Soil permeability is the property of the soil to transmit water and air. The coefficient of permeability (or permeability) in soil mechanics is a measure of how easily water can flow through a porous soil medium. The more permeable the soil is, the greater is the seepage of water through it. Some soils are so permeable and corresponding seepage is so high that it is not possible to build a pond without special construction techniques. Permeability is closely related to porosity. It reflects the capacity of soil sediments to transmit water and is controlled by the size of the pores and the degree to which they are interconnected.
Generally, materials of larger particle size which are consistently sorted will be more permeable. A soil can have a high porosity, but low permeability if the open spaces are not well connected. The sandy soils are often quite porous, since there are a relatively high percentage of void spaces between the sand grains. These soils are also very permeable, because the pore spaces are usually large and interconnected, allowing water to flow through them more readily. While clay may have a higher porosity than sand, clay particles are much finer and the spacing between them is very small. Clays and silts may not pack together well due to irregular grain shapes and the fact that certain clay minerals have electrostatic charges which repell each other. In addition, molecular attraction on the water trapped in the tiny pore spaces between clay particles is much stronger than that found in the larger sand grain openings. In soil, permeability depends on the average size of the pores and is related to the distribution of particle sizes, its shape, and structure.
Permeability of rock masses is controlled by discontinuity geometry, which includes spacing, direction, discontinuity width and form, as well as the degree of infilling and the roughness of the discontinuity surfaces. These factors are all variable over small distances and make both observation and analysis difficult.
Permeability of soil can be measured either in the laboratory or the field. The followings are some of the methods used in the laboratory to determine permeability.
1. Constant head permeameter
2. Falling or variable head permeameter
3. Direct or indirect measurement during an Oedometer test
4. Horizontal capillarity test.