Nature and Phase Properties of Unsaturated Soil - Unsaturated Soil Mechanics in Engineering Practice - page 32

of mass is maintained. Conservation of mass can also be
written in terms of a volumetric requirement that must be
maintained for a referential element.

2.1.7 Typical Profiles of Unsaturated Soils
Climatic conditions in an area constitute one of the main
factors causing a soil deposit to be unsaturated. Unsaturated
soils or soils with negative pore-water pressures can occur in
essentially any geological deposit. An unsaturated soil can
be a residual soil, a lacustrine deposit, a bedrock formation,
or any other soil or rock type. However, there are certain
geological soil categories that are commonly found to have
negative pore-water pressures. A few examples will illustrate
some of the features common to these deposits.

2.1.7.1 Tropical Residual Soil Profile
Tropical residual soils possess unique characteristics related
to their composition and the environment under which they
have been formed. Most distinctive is the microstructure
which changes in a gradational manner with depth (Vargas,
1985; Brand, 1985). The in situ water content of residual
soils is generally greater than its optimum water content for
compaction. The density, plasticity index, and compressibil-
ity of residual soils are likely to be less than the correspond-
ing values for temperate zone soils with comparable liquid
limits. The strength and permeability of residual soils are
likely to be greater than those of temperate zone soils with
comparable liquid limits (Mitchell and Sitar, 1982).
Behavioral concepts in classical soil mechanics have
mainly emerged in regions where the climate is temperate.
Geotechnical engineers have often encountered difficulties
in modeling residual soil behavior in a reasonable manner.
Engineers are now recognizing that negative in situ pore-
water pressures play an important role in understanding the
behavior of residual soils. Much of the unusual behavior
of residual soils is related to the suction changes imposed
during laboratory testing (Fredlund and Rahardjo, 1985).
A typical deep, tropical weathering profile is shown in
Fig. 2.2 (Little, 1969). Boundaries between layers and
changes with depth are generally not clearly defined.
Numerous systems of classification for residual soils have
been proposed based mainly on the degree of weathering
and engineering properties (Deere and Patton, 1971; Tuncer
and Lohnes, 1977; Brand, 1982).
Zones of completely weathered or highly weathered rock
that contain particulate soil but retain the original rock struc-
ture are termed saprolite. Once the deposit has essentially
no resemblance to the parent rock, it is termed a lateritic or
residual soil. Figure 2.3 shows the profile and soil proper-
ties for a porous saprolite soil from basalt in Brazil (Vargas,
1985). The region has a hot, humid summer and a mild,
dry winter climate with an annual rainfall of less than 1500
mm. The structure is highly porous and in some cases may
be unstable, resulting in collapse upon saturation. The soil

Figure 2.2 Schematic diagram showing typical tropical residual
soil profile (after Little, 1969).

deposits are often unsaturated and the in situ pore-water
pressures are negative.

2.1.7.2 Expansive Soil Profile
Expansive soil deposits commonly occur as lacustrine
deposits or as bedrock shale deposits. In general, expansive
soils have high plasticity (i.e., high liquid limit) and
are relatively stiff or dense as a result of compression
or desiccation. The above description is typical but not
exclusive. The expansive nature of the soil is most obvious
near ground surface where the profile is subjected to
seasonal, environmental changes. The pore-water pressure
may be initially negative and the deposit may be unsaturated.
These soils often have montmorillonite clay minerals as
part of the clay size particles. Monovalent cations absorbed
to the clay mineral (e.g., sodium) can render the soil
highly expansive. Expansive soil deposits and their related
engineering problems have been reported and studied in
many countries.
Extensive areas of western Canada are covered by
preglacial, lacustrine clay sediments that are expansive
in nature. Regina, Saskatchewan, is located in a semiarid
climate where the annual precipitation is approximately
350 mm. A typical soil profile is shown in Fig. 2.4
(Fredlund, 1973a). The average liquid limit of the soil is
75% and the average plastic limit is 25%. The shrinkage
limit is typically 15%. The lacustrine clay is classified as