Permeability : Permeability is a measure of the ease with which water flows through soils and/or rocks.
No soil is completely impervious but some are relatively impervious while others are pervious.
Pervious Soil :
A soil is said to be pervious soil if it offers minimum resistance to the flow of water through it.
For examples, all clean coarse grained soils such as gravels, sand and gravel sand mixtures are pervious soil.
The pervious soil have permeability in the range of > 10-2 to 10-5 m/sec.
Impervious Soil :
The soil which offers maximum resistance to the flow of water through it is known as impervious soil.
For example, Silts, clay and their mixtures.
The impervious soil have permeability < 10-8 m/sec.