Permeability can be defined as the property of the soil which enables the flow of water through it. There are voids in the soil, which if are interconnected to each other, provides the path for the flow of the water through it. It is very important to study the permeability of the soil because,
(1) Almost all the civil engineering structures are constructed on the soil and if the soil below them is pervious, may result in the percolation of the water, and may also result in the piping action, which will reduce the strength of the soil to take the structural weight.
(2) Soil embankments are likely to get failed if the soil used to construct them is of high permeability, because it will reduce their shear strength. So it important to study the permeability of the soil to be used for the pavement construction.
(3) The canals are likely to get failed if there is any leakage from the embankments, so it is important to study the soil used to construct them.
(4) The earth dams use the material which has zero permeability to store the water.
(5) The gravity dams fails if there is any piping action taking place through the soil used below.
Hydraulic gradient: Hydraulic gradient may be defined as the loss of the head per unit length of the soil.
H.Darcy gave a law for the flow of the water through the soil. According to Darcy, the velocity of the water flowing through the soil is directly proportion to the hydraulic gradient of the soil.
v = ki
k=co-efficient of permeability.
i= hydraulic gradient.
now discharge q= kiA = vA.
In laboratory we can find the co-efficient of permeability by two methods:
(a) Constant head permeameter
(b) Falling head permeater.
In the constant head permeater the available head for the flow of water through soil is kept constant by the arrangement of the steady inflow of the water in the storage tank. In the falling head permeater the head of the water is not constant. The initial head and final head are noted with respect to the time taken by it.