Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 240

an unsaturated soil. An unsaturated soil in the field is often
subjected to significant and frequent changes in matric suc-
tion which result in changes in the unsaturated soil proper-
ties. Changes in soil suction are often more significant than
changes in total stress. The soil undergoes processes of dry-
ing and wetting because of the imposed climate. On the other
hand, the applied total stress on the soil is seldom altered. It is
important to know the SWCC of an unsaturated soil in order
to predict soil behavior associated with drying or wetting.
Croney and Coleman (1954) show SWCCs which compare
the behavior observed for incompressible and compressible
soils. Figure 5.78 compares the SWCCs of soft and hard
chalks, which are considered relatively incompressible. The
drying curves of the incompressible soils exhibit essentially
constant water contents at low matric suctions and rapidly
decreasing water contents at higher suctions. The point where

the water content starts to decrease significantly indicates the
air-entry value of the soil. The data show that the hard chalk
has a higher air-entry value than the soft chalk. The high pres-
sure applied during the formation of the hard chalk bed results
in smaller average pore sizes than in the soft chalk.
Another noticeable characteristic is that the drying curves
for both hard and soft chalks become identical at high matric
suctions (Fig. 5.78). This indicates that under high-suction
conditions both soils have similar pore size distributions.
There is marked hysteresis between the drying and wetting
curves for both soils.
The effect of initial water content on the drying curves
of incompressible mixtures is demonstrated in Fig. 5.79. An
increase in the initial water content of the soil results in a
decrease in the air-entry value. The decrease can be attributed
to the larger pore sizes in the high-initial-water-content mix-
tures. The soils with the high initial water contents drain
quickly at relatively low matric suctions. As a result, the
water content in the soil with the large pores is less than the
water content in the soil with small pores at matric suctions
beyond the air-entry value. The soils with low initial water
content (i.e., small pore sizes) require larger matric suction
values to commence desaturation. There is a slower rate of
water drainage from the pores at higher soil suctions.
The initial dry density of incompressible soils has a sim-
ilar effect on the SWCC, as was illustrated by the initial
water contents. As the dry density of a soil increases, the
pore sizes become small and the air-entry value becomes
higher, as illustrated in Fig. 5.80. The high-density speci-
mens desaturate at a slower rate than the low-density spec-
imens. As a result, the high-density specimens have higher
water contents than the low-density specimens at matric suc-
tions beyond their air-entry values. Hysteresis associated

30

25

Drying from
saturation

Soft chalk
Wetting from
oven dryness

20

D

C

15

Drying

A

B

10

Hard chalk

Wetting

5

0

0.1 1 10 100 1000 10,000

Matric suction ( u a - u w ), kPa

Figure 5.78 SWCCs for soft and hard chalks with incompressible
soil structures (after Croney and Coleman, 1954).

Figure 5.79 Effects of initial water content on the drying curves of incompressible mixtures
(after Croney and Coleman, 1954).