Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 6

to negative pore-water pressures has become widely referred
to as the unsaturated soil zone in geotechnical engineering.
The unsaturated soil zone forms a transition between the
water in the atmosphere and the groundwater (i.e., positive
pore-water pressure zone).
The pore-water pressures in the unsaturated soil zone can
range from zero at the water table to a maximum tension on
the order of 1,000,000 kPa under dry soil conditions (Croney
et al., 1958). The degree of saturation of the soil can range
from 100% to zero. The changes in soil suction result in
distinct zones of saturation. The zones of saturation have
been defined in situ as well as in the laboratory (i.e., through
the SWCC; Fig. 1.3). Table 1.1 compares the terminologies
commonly used to describe saturation conditions in situ and in
the laboratory. Soils in situ start at saturation at the water table
and tend to become unsaturated toward the ground surface.
Soils near the ground surface are often referred to as
problematic soils, but it is the handling of highly negative
pore-water pressures that tends to present the most serious
problem for geotechnical engineers. Common problematic
soils are expansive soils, collapsible soils, and residual soils.
Any of the above soils, as well as other soil types, can also
be compacted, once again giving rise to a material with
negative pore-water pressures.

Table 1.1 Terminology Commonly Used to Describe
Degrees of Saturation in Field and Laboratory

Degree of Field Laboratory

Saturation, S , % Description Description

∼ 100 Saturated Saturated

∼ 90-100 Capillary zone Boundary effect zone

∼ 15-90 Two-phase zone Transition zone

∼ 15-0 Dry zone Residual zone

soil either by evaporation from the ground surface or by
evapotranspiration from a vegetative cover (Fig. 1.4). These
processes produce an upward flux of water in the form of
vapor from the soil. On the other hand, rainfall and other
forms of precipitation recharge the soil through a downward
liquid flux. The difference between these two flux conditions
on a local scale largely dictates the pore-water pressure con-
ditions in the soil profile.
A net upward moisture flux produces a gradual drying,
cracking, and desiccation of the soil mass whereas a net
downward flux tends to wet a soil mass. The depth of the
water table is influenced, among other things, by the net sur-
face flux. A hydrostatic line relative to the groundwater table
represents an equilibrium condition representing zero mois-
ture flux at the ground surface. During dry periods, the pore-
water pressures become more negative than those represented

1.2 MOISTURE AND THERMAL FLUX
BOUNDARY CONDITIONS

Climate plays an important role in determining whether a
soil is saturated or unsaturated. Water is removed from the

35

Transition
zone

30

Air-entry
value

25

Inflection point

20

15

Boundary
effect zone

Residual zone

10

5

Residual
conditions

0

0.1 1 10 100 1,000 10,000 100,000 1,000,000

Soil suction (kPa)

Figure 1.3 Zones of desaturation defined on SWCC.