Geoscience Reference
In-Depth Information
expansive clays in the upper soil layers increases the
effect of swelling and shrinking. Large buildings posi-
tioned on expansive clays will not usually be affected
because the weight of the structure can prevent expan-
sion. Extensive surficial cracking due to desiccation is
also indicative of expanding clays. In Australia, the
presence of
gilgai
(undulations in the soil surface with
wavelengths of 1-2 m) is indicative of the problem.
People can enhance the effect of expanding clay by
disrupting ground drainage and modifying vegetation.
Drainage from a house gutter or septic tank into an
area of expansive clays can cause that material to
expand, while the drier material under the house does
not. This differential expansion can affect foundations
closest to the wetted soil. The removal of vegetation
can also result in increased soil moisture because
evapotranspiration from trees or shrubs is negated. In
built-up areas where rainfall variation is high, expand-
ing clays can undergo repetitive cycles of swelling and
shrinking, hence putting unequal stress on foundations
over time. The process is quite common in Australia
where drought and flooding rains alternate between
El Niño and La Niña events. This is illustrated
schematically for my house in Figure 12.8. During
droughts, the soil underneath my house will dry out
more slowly than that in the open. The exposed soil
shrinks and the outer foundations of the house tend
to settle before the inner ones. After rainfall, the
exposed soil is wetted and expands. A wetting front
then proceeds under the house causing expansion over
time towards the centre of the house. Trees around a
house can accelerate the drying effect during drought
because of their higher evapotranspiration, while lawn
watering can accelerate the wetting effect. Cracking
of interior walls, especially during a change from
drought to wet or vice versa, is very common. Floors
can become creaky as pilings sink differentially due to
variations in clay or moisture content.
Normal
Conditions
Drought
cracks
appear
high
evaporation
foundations
settle
soil still wet
soil shrinks
soil dries
Wet
more cracks!
soil swells
gilgai surface
water infiltrates
soil still dry
wetting
front
Schematic representation of the effect on a house of alternate wetting and drying of expansive soils.
Fig. 12.8
