Civil Engineering Reference
In-Depth Information
Figure 6.3
Pore pressures and suctions in the ground.
high-tide mark. Immediately above the water table the soil remains saturated because
of capillary rise in the pore spaces. In this zone the pore pressures are negative and are
given by
u
=−
γ
w
h
w
(6.5)
Between the dry and saturated zones there is a zone of unsaturated soil which con-
tains soil grains, water and gas, usually air or water vapour. In this soil the pore
water and gas exist at different pressures and the pore water pressures may increase
or decrease as indicated in Fig. 6.3. There is, at present, no simple and satisfactory
theory for unsaturated soils and in this topic I will deal mostly with saturated or dry
soils. Unsaturated soils will be considered further in Chapter 26.
In practice soils in slopes, foundations, retaining walls and other major civil engi-
neering works are usually saturated, at least in temperate or wet climates. Unsaturated
soils may occur very near the surface, in compacted soils and in hot dry climates.
6.4 Suctions in saturated soil
Saturated soils may very well have negative pore pressures or suctions. This implies
that the water is in tension and the rise of water in soil above the water table is like
the rise of water in a capillary tube with a diameter equivalent to the size of the pore
spaces in the soil.
Figure 6.4(a) shows water rising to a height
h
w
in a capillary tube with diame-
ter
d
t
. (Note that
h
w
is negative because the head of water in a standpipe is positive
downwards in Fig. 6.3.) The suction just inside the meniscus is related to the tube
−
