Page 203 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
198 Geotechnical Engineering

Permanent saturation tends to cause gray or even a bluish or green soil color
indicative of reducing conditions. During periods when the groundwater table is
low, infiltrating water carries oxygen into the gray soil so that the soil becomes
mottled with brown colors. Water infiltrating through root channels draws
soluble iron compounds toward the channels where the iron oxidizes and becomes
concentrated as rust minerals. This also occurs where root channels penetrate
a zone of capillary saturation. Soil color therefore is an important indicator
of former levels of a groundwater table that may be experienced again in the
future.

For example, landslides usually stop during a dry season and give a false sense of
security when all that is needed for them to start again is more rain. The design of
corrective measures should be based on the highest anticipated level of the
groundwater table, which can be indicated by the soil color.

10.3 CLASSIFICATION OF SOIL WATER

Gravitational water, or free water, is water that flows in response to gravity. It is
this water that flows into wells or drains.

Capillary water is held by surface tension forces in small pores or capillaries. One
characteristic of capillary water is negative pore pressure relative to atmospheric
pressure. Negative pore pressure equals suction, since it in effect sucks water up
above the phreatic surface.

All capillary water, including that which occurs in the zone of capillary saturation,
is said to be in the capillary fringe. The upper limit of the capillary fringe is
indistinct and may be at the ground surface. It is emphasized that the water itself
is not different; only the dominant forces retaining the water are different.

Hygroscopic water or adsorbed water is that which is held by adsorptive forces on
mineral surfaces. Careful measurements indicate that sliding friction between
mineral grains involves adsorbed water. This is reasonable because of the array of
positive and negative forces exposed at the surface of an ionic crystal. Oxygen ions
dominate mineral surface areas, which therefore tend to carry a negative charge.
In clay minerals the negative charge is greatly increased by ionic substitutions
within the crystal structure.

Hygroscopic water may be defined as water that is retained after air-drying and
lost upon oven-drying at 1058C. The amount of hygroscopic water therefore
varies depending on temperature and relative humidity.

Adsorbed water can be removed by heating or by a high vacuum. After
evacuation incremental amounts of water are allowed back into the system,

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.