The falling head permeability test is a common laboratory testing method used to determine the permeability of 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 can be carried out in a Falling Head permeability cell or in an oedometer 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 standpipe reaches a given lower limit. The time required for the water in the standpipe to to drop from the upper to the lower level is recorderd. 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.
On the basis of the test results, the permeability of the sample can be calculated as
K=[2.3 a.L / (A.Δt)].Log(h_U / h_L)in which we have
L: the height of the soil sample column
A: the sample cross section
a: the cross section of the standpipe
Δt: the recorded time for the water column to flow though the sample
h_U and h_L : the upper and lower water level in the standpipe measured using the same water head reference
Figure 1 . Falling head permeability test setup (Ref.)
