Fallinghead

Falling-head permeability test, sometimes referred to as the falling-head method, is a laboratory procedure used to determine the hydraulic conductivity (permeability) of soils by observing the rate at which water drains from a soil specimen as the head in a connected standpipe falls. It is one of the common methods for measuring permeability in geotechnical testing, typically used for soils with low to moderate permeability.

Principle and formula

During the test, a saturated soil specimen of cross-sectional area A and length L is placed in a permeameter with a vertical standpipe of cross-sectional area a connected to the outlet. The standpipe is initially filled to a water level h0 and the water surface is allowed to fall to h1 as water flows through the specimen. The time t required for the head to drop from h0 to h1 is recorded. The hydraulic conductivity k can be calculated from k = (a L)/(A t) ln(h0/h1). If several head intervals are used, the results can be averaged to improve accuracy.

Procedure and equipment

Typical equipment includes a permeameter housing the soil specimen and a vertical standpipe with a valve to control flow. Steps: saturate the specimen, assemble the apparatus to minimize leaks, fill the standpipe to h0, start timing as the head falls to h1, and record the time t. Repeat for multiple head changes to obtain a reliable k value.

Applications and limitations

The falling-head method is well suited for soils with low to moderate permeability and offers a relatively quick, straightforward way to estimate k in the lab. Limitations include sensitivity to air leaks, incomplete saturation, temperature effects, and end-blocking or boundary effects, which can bias results if not carefully controlled.