Page 64 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING

4 Soils That Are Sediments

4.1 SEDIMENTS IN ENGINEERING

4.1.1 Overview
Most soils used in engineering actually are sediments instead of residual soils that
have been developed in place by weathering. Their properties therefore relate to
sedimentation processes instead of weathering.

Sediments are geologically young compared with sedimentary rocks, and have
not undergone significant lithification or hardening. Sand is a sediment, sandstone
a rock. Sedimentary soils may subsequently be altered by weathering, which is
the topic of the next chapter.

Sand and gravel deposited in river bars are readily recognized as sediments,
as is sand that has been shaped by wind into dunes. However, other sediments that
are less obvious are much more plentiful and play a more significant role in
engineering. These include broad swaths of soils left plastered across the northern
parts of the continents by continental glaciers and blankets of dust that were
carried and deposited by winds.

An understanding of sedimentary processes helps to define and explain differences
that impact engineering uses. A sediment that has been crushed under the weight
of a continental glacier most likely will be an excellent foundation soil, whereas
the sand that is loosely deposited in a dune is not. Some soils make excellent fill
materials; others do not. Some soils expand and other soils collapse when wet
with water. Sediments are the most variable of all engineering materials, and
geotechnical engineers must be able to recognize, know, and appreciate their
properties—the good, the bad, the indifferent, and the impossible. In some cases
the best way to deal with a difficult soil is to replace it.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) 59
Copyright © 2007 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.