Page 122 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 117

from above so that they cannot close when the clay re-expands. The resulting high
horizontal stresses surpass the strength of the soil, so it fails by shearing along
inclined shear planes. Smearing of clay (slickensides) along the shear planes
imparts permanent weakness. Mixing prevents the development of a normal soil
profile and arrests weathering at the smectite stage instead of allowing it to
proceed to more stable, less expansive clay minerals that are more in keeping with
the climate.

The disruption of the soil mass by shearing sometimes creates a hummocky
microrelief called gilgai, tilting trees and fenceposts. High lateral soil stresses make
basements uneconomical because walls push in, and can exhume graves so that
burials are in soil mounds placed above the natural ground surface. The parent
material for Vertisols typically is alluvial clay or highly weathered limestone or
basalt. The climate may be temperate to tropical, but must include a dry season
for cracking and vertical mixing to occur. Severe engineering problems include
tilting and tearing apart of buildings, upward pulling of pile foundations, and
distress to highways. In the older literature these soils are called Grummosol,
Regur, or Black Cotton Soils.

Spodosol means ‘‘wood ash (Greek, spodes) soil’’ and contains an accumulation of
free oxides or humus or both. Formerly these were called Podzols, Brown
Podzolics, and Ground Water Podzols. They have an ash-colored E horizon
(hence the name). Spodosols are soils of the humid, usually coniferous forest
areas. They typify sandy parent materials in these locations, and do not develop
on parent material containing more than about 30 percent clay. Suborders are
defined for wet conditions (Aquods), humus accumulation (Humods), iron plus
humus accumulation (Orthods), and iron accumulation (Ferrods). The humic
acids in Spodosols can prevent setting of Portland cement.

Ultisol means ‘‘ultimate soil’’ and has a variable A horizon over a red or yellow
clayey B, which often contains iron-emented nodules or an iron-cemented layer
called plinthite (hard layers also are called laterite). Ultisols are highly weathered
acidic soils developed under forest, grass, or marsh in pre-Pleistocene deposits.
B horizons are very well developed, and contain nonexpansive kaolinite clay
mineral plus some montmorillonite.

Oxisol means ‘‘oxide soil’’ characterized by a red B horizon of low density, weakly
cemented silt, and clay that is mostly aluminum and iron oxide and hydroxide
minerals plus kaolinite clay. More readily weathered minerals including quartz and
montmorillonite have been degraded or removed. The occurrence of Oxisols is
restricted to tropical and subtropical climates and older (pre-Pleistocene) land
surfaces. They often have a soft or hard iron accumulation layer or plinthite.
Suborders are defined for wet conditions (Aquox), extreme weathering (Acrox),
humid climate (Udox), hot, dry climate (Ustox), and arid climate (Idox).
Aluminum ore, or bauxite, mainly derives from Oxisols. The weathering stage is
kaolinite or gibbsite.

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