Geoscience Reference
In-Depth Information
There is an exception to the above occurrence of
liquefaction. Some sodium cation -rich clays - termed
quick clays - will liquefy if salt is leached by fresh
water. This scenario often occurs with glacially
deposited marine clays that are subsequently elevated
above sea level and flushed with fresh water. The salt
or sodium chloride deposited with the clays acts as
electrolytic glue, adhering to the clay particles and pro-
viding cohesiveness and structure to the clay matrix. If
the sodium is leached out or replaced by calcium - a
procedure that can be performed even by the mobi-
lization of free calcium in cement foundations into the
surrounding clay subsoil - the clay dramatically loses
its cohesiveness. These clays then become very respon-
sive to shock waves and liquefy during moderately
intensive earthquakes. Such a process occurred in the
Bootlegger Cove clay underlying Anchorage during
the Alaskan earthquake of 1964 (see Figure 12.4
for the location of major placenames mentioned in this
chapter). The process can also be instigated by the
flushing of evaporite lake clays or highly weathered
clays such as montmorillonite, and is certainly
exacerbated by irrigation, pipe leakage, or simple lawn
watering. The latter circumstances exist today in many
parts of southern California including the San Joaquin
Valley, southern coast ranges, Mojave Desert, and Los
Angeles Basin - all regions that are seismically active.
elastic solids, as plastics, or as fluids. The process of
liquefaction determines the point at which a soil body
behaves as a fluid. The concept of a rigid solid is also
easily explained. No matter how much stress is applied
to a solid, that solid is considered rigid until the stress
exceeds the strength of the material. At this point, the
solid either deforms or fractures. The rate at which
the stress is applied determines how the solid will
behave. For instance, toffee candy, when given a
sharp jolt, will fracture. In this case, the toffee can be
considered a rigid solid. If, however, gentle pressure is
applied to the toffee, it may simply deform. In this
case, the toffee is not rigid under this type of stress. A
solid may be considered elastic if a stress is applied that
results in slow, continuous deformation proportional to
the applied stress before fracturing. The deformation
at this stage is reversible, hence the description of the
body as an elastic. Earth materials behave elastically as
long as the stress changes applied are small enough.
Plas tic solids
The relationship between the moisture content of a
soil and its volume permits three factors to be deter-
mined: shrinkage, plasticity, and liquid limits. These
three terms are labelled Atterberg limits . They refer
mainly to properties of clays and form a continuum,
illustrated schematically in Figure 12.5. The shrinkage
limit is defined as the moisture content of a soil at
which point the soil stays at a constant volume upon
drying. Above the shrinkage limit, soil material will
behave as a rigid or elastic solid because it consists of
Rigi d and elastic solids
Depending upon how stress and strain are related, soil
bodies may behave in four ways: as rigid solids, as
Anchorage
Norway
North Sea
Russian Steppes
Rogers Pass
Aberfan
Wellington
Switzerland
St. Jean-
Vianney
Alps
Wyoming
Kansu
Shensi
Colorado
St. Lawrence Valley
Khait
Usoy
Vaiont Valley
Tokyo
San Francisco
West Virginia
Kobe
San Joaquin Valley
Chiavenna Valley
Great Plains
Kure
Los Angeles
Mojave Desert
Mississippi Valley
Hong Kong
Caracas
Panama
New Britain
Fiji
Solomon
Islands
Mt. Huascazrán
Rio de Janeiro
Australia
Australia
Burning Mt.
Wollongong
Location map.
Fig. 12.4
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