Effective Stress (Concept only)

Any soil sample in its natural state is loaded with its self weight and it may also be loaded with any surcharged weight or external weight. So there are two possible loading :
(a) Self weight.
(b) Surcharged weight (external weight).
Now in case there is the water present in the saturated soil also applies a pore water pressure to the soil grains. Its value is directly proportional to the head of the water.
So now there are two main loading on any soil particle of a saturated soil sample. One is the external +self weight and another is pore water pressure. The external weight + Self weight can be easily determined by knowing the bulk density of the soil. This load can be termed as the total load, and the total load per unit area is termed as the total stress.  This load tries to compress the solid grains together and also it reduces the void ratio, be getting deformed or rearranged. The pore water pressure applied by the pore water tries to separate out the grains, so it tries to increase the volume of the voids. So it is not responsible for the compression of the soil or the reduction of the permeability of the soil. So the stress which is effective in the compression or the reduction in the permeability of the soil is the difference  of the total stress and pore water pressure and it is termed as the effective stress.
So,
Total Stress = Effective stress + Pore water pressure.

Or
 Effective stress = Total stress - Pore water pressure.

Effective stress is a term which can not be measured directly but it can be calculated after measuring the total stress and the pore water pressure.