Page 113 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
108 Geotechnical Engineering

‘‘t’’ means age of the parent material as modified by the rate of erosion. For
example, an active sand dune has excellent drainage and no time for development
of a soil profile, so the profile is nil. A bog may be thousands of years old but have
no soil profile because continuously wet conditions have prevented oxidation and
weathering.

Question: What are the soil parent materials in your area? Information that is not
shown in Fig. 5.4 is available from geological maps and from the internet.

5.4 IMPLICATIONS FROM BURIED SOIL PROFILES (PALEOSOLS)

5.4.1 Origin and Occurrence
Paleosols (ancient soils) are soil profiles that have been buried underneath more
recent deposits. The clay layer that occurs underneath a layer of coal, referred to
by miners as ‘‘underclay,’’ is an ancient paleosol. More common and closer to the
ground surface are paleosols that separate different episodes of sediment depo-
sition from wind, water, or ice. Paleosols are important geological markers
because they separate stratigraphic units of different ages. They also are impor-
tant in engineering because they create perched groundwater tables in otherwise
well-drained soils and can cause landslides. They also can have their good points,
as a perched groundwater table can reduce or prevent seasonal cycling of
expansive clays.

The time interval during which a paleosol has developed may be much longer
than for the modern soil profile, so as a result paleosols tend to be more highly
weathered and have a more pronounced soil profile. The same horizon desig-
nations are used, but with a subscript b: A b ,B b ,C b , indicating that this is a
buried soil.

As in the case of modern soils, the properties of the paleosols depend on factors
involved in their origin, which can be quite different from those that exist today.
For example, a paleosol developed in basalt and subsequently covered by a later
lava flow will be baked like a clay pot. A lateritic paleosol formed under tropical
conditions can occur in today’s temperate climate or even in Antarctica. Paleosols
in alluvial deposits often show a marked increase in the rate of deposition in
recent decades following the introduction of agriculture.

5.4.2 Perched Groundwater
Perched groundwater conditions result when downward percolating rain water
encounters an obstacle such as a clay-rich layer in a paleosol. However, the
perched water will drain out where a paleosol is exposed on a slope by erosion,

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.