Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 735

Conversions between compressibility and elasticity soil
properties for other loading conditions can be found in
Chapter 13. In each case, the elasticity parameter H is not a
constant but forms a functional relationship with changes in
stress state. The functional form can be used to undertake
nonlinear stress-deformation numerical modeling analyses.
The above explanations apply to the limiting planes of the
volume-mass constitutive surfaces. It is also possible to use
the more rigorous volume-mass constitutive model proposed
by Pham and Fredlund (2011a) to define the elasticity
parameters associated with a variety of stress paths.

changes upon wetting cause extensive damage to structures,
in particular light buildings and pavements. In the United
States alone, the damage caused by shrinking and swelling
soils amounts to about $9 billion per year, which is greater
than the combined damages from natural disasters such
as floods, hurricanes, earthquakes, and tornadoes (Jones
and Holtz, 1973). Therefore, the problems associated with
swelling soils are of enormous financial proportions.
The heave potential of a soil depends on soil properties
such as clay content, plasticity index, and shrinkage limit.
Heave potential also depends on the initial dryness or matric
suction in the soil. Several empirical methods have been
proposed that correlate the swelling potential of a soil to
classification soil properties (Table 14.2). Most correlations
make use of the plasticity characteristics of a soil along with
the percent clay sizes to assess the potential expansibility of
a soil. The words “potential expansibility” suggest that no
expansion will be realized by the soil mass unless water
is also made available to the soil. Van der Merwe (1964)
combined the plasticity index of a soil and the percent clay
size particles to provide a classification of expansibility, as
shown in Fig. 14.2. The classification system is useful for
identifying the swelling potential of a soil. In other words,
the correlation reflects one component of the potential for
heave to occur.
Total heave can be written as a function of the difference
between the initial (or present) in situ stress state and some
possible future stress state. The pathway between the initial
and final stress state is defined by a soil property, namely,
the swelling index, C s . Generally the net normal stress state
variable remains constant while the matric suction stress
state variable changes during the swelling process. Changes
in matric suction result in changes in overall volume, which
in turn result in changes in water content. Total heave can be
computed by measuring the in situ matric suction conditions
and estimating (or predicting) possible future matric suction
in the field under specified environmental conditions.
There are several heave prediction formulations that have
been proposed. These formulations differ primarily in the

14.3 APPLICATION TO PRACTICAL
STRESS-DEFORMATION PROBLEMS

An unsaturated soil will undergo volume change when net
normal stress or soil suction changes in magnitude. An
unsaturated soil will experience swelling and shrinking
as a result of matric suction variations arising from envi-
ronmental changes under constant total stress conditions.
Similarly, structural collapse may occur in collapsible soils
when matric suction decreases in the soil.
The methodology for the prediction of heave in a swelling
soil can be described for one-dimensional, two-dimensional,
and three-dimensional situations. The stress history of a soil
is an important factor to consider in understanding swelling
soil behavior. Formulations and example problems of heave
prediction are presented along with case histories. The
factors influencing the amount of heave are also discussed.

14.3.1 Problems Associated with Expansive Soils
Expansive soils are found in many parts of the world,
particularly in semiarid areas. An expansive soil is often
unsaturated due to desiccation drying. However, even under
saturated soil conditions it is possible to have highly
negative pore-water pressures in an expansive soil.
Expansive soils usually contain clay minerals that
undergo large volume changes upon wetting. Large volume

Table 14.2 Probable Expansion as Estimated from Classification Test Data a

Probable Expansion as %

Degree of of Total Volume Change Colloidal Plasticity Shrinkage

(Dry to Saturated Condition) b

Expansion Content (% < 1 μ m) Index, PI Limit, W S

Very high > 30 > 28 > 35 < 11

High 20-30 20-31 25-41 7-12

Medium 10-20 13-23 15-28 10-16

Low < 10 < 15 < 18 > 15

Source: From Holtz and Kovacs (1981).
a After Holtz (1959) and U.S.B.R. (1974).
b Under a surcharge of 6.9 kPa (1 psi).