Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 241

30

0.9

0.8 Desaturation

point

25

0.7

Slope =
de
dw

Natural soil a m

b m

0.6

=

20

0.5

0.4

15

0.3

High initial
dry density

Saturation line

0.2

Low initial
dry density 10

0.1

Assumed G m = 2.7

04 8 2 6 0 4 8 2
Water content, w (%)

5

Figure 5.82 Shrinkage curve for London clay (after Croney and
Coleman, 1954).

0

0.1 1 10 100

Matric suction ( u a - u W ), kPa

Figure 5.80 Effect of initial dry density on SWCCs of compacted
silty sand (after Croney and Coleman, 1954).

an increase in matric suction are essentially equal until the
water content reaches 22%. The shrinkage curve above a
water content of 22% is parallel and close to the saturation
line indicating essentially a saturation condition. The soil
starts to desaturate when the water content goes below 22%,
causing the shrinkage curve to deviate from the saturation
line. The void ratio of the soil reaches a limiting value (i.e.,
e

with the high-density specimens is less than the hysteresis
exhibited by the low-density specimens.
Croney and Coleman (1954) used the SWCC for London
clay (Fig. 5.81) to illustrate the behavior of a compressible
soil upon wetting and drying. The gradual decrease in water
content upon drying does not provide a clear indication of
the air-entry value. In this case, the shrinkage curve of the
soil (Fig. 5.82) must be used together with the SWCC in
order to determine the correct air-entry value of the soil. The
shrinkage curve clearly indicates the compressible nature of
the soil. The total and water volume changes caused by

0.48), corresponding to a water content of 0%. A water
content of 22% in the natural soil corresponds to a matric
suction of approximately 1000 kPa according to the SWCC
(Fig. 5.81).
Some irreversible structural changes causing an irreversible
volume change occur primarily during the first drying process,
as indicated by curve A in Fig. 5.81. Subsequent wetting and
drying cycles follow curves B and C (Fig. 5.81), respectively.

=

110

100

Continuously disturbed soil
(curve G )

90

80

Drying curve E

70

Initially slurried soil (curve D )

60

Wetting curve F

50 Natural soil

(curve A )

40

Plastic limit
Air-entry value
of natural soil

30

Curve C

20

Curve B

10

0

10 6 0.1 1 10 100 1,000 10,000 100,000

Matric suction ( u a - u w ), kPa

Figure 5.81 SWCCs for London clay (after Croney and Coleman, 1954).