Abstract
This study investigates the permeability characteristics of unsaturated compacted loess by focusing on the anisotropy parallel and perpendicular to the compaction. Three tests are conducted on compacted loess: permeability test, soil-water characteristic curve test, and scanning electron microscope (SEM) test. Samples are maintained and tested at different dry densities under optimal moisture conditions. Test results showed that the horizontal saturated permeability coefficient of compacted loess was larger than in the vertical direction, showing obvious anisotropy. Based on the saturated permeability coefficient, the permeability coefficient of unsaturated compacted loess can be predicted according to the soil-water characteristic curve fitted using the van Genuchten model combined with the Childs and Collis-Geroge model. The relational formula was established between the unsaturated permeability anisotropy ratio and the matrix suction through introducing a pore connectivity-tortuosity parameter, which represents the relationship between unsaturated permeability anisotropy ratio and matrix suction. The difference of pore characteristics between horizontal and vertical directions of compacted loess is the main reason for the permeability anisotropy.
Highlights
By conducting permeability test and soil-water characteristic curve test on compacted loess of different dry densities, this study investigated the permeability anisotropy of unsaturated compacted loess and drew the following conclusions: (1) e vertical and horizontal permeability coefficients of saturated compacted loess significantly decreased with the increase in dry density. e horizontal saturated permeability coefficient of compacted loess was larger than that in the vertical direction, showing obvious anisotropy
Flake and needle particles rotated to be parallel with the compaction surface after vertical compaction, which weakened the porosity and connectivity of the pores, led to a reduction in vertical saturated permeability coefficient. e porosity and connectivity of pores parallel to the compaction surface were improved to make the horizontal permeability coefficient larger
Based on the vertical and horizontal saturated permeability coefficients, the permeability coefficient of unsaturated compacted loess was calculated using the soil-water characteristic curve fitted using the van Genuchten model and Childs and Collis-Geroge model. e permeability anisotropy of unsaturated compacted loess could be expressed by the ratio of unsaturated permeability anisotropy
Summary
According to laboratory compaction tests following the Chinese National Standards [28], the optimal soil sample moisture content was 18.9%, and the maximum dry density was 1.73 g/cm. Wet soil was weighed based on the dry density and moisture content and was vertically compacted into five layers. Samples for the soil-water characteristic test were cut out of the compacted loess using a cutting ring with an inner diameter of 70 mm and a height of 19 mm. E samples for the scanning electron microscope (SEM) test were cut into 5 mm × 5 mm x 10 mm blocks with dry densities of 1.40 g/cm3, 1.50 g/cm3, 1.60 g/cm, and 1.70 g/cm. Four vertical and horizontal samples were used in the scanning electron microscope test
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