Total and Effective Stress - Elements of Soil Mechanics - page 76

ground surface

z

σ z

σ x σ x

σ z

Fig. 3.3 Total stress in a homogeneous soil mass.

ground surface

water table

z w

u

Fig. 3.4 Hydrostatic pore water pressure.

σ = γ z

z

where γ = unit weight of the soil.

If the soil is multi-layered, the total vertical stress is determined by summing the stresses induced by each
layer of soil (see Examples 3.1 and 3.2 ).

3.3 Pore pressure

The term pore water was introduced in Section 2.2.1. Pore water experiences pressure known as the pore
pressure or pore water pressure, u. The magnitude of the pore pressure at a point in the soil depends on
the depth below the water table and the flow conditions. In the case of a horizontal ground water table,
we may be able to assume that no flow is taking place and the pore pressure at a point beneath the
ground water table can be established from the hydrostatic pressure acting. The magnitude of the pore
pressure at the water table is zero.
In Fig. 3.4 the pore pressure is given by the hydrostatic pressure.

u = γ w w

where

z w = the depth below the water table .