Environmental Engineering Reference
In-Depth Information
Figure 6.53.
Effect of structure on permeability (Lambe and Whitman, 1981). Reprinted by permission
of John Wiley and Sons Inc.
to the soil. In the latter the dispersant breaks up the flocculated structure, resulting in a
lower permeability.
The water content at which a clay is compacted affects its permeability. In particular a
cohesive soil compacted dry of optimum results in a less oriented structure, with poten-
tially aggregates of soil separated by voids, leading to relatively high permeability. An
example of this behaviour is given in
Figure 6.54
where soils are compacted with the same
compactive effect at different water contents.
It is virtually impossible to simulate in the laboratory the effect of different degrees of
compaction at the top and bottom of layers and other factors, such as desiccation at the
surface of layers during construction. In the dam, the ratio of k
H
/k
V
is likely to be high,
possibly as high as 100, with the laboratory permeability probably best representing the
vertical permeability k
V
.
Nature of the permeating fluid.
The permeability (coefficient of permeability k) is
dependent on the properties of the permeating fluid. It is related to the absolute (also
known as intrinsic, or specific) permeability, K, by:
Kg
k
(6.45)
where
fluid viscosity.
For most dam projects water is the permeating fluid and the small variations in viscos-
ity and density, resulting from differences in temperature and dissolved salts content, are
not significant compared to other variables. However the chemical composition of the
fluid density; g
acceleration due to gravity;
