Permeability of Soils

Permeability of a soils is the rate at which water flows through it under the action of (unit) hydraulic gradient. The passage of moisture through the interspaces or pores of the soil is called “percolation”. Soils porous enough for percolation to occur are termed “pervious” or “permeable” while those which do not permit the passage of water are termed “impervious” or “impermeable”. In the majority of materials the rate of flow is directly proportional to the head of water, and the permeability is therefore a constant for the particular material. Permeability is a property of the soil mass and not of individual particles, and varies as the square of the diameter of the grains of the soil the ratio of the fine material and with the arrangement of the grain particles of the soil mass. The permeability of cohesion soils is, in general, very small. Sands drain readily whilst silts and clays are difficult or impossible to drain. A knowledge of permeability is required not only -for seepage, drainage and ground water problems but also for the rate of settlement of structures on saturated soils. Soils yield under pressure when moisture content is increased. Ground water level depends upon a combination of the permeability of the strata and the causing the water to flow.