DHARM
132 GEOTECHNICAL ENGINEERING
Since k has the units L/T, K has the units L^2. K is also expressed in Darcy’s; 1 darcy
being equal to 0.987 × 10–8 cm^2. K has the same value for a particular soil, for all fluids and at
all temperatures as long as the void ratio and structure of the soil remain unaltered.
Viscosity and unit weight are considered to be the only variables of the permeant fluid
that influence the permeability of pervious soils; however, other permeant characteristics can
have a major influence on the permeability of relatively impervious soils. The effects of viscosity
and unit weight may be eliminated by expressing the permeability in terms of the absolute
permeability. It has been found by Michaels and Lin (1954) that the values of absolute
permeability of Kaolinite varies significantly with the nature of the permeant fluid, when the
comparisons are made at the same void ratio. Further, they found that the variation was large
when the kaolinite was moulded in the fluid which was to be used as the permeant than when
water was used as the moulding fluid and initial permeant, each succeeding permeant displacing
the preceding one. These differences in permeability at the same void ratio have been attributed
to the changes in the soil fabric resulting from a sample preparation in the different fluids.
The effect of the soil fabric will be discussed in next sub-section.
5.6.2 Soil Characteristics
The following soil characteristics have influence on permeability :
- Grain-size
- Void ratio
- Composition
- Fabric or structural arrangement of particles
- Degree of saturation
- Presence of entrapped air and other foreign matter.
Equation 5.31 indicates directly only grain-size and void ratio as having influence on
permeability. The other characteristics are considered indirectly or just ignored. Unfortunately,
the effects of one of these are difficult to isolate in view of the fact that these are closely
interrelated; for example, fabric usually depends on grain-size, void ratio and composition.
Grain-size
Equation 5.31 suggests that the permeability varies with the square of particle diameter. It is
logical that the smaller the grain-size the smaller the voids and thus the lower the permeabil-
ity. A relationship between permeability and grain-size is more appropriate in case of sands
and silts than that of other soils since the grains are more nearly equidimensional and fabric
changes are not significant.
As already stated in sub-section 5.5.6, Allen Hazen proposed,
k = 100 D 102 where D 10 is in cm and k is in cm/s.
Void Ratio
Equation 5.31 indicates that a plot of k versus e^3 /(1 + e) should be a straight line. This is more
true of coarse grained soils since the shape factor C does not change appreciably with the void
ratio for these soils.
Other theoretical equations have suggested that k versus e^2 /(1 + e) or k versus e^2 should
be a straight line. It is interesting to note that, as indicated in Fig. 5.9, a plot of log k versus e
approximates a straight line for many soils within a wide range of permeability values.
