Page 118 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 113

5.5.9 Color Standards
Most geotechnical reports describe soil color in everyday terms based on visual
impressions, such as light or dark gray, black, tan, brown, buff, red-brown, etc.
A more systematic notation was introduced by Albert Munsell in 1905 and is
widely used in many different fields. Geotechnical engineers should at least be
familiar with the shorthand of the Munsell system, and may wish to use it.

Colors are characterized by letters and numbers indicating (1) a number and
capital letter for hue, or spectral color, (2) a number for value, for the degree of
lightness or darkness, and (3) separated by a slash, a number for chroma, or
intensity of the color. In the Munsell system the three components are sequential:
hue, value, and chroma.

Hues most applicable for soils are designated by R for red, YR for yellow-red, and
Y for yellow, all indicative of oxidizing conditions. GY signifies green-yellow, G is
for green, BG is for blue-green, B is for blue, and N is for neutral or colorless. G,
B, and N hues indicate unoxidized or gleyed soils that have been influenced by a
groundwater table. An example is 10YR 5/4, for a color on the 10YR (yellow-red)
chart with a medium value of 5 and a medium chroma of 4.

The color of a soil will change as much as two or three steps in value and
chroma depending on the moisture condition. For this reason, and because of
the possibility for change after more than a few minutes exposure to air, color
identifications should be made on field-moist samples prior to air-drying.
Dry soils are moistened prior to a color determination but may not regain their
color.

5.6 AGRONOMIC SOIL MAPS

5.6.1 Availability of Soil Maps

Governmental agencies in many countries are actively engaged in mapping soils as
an aid to land evaluation and to improve soil uses in agriculture and in
engineering. In the U.S., soil surveys and maps are prepared on a county-
by-county basis and are available from state universities or county or state offices
of the USDA-NRCS (U.S. Department of Agriculture, Natural Resources
Conservation Service). Most soil surveys stop at city limits, although cities may
contract the studies as an aid to planning. County soil surveys nevertheless show
what can be expected in a given area.

5.6.2 Soil Series
The basic agronomic soil mapping unit is the soil series. Each soil series is given
a name, usually the name of a lake, city, stream, or other geographical entity near

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