Page 107 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
102 Geotechnical Engineering

usually exhibits undesirable engineering characteristics such as high compress-
ibility and rebound, low shear strength, resistance to compaction, and variable
plasticity. In construction operations the A horizon generally is removed and
saved for topping-out and seeding, sodding, or planting for erosion control or
aesthetic purposes.

The A horizon also is the site for intensive weathering to form clay minerals, but
these tend to be carried downward by percolating water to be deposited in the
next lower layer, the B horizon. This process is called eluviation.

In engineering the A horizon is the layer to avoid. A horizon soil that is left on
a slope and buried under fill can cause a landslide, no matter how well the fill soil
has been compacted. Acceptable construction practice involves removing all
vegetation and A horizon material, and cutting horizontal steps prior to placing
soil fill on a hillside. Exploration borings must penetrate all of the way through fill
material to ensure that there is no buried topsoil, trash, or organic material that may
cause future problems. Any construction on such a site also must bear an
additional cost for removal of the poor material. Structures that are built on top
of weak or compressible layers may suffer severe damage and require expensive
remedial measures.

5.2.3 B Horizon

In humid or subhumid climates, clay minerals are concentrated in the B horizon
by eluviation. Most clay-rich B horizons form a valuable reservoir for water for
plants during periods of drought, and clay minerals act as temporary holding sites
for fertilizers.

Two expansive clay B horizons are shown in Fig. 5.1, the highly expansive
Houston and the moderately expansive Holdrege soil series. The Holdrege also
has a transitional BC horizon.

As shown in Fig. 5.2, because soil horizons follow contours of hills, they are
transected by horizontal floors and foundations, resulting in differential uplift
or settlement. Expansive clay B horizons often are removed and replaced with
nonexpansive soil. This can be the same clay stabilized by mixing with a few
percent hydrated lime. Expansive B horizon clay can readily push a basement wall
off its foundation, as shown at the bottom of Fig. 5.3. A failure to recognize and
deal with expansive clay is an invitation for future problems and lawsuits.

Blocky Structure
Soil containing expansive clay typically develops vertical shrinkage cracks during
periods of dry weather. Such cracks can extend a meter or more deep, extending to
a nonexpansive soil layer or to a depth below which the moisture content remains
stable.

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.