Structural characteristics and hydraulic conductivity of an eluvial-colluvial gravelly soil

Gravelly soil is a typical heterogeneous porous medium with a multiscale structure and hydraulic conductivity that is challenging to quantify. The aim of this study is to examine the structure of an eluvial-colluvial gravelly soil at different scales and link the structural characteristics to the hydraulic conductivity. To this end, large gravelly soil samples and small fines-sand mixture samples were prepared and then characterized by X-ray computed tomography (CT) and optical microscopy, respectively. Through image analyses, the pore structural characteristics at the gravel scale (≥ 1.0 mm) and sand scale (0.01–1.0 mm) were identified. Constant-head tests were performed on the large gravelly soil samples to measure the saturated coefficients of permeability (k). The results show a relatively small gravel-scale porosity and a large sand-scale porosity in compacted gravelly soils. The dominant sizes of gravel-scale pores and sand-scale pores are several millimeters and approximately 0.06 mm, respectively. An evaluation of ten existing permeability equations indicates that most of the previous empirical equations proposed for sand and gravel are not applicable to well-graded gravelly soils. For this reason, the existing permeability equations were improved. In addition, novel empirical equations for estimating the k value of gravelly soils were proposed based on the structural parameters of pores, as well as the concepts of the effective porosity and the effective grain size.