Page 117 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
112 Geotechnical Engineering

Mottled or mixed soil colors often indicate a fluctuating groundwater table, in
which case the highest elevation of the gray represents the highest former ground-
water level. This is particularly useful for determining a need for tile drains, for
example to prevent a landslide or flooded basements.

Question: What color sequence might be expected with a perched groundwater
table?
Hint: Is the soil below a perched table saturated?

5.5.7 Minerals that Give Soil Its Color

Soil colors mainly depend on the colors of the coatings of the mineral grains
rather than the colors of the grains themselves. Humic acids give the A horizon
a dark color. Iron oxide coatings give soil a red, red-brown, or yellowish-brown
color. Only a small percentage of iron compounds is sufficient to give a red or
yellow-brown color. Red-brown color is attributed to an oxide mineral, hematite,
and yellowish brown to a hydroxide mineral, goethite. Reddish-brown colors
indicate another iron hydroxide mineral, lepidocrocite. Black specks are mainly
manganese oxides.

Reduction of iron from the ferric to ferrous state requires dismantling the mineral
crystal structure and removal of oxygen, which can occur only in a strong
reducing environment. That is why red colors can persist in rocks for thousands
or millions of years. Reduction can occur rapidly in the presence of organic
matter, and occurs under asphalt pavements and in the presence of natural gas
leaks. This can have forensic significance in the case of a fire or explosion, by
indicating whether a gas pipe has recently been broken or if it has been leaking
for some time.

Gray or white colors indicate that a soil has been leached of its iron oxide
and hydroxide coatings so that one sees mineral colors that normally are dominated
by quartz.

In arid and semiarid areas a white, crusty appearance indicates secondary deposits
of calcium carbonate or, in arid climates, gypsum or rock salt. These deposits also
can develop artificially in irrigated areas such that they eventually become sterile
for agriculture.

5.5.8 Color Changes During Soil Sampling
Gray, greenish, or bluish hues indicating reducing conditions rapidly change to
light gray or brown when exposed to air. Color identifications therefore should be
made immediately after sampling in the field. Samples that are color-coded in the
laboratory should be broken open to examine the interiors.

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.