Page 97 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
92 Geotechnical Engineering

Answer: R ¼ 180/10 ¼ 18 m/y. To go 200 m will require 200/18 ¼ 11 years. The ‘‘For Sale’’
sign probably will appear after about 10 years, as soon as they get the drift.

4.8.8 Stabilizing Sand Dunes
Dune migration is a challenge. One response that is used to keep roads open
is to keep a road grader handy, push the sand off, and wait for more. A more
reasoned approach is to stop encroachment by cutting off the source of sand.
This usually involves anchoring the sand with vegetation that can grow to keep
pace with the rate of sand accumulation. Migration also can be slowed by
covering the windward side to prevent erosion or creep of the surface layer. If all
else fails, a cover can be constructed over a road or railroad so that the dune can
walk over the top.

4.9 EOLIAN SILT, OR LOESS

4.9.1 Definition

Loess is eolian dust that, as shown in the background of Fig. 4.9, still may be seen
blowing off glacial outwash to be deposited on nearby terraces and upland. Loess
is mainly silt, having grain sizes that are finer than sand and for the most part are
coarser than clay. The name is Anglicized from the German lo ¨ ss, which literally
means loose. The German pronunciation is approximated by ‘‘lerse’’ but more
common pronunciations are ‘‘luss,’’ ‘‘less,’’ and ‘‘lo-ess.’’

While loess is mostly silt, it also can contain some clay and minor amounts of
fine sand. The silt and sand are mainly quartz and feldspars, and the clay fraction
often consists of expansive clay minerals that usually are not in a sufficient
amount to make the soil expansive. However, weathering processes discussed
in the next chapter can turn it into expansive clay.

4.9.2 Geography of Loess
Silt and clay are carried aloft as clouds of dust that is carried in suspension
and spread across many tens of kilometers. In the U.S. and in Europe, most loess
was derived by winds blowing across exposed bars of braided, outwash-carrying
rivers during Pleistocene continental glaciation. The loess was deposited during
the Pleistocene between 25,000 and 10,000 years ago and therefore was
witnessed by early man. A native American name for the Missouri River valley
is ‘‘valley of smoke.’’

Loess deposits are thickest close to source areas and thin exponentially with
distance. A common assumption is that this reflects a prevailing wind direction,
but that does not explain deposition on both sides of a source. A more logical

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