Abstract: In order to realize the visualization of unsaturated preferential flow migration of soil, a column device was designed to carry out unsaturated seepage tests. The transparent soil and digital image processing technology were used to establish the relationship between the normalized pixel intensity and the saturation of transparent soil. On this basis, the effects of the connectivity of the preferential flow path and the rotation angle of the adjacent preferential flow path on the preferential flow migration in unsaturated soils were studied by conducting laboratory model tests. The results showed that it was feasible to characterize the saturation of unsaturated transparent soils based on the intensity of image gray pixels. The profile of fully connected preferential flow (O-O type) and upper connected preferential flow (O-C type) presented T-shape. The saturation of the central axis profile was obviously different from that of the central edge profile. In the lower connected preferential flow path (C-O type), the soil oil pressure and matrix potential could not make the fluid enter the preferential flow path to form the preferential flow, in which the change of infiltration was consistent with the uniform flow. The stable infiltration rate and the wetting front moving velocity of O-C preferential flow were 1.5 times and 1.4 times of those of the C-O type, respectively. A new preferential flow was formed in the area between adjacent O-C preferential flows. The soil in that area reached higher saturation and the growth rate of saturation decreased with the increase of rotation angle. When the preferential flow rotation angle was 90?, 60? and 30?, the stable infiltration rate was respectively 1.5 times, 1.3 times and 1.2 times that of the uniform flow. As the fluid was affected by gravity, the preferential flow with a small rotation angle only infiltrated horizontally along one side of the path. However, the moving velocity of the wetting front was 1.3 times, 1.4 times, and 1.5 times that of uniform flow, respectively. The interaction between adjacent preferential flows was weakened, so it was difficult to form the new preferential flow.

Key words: transparent soil, normalized pixel intensity, saturation, unsaturated preferential flow, wetting front