此，在建立SWCC模型时，应以孔隙比而不是应力状态作为变量；在含水率-基质吸力-孔隙比三维空间中，建立了考虑膨胀土体变影响的土-水特征曲面及其函数方程；.（4）基于非饱和土理论，提出简单实用的膨胀土地基增湿变形的计算理论和方法；.（5）提出了膨胀土“残余膨胀力”的概念，并建立了反映蒙脱土含量和NaCI溶液浓度影响的残余膨胀力预测模型；.（6）基于土的土-水特征曲线（SWCC）和收缩特征曲线（SSCC）的特点以及二者之间的联系，采用三直线的分段 SSCC模型，结合非饱和土的三相比例关系，推导出了非饱和土孔隙比与基质吸力之间关系的通用模型。.（7）提出了预测 van Genuchten（VG）模型参数 n的一个土壤转换函数（pedotransfer function, PTF），该函数是 一个非线性回归方程，与初始孔隙比 e0 的回归方程结合起来，用以预测不同初始孔隙比土的土-水特征曲线 （SWCC）。