The Civil Engineering Contractor June 2019 | Page 27

FEATURE
Limited access drilling.
N
o contractor wants to
see an adjacent 17-floor
building full of people
collapse into their recently dug
basement excavation, due to faults
with the piling and lateral support.
It’s not neighbourly.
The client is responsible for
seeing that a geotechnical report
is undertaken and supplied to
the foundation contractor for
design and pricing. It should be
undertaken as early as possible before
proceeding with a development as
it is fundamental to the design and
viability of the entire project. It may
not even be an exorbitant expense
– it may be as low as R100 000 to
R150 000 for a geotechnical
investigation in straight-forward
geology for large multi-level office
development in Sandton or Rosebank
– and it saves money in the long run.
However, it is often not done
properly. Shaun Nell, MD of
geotechnical contractor Terra Strata
Construction, says a way to resolve this
www.civilsonline.co.za
is for civil and structural engineers to
play a larger role in developments by
laying down engineering parameters
at the time of tender rather than have
decisions made purely on commercial
considerations.
“Or the quantity surveyor should
advise the client of the appropriate
standards of what is required for a
project before going out to tender.
These professionals are closely
involved in the costing and feasibility.
The contractor will never be able to
influence the client’s thinking during
the feasibility and tender stages – but
the consulting professionals can.”
Trevor Green, geotechnical
engineer and head of the Geotechnical
Department at Jones & Wagner, says
having the geotechnical report early
enables better decisions to be taken on
the development by the technical team.
“You know where the rock is, how
easy it is to get the rock out. Done
the other way round, you may be
planning a multi-storey building with
basements and only when you’ve dug
out the basement do you find out
the rock is inconveniently placed.
This entirely changes the costing,
affecting the viability of the project.
Poor information also makes pricing
difficult: is piling a R1-million or a
R10-million portion of the budget?”
Green says his biggest fear in this
regard is with lateral support. A lateral
support failure can have catastrophic
consequences and could easily
result from poor or inappropriate
geotechnical information being used
for the design.
When an engineer receives a poor
geotechnical report on which to base
the design, that design is consequently
based on assumptions. The design
is then qualified with caveats and
provisos based on the fact it isn’t
clear what is below the ground. Piling
contractors experience problems
usually because of incomplete
information in the geotechnical
report due to an insufficient number
of auger holes or boreholes being
drilled, or drilled to insufficient
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