Which type of soil is most likely to require a Falling Head Test for permeability determination?

The Falling Head Test is typically used for fine-grained soils when determining permeability. This methods effectiveness arises from its ability to measure the permeability of soils that have low hydraulic conductivity, which is characteristic of fine-grained soils such as silts and clays.

Fine-grained soils can retain moisture and exhibit lower permeability due to their smaller particle sizes and greater surface area, which affects how water flows through them. The Falling Head Test applies a small hydraulic gradient, minimizing disturbances to the soil structure, and allows for accurate measurement of the very slow flow rates typical of these soils.

By comparison, coarse-grained soils, like sands and gravels, generally possess higher permeability and are more commonly tested using the Constant Head Test, which is more suited to their rapid flow characteristics. Well-graded and poorly graded soils can contain a variety of grain sizes; however, the finer particles predominantly define their permeability characteristics, making the Falling Head Test the preferred method for fine-grained soils.