Instability
events and soil clays: a comment
For many years now the Hong Kong Geotechnical Engineering Office
has been examining active landslide sites in an attempt to identify
various predisposing factors - those that might collectively trigger
instability events. The role of soil clays in triggering instability
in both natural and man-modified slopes is rarely examined in textbooks,
but is now a very 'hot topic' in Hong Kong. Why?
A Brief Background to the Clay Minerals
The sand and silt fractions of soil consist mostly of crystals
weathered out of the rock. Clay minerals represent the finest soil
fraction (<32um = 0.032mm), but unlike the sand and silt fractions,
they are not usually produced by physical weathering of rock. Most
clay minerals form from the chemical weathering of the primary
silicate minerals or crystals which make up the coarser silt
and sand fractions of soils. Feldspars, augites, and micas are examples
of primary minerals which weather to produce clay minerals. Volcanic
glass which is the main component of volcanic ashes also weathers
to produce clays. We term minerals such as feldspar, augite, and
mica "primary minerals", and the clay minerals which form from them
"secondary minerals" or "phyllosilicate clays" (with a crystalline
structure) and "short range order clay minerals" (those with a non-crystalline
structure, i.e. they are amorphous).
Generally speaking, different primary minerals will weather to produce
different types of clays. In addition to the chemical composition
of the primary mineral, soil pH and the soil weathering and leaching
regimes will influence the kinds of clay minerals that form in soils
or residual sediments. Clay minerals are classified according to
their structure, which consist of sheets of silica and aluminium
atoms, with interlayer cations. Common kinds of clay minerals found
in soils are Kaolin (1:1 structure), Halloysite (1:1
structure), Vermiculite (2:1 structure), and Smectite
(2:1 structure). Kaolin, Halloysite, Vermiculite and Smectite are
examples of crystalline clay minerals.
The montmorillonite group of clay minerals (which includes smectite)
is a special group of minerals with an unusual property. These clays
are often called "swelling clays" because of their ability to absorb
large quantities of water in relation to their mass. The water is
absorbed into their sheeted structures and may result in a doubling
of water-holding capacity and consequent increases soil volume.
Montmorillonite clays have been implicated in a number of large
slope failures, but interestingly enough, recent investigations
in Hong Kong indicate that Kaolin clay, a non-swelling clay, may
also be important. Where kaolin clays infill joints or occur as
a laterally-continuous clay pan, they may perch water, so restricting
natural infiltration and drainage. This leads to elevated water
pressures within overlying sediments which may contribute to soil
failure principally through the reduction of the shear strength
of the overburden.
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