To determine the coefficient of permeability of a given soil sample by
Variable head permeability test.
The passage of water through porous material is called seepage. A
material with continuous voids is called a permeable material. Hence
permeability is a property of a porous material which permits passage of
fluids through inter connecting conditions. Hence permeability is
defined as the rate of flow of water under laminar conditions through a
unit cross-sectional area perpendicular to the direction of flow through
a porous medium under unit hydraulic gradient and under standard
temperature conditions.
IS 2720-17 (1986): Methods of test for soils, Part 17. This test is used for fine grained soils with intermediate and low permeability such as silts and clays. This testing method can be applied to an undisturbed sample.
The falling head permeability test involves flow of water through a relatively short soil sample connected to a standpipe which provides the water head and also allows measuring the volume of water passing through the sample. The diameter of the standpipe depends on the permeability of the tested soil. The test is carried out in falling head permeameter cell.
Before starting the flow measurements, the soil sample is saturated and
the standpipes are filled with de-aired water to a given level. The test
then starts by allowing water to flow through the sample until the water
in the stand pipe reaches a given lower limit. The time required for the
water in the stand pipe to drop from the upper to the lower level is
recorded. Often, the standpipe is refilled and the test is repeated for
couple of times. The recorded time should be the same for each test
within an allowable variation of about 10% (Head 1982) otherwise the
test is failed.
The below equation can be used:
k = ((2.3×a×L)/(A×(t2-t1)))×log10(h1/h2)
Where,
L = length of soil sample column
A = Sample cross-section
a = the cross-section of the standpipe
(t2-t1) = the recorded time for the water column to flow though the
sample
h1 and h2 = the upper and lower water level in the standpipe measured
using the same water head reference