应力路径试验前后不同黄土的孔隙分形特征

为了解不同黄土孔隙形状复杂程度的差异和应力路径对孔隙形状复杂程度的影响,对两种黄土应力路径前后孔隙分形特征进行了研究。首先比较了3种分维模型所得孔隙分形维数的可靠性,然后选用热力学关系模型,由进汞、退汞试验得到地裂缝区黄土、充填黄土初始样和三轴应力路径试验后的孔隙分布,据此分析了两种土体进汞孔隙和退汞孔隙分形维数的差异和受载后的分形维数变化,根据退汞过程仅管形孔内的汞流出及孔隙由管形孔和球形孔组成的假定,得到了应力路径试验前后孔隙形状的改变。结果表明,热力学关系模型得到的孔隙分形维数合理、可靠。初始状态,原状和重塑充填黄土的孔隙形状比原状和重塑黄土复杂;常规三轴试验后试样孔隙比减少,孔隙分形维数增加,球形孔向管形孔转变;相对于常规三轴压缩试验,减围压三轴压缩试验后试样的孔隙分形维数较小,管形孔占总孔隙的体积比例较少;总体上管形孔的分形维数比球形孔大,且基本不受应力路径的影响,其占总孔隙的体积比例随着试样宏观孔隙比的减少而增加。

Pore fractal features of different kinds of loesses before and after stress path tests

In order to study the difference of the pore shape complexity among different loesses and the influence of stress path on the pore shape complexity, the pore fractal features are investigated on two kinds of loesses before and after stress path tests. Firstly, the reliabilities for calculating the fractal dimensions of pore by three fractal models are checked. Then the thermodynamic model is adopted to study the difference of pore fractal dimensions of two different kinds of loesses and the change of pore fractal dimensions after stress path according to the pore distributions of loess and filling loess around ground fissures, which are determined by mercury intrusion and withdrawal test. The change of pore shape is obtained based on the assumption that the pore is composed of sphere-shaped pore and tube-shaped pore. The results indicate that the pore fractal dimensions obtained by thermodynamic model are reasonable and reliability. The initial natural filling loess or initial remoulded filling loess has more complex pore structure compared with natural loess or remoulded loess. The void ratio decreases, the pore fractal dimensions increase and a part of sphere-shaped pores turn into tube-shaped pores after conventional triaxial compression tests. Compared with conventional triaxial compression test, the pore fractal dimensions of the sample and the ratio of the volume of tube-shaped pores to the total volume of pores after reduced confining pressure in triaxial compression test are smaller. In general, the tube-shaped pores possess larger fractal dimensions, the fractal dimensions are almost independent of stress path, compared with sphere-shaped pores, the ratio of the volume of tube-shaped pores to the total volume of pores increases with the decrease of void ratio.