Environmental Engineering Reference
In-Depth Information
Figure 6.51.
Permeability and laboratory testing method for the main soil types (Head, 1985).
6.3
PERMEABILITY OF SOILS
6.3.1
General principles
The “permeability”, or more correctly “permeability coefficient” or “hydraulic conduc-
tivity” of the soil in a dam embankment or foundation, is not a fundamental property of
the soil but depends on a number of factors. Head (1985) outlines these as: particle size
distribution; particle shape and texture; mineralogical composition; void ratio; degree of
saturation; soil fabric; nature of fluid; type of flow and temperature.
In embankment dam engineering these factors have varying degrees of influence:
Particle size distribution.
The permeability is dependent on the particle size distribution
of the soil with fine grained clay soils having permeabilities several orders of magnitude
lower than that of coarser soils, i.e. sands and gravels. Figure 6.51 shows in general terms
the range of permeabilities which can be encountered.
It has been recognised that the finer particles in a soil largely determine its permeabil-
ity, and granular soils are often compared by their “effective grain size”, D
10
, which is the
particle size for which 10% of the soil is finer.
Particle shape and texture.
This affects permeability to a lesser extent. Elongated
(“platey”) particles tend to have a lower permeability than rounded, and rougher textured
particles a lower permeability than smooth.
Mineralogical composition.
This is a factor in clay soils. Montmorillonite clays, for
example, are finer grained and have a greater tendency to adsorb water (and hence a
lower permeability) than say a kaolin clay. The mineralogy of sand and gravel has little
effect except where it results in elongated particles.
Void ratio.
The void ratio of a soil has an important effect on permeability. Cohesive
soils which are compacted to a high density ratio (e.g. 98% of standard maximum dry
density) will have lower permeability than those compacted to a low density ratio (e.g.
90%). The difference may be orders of magnitude.
Figure 6.52
shows the effect of void
ratio for several clays.
The void ratio also has an effect on the permeability of granular soils, with soils com-
pacted to a small void ratio (dense) having lower permeability than those with a high void
ratio (loose). This effect is allowed for in the Kozeny-Carman formula discussed below.
