When a load is applied to soil, it is carried by the solid grains and the water in the pores.
The total vertical stress acting at a point below the ground surface is due to the weight of everything that lies above, including soil, water, and surface loading. Total stress thus increases with depth and with unit weight.
Vertical total stress at depth z, sv = g.Z
Below a water body, the total stress is the sum of the weight of the soil up to the surface and the weight of water above this. sv = g.Z + gw.Zw
The total stress may also be denoted by sz or just s.
It varies with changes in water level and with excavation.