Falling-head test

To determine the permeability of fine-grained soils, e.g. clay, silt or peat, the falling-head method is used; the fine particles in the sample result in slow hydraulic conductivity. This test is can be conducted on an undisturbed sample. During the test a small saturated soil sample is placed in a container and attached to a standpipe (piezometric tube) filled with a specific level of de-aired water. The water is then allowed to permeate the sample. When the water in the standpipe reaches a pre-determined level the permeability coefficient can be calculated using the sample size, cross-sectional area of the standpipe and the time it took to change the water level.