Geotechnical Engineering

DHARM

124 GEOTECHNICAL ENGINEERING

This can be derived as follows :
Let – dh be the change in head in a small interval of time dt. (Negative sign indicates
that the head decreases with increase in elapsed time).

From Darcy’s law,

Q = (– a.dh)/dt = k.i.a

• adh/dt = K.A.h/L

∴ (kh/L).A = −adh

dt

.

or (kA/aL). dt = – dh/h

Integrating both sides and applying the limits t 0 and t 1 for t, and h 0 and h 1 for h,

kA

aL

dt dh

h

dh

t h

t

h

h

h

h

0

1

0

1

1

0

zzz=− =

∴ (kA/aL)(t 1 – t 0 ) = loge (h 0 /h 1 ) = 2.3 log 10 (h 0 /h 1 ).

Transposing the terms,

k =

2 303

10

.

()

aL

At t−

.log 10 (h 0 /h 1 )

which is Eq. 5.17.

The ‘Jodhpur permeameter’ developed at the M.B.M. Engineering College, Jodhpur,
may be conveniently used for conducting the falling head as well as constant head tests on
remoulded as well as undisturbed specimens. Remoulded specimens may be prepared by static
or dynamic compaction. The apparatus has been patented and manufactured by ‘AIMIL’. (M/s.
Associated Instrument Manufacturers India Limited, Bombay). The detailed description of
the apparatus and the procedure for the permeability tests are given in the relevant Indian
standards. [(IS : 2720 Part XVII—1986) and (IS : 2720 Part XXXVI—1987)].

5.5.3 Direct or Indirect Measurement During an Oeodometer Test

As discussed in Chapter 7, the rate of consolidation of a soil depends directly on the permeabil-
ity. The permeability can be computed from the measured rate of consolidation by using ap-
propriate relationships. Since there are several quantities in addition to permeability that
enter into the rate of consolidation-permeability relationship, this method is far from precise
since these quantities cannot easily be determined with precision. Instead of the indirect ap-
proach, it would be better to run a constant-head permeability test on the soil sample in the
oedometer or consolidation apparatus, at the end of a compression increment. This would yield
precise results because of the directness of the approach.

5.5.4 Permeability from Horizontal Capillarity Test

The ‘Horizontal capillarity test’ or the ‘Capillarity-Permeability test’, used for determining the
capillary head of a soil, can also be used to obtain the permeability of the soil. This is described
in detail in a later section dealing with the phenomenon of ‘Capillarity’.

The laboratory measurement of soil permeability, although basically straightforward,
requires good technique to obtain reliable results. The reader is referred to Lambe (1951) for
an exhaustive treatment of measurement of permeability.