Environmental Engineering Reference
In-Depth Information
FIGURE 7.68
Topograpnic expression of ancient sand sheet. (Sand Hills region of Antioch, Nebraska; scale 1:62,500.)
(Courtesy of USGS.)
Engineering Characteristics
Wind-deposited sands are almost always in a loose state of density that results from their
uniform gradation (uniformity coefficient about 1.0-2.0), grain diameters (0.15-0.30 mm),
and relatively gentle modes of deposition.
7.5.3
Loess
Origin and Distribution
Loess originates as eolian deposits in the silt sizes, transported in suspension by air cur-
rents. Ancient deposits were derived from silt beds of glacial outwash, or shallow streams
when the last glaciers were beginning their retreat. Modern deposits originate in desert
environments or are associated with glacial rivers.
The most significant occurrences are those associated with the retreat of the Wisconsin
glaciers. In the United States, loess covers large areas of Nebraska, Kansas, Iowa, Missouri,
Illinois, and Indiana as well as other states in the Mississippi and Missouri River valleys,
Idaho, and the Columbia River plateau of Washington, as shown in
Figure 7.1.
In Europe,
loess deposits occur along the Rhine, Rhone, and Danube valleys and throughout the
Ukraine in Russia as shown in
Figure 7.69.
Substantial deposits also cover the plains regions of Argentina, Uruguay, and central
China. The effects of the disastrous earthquake of 1556 in the loess region of the Shensi
province of China are described in
Section 11.3.4.
Along the Delta River in Alaska today,
winds carry silt grains from the floodplain during dry spells, depositing them as loess.
Silts of similar origin occur around Fairbanks as a blanket ranging from 10 to 100 ft thick
on hilltops to more than 300 ft thick in valleys (Zumberge and Nelson, 1972).
