Page 227 - Geotechnical Engineering Soil and Foundation Principles and Practice

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

potential in terms of the height of a column of liquid such as meters or feet of
water. Conversions are:
2
2
1 bar ¼ 100 kPa ¼ 100 kN=m ¼ 14:5lb=in: ¼ 1:02 kg=cm 2
¼ 750 torr ¼ 750 mm Hg ¼ 29:5in:Hg ¼ 10:2mH 2 O ¼ 33:5ftH 2 O
2
¼ 10 dyne=cm ¼ 0:987 atmosphere
Another term, pF, is no longer widely used, and is the logarithm of the height
of capillary water in centimeters. The p is analogous to pH and F signifies free
energy.

Example 11.6
The zone of capillary saturation in a soil under equilibrium conditions extends 1 m above
the water table. What is the indicated matric potential in meters of water?
Answer: 1m.

11.5.4 Movement of Capillary Water

When capillary potential is in balance with gravitational potential, the soil
moisture is in a state of static equilibrium so there is no upward or downward
flow. Ordinarily, the matric potential is more negative in dry soils than in wet
ones. However, this is not necessarily true for soils that are texturally different, as
moisture may flow from a relatively dry sandy soil into a wetter clay if that is
dictated by a potential gradient. In other words, the criterion for unsaturated
moisture flow from one soil region to another is the relative potential of the two
regions, and not their respective moisture contents.

In addition, moisture in a field soil seldom reaches a state of equilibrium because
of the relatively slow rate at which capillary water moves in the soil compared
with changing weather conditions. When a capillary tube is inserted into a vessel
of water, the liquid rises in the tube almost immediately, but in the soil,
considerable time is required for a similar movement to take place. Consequently,
when rain falls on the surface, that portion of the precipitation that soaks into
the soil causes a wave of wetness that migrates slowly downward. Before an
equilibrium distribution of moisture is established, however, evaporation may dry
soil near the surface so that the matric potential is reduced and moisture starts to
move back up. Alternating rains and periods of evaporation, plus the effects of
transpiration and variations in barometric pressure and soil temperature, keep
capillary water in motion.

11.5.5 Moisture Contents Stabilized by an Impervious Cover
By preventing normal evaporation and transpiration, a building or pavement is
conducive to establishing equilibrium with the water table below, and as a result,
capillary moisture tends to accumulate in a pavement subgrade after the pavement

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.