The soils in railway engineering always stay unsaturated, and the soil water characteristic curve is one of the important relationships for reflecting the engineering characteristic of unsaturated soils. It has important theoretical significance for analyzing the settlement characteristics to study the soil water characteristic curve. In this paper, the theoretical solution of air entry value of soil water characteristic curve was obtained based on the Van Genuchten model. In combination with the fractal model of pore volume of unsaturated soils, a method for directly solving the fractal dimension based on the parameters of the Van Genuchten model was obtained. By using the pressure plate apparatus, soil water characteristic curve tests of unsaturated clay soil were conducted by considering four different dry density and initial water content levels, and the influences of dry density and initial water content on fractal dimensions of unsaturated clay soil were analyzed. Based on the analysis of test results, it shows that the method for calculating fractal dimensions proposed in the paper is reasonable. It showed approximate linear relationship between fractal dimension and dry density, whereas the fractal dimension increased with increasing dry density. Meanwhile, the relationship between fractal dimension and initial water content was also approximately linear, but the fractal dimension decreased with increasing initial water content.%铁路工程中地基土往往处于非饱和状态,而土水特征曲线是表征非饱和土工程特性的重要关系曲线,进行铁路地基土土水特征曲线的研究,对分析其沉降特性具有重要理论意义.基于Van Genuchten模型,得到非饱和土土水特征曲线空气进气值的理论解;并结合非饱和土孔隙体积的分形模型,得到一种利用Van Genuchten模型参数直接求解分维数的方法.通过非饱和土压力板仪测量4种不同干密度和初始含水率情况下的非饱和黏性土土水特征曲线,分析干密度和初始含水率对非饱和黏性土分维数的影响.研究结果表明:建议的分维数计算方法是合理的;非饱和地基土分维数与干密度近似呈线性关系,并随着干密度的增大而增大;非饱和地基土分维数与初始含水率也近似呈线性关系,但随初始含水率的增大而减小.
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