Geoscience Reference
In-Depth Information
Land Instability
as a Hazard
wetting of expansive clays. This can lead to surface
deformation and eventual destruction of structures
with insufficient foundations.
In this chapter, a broad overview of the wide range
of land instability hazards will be presented. Firstly, the
basic principles of soil mechanics will be described to
show how surface material becomes unstable, and
to point out what factors exacerbate ground failure.
Secondly, each of the main types of land instability will
be examined in turn, together with a description of
some of the major disasters. To limit the coverage
of such a wide topic as land instability, direct, human-
induced and ice-related ( cryogenic ) factors will be
discussed only at a cursory level.
One of the most widespread natural hazards is the
unexpected and sometimes unpredictable movement
of unconsolidated weathered material ( regolith ) or
weathered rock layers near the Earth's surface. Land-
slides and avalanches, while historically not renowned
for causing as large a death toll as other natural disas-
ters such as tropical cyclones or earthquakes, have had
just as dramatic an impact on property and lives. The
sudden movement of slope material is as instantaneous
as any earthquake event but it is a more widespread
problem. In any moderate- to high-relief region
subject to periods of high rainfall, slippage of part or all
of the regolith downslope is probably the most
common hazard. Nowhere is this problem more preva-
lent than in cold regions underlain by permafrost or
ground ice. Of a slower nature, and just as widespread
a hazard, is land subsidence . While much of a land
surface may be stable or even flat, there is a wide range
of natural processes that can generate ground collapse.
Another important aspect of land instability is the
multitude of factors that can trigger ground
movement. Almost all of the hazards presented in this
book can generate secondary land instability problems.
In many cases, associated landslides have contributed
significantly to the large death toll from earthquakes
and cyclones. Even droughts can exacerbate ground
instability through the process of repeated drying and
(Young, 1972; Chowdhury, 1978; Finlayson & Statham,
1980; Goudie, 1981; Aune, 1983; Bowles, 1984)
Stre ss and strain
Consider a body of soil with mass
sitting on a slope
of angle
. This mass is affected by gravity and tends
to move downslope. The effect of gravity is directly
related to the angle of the slope as follows:
effect of gravity on a slope =
= mass
= the slope angle
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