Solving Stress-Deformation Problems with Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 755

1.05

Surcharge pressure
600 kPa
800 kPa
1600 kPa
2600 kPa

1.00

Unloading curve

0.95

0.90

0.85

Inundation

e 0

0.80

(Initial void ratio)

0.75

Natural water content curve

0.70

1 10 100 1000 10,000

Vertical pressure (kPa)

Figure 14.32 Swelling pressures measured when using unloading test procedure from surcharge
pressures of 600, 800, 1600, and 2600 kPa (from Feng et al., 1998).

1200

P 2-2 = swell under load

1000

P 3-2 = C V, corrected

800

P 3-1 = CV, uncorrected
P 1-2 = loading after swell

600

P 1-1 = loading after swell

P 2-1 = swell under load

400

P 4 = Unloading

200

0

1 10 100 1000 10,000

Surcharge pressure (kPa)

Figure 14.33 Relationship between swelling pressure and surcharge pressure when using four
different test procedures (from Feng et al., 1998).

test procedure 2 (i.e., swell under load) gives an upper limit
value of 1050 kPa, and the swelling pressure P 4 from test
procedure 4 (i.e., unloading method) gave the lowest value,
which was between 310 and 400 kPa. For the same surcharge
pressure, the corrected swelling pressure, P 3 − 2 , when using
test procedure 3 (i.e., constant volume) was always larger
than the other swelling pressures except swelling pressure
P 2 − 2 . The swelling pressures measured by the four testing
methods ranged from 340 to 1050 kPa.
Research studies to date have not isolated a single test
procedure for the measurement of swelling pressure. How-
ever, it would appear that the preferred test procedure should

correspond to the methodology that yields an upper bound
value for swelling pressure.

14.4.12 Unloading Soil after Compression
The rebound curve associated with unloading of the soil
can be measured once the soil has been loaded well beyond
the swelling pressure (Fig. 14.34). The rebound curves are
approximately parallel to one another and can generally
be represented as the slope on a semilogarithmic scale
(Schmertmann, 1955; Holtz and Gibbs, 1956; Gilchrist,
1963; Noble, 1966; Lambe and Whitman, 1979; Lidgren,
1970; Chen, 1975).