Page 217 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
212 Geotechnical Engineering

remotely connected groundwater. Artesian pressure is analogous to pressure in
a water faucet that is remotely connected to a water tower.

11.3 NEGATIVE PORE WATER PRESSURE

11.3.1 Capillary Rise
A simple demonstration of negative pore water pressure is by analogy to water
retained by capillary forces in a small clean glass tube, as shown in Fig. 11.1.
Generally, the smaller the diameter of the tube, the higher the water will rise.
A similar situation exists with soils; the finer the soil and the smaller the void
diameter, the higher the capillary rise. As atmospheric pressure at the phreatic
surface is the base for measurement, pressure in the capillary is negative and
decreases with height above that surface, as shown in the figure.

11.3.2 Surface Tension
Capillary rise is attributed to surface tension of the water, which is the result of
unbalanced attractive forces at the interface between water and air. This is
illustrated in Fig. 11.2, where the low attractive force between water and air
molecules results in higher dipolar attractions in the water surface.

Surface tension exists at any interface between solid, liquid, and gas, but its effects
are most obvious at liquid-gas interfaces, where surface tension causes the surface
of a liquid to behave as if it were covered with a tightly stretched membrane. The
surface sometimes is considered a separate phase called a ‘‘contractile skin.’’
A membrane analogy is an appropriate and useful concept in helping to explain

Figure 11.1
Capillary rise and
negative pore
pressure.

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