Civil Engineering Reference
In-Depth Information
Figure 6.1
Total stresses in the ground.
6.3 Groundwater and pore pressure
The water in the pores of saturated soil has a pressure known as the pore pressure
u
.
This is conveniently represented by the height of water
h
w
in a standpipe, as shown in
Fig. 6.2. When everything is in equilibrium the pressures of water just inside and just
outside the pipe are equal and so
u
=
γ
w
h
w
(6.4)
When the level of water in the pipe is below ground, as in Fig. 6.2(a), it is known as
the water table or the phreatic surface. If the water in the soil is stationary the water
table is horizontal like the surface of a lake. However, as we will see later, if the water
table is not level there will be seepage as the groundwater moves through the pores of
the soil. From Fig. 6.2(a) pore pressures at the water table are zero and positive below
and a question is: what is the pore pressure above the water table?
Figure 6.3 illustrates the variation of pore pressure in the region between the ground
level and the water table. There may be a layer of dry soil at the surface where pore pres-
sures are zero. This is actually relatively rare but can be found on beaches above the
Figure 6.2
Pore water pressures in the ground.
