Page 230 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 225

Figure 11.9
Moisture contents
sucked up by
different soil layers
in a tube with the
bottom in water to
simulate a
groundwater table.
Dashed lines are
from adsorption
measurements for
the two soils.

11.6.2 Hysteresis

If a sorption experiment such as that outlined above is performed from the
wet side down instead of the dry side up, different curves are obtained because
of the lag effect or hysteresis. A simple model for capillary hysteresis is
called the ‘‘ink-bottle effect’’ and is illustrated in Fig. 11.10. In the left part
of the figure, water rising into a capillary stops where the capillary is
enlarged at the ‘‘bottle.’’ However, if the bottle already is filled, as shown
at the right, capillary attraction allows it to retain water. The ink-bottle there-
fore is reluctant to take in water, but one that is full also is reluctant to
give it up.

The ink-bottle analogy also applies to expansive clay mineral particles as edges of
particles pinch together during drying so that the interior tends to retain water,
and then act to slow down re-entry of water during re-wetting.

11.6.3 Adsorption and Desorption Curves
Capillary hysteresis also becomes apparent by comparing the sorption curve
obtained during draining, called a desorption curve, with that obtained during
wetting, called an adsorption curve. Examples are shown in Fig. 11.11, where it
also will be seen that upon repeating the experiment the second cycle does not
exactly track the first. This is reasonable since the first adsorption-desorption

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