Page 121 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
116 Geotechnical Engineering

can expect severe engineering problems, namely wet conditions and highly
compressible soil such as peat or muck.

Entisol means ‘‘recent soil.’’ Entisols have little or no genetic horizonation—that
is, they have an AC profile, and sometimes a very weak B horizon. Suborders are
defined for wet soils (Aquents), sandy soils (Psamments), arid soils (Ustents), and
humid soils (Udents). Under Aquents, the great group Cryaquent (cold-wet-
recent) includes tundra soils, indicative of permafrost, with attendant engineering
problems.

Inceptisol means ‘‘beginning soil.’’ Horizons in Inceptisols mainly reflect changes
in color and pH, but there is little textural change due to clay movement.
Suborders are defined for wet soils (Aquepts), predominantly volcanic ash or
allophane (amorphous clay) soils (Andepts), dark-colored A horizon soils
(Umbrepts), and light-colored A horizon soils (Ochrepts). Most important in
engineering are Aquepts because they are wet, and Andepts because they may be
sensitive to shock or vibrations. Cryaquepts (cold-wet-beginning soil) are on
permafrost.

Aridisol means desert soil. Aridisols have light-colored A horizons and a variety of
B horizons containing secondary deposits of salt, gypsum, or calcium carbonate.
Suborders are defined for nonclayey (Orthid) and clayey (Argid) soils; the latter
contain highly expansive clays that afford severe engineering problems.

Mollisol means ‘‘soft soil,’’ in reference to the dark-colored, crumbly A horizon
that is excellent for agriculture. Mollisols develop on grasslands and often
have clay-rich B horizons containing expansive clay minerals. Suborders indicate
if they have developed on limestone (Rendolls), or have a white A2 horizon
(Albolls), show wetness plus gleying or mottling (Aquolls), develop in cool or high
altitudes (Altolls), develop in humid climates (Udolls), or in hot, dry climates
(Ustolls and Xerolls). For engineering purposes, the A horizon is stripped away
and saved, and the B horizon should be recognized as probably being expansive.
Soils in this order formerly were called Brunizem, Chernozem, Chestnut, and
related soils.

Alfisol means ‘‘aluminum-iron soil.’’ These soils have a light-colored A2 horizon
over a clayey B. They develop under boreal or deciduous forest, usually in
calcareous deposits of Pleistocene age (i.e., those containing calcium carbonate).
Suborders are defined for wet conditions (Aqualfs), high, cool climate (Altalfs),
humid climate (Udalfs), and hot, dry climate (Ustalfs and Xeralfs). The B hori-
zons are well developed, blocky, and montmorillonitic or expansive. Formerly
these were called Gray-Brown Podzolic, Gray Wooded, and related soils.

Vertisol means ‘‘inverted soil,’’ that is, turned upside down due to churning action
of expansive clay. Vertisols have a black A1 horizon that sometimes extends to
1 to 1.5 m (3 to 5 ft) depth. Vertical mixing occurs when shrinkage cracks fill in

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