Environmental Engineering Reference
In-Depth Information
conductivity depends on many factors, including the pore-size distribution of the
porous medium, and the tortuosity, shape, roughness, and degree of interconnected-
ness of pores. The hydraulic conductivity decreases considerably as soil becomes
unsaturated and less pore space is filled with water. The unsaturated hydraulic con-
ductivity function gives the dependency of the hydraulic conductivity on the water
content,
K
(
), or pressure head,
K
(
h
). The conceptual model that views the soil
as a bundle of capillaries of different radii, as used earlier to explain water reten-
tion properties (Fig.
18.3
), may be used also to evaluate the hydraulic conductivity
function. The ability of each capillary tube to conduct water can be calculated using
Poiseuille's flow equation. By adding the conductivity of all capillaries that are filled
with water at a particular water content or pressure head, one obtains the hydraulic
conductivity of the complete set of capillaries, and consequently of the soil itself.
Figure
18.6
presents examples of hydraulic conductivity functions for the sand,
loam, and clay textural classes, again using the Carsel and Parish (1988) parameter
values as listed in Table
18.1
. The hydraulic conductivity curves in Fig.
18.6
are pre-
sented as functions of both the pressure head (left) and water content (right). Notice
that the hydraulic conductivity at saturation is significantly larger for coarse-textured
soils (sands and gravels) than for loams and clays. This difference is often sev-
eral orders of magnitude. Also notice that the hydraulic conductivity may decrease
several orders of magnitude as the soil becomes unsaturated. This decrease, when
expressed as a function of the pressure head (Fig.
18.6
, left), is much more signif-
icant for sands than for loams, and even more so than for clays. The decrease for
coarse-textured soils (sands and gravel) is so large that at a certain pressure head the
hydraulic conductivity becomes smaller than for heavy-textured soils (clays). These
properties of the hydraulic conductivity function are often used in the design of
θ
4
4
Sand
Loam
Clay
2
2
0
0
-2
-2
-4
-4
-6
-6
Sand
Loam
Clay
-8
-8
-10
-10
-2
0
2
4
0
0.1
0.2
0.3
0.4
0.5
Log
10
(Matric head [m])
Water content [-]
Fig. 18.6
Example of hydraulic conductivity curves for the sand, loam and clay soil textural
classes (based on Carsel and Parish (1988) soil hydraulic parameters, Table
18.1
)
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