Page 123 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
118 Geotechnical Engineering

5.6.6 Deciphering the Names
Soil taxonomy names start with a prefix designating some key soil property used
in classification, followed by more general names. For example, ‘‘Argalboll’’
should be read as Arg-al-boll, for having a clay layer (arg), a white layer (alb),
in a soft (Mollisoll soil order) soil. The taxonomic name therefore carries more
information than the former name for this soil, which was ‘‘Planosol,’’ or flat soil.

Soils are not isolated species, but form a continuum and are transitional one to
another. Subgroup names are added in front to indicate whether a soil fits the
central concept for that group, in which case it is called ‘‘Orthic.’’ If it represents
an intergrade into another great group, that group will be indicated. This kind of
detail is required to satisfy the sensibilities of soil scientists but ordinarily is of
little concern in engineering.

5.6.7 Important Soil Orders in Engineering

The most critical soil orders for engineers are as follows:

Histosol, because it designates peat that is weak, usually wet, highly compressible,
and a nuisance in engineering.

Vertisol, because it designates a considerable depth of highly expansive clay that
can cause severe foundation and other problems.

Aridosol, because it means desert conditions and may include highly expansive
clay concentrated in shallow lakebeds or ‘‘playas,’’ and may include a layer
cemented by secondary carbonates, ‘‘caliche.’’

Mollisols and Alfisols, because they are likely to contain expansive clay B horizons
(subsoils).

Oxisols and Ultisols, because they are leached to such a low density that they may
collapse under a foundation load, the clay mineral is not expansive but nevertheless
can contribute to landslides, and there may be a surficial hard, laterite layer.

5.6.8 Soil Database
Identifications, descriptions, and engineering data for soil series are included in
maps published by the USDA-NRCS. A soil series database is maintained at
http://www.statlab.iastate.edu/soils/index/html/

5.6.9 The Geotechnical Report
Information, data, analyses, recommendations, and conclusions resulting from an
investigation ordinarily are presented in a written report that is discussed in more

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