- Total Stress: 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
Pore Water Pressure: The pressure of water in the pores of the soil is called pore water pressure (u). The magnitude of pore water pressure depends on:
- the depth below the water table.
- the conditions of seepage flow.
Under hydrostatic conditions, no water flow takes place, and the pore pressure at a given point is given by u = gw.h where h = depth below water table or overlying water surface.
- The natural level of ground water is called the water table or the phreatic surface.
- The principle of effective stress was enunciated by Karl Terzaghi in the year 1936. This principle is valid only for saturated soils, and consists of two parts:
- At any point in a soil mass, the effective stress (represented by or s‘ ) is related to total stress (s) and pore water pressure (u) as = s – u.
All measurable effects of a change of stress, such as compression and a change of shearing resistance, are exclusively due to changes in effective stress.
Shear Strength =
- Above the water table, when the soil is saturated, pore pressure will be negative (less than atmospheric). The height above the water table to which the soil is saturated is called the capillary rise, and this depends on the grain size and the size of pores. In coarse soils, the capillary rise is very small.
- In contrast, an upward flow opposes the force of gravity and can even cause to counteract completely the contact forces. In such a situation, effective stress is reduced to zero and the soil behaves like a very viscous liquid. Such a state is known as quick sand condition.
where icr = critical hydraulic gradient