Page 204 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 199

and highly sensitive weight measurements show a stepwise weight gain as
surfaces become covered with discrete layers of molecules. Since the size of the
water molecule is known, a simple calculation gives the surface area of the sample.
This is the Brunnaer-Emmet-Teller or BET method and is used in the study of
clays. Teller later was in charge of development of the hydrogen bomb.

Interlayer water is specific to expansive clay minerals, being water that is
adsorbed between clay mineral layers. Considerable interlayer water remains
in place after oven-drying at 1108C, which is sufficient to remove hygroscopic
water. Temperatures in excess of 2008C may be required to remove all interlayer
water.

Structural OH water is not water until clay minerals are heated sufficiently to
break down their crystal structure, normally in the range 500–7008C, depending
on the clay mineral.

Crystalline water is ice.

Evaporated water is part of the soil air, in which the relative humidity may be 100
percent. Drying at the ground surface creates a gradient that draws water up from
below.

10.4 MOVEMENT OF WATER

Gravitational water moves only in response to gravity, which is vertically
downward in an unsaturated medium because gravity has no horizontal
component. As illustrated in Fig. 10.2, when gravitational water encounters a
groundwater table that impedes downward seepage, it moves downward laterally
toward an outlet that is lower in elevation. If a potential outlet is at the same
elevation, gravitational water will not move.

Capillary water may move in any direction including horizontal in response to a
gradient in capillary pressure, referred to as capillary potential or matric potential.
Therefore when a gradient is induced by drying at the ground surface, capillary
water is attracted toward the drier zone. Capillary water also moves in response to
ground freezing, which by lowering the vapor pressure of water in effect dries a
soil out. This movement of capillary water upward into a freezing zone results in
the formation of pure ice crystals that cause frost heave, discussed in the next
chapter.

Hygroscopic and interlayer water move in response to changes in temperature
and relative humidity. Freezing therefore can simultaneously cause shrinkage of
expansive clay by drawing out interlayer water, and expansion by attracting
capillary water that contributes to frost heave.

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.