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CONSTRUCTION WORLD
AUGUST
2015
PROJECTS AND CONTRACTS
The project
In the words of Franki senior contracts
manager Jim Oldknow: “Having carried
out numerous basements in the Claremont
area, we are able to anticipate the condi-
tions and create the most cost-effective
solutions. Because of this we are often the
recommended tenderers or we are requested
to negotiate the contract to the financial
budget, as was the case with the Citadel
contract,” he says.
Challenges
Oldknow adds that the type of basement
construction on this project posed a number
of challenges both in the general design and
in the design of the lateral support require-
ments. “The soils on this site consist of a 3 m
– 4 m layer of transported silty sand beneath
which is high-weathered decomposing
granite consisting of weak clay in the form of
kaolin. This extends to a considerable depth
of 20 m. A perched water table rests on top
of these kaolin clays and is visible within the
first 2 m from natural ground level.
“The basement, consisting of four levels,
is approximately 50 m x 50 m with a depth
of 12 m at the deepest wall height. One of
the major difficulties on this project was
the restricted access to the site with only
two access points for materials and plant in
and out and both exiting onto the very busy
Cavendish and Warwick roads in the Clare-
mont CBD. This was exacerbated by the small
footprint of the site combined with the depth
of the basement. All this severely limited our
ability to provide suitable access ramps for
the heavy plant,” says Oldknow.
“From a geotechnical perspective the
central challenge was that the very low
shear strength of the kaolin, in which ground
anchors had to be installed on this project,
has a limiting effect on the maximum
possible anchor loads. This, combined with
the disintegrating qualities of the kaolin,
when exposed to ground water, results in
nominal maximum anchor loads of 450 kN,”
he says.
It is well-known that lateral support
movements can be considerable in these
soils. And the perched water table certainly
didn’t make things any easier. This had to be
constantly managed to ensure that the in-situ
soils did not become saturated and make the
working platform inaccessible or unstable.
As the lateral support progressed the perched
water table was caught behind the gunite
arches and transported to excavation levels
by drains. This required constant de-watering
in order to prevent the disintegration of the
kaolin in slurry/clay. “Key to the success of
the project was the creation of a suitably
hard standing area and de-watering system
CHALLENGING
Cavendish project
Over the years Keller’s
Franki Africa (Franki)
has performed some
landmark geotechnical
work in Southern Africa
and beyond, providing
interesting, innovative
and often brilliant
solutions for a vast range
of challenging geological
conditions. Such
conditions contributed to
Franki being awarded the
Citadel Basement Parking
Project in Claremont,
Cape Town.
Quantity summary
• 1 870 m²of lateral support
• 147 No. 500 mm Ø Soldier piles
• Foundation Piling 210 No. 600mm
Ø Franki DCIS Piles
• 21 000m³ Bulk Earthworks
The four level basement measures
approximately 50 x 50 m with a depth
of 12 m at the deepest wall height.