Permeability refers to a soil’s ability to allow water to flow through its interconnected voids. This property is critical in engineering because it influences how quickly saturated, compressible soil layers settle and how much water an aquifer can supply. Permeability is considered in various applications, such as pumping groundwater, spacing well points for dewatering excavation sites, designing reservoirs and dams, and selecting soils for different parts of embankments in dams and reservoirs.

The coefficient of permeability can be estimated using factors like the effective diameter of soil particles, porosity, specific surface area, or data from consolidation tests. However, since permeability depends on many variables, direct measurement in a laboratory using a device called a Permeameter is considered more reliable. Permeameters test small soil samples, require less time, and provide accurate results.

There are two main types of Permeameters: Constant Head Permeameters and Falling Head Permeameters. Constant Head Permeameters are used for coarse-grained, cohesion less soils, while Falling Head Permeameters are suitable for fine-grained soils (either remolded or undisturbed) with a coefficient of permeability less than 10⁻² cm/sec.