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Air-entry point
1
2
qs: water content
at saturation
e: water content at
maximum swelling
a: water content at
air-entry point
3
q
Air
Liquid
q
Solid
Water
content
0
qa
qe
qs
Fig. 6.7 Soil Shrinkage Characteristic Curve, SSCC, established in a swelling-clay soil
based on the Three Straight Lines Model , TSLM (Crescimanno and Provenzano 1999). There
are more refi ned models but they are not considered here. In a Vertisol, the void ratio goes
from 1.6 (expressed as the ratio to the solid mass) to 0.7 or 0.4.
￿ Phase 1: structural shrinkage. Water first leaves the cracks and
coarse pores and is replaced by air. The reduction in volume
of the sample is slight. Actually this phase of shrinkage is
often absent in samples without cracks: qe and qs are then the
same and the model is reduced to two straight lines described
below.
￿ Phase 2: normal shrinkage. The clay then loses the water that sur-
rounds the particles and is held to them by electrostatic forces.
The reduction of the quantity of water leads to a corresponding
reduction in volume of the soil sample.
￿ Phase 3: zero shrinkage. The particles, till now separated by
continuous water films, are locally brought in contact with one
another. From here on ( air-entry point ), the material has lost
almost all possibility for further shrinkage. The water lost is
replaced by air. The total volume of the sample remains constant.
Actually, this third stage, typical of kaolinites, is much more
subdued in smectites, which shrink more.
Shrinkage, if great and caused by strong drying, becomes partially
irreversible. Rewetting does not enable return to the original volume.
Slickensides correspond to the fracture lines, a kind of reverse faults
generated when the soil tends to swell in all three directions, but
with only one possibility for the final expansion, upward, as shown in
Fig. 6.8 (Wilding and Tessier 1988). The clay is found smoothed on these
Swelling-shrinkage at the profile scale
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