Geotechnical Engineering

DHARM

SOIL MOISTURE–PERMEABILITY AND CAPILLARITY 123

The constant head permeameter is widely used owing to its simplicity in principle. How-

ever, certain modifications will be required in the set-up in order to get reasonable precision in

the case of soils of low permeability.

5.5.2 Falling or Variable Head Permeameter

A simple set-up of the falling, or variable head permeameter is shown in Fig. 5.5.

Constant head

chamber

Overflow

Porous stone

Screen

Soil sample

(Cross-sectional

LL area A)

hh 11

hh 00

Stand pipe

or burette

(Cross-sectional area a)

Fig. 5.5 Falling, or variable, head permeameter
A better set-up in which the top of the standpipe is closed, with manometers and vacuum
supply, may also be used to enhance the accuracy of the observations (Lambe and Whitman,
1969). The falling head permeameter is used for relatively less permeable soils where the
discharge is small.
The water level in the stand-pipe falls continuously as water flows through the soil
specimen. Observations should be taken after a steady state of flow has reached. If the head or
height of water level in the standpipe above that in the constant head chamber falls from h 0 to
h 1 , corresponding to elapsed times t 0 and t 1 , the coefficient of permeability, k, can be shown to
be :

k =

2 303

10

.

()

aL

At−t

. log 10 (h 0 /h 1 ) ...(Eq. 5.17)

where
a = area of cross-section of standpipe
L and A = length and area of cross-section of the soil sample and the other quantities as
defined.