Permeability is defined as the property of a porous material which permits the passage or seepage of water (or other fluids) through its interconnecting voids. A material having continuous voids is called permeable. Gravels are high permeability while stiff clay is the least permeable, and hence such a clay may be termed impermeable for all practical purposes.
The flow of water through soils may either be a laminar flow or a turbulent flow. In laminar flow, each fluid particle travels along a definite path which never crosses the path of any other particle; In turbulent flow, the paths are irregular and twisting, crossing at random. (Taylor, 1948). In most of the practical flow problems in soil mechanics, the flow is laminar.
The study of seepage of water / Permeability through soil is important for the following engineering problems:
1. Determination of rate of settlement of a saturated compressible soil layer.
2. Calculation of seepage through the body of earth dams, and stability of slopes.
3. Calculation of uplift pressure under hydraulic structure and their safety against piping.
4. Ground water flow towards wells and drainage of soil.