Geoscience Reference
In-Depth Information
6.3 FUNCTIONING
The phenomena of swelling and shrinkage depend above all on water
content but are also influenced by:
Emphasis on swelling-shrinkage
clay content and type of mineral; smectites swell more than
illites; as for kaolinites, their volume per unit weight scarcely
changes,
the nature of adsorbed cations; Na-smectites swell more than
Ca- or Mg-smectites,
Natural cements present (Ca, Fe…).
The change in volume of the sample when the water content falls on
drying is followed by optical devices (Hallaire 1991) or mechanical
methods using displacement sensors (Michel
et al.
2000). The
coefficient
of linear extensibility
(COLE) is then determined.
Measurement of swelling and shrinkage on a sample
COLE
= (moist length - dry length)/dry length
The moist state is defined by the field capacity (-340 cm water
column) and the dry state by the wilting point (-15,000 cm water).
The COLE may be as high as 0.13, that is 13 per cent. On remoistening,
swelling will restore approximately the same length at least if the earlier
desiccation was limited (reversibility).
Although the results reported by scientists are variable, we must
agree that shrinkage or swelling of an isolated soil aggregate is
isotropic
,
that is, the same in all directions. Let us assume a COLE of 0.09. This
represents a volume expansion of (1 + COLE)
3
- 1 or almost 30 per
cent. Thus, considerable pressures develop in the soil! They act laterally
(closing of cracks) and in the upward direction; the soil cannot expand
downwards.
The swell-shrink tendency has been modelled on the basis of an
index taking into account the following properties: clay content, CEC,
liquid limit and mineralogical composition (Thomas
et al.
2000).
Working on a sample starting from water-saturated state, we distinguish
three phases in the drying process (Fig. 6.7):
Relationship of shrinkage and water content
