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CONSTRUCTION WORLD
FEBRUARY
2016
PROJECTS AND CONTRACTS
One of these, the basement for
the new Old Mutual building on
the corner of West and Rivonia
streets (opposite the Gautrain
station) turned out to be one of Franki Africa’s
most challenging projects in the area. While
there were several reasons for this, the three
main challenges were the extensive diabase
dyke on the south west side of the site, the
presence of the Gautrain tunnel, which runs
directly under the site and the integration of
the adjoining Sandown Mews development.
The Gautrain necessitated a challenging
approach for the excavation and ‘expected’
rock profile works directly over its tunnel
alignment along the entire Rivonia Road face.
No blasting was permitted in this region and
there were some anxious moments during
the installation of the soldier piles. As it
turned out the soldier piles were installed
deeper than the final excavation level thereby
providing the first signs that rock might not
be present over the Gautrain alignment.
“This proved to be the case,” says Franki
engineer, Paulo Alves, “with numerous bene-
fits accruing to the client with respect to the
management of the risk in this area.”
He adds that even when blasting in
areas not directly over the Gautrain tunnel,
extreme caution had to be exercised. This was
achieved by constant physical monitoring of
all blasting vibrations in the tunnel.
“In terms of the Sandton Mews 3-level
basement, which also had soldier piles and
anchors as a lateral support solution, while
we were able to remove the piles, and did
so, our overall solution had to include the
geotechnical ‘marriage’ of the two sites i.e.
the physical integration of the Sandown
Mews site into the Old Mutual foundations,”
Alves says.
He adds that the third main challenge,
the diabase dyke, has been a perennial
geotechnical issue in the Sandton area.
“The main problem is that it has weathered
so differently to the rest of the site, which
is granite. When the diabase dyke weathers
it forms a much more clayey material, the
bedrock is generally deeper, it retains more
water and has a localised, elevated phreatic
surface. Also its weathering is very hard and
spheroidal,” says Alves.
Franki is the main contractor working
with Zero Azania as the preferred earthworks
specialists and Alves says that although a
geotechnical report had been presented,
Franki’s experience of ground conditions
when it comes to Sandton basement work,
indicated that an extension of the geotech-
nical investigations was required – a decision
which paid off.
“This revealed the diabase dyke in the
south-west corner of the site, situated directly
over the Gautrain tunnel and confirmed the
Sandton granite profile over the remaining
site footprint. Granitic rock on the north-west
corner was detected at 8 m below natural
ground level, where the basement excavation
extends to its deepest point of a total of
22 m. The shallowest point is situated on the
south-east corner where the excavation is
only 14 m deep.
The design comprised 184 soldier piles
of 600Ø drilled to refusal, 12 500 m
2
of gunite
walls, 860 strand anchors of 600 kN and 750 kN
capacity and 920 soil nails / rock-bolts varying
in length from 3 m to 12 m. The total excava-
tion volume was calculated to be 380 000 m
3
,
of which 80 000 m
3
was in hard rock.
Handover
Site access was granted on 20 October 2014
MOST CHALLENGING PROJECT
In recent months, leading geotechnical solutions provider,
Keller’s Franki Africa, has enhanced its reputation with
several history-making basements in the Sandton area.
>
Completion of inner gunite walls over the Gautrain servitude.