Permeability of Soil element

Pressure head, Elevation head and Total Heads

Permeability : In a soil element, the interconnected pores or drainage path provide passage for water to drain. So many number of such flow paths when act together, and the average rate of flow of water is termed as the coefficient of permeability, or permeability of soil. It is a process of measurement of that the soil provides to the flow of water through its internal pores.

At point considered i.e A, the pore water pressure (u) can be measured from the height of water in a standpipe located at that particular point.

The height of the water column head is known as the pressure head (hw).
hw = u/gw

For identification of any significance difference in pore water pressure at different head of points, it is very much necessary to eliminate the effect of the point of measurement. With this view, a datum is also required from which locations are also measured.

The elevation head (hz) in soil element at any point is, its height above the datum line. The datum height of water level in the standpipe zone above the datum is the piezometric head (h).

h = hz + hw  

The Total head consists of basically three components : 

Elevation head,

Pressure head,

velocity head.

The seepage velocity in soil mass is normally low, velocity head may be ignored, and the total head becomes equal to the piezometric head. Due to the low seepage velocity of water and small size of water pores, the seepage flow of water inside the pores is steady and laminar in most of the cases. Water flow always takes place between two points in soil mass due to the difference in total heads.