Geoscience Reference
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layer sandwiched between two silica layers. The layers
are bound together by Van der Waals' bonds. The clay
minerals have a large residual negative charge because
of a charge imbalance in their structure. This leads to
the extensive addition of water molecules to the
structure of the clay. As a result, the cation exchange,
plasticity, and swelling capacity of such clays are high,
making them especially susceptible to deformation and
swelling. As the clays hydrate through progressive
chemical weathering, they begin to lose these
characteristics. Illite [KAl 2 (OH) 2 (AlSi 3 (O,OH) 10 )],
a daughter product of montmorillonite, contains a
higher proportion of silicon atoms, inducing a high, net
negative charge between layers. Potassium ions are
attracted to these sites in the crystal lattice and bind
the silica layers together, thus reducing their swelling
capacity and plasticity. Kaolinite [Al 2 O 3 .2SiO 2 .2H 2 O]
is a highly weathered clay mineral in which all potas-
sium ions have been stripped from the lattice. These
clay minerals consist of a layer of silica and alumina
bonded together by Van der Waals' bonds or hydrogen
ions. Both bonds are strong. There is little substitution
of other atoms or molecules possible in the mineral;
hence, it has low plasticity and low swelling capacity.
content (%)
Schematic representation of Atterberg limits (after Goudie,
Fig. 12.5
interlocked assemblages of grains and particles that
possess a finite strength, termed the yield limit .
However, if a force is applied to that material such that
the rate of deformation is proportional to the amount
of applied stress above this yield limit, then the
material is termed a 'plastic solid'. The difference
between an elastic solid and a plastic solid depends
upon how the material reacts after the stress is
released. In an elastic solid, yield strength has not been
exceeded, and the solid will tend to return to its
original configuration after deformation. In a plastic
solid, the deformation is irreversible because the yield
limit has been exceeded. The yield or plastic limit is
defined by the minimum moisture content at which
point that material, usually clay, can be molded. It is
also the point where the angle of shearing resistance in
clays approaches zero as moisture content increases.
This limit defines plasticity and, for clays, this state can
be maintained over a wide range of moisture contents.
As long as clay has a moisture content below the plastic
limit, it will support objects. However, when the plastic
limit is reached, then the bearing strength of the clay is
greatly reduced and it will begin to deform. The liquid
limit defines the moisture content at which the clay
flows under its own weight. When the liquid limit is
reached, clay behaves like a fluid and easily flows
downslope. This point also defines when liquefaction
occurs, because the material has no shear strength.
Plastic and shrinkage limits are very much a function
of the type of clay material. There are three main types
of clays, dependent upon the degree of weathering of
feldspars or other easily weatherable minerals.
Feldspars progressively weather to montmorillonite ,
illite , and finally kaolinite . Montmorillonite [(Mg,Ca)
O.Al 2 O 3 5SiO 2 .nH 2 O] is composed of one alumina
Intro duction
(Sharpe, 1968; Varnes, 1978; Finlayson & Statham, 1980;
Crozier, 1986)
Land instability can best be described by setting the
various types of failure into a general classification. At
present, land subsidence can be considered as a special
case of land instability because it is more related to
the behavior of the substratum than it is to the stress
applied to material on a slope. Most classifications of
land instability are based upon the type of material
and the type of movement. There are five types of
movement: falls, topples, slides, lateral spreads and
flows. Material can consist of bedrock, consolidated soil
and regolith, loose debris, various mixtures of sediment
and water, and pure water in the form of snow or ice.
Unfortunately, many of the classifications vary in their
emphases. Some are based on geotechnical aspects,
reflecting an engineering orientation; while others
center on processes and morphology, reflecting a
geomorphological perspective. Each classification has
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