DHARMSOIL MOISTURE–PERMEABILITY AND CAPILLARITY 131through a circular capillary tube. The flow through a porous medium is considered similar to a
flow through a bundle of straight capillary tubes. The equation is :k = D e
e
s^2 C3
1..
()γ.
μ+...(Eq. 5.31)in which,
k = Darcy’s coefficient of permeability
Ds = effective particle-size
γ = unit weight of permeant
μ = viscosity of permeant
e = void ratio
C = shape factor
This equation helps one in analysing the variables affecting permeability. The charac-
teristics of the permeant are considered first and those of the soil next.5.6.1 Permeant Fluid Properties
Equation 5.31 indicates that the permeability is influenced by both the viscosity and the unit
weight of the permeant fluid. In the field of soil mechanics, the engineer will have occasion to
deal with only water as the common permeant fluid. The unit weight of water does not signifi-
cantly vary, but its viscosity does vary significantly with temperature. It is easy to understand
that the permeability is directly proportional to the unit weight and inversely proportional to
the viscosity of the permeant fluid.
It is common practice to determine the permeability at a convenient temperature in the
laboratory and reduce the results to a standard temperature; this standard temperature is
27°C as per I.S. Code of practice. (IS : 2720 Part XVII-1966 and its revised versions). This is
done by using the following equation :k 27 = kT.μT
μ 27...(Eq. 5.32)where kT and μT are the permeability of soil and the viscosity of water at the test temperature
of t°C and, k 27 and μ 27 are the permeability and viscosity at the standard temperature, i.e.,
27°C.
According to Muskat (1937), these two permeant characteristics, that is, viscosity and
unit weight, can be eliminated as variables by defining a more general permeability, K, as
follows :
K =k.μ
γ ...(Eq. 5.33)where
K = specific, absolute, or physical permeability
μ = viscosity of the permeant
γ = unit weight of the permeant
k = Darcy’s coefficient of permeability.