Page 105 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
100 Geotechnical Engineering

The time required for development of a soil profile is measured in hundreds of
years. Erosion not only strips away valuable topsoil, it fills nearby road ditches
and increases the rate of accumulation of sediments on floodplains and in lakes
and deltas. Agricultural fertilizers that are carried along with eroded topsoil
can impact aquatic life, and have created an infamous ‘‘dead zone’’ where the
Mississippi River enters the Gulf of Mexico.

5.1.3 Relation to Climate
Weathering and soil profiles are closely related to climatic conditions, for example
being weakly developed in the Arctic, and in areas where there is little rain. On the
other hand, soil profiles developed in tropical climates can extend tens of meters
deep into the in-place rock or sediment.

Soil profiles in temperate zones frequently contain an expansive clay layer subject
to volume changes during normal cycles of wetting and drying. Such soils usually
are easily recognized upon close inspection from crack patterns in the soils.
Expansive clays exert pressures sufficient to lift pavements, floors, and foun-
dations, so their recognition is of paramount importance in geotechnical engi-
neering. Expansive clays in soil profiles have for the most part been ignored in the
engineering literature, and on maps that are indicated to show the distribution
of expansive clays. Such maps may show only geological occurrences such as in
shale, alluvium, and coastal plain deposits. Ironically, the latter occurrences
owe their origin to expansive clays developed in weathered soil profiles that have
been eroded, transported, and deposited many hundreds of kilometers from their
places of origin. Therefore tropical areas where expansive clay is not part of the
weathered soil profile often have extensive deposits in alluvial deltas and rice
paddies.

5.2 DEVELOPMENT OF A SOIL PROFILE

5.2.1 Weathering

Weathering almost inevitably involves chemical changes that contribute to
expansion and disintegration. For example, exfoliation or flaking-off of the
surface of a granite boulder, and its eventual disintegration into sand, are the
result of hydration reactions that cause a volume expansion of the grains
of feldspar.

Chemical weathering converts rock minerals into new minerals, in particular
a distinct class of minerals called clay minerals. Water plays a key role in the
conversions, as OH ions are essential building blocks in clay mineral crystal
structures. As indicated in the preceding chapter, clays can be concentrated by
erosion and deposition as sediments, but with rare exceptions their ultimate
genesis is by weathering.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.