Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 739

imperative for the geotechnical engineer to understand the
relationship between the various test procedures when using
the test results for solving practical swelling soil problems.
The prediction of heave in a swelling soil is used as an
example problem. However, the methodology for predicting
settlement and collapse in unsaturated soils is similar.

Evaporation and
evapotranspiration

Cracks and fissures
(unsaturated)

a ′

Pore-water
pressure

14.4.1 Past, Present, and Future States of Stress
The prediction of one-dimensional heave requires informa-
tion on possible changes in the stress state and the soil
property known as the swelling index C s . Oedometer tests
are commonly performed to determine the present in situ
state of stress as well as the swelling index of the soil. The
oedometer test results provide information on the present in
situ state of stress which is translated onto the net normal
stress plane. The in situ total stress in the soil is referred to
as the “corrected” swelling pressure.
The corrected swelling pressure represents the sum of
the overburden pressure and the “matric suction equiva-
lent.” The matric suction equivalent is the matric suction of
the soil translated onto the net normal stress plane. There-
fore, the oedometer test measures the in situ state of stress
(on the net normal stress plane) without having to mea-
sure the individual components of total stress and matric
suction.

b ′ Saturated

c ′

(a)

Saturated

Evaporation and
evapotranspiration

Overburden
Loading

a

b

c

a ′

b ′

c ′

Matric suction
( u a - u w )

14.4.2 Stress State History
The development of an expansive soil over time can be
visualized in terms of changes in the stress state in the lacus-
trine deposit. Changes in stress state occur during geological
deposition, erosion, and environmental changes related to
precipitation, evaporation, and evapotranspiration. The fol-
lowing example illustrates the stress history of preglacial
lake sediments that have evolved to become an expansive
soil over geological time.
Consider a preglacial lake deposit that was initially con-
solidated under its own self-weight. The drainage of the
lake and the subsequent evaporation of water from the lake
sediments result in the drying or desiccation of the sedi-
ments. The term “desiccation” refers to the drying of soils
by evaporation and evapotranspiration. The water table is
drawn below the ground surface over a period of time.
As a result, the pore-water pressures above the water table
decrease and become negative in value while the total stress
in the deposit remains almost constant. In other words, con-
solidation occurs as the effective stresses in the soil increase.
The negative pore-water pressures act in all directions (i.e.,
isotropically), resulting in a tendency for the soil to crack.
With time the water table recedes and overall desaturation
occurs in the upper portion of the profile (Fig. 14.8).
The soil is further desiccated as a result of the growth of
grass, trees, and other plants on the ground surface. Most
plants are capable of applying as much as 1000-2000 kPa
of tension on the water phase prior to reaching the wilting
point of the plants. A high tension in the water phase (i.e.,

(b)

Figure 14.8 Stress state representation of lacustrine sediments
subjected to evaporation and evapotranspiration: (a) pore-water
pressures during drying of lacustrine deposit; (b) stress path fol-
lowed during drying of lacustrine deposit.

high matric suction) causes the soil to have a high affinity
or thirst for water (Fig. 14.8a).
The surface deposit can also be subjected to varying envi-
ronmental conditions. Changing water fluxes (i.e., wetting
and drying) at the surface results in swelling and shrink-
ing of the upper portion of the deposit. Seasonal volume
changes might extend to depths in excess of 3 m, causing
the surface deposit to be highly desiccated.
Figure 14.9 illustrates the changes in the stress state of the
surface deposit during wetting and drying associated with
water infiltration and water evaporation at ground surface,
respectively. Stress state changes occur mainly on the matric
suction plane under almost constant net normal stresses. The
stress paths followed during the wetting and drying pro-
cesses can be illustrated as a series of hysteresis loops on
the soil suction plane.
The natural water content of a deposit in arid and semiarid
regions tends to decrease gradually over time. Low-water-
content conditions in an unsaturated clay deposit are the
first indication that the soil may have a high swelling poten-
tial. Unsaturated soils with a high swelling index C s in a
changing moisture flux environment are referred to as highly