Page 106 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 101

Figure 5.1
Three examples of soil profiles. In all cases, A horizons should be removed prior to use in
engineering. The Houston Black is an expansive clay with a B horizon that extends to a depth of
about 2.6 m (9 ft) and causes extensive engineering problems. A somewhat shiny appearance is
caused by slickensides or smearing of clay along shear surfaces. Tunbridge is officially designated
the Vermont state soil. R denotes rock. (From a calendar publication of the Soil Science Society of
America, 2002.)

Because weathering is most intense at the ground surface, there is a transition from
highly weathered soil to unconsolidated ‘‘parent material’’ that may be either a
weathered rock or a sediment. Different layers in the weathered zone constitute the
soil profile. The layers are called ‘‘horizons’’ and are designated by capital letters,
the most common from the ground surface down being A, B, and C. Various
subscripts are used to designate specific properties. For example a subscript A 1
designates a dark-colored topsoil that develops under grassy vegetation whereas
A 2 signifies a light gray or white layer that develops under forest. Some examples
of soil profiles are shown in Fig. 5.1, where the Tunbridge series shows a strong A 2
horizon. The zone of maximum development of the B horizon is called the B 2 .

5.2.2 The A Horizon

The A horizon is the common ‘‘topsoil.’’ (The scientific name is ‘‘epipedon,’’ which
means over-soil.) A horizons often are rich in humus and organic plant residues
and have a loose, loamy texture preferred by gardeners. An A horizon usually is
darker in color than the underlying soil except under forest conditions, where
intense leaching by humic acids derived from forest litter creates an ash-gray or
white layer. This is particularly true under conifer forest, where Russian soil
scientists who pioneered the classification of soils named these ‘‘podzols,’’ which
means ash. A white or ashy layer therefore is diagnostic of development under
forested conditions and may be designated an E (for eluviated) horizon.
An example is shown at the right in Fig. 5.1.

The thickness of the A horizon typically ranges from a few centimeters (inches) to
about 0.6 m (2 ft). Because of its high content of organic matter the A horizon

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