Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 133

This suggests a simple method for the permeability of a soil at any void ratio when
values of permeability are known at two or more void ratios. Once the line is drawn, the per-
meability at any void ratio may be read directly.

Increase in the porosity leads to an increase in the permeability of a soil for two distinct
reasons. Firstly, it causes an increase in the percentage of cross-sectional area available for
flow. Secondly, it causes an increase in the dimension of the pores, which increases the aver-
age velocity, through an increase in the hydraulic mean radius, which enters the derivation of
Eq. 5.31, and which, in turn, is dependent on the void ratio.
Composition

The influence of soil composition on permeability is generally of little significance in the case
of gravels, sands, and silts, unless mica and organic matter are present. However, this is of
major importance in the case of clays. Montmorillonite has the least permeability; in fact, with
sodium as the exchangeable ion, it has the lowest permeability (less than 10–7 cm/s, even at a
very high void ratio of 15). Therefore, sodium montmorillonite is used by the engineer as an
additive to other soils to make them impermeable. Kaolinite is a hundred times more perme-
able than montmorillonite.
Fabric or Structural Arrangement of Particles

The fabric or structural arrangement of particles is an important soil characteristic influenc-
ing permeability, especially of fine-grained soils. At the same void ratio, it is logical to expect a
soil in the most flocculated state will have the highest permeability, and the one in the most
dispersed state will have the lowest permeability. Remoulding of a natural soil invariably
reduces the permeability. Stratification or macrostructure also has great influence; the per-
meability parallel to stratification is much more than that perpendicular to stratification, as
will be shown in a later section.

100 200 300 400 500 1000

0.4

0.5

0.6

0.7

Permeability (k × 10 mm/s) (log scale)

(b)

5

Void ratio, e

0 0.1 0.2 0.3 0.4 0.5

Void ratio function

(a)

1000

800

600

400

200

e /(1

+

e)

(^33) e /(1+e)e /(1
+e)
2
e /(1+e)
2
e
2
e
2
Permeability (k × 10 mm/s)
5
Fig. 5.9 Permeability-void ratio relationships