The Civil Engineering Contractor June 2019 | Page 28

FEATURE
depth, or only a portion of the site
was investigated.
“There are some sites where the
geology and geotechnical conditions
vary dramatically. Johannesburg CBD
is one such case, with an extremely
weathered zone running through the
middle of the CBD called the Grabon
which can make it a horrible place
to build. The rock can be up to 50m
depth with weathered rubbish on
top,” says Green.
Another common error is for
people to make assumptions rather
than investigate. “They’ve done work
on the neighbouring site and assume
the geology will be the same. Each
individual site needs a separate site-
specific and thorough geotechnical
investigation because geologies do
vary. We don’t know everything, and
oddities can pitch up for no reason.”
In Sandton the geology is typically
made up of granite with dolerite
intrusions caused by ancient volcanic
activity. In Pretoria East, you often
find shallow shale rock intruded
by much more deeply weathered
dolerite. You cannot predict exactly
where and how extensive the
intrusions are, explains Green. The
effect of this variation stems from
the fact that different rocks weather
at different rates. “We had one site
in Pretoria where one half of a site
was rock at three or four metres and
the other half residual dolerite clay
down to 20m, with literally a line
drawn half way through the site. Each
side required a completely different
treatment in terms of lateral support
and foundations.”
Piles can go as deep as 80m in
extreme cases such as some of the
massive buildings being built in
Dubai, but in South Africa 20m to
30m is regarded as deep. The cost of
piling to such depths, however, rises
exponentially, says Green – hence the
impact on costing.
Cutting costs in the
wrong places
To try save on costs is admirable,
especially in such tough economic
26 | CEC June 2019
times, but Green cautions that often
people try to save on costs in the
wrong places. The geotechnical report
is one instance, because it amounts to
a tiny percentage of the total cost of a
project, particularly when you factor
in the full lifecycle costs of a project
including maintenance. If something
goes wrong for such a small overall
saving, it will, in hindsight, have been
an extremely poor decision.
“Geotechnical reports are not
always widely understood or
appreciated, and of late appear to have
come to be thought of as an area which
can be readily cut to make savings
by skimping on the geotechnical
report or hiring someone cheap.
Geotechnical investigations are all
about what’s below the ground, and
the only time some people even think
about this aspect of a project is when
something goes wrong,” says Green.
However, this is when things get
really expensive.
“It’s often open to interpretation
what is required: I might think three
boreholes are required while someone
else says two will suffice and a third
person wants 10. It’s highly subjective,
and in every industry there are people
who will come in cheap to get work.
The difficulty with geotechnical
investigations is that it’s difficult to
quantify the benefits up front – on
three jobs the client may be lucky
and have no problems even with sub-
standard geotechnical investigations,
but on the fourth project it may be a
complete disaster, more than wiping
out the benefit of the other three.
At that point everybody says they
wished they’d spent more money on
the geotechnical report.
“The business case for a proper
geotechnical investigation is that the
development goes ahead smoothly
with no cost or time overruns.
There was a development in Sandton
recently where the contractor dug
the basement, and based on the
geotechnical report was expecting
rock at basement level. The project
was costed based on straightforward
conventional foundations on rock.
When this turned out not to be the
case, they ended up over-excavating
and pouring millions of rands of
mass concrete into that basement
to fill up softer areas and allow the
original foundation solution to be
implemented. This slowed them up by
months. There can be severe penalties
for time overruns and these add up
quickly. This can also result in claims
by other contractors,” says Green.
“For a typical development, the
professional team and contractor costs
and programmes the works according
to the geotechnical information – for
instance, if 30 000m 3 of rock was
expected to be moved and becomes
80 000m 3 and takes an extra three
months for the earthworks, this has
massive financial implications and
causes huge fights.” The client can
often eliminate this risk by paying for
a decent geotechnical investigation.
Geotechnical investigations in the
rest of Africa are not theoretically
any different to South Africa, but
Green notes they are extremely
difficult for logistical reason.
Facilities such as quality commercial
labs, which are readily available in
South Africa, are typically scarce,
and shipping large amounts of soil
down to a lab in South Africa can
be problematic at border customs.
The same obstacles apply to getting
work permits and equipment into the
country. Jones & Wagner has done
investigations in much of Africa, but
does rank some countries as ‘virtually
impossible’ due to safety concerns,
excessive logistical difficulties and
infrastructure problems.
Design and construct
In the 1970s and 1980s it was
common for a client contemplating
a development to hire an engineer
to design everything – including
foundations and building – and
put it out to tender to be priced
according to the engineer’s design.
In that model, the engineer had
complete control over the whole
process. Later, specialisation became
more commonplace and geotechnical
contractors came up with their
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