Environmental Engineering Reference
In-Depth Information
70
High natural moisture,
high density
60
50
40
Low natural moisture,
low density
30
20
Wetted, medium density
Wetted, low density
FIGURE 7.73
Typical shear envelopes for loess from
Missouri River basin. (From Clevenger,
W.A.,
Proc. ASCE, J. Soil Mech. Found. Eng.
Div.,
82, 1958. With permission.)
10
0
0 0 0 0 0 0 0 0 0 0
Normal stress (psi)
7.5.4
Volcanic Clays
Origin
Volcanic ash and dust are thrown into the atmosphere during volcanic eruptions and can
be carried hundreds of miles leeward of the volcano. The eruptions of recent history are of
too short a duration to expel substantial quantities of dust into the air and only very thin
deposits settle to the earth, except when close to the source. The ashfall from Mount St.
Helens during the eruption of May 18, 1980 was reported in National Geographic, January
1981, to range from 70 mm near the volcano to 2 to 10 mm about 600 km distant.
In older geologic times, however, eruptions of long duration threw vast amounts of ash
and dust into the atmosphere, which came to rest as blankets of substantial thickness.
These deposits were often altered by weathering processes into montmorillonite clay, one
form of which is bentonite.
Distribution
Bentonite is found in most states west of the Mississippi as well as in Tennessee, Kentucky,
and Alabama. On the island of Barbados, clays thought to be of volcanic origin cover the
surface to depths of a few feet or more. The extensive volcanic clays washed from the
mountains into the basin of Mexico City are discussed in
Section 7.4.5.
7.6
Glacial Deposits
7.6.1
Glacial Activity
General
Glaciers are masses of ice, often containing rock debris, flowing under the force of gravity.
During long, cold, moist periods, vast quantities of snow accumulate and change to ice.
Gravity acting on the mass causes it to undergo plastic flow. The tremendous force of the
moving glacier causes changes in the landscape over which it passes, and leaves many
unique forms of deposition.
