Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 194

5.2.2 Deformable and Nondeformable Soils
The terms “deformable” and “nondeformable” are being used
mainly with respect to the material response to changes in
soil suction. Sands are essentially nondeformable materials
because of their low compressibility. Sands are essentially
nondeformable even when prepared as slurry. Clays can be
either deformable or essentially nondeformable depending
upon the initial water content, stress history, and nature of the
clayminerals. An initially slurry claywill be highly compress-
ible while compacted, low-activity clay may have relatively
low compressibility. An undisturbed clay soil can have low
to high deformability.
The influence of the applied net normal stress should also
be taken into consideration. It is possible for soils to undergo
volume decrease (i.e., collapse) upon wetting and there is
also the possibility for soils to increase in volume (i.e.,
swelling) upon wetting at the start of the SWCC test. Vol-
ume changes during the initial stages of the SWCC test setup
also need to be taken into consideration in the interpretation
of the final test results. There is justification for using slight
variations in the SWCC test procedure depending upon the
material being tested.
The initial state of the soil specimen should be recorded
whenever the SWCC is measured. The initial state of the soil
has an influence on data reduction and data interpretation.
SWCC datasets should be classified into one of the following
categories: slurry, compacted, or undisturbed (i.e., natural
structured soils or “real” soils).
Figure 5.8 suggests a categorization system for SWCC
datasets. A particular soil initially prepared in different ways
is expected to result in SWCCs with distinctly differing
shapes. The initial state should be recorded and in this way
the dataset will be of increased value when subsequently
used as part of a knowledge-based system (Fredlund et al.,
1997b). Compaction and other complex stress histories make
the estimation of a representative SWCC difficult. Estima-
tions of SWCCs from grain-size distribution data is most
feasible for silt and sand soils as well as soils prepared in
an initial slurry condition (Fredlund et al., 1997a).

Varying degrees of saturation

Initially slurry clay

Natural or
compacted clay

Initially slurried sand

Natural or compacted sand

Water content, w

Figure 5.9 Shrinkage curves corresponding to various initial con-
ditions of soil specimen.

Shrinkage curves provide information on the amount of
deformation or volume change that is likely to occur as a soil
dries. Figure 5.9 illustrates the shrinkage characteristics for
various soils. A primary differentiation can be made between
soils where there is essentially no deformation as soil suction
increases and soils where large deformations take place as
soil suction increases. Figure 5.10 illustrates differences that
can be observed in SWCCs for a single soil with differing
stress histories and methods of specimen preparation.
Theoretical formulations for the flow of water through an
unsaturated soil are generally written in terms of volumetric
water content because a referential element is used during
the derivation of the partial differential flow equation. The
water storage input data should be in terms of volumetric
water content referenced to the instantaneous overall vol-
ume. The numerical modeler should be aware of the need

Soil-water characteristic curve

Change in volume as suction is increased

Essentially no
volume change

Significant
volume change

Initial preparation of soil specimen Initial preparation of soil specimen

Slurry Compacted Natural,

structured soil

Slurry Compacted Natural,

structured soil

Figure 5.8 Classification of SWCCs based on amount of volume change that occurs as soil
suction is increased.