Page 141 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
136 Geotechnical Engineering

6.4 PROBLEMS WITH AIR-DRYING SOIL SAMPLES

6.4.1 Hysterical about Hysteresis

As shown in Fig. 6.7, there is a considerable hysteresis or lag effect between
expansion on wetting and shrinkage on drying of expansive clay. This can be
attributed to drying constricting the interlayer spacing so that re-entry of water is
slow. If the clay does not fully rehydrate, test results will be biased toward lower
moisture contents. For example, the optimum water content for compaction can
be 6 to 8 percent too low, which can have serious consequences.

Air-dried soils therefore should be thoroughly mixed with water and allowed to
age overnight prior to testing. Atterberg limits used to classify soils also will be
too low and can result in an erroneous classification that is on the unsafe side.

6.4.2 A Special Problem Clay
X-ray diffraction shows that special precautions are necessary to prevent drying of
soils containing ‘‘halloysite’’ clay mineral. This is a kind of kaolinite that has
formed in a continuously wet environment, and has a layer of water sandwiched
˚
between AB layers. This increases the basal spacing to 10 A, the same as the ABA
spacing of mica and illite, but when the clay is air-dried the spacing permanently
˚
decreases to 7.2 A and the crystals roll up like tiny newspapers. Re-wetting
therefore cannot replace the lost water.

Soils containing this mineral therefore should not be air-dried prior to testing.
The problem can be detected by comparing Atterberg limit tests on a soil
before and after air drying: if the limits are different, soils from the site should

Figure 6.7
Stepwise
hydration and
dehydration of Ca-
montmorillonite
and relation to
layers of water.

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