Geoscience Reference
In-Depth Information
the other classifications, but because it emphasizes
time and includes expansive soils - the latter in the
long term causing the most expensive type of land
instability.
10 per cent developing serious damage over their life
span. During the drought of 2002-2003, the damage
bill reached $US600 million.
Expansive soils are produced mainly by clays
derived from two major groups of rocks. The first
group consists of aluminum silicate minerals in
volcanic material that decomposes to form montmoril-
lonite. The second group consists of shales containing
this clay mineral. In Australia, both these rock types
are very prevalent because large areas of the eastern
half of the continent were subject geologically to vol-
canism. Additionally, slow evolution of the Australian
landscape has permitted the widespread accumulation
of large quantities of montmorillonite clay as
cracking
clays
on inland river systems.
Expansion usually takes place when water penetrates
the lattice structure of clay minerals. Two conditions
must be met before swelling can take place. First, it
must be possible for a volume change to occur and,
second, water must be present. An increase in soil
moisture content of only 1-2 per cent is sufficient to
cause expansion. For the volume change to be made,
sufficient clay must be present. A thick layer of
expansive clay has greater potential volume change than
a thin layer. If the load on the clay is high, then com-
pressive forces may exceed the expansive force exerted
by clay upon wetting. Because this compressive load
decreases towards the ground surface, the presence of
Exp
ansive soils
(Hays, 1981)
Expansive soils annually cause more than $US3000
million damage (1989 dollar value) to roads and
buildings in the United States. This cost exceeds the
combined, annual damage bill from all climatic hazards
in this country. Because the process works so slowly,
damage is not always obvious. The actual cost to society
may in fact exceed $US10 000 million annually.
Fifty per cent of the damage occurs to highways and
streets, while 14 per cent occurs to family dwellings or
commercial buildings. Of the 250 000 homes built on
expansive clays in the United States each year, about
10 per cent will undergo significant damage and
60 per cent will undergo minor damage in their life
span. Figure 12.7 maps the extent of expansive soils in
the conterminous United States. Over a third of the
country is affected. The extent of the problem is not
unique to the United States: it occurs on a similar scale
in Australia and on the Russian steppes, where soil
and geological conditions are similar. In Australia,
38 per cent of homes in the southern half of the
continent show evidence of cracking, with again
Great Plains
Great Lakes
Appalachian
Mountain &
Basin
Coastal Plain
0
1000 km
The extent of expansive soils in the United States (adapted from Hays, 1981).
Fig. 12.7
