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 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. >

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

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

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-

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Completion of inner gunite walls over the Gautrain servitude.

CONSTRUCTION WORLD FEBRUARY 2016