Theory to Practice of Unsaturated Soil Mechanics - Unsaturated Soil Mechanics in Engineering Practice - page 7

by the hydrostatic line. The opposite condition occurs during
wet periods.
Grasses, trees, and other plants growing on the ground
surface dry the soil by applying a tension to the pore-water
through evapotranspiration (Dorsey, 1940). Most plants are
capable of applying 1000-2000 kPa (10-20 atm) of tension
to the pore-water prior to reaching their wilting point (Taylor
and Ashcroft, 1972). Evapotranspiration dries the soil result-
ing in desaturation and cracking and an overconsolidation
of the soil mass.
The tension applied to the pore-water acts in all directions
and can readily exceed the lateral confining pressure in the
soil. When this occurs, a secondary mode of desaturation of
the soil mass commences (i.e., cracking). Year after year,
the deposit is subjected to varying and changing environ-
mental conditions. The varying climate produces changes in
the pore-water pressure distribution, which in turn results in
shrinking and swelling of the soil deposit. The pore-water
pressure distribution with depth can take on a variety of
shapes as a result of environmental changes (Fig. 1.4).
Significant areas of the earth's surface are classified as
arid and semiarid. The annual evaporation from the ground
surface in these regions exceeds the annual precipitation.
Figure 1.5 shows the climatic classification of the extremely
arid, arid, and semiarid areas of the world. Meigs (1953)
used the Thornthwaite moisture index (Thornthwaite, 1948;
Thornthwaite and Mather, 1955) to map these zones while
excluding the cold deserts. About 33% of the earth's surface
is considered arid and semiarid (Dregne, 1976).
Arid and semiarid areas usually have a deep groundwa-
ter table. Soils located above the water table have negative
pore-water pressures. The degree of saturation of the soils
is reduced when evaporation and evapotranspiration exceed
precipitation. Climatic changes highly influence the water

content of the soil in the proximity of the ground surface.
The pore-water pressures increase upon wetting, tending
toward positive values. As a result, the volume and shear
strength of the soil are changed. Many soils exhibit severe
swelling or expansion when wetted while other soils exhibit
significant collapse when wetted. Many soils are known for
significant loss of shear strength upon wetting.
Changes in the negative pore-water pressures associated
with heavy rainfalls are the cause of numerous slope failures.
Reductions in the bearing capacity and resilient modulus of
soils in roadways are also related to an increase in the pore-
water pressures. These phenomena indicate the important
role that negative pore-water pressures play in controlling
the mechanical behavior of unsaturated soils.

1.2.1 Quantification of Moisture and Thermal
Boundary Fluxes
Soil mechanics textbooks have been particularly silent on
how thermal and moisture fluxes at ground surface are to
be calculated. Problems involving the flow of water through
soil have generally required that either a “hydraulic head”
boundary condition be imposed or else a “zero flux” (i.e., an
impervious boundary condition) be imposed when solving
seepage problems. However, the ground surface is a bound-
ary across which there is continuous moisture movement.
Moisture is either going up to the atmosphere in the form
of evaporation or evapotranspiration or coming down in the
form of precipitation (i.e., rainfall, snowfall, or irrigation).
Weather stations have dotted the globe collecting large
amounts of data on key variables related to moisture and ther-
mal fluxes; however, little usage has historically been made
of these data in geotechnical engineering. The design of soil
cover systems (i.e., also called alternative covers) and other
near-ground-surface engineered structures has provided an

Evaporation Evapo transpiration

Equilibrium with
water table

Flooding of
desiccated soli

Fissure
desaturation

At time of
deposition

Excessive
evaporation

Saturation

Total stress
(

Pore-air
pressure
(u a )

Pore-water
pressure
(u w )

s )

Figure 1.4 Total stress, pore-air pressure, and pore-water pressure distributions in unsatu-

rated soil.