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

  • Qian-Feng Gao
  • Hui DongEmail author
  • Runqiu Huang
  • Zhi-Fei Li
Original Paper


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.


Gravelly soil Structural characteristics Hydraulic conductivity Computed tomography (CT) Optical microscopy 



Pore area

a, c

Fitting constants in Eq. (18)

Cc, Cu

Coefficient of curvature and coefficient of uniformity, respectively

CT, C0, Cs

Constants in previous permeability equations


Coefficient of variation

Df, DfG, Dfs

Fractal dimensions of total pores, gravel-scale pores, and sand-scale pores, respectively


Pore equivalent size

deG, des

Dominant equivalent sizes of gravel-scale pores and sand-scale pores, respectively


Effective grain size considering the entire grain size distribution

di (i = 10, 20, 50)

Grain size at which i% of the grains are finer

d*j (j = 1, 2, 3,..., m)

Grain size of the jth grain class


Coefficient of porosity variation

e, emax

Void ratio and maximum void ratio, respectively


Variable as a function of the void ratio in the NAVFAC permeability model

G, S, F

Gravel content, sand content, and fines content, respectively


Specific gravity


Acceleration of gravity


Saturated coefficient of permeability

kc, km

Calculated value and measured value of the saturated coefficient of permeability, respectively


In situ saturated coefficient of permeability

k (di, e, n, Cu, Cc)

Previous permeability equations

L, W

Window level and window width, respectively

M, N

Fitting constants in Eq. (10)


Gravel number (the number of gravel particles in a sample cross-section)


Total porosity measured by the conventional method


2D effective porosity

n2G, n2S

2D gravel-scale porosity and 2D sand-scale porosity of gravelly soils, respectively

n2s, n2st, n2sb

2D sand-scale porosity, 2D sand-scale porosity on the sample top surface, and 2D sand-scale porosity on the sample bottom surface of fines-sand mixtures, respectively


Reference 2D sand-scale porosity of fines-sand mixtures


Pore perimeter

Pj (j = 1, 2, 3,..., m)

Cumulative mass percentage of grains

w, wopt, wsat, w0

Water content, optimum water content, saturated water content, and natural water content, respectively


Total area of gravels, total imaging areas, total area of gravel-scale pores, and total area of sand-scale pores in gravelly soils, respectively

St, Svs

Total imaging areas and total area of sand-scale pores in fines-sand mixtures, respectively

X, Y

Fitting constants in Eq. (12)


Kinematic viscosity of water


Constant for the determination of the fractal dimension

μ, μT

Dynamic viscosity of water and dynamic viscosity of water at T °C


Density of water

ρd, ρdmax, ρd,re

Dry density, maximum dry density, and reference dry density of soil, respectively


Gravel area ratio



This study was funded by the National Natural Science Foundation of China (no. 51108397); the Natural Science Foundation of Hunan Province, China (no. 2015JJ2136); the Scientific Research Project of the Education Department of Hunan Province, China (no. 16B255); and the Hunan Key Laboratory of Geomechanics and Engineering Safety, China (no. 16GES04).


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Hunan Key Laboratory of Geomechanics and Engineering SafetyXiangtan UniversityXiangtanChina
  2. 2.Laboratoire d’Etude des Microstructures et de Mécanique des Matériaux, CNRS UMR 7239Université de LorraineMetzFrance
  3. 3.State Key Laboratory of Geohazard Prevention and Geoenvironment ProtectionChengdu University of TechnologyChengduChina

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