Civil Engineering Reference
In-Depth Information
Behaviour of unsaturated soils
26.1 Introduction
Throughout most of this topic I have considered the behaviour of soils which are either
saturated or dry. In dry soils the pore spaces are filled with air and in saturated soils
they are filled with water. In an unsaturated soil the pore spaces contain both water
and a gas, which is usually air, but which may be water vapour or methane.
The behaviour of unsaturated soil is very complicated and there is, at present, no
simple theory which adequately describes the engineering behaviour of unsaturated
soils. Amajor difficulty is that the behaviour of the soil as it is drying and the proportion
of air is increasing is different from the behaviour as it is wetting and the proportion
of air is decreasing. All I can do in this short chapter is outline the basic features of
unsaturated soils.
26.2 Occurrence of unsaturated soils
As illustrated in Fig. 6.3 soil below the water table and for some distance above it is
saturated and soil above that is unsaturated. There may be dry soil at the surface but,
in practice, this is rare. Soils which were initially saturated may become unsaturated
when the pore pressure falls below a critical value and air is able to enter the pores.
Alternatively, soils may be formed in an initially unsaturated condition. Unsaturated
soils may become saturated if the water table rises.
Figure 26.1 illustrates different ways in which unsaturated soils are formed. In
Fig. 26.1(a) desaturation of an initially saturated soil occurs as the water table falls
naturally or by pumping. As discussed in Sec. 6.3 there will be a zone of saturated soil
above the water table the height of which depends on the grain size of the soil. Water
contents in soil near the surface may be reduced further by vegetation and evaporation.
In Fig. 26.1(b) residual soils which are formed by weathering of rock
in situ
are often
unsaturated from the start. In Fig. 26.1(c) excavated soil lumps, which themselves may
be saturated or unsaturated, have been compacted into a fill.
26.3 Degree of saturation and specific volume
Figure 26.2 illustrates the volumes and weights of soil grains, water and air (or gas) in
an unsaturated soil; it is similar to Fig. 5.3 which was for saturated soil.
