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CONSTRUCTION WORLD
APRIL
2016
GEOTECHNICAL ENGINEERING
In February 2015, the CTICC East
contract was awarded to Franki
Africa by Aveng Grinaker-LTA
Building Cape, following on from
the basement lateral support for the first
phase of the CTICC extension which had been
successfully completed in 2014, with WBHO as
the main contractor.
The CTICC East contract consists of lateral
support in the form of Continuous Flight
Auger (CFA) piles with gunited arches for the
underpass, connecting to the existing CTICC,
together with piling for the access ramp from
the basement, which required piled lateral
support and anchors.
Jet grouting gravity wall
However, Franki Cape Town’s Anton Stoll
points out that conventional lateral support
could not be installed everywhere due to the
highly sensitive service lines feeding the city
centre and overpass headroom restrictions.
“In order to create the necessary lateral
support under this major servitude of cables,
a jet grouting gravity wall was considered to
be the safest solution, with the lowest risk of
damaging these vital services,” he says.
He adds that the existence of all the
electrical cables, water ducts and optic fibres
feeding Cape Town posed the greatest chal-
lenge on this project.
“These lines were so closely laid that
piling between them for lateral support was
just not possible. However, jet grouting allows
the installation of soil cement columns (with
diameters of up to 3,5 m) through a small
diameter borehole of less than 200 mm,
which makes this technology exceptional for
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CTICC
EXTENSION
Maintaining its global competitiveness relative to
other international convention venues, the Cape
Town International Convention Centre (CTICC), which
has reached its capacity, needed to make structural
enhancements. The extension plans were announced
in 2013. Franki Africa was contracted to implement a
range of geotechnical works in various phases.
working around utilities without having to
disrupt their operations,” he says.
With the aid of high pressure cutting
jets, the soil around the borehole is eroded,
rearranged and mixed with the grout. The
columns can then be arranged and combined
in any way to form each type of element.
All service lines had to be identified and
completely exposed beforehand, and to
prevent possible damage during the instal-
lation of the jet grouting columns, they were
wrapped and encased in wooden boxes.
Franki Africa recommended that tempo-
rary 165 mm diameter ducts be installed to
guide the jet grouting process and protect the
service lines during drilling and jetting. These
sleeves were placed at a minimum depth of
500 mm below the service lines.
Thereafter, the exposed lines were
backfilled and compacted up to the existing
ground level to create access for the jet grout
rig. After all the preparatory work, the instal-
lation of the gravity wall could start with the
execution of three test columns to verify the
pre-determined jetting parameters.
“Thanks to the additional support from
Ralf Junker, a highly-experienced super-
visor from Keller Germany, we were able
to complete the trial phase and prove the
maximum required column diameter of
2 600 mm, after which the actual execution
of the gravity wall could begin,” Stoll says.
The drilling rod was guided within the
sleeves and overlapping columns were plotted
and set out to create a mass gravity wall below
the service lines.
During the process, a soil-cement mix
(sludge) was partly flushed out to the top
TOP TO BOTTOM:
Installation of the sleeves to protect services
from damage.
Layout of jet grouting columns.
Exposed trial column with a diameter of 2 600 mm.
The different sequences of the jet grouting process.
Boxed services suspended between the gravity walls awaiting slab construction.
of the borehole through the annular space
between the jet grouting rods.
Stoll says that pumping the sludge to a
temporary storage facility and then disposing
of it always poses a challenge, especially
when working in areas with restricted space.
“Sludge management is often crucial for a
successful jet grouting site, as it can dictate
the production rate. This also applied to the
CTICC project. However, after two weeks we
managed to complete the two gravity walls,
on which a slab could be cast, as well as span-
ning the walls and forming the roof of the exit
and entry ramps on which the service lines
could be supported,” he concluded.
The entire contract consists of piling, jet
grouting and lateral support, which include
oscillator piles for future developments, CFA
soldier pile walls, a Berlin Wall for temporary
support and bored piles for cranes.
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