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HG MY IDEAL HOUSE
74 | AUSTRALIAN HOUSE & GARDEN
W
ell before the slab for any
new home is poured, strong
consideration must be given
to the nature of the ground beneath
it. And this is where the skill of a
geotechnical engineer comes into play.
“There are generally two instances
that lead to prospective homeowners
relying on a geotechnical professional,”
says Andrew Levanthal, a former
national chairman of the Australian
Geomechanics Society. “The first is
building in certain areas of Australia,
including Sydney’s west, around Wagga
Wagga and Adelaide, and in parts of
Melbourne, where the soils are clay-
based and are what we classify as
reactive soils.” This means they’re at
risk of moisture-driven movement,
which could potentially cause a
construction to sink, crack or buckle.
“The second is if the area is prone to
landslide or slippage,” says Levanthal.
“This is a situation we’re seeing more
of, as population pressures cause us to
move further up slopes on the outskirts
of our cities.”
Sometimes homeowners will take
a punt on building a new house on
what may be an unstable site, perhaps
because the view or the privacy
achieved makes it a risk worth taking.
“A geotechnical engineer will come
in and make an assessment that takes
into account the soil structure, geology
and geomorphology of the site,” says
Levanthal. “They will identify any
hazards and then determine how
these hazards can be managed.”
They will then prepare advice for the
building engineer, so the appropriate
foundations or footings can bedeveloped. On unstable sites, deep piers
may be required under the concrete slab
or strip footings. This, of course, adds to
the build cost, so if you have concerns, it
can be a good idea to have a geotechnical
engineer check out a site before you
buy. It may turn out that you’d be better
off purchasing a different block.
A geotechnical engineer may also
be required to determine the best
foundation or substrate requirements
for retaining walls and driveways on
problematic sites. These professionals
look at the appropriate foundations
for everything from houses to oil
rigs and skyscrapers in earthquake
zones. Clearly, the science has come
a long way since the Leaning Tower
of Pisa was built.
Soil samples are collected by boring
or excavation and the geotechnical
engineer will have them analysed in a
laboratory. They’ll also sometimes use
a magnetometer or ground-piercing
radar to find out what is beneath the soil.
So, what what would the services of
a geotechnical engineer cost? It
depends entirely on the site, as well
as on the qualification of the person
who does your report. “Costs could
vary between $1000 and $5000,” says
Leventhal, “which is not exorbitant in
the context of securing the site for, say,
a million-dollar build.” #Will the earth move for you?
Sometimes a building site that looks stable may not be, and it takes
a highly trained professional to identify geotechnical risk, says
Andrew Levanthal. “It’s only when someone with experience walks
on site and interprets what’s beneath the surface that you get an
accurate picture of site stability,” he says. “The site may seem fine
but there could be hazards uphill or erosion issues below.”Property
SOLID ADVICE
If you have grounds for concern about a site’s stability,
call in a geotechnical engineer, writes Harvey Grennan.