“This is not just another building,” says Franki Africa’s Paulo Alves of the Sasol premises. “The building will have a massive footprint of about 6 500 m2 and will be 67 000 m2 in total.”
“Our main challenge on this job was to excavate 170 000 m3 – of which 100 000 m3 were decomposed granite rock – to a depth ranging between 14 m and 18 m, and still complete the job in seven months, which is extremely tight from a time perspective. The granite required drilling and blasting, which is challenging in such a densely populated area as Sandton. We were of course considerate of our neighbours who were notified well in advance every time there was to be a blast.”
Alves adds that this was a two-phase contract, which meant that Franki Africa did not have immediate access to the entire site. “It is fairly difficult to work under partial-access circumstances, which were in this case exacerbated by the fact that there were also five large buildings on the site that had to be demolished. We had to work while the demolition was taking place, and on only half the site at a time. This left us very little room to manoeuvre.”
But ultimately this project went off smoothly, starting with the fact that the original soil investigations were, according to Alves, “pretty accurate”, with “no surprises”. The scope of works included 170 000 m3 of excavation, 6 600 m2 of lateral support, 169 (no) 600-diameter soldier piles, 550 strand anchors, 400 rock-bolts and a 6 600 m2 gunite face area. The works also included approximately 85 (no) structural piles and the diversion of existing stormwater and sewerage lines that ran through the site. The Sasol project began in May 2013 and was completed by the December 2013 annual industry shutdown.
The other basement, that of Webber Wentzel’s new head office, while considerably smaller than the Sasol excavation, turned out to be much more challenging. Of the total excavation of 60 000 m3, no less than 20 000m3 were extremely hard blue granitic rock, which required extensive drilling and blasting. According to Alves, the biggest challenge on this job was the diabase dyke that they came across along the Rivonia Road face when they were busy installing the soldier piles.
“This was unexpected and required a completely different piling application and technique, which entailed replacing 40 (no) auger drilled soldier piles with 610 mm diameter percussion-drilled piles, which took in excess of a month to install.
“This, too, was a tight job with respect to time, and in this regard we were put under pressure with the discovery of the dyke and when the demolition of the existing structure took longer than had been anticipated, thus limiting the lateral support and earthworks access on site.” Alves nevertheless feels that the contract, which was started in June 2013 and was completed by mid-November 2013, was still completed in an acceptable timeframe. The scope of works included 60 000 m3 of excavation to a depth ranging between 8 m and 14 m, 96 (no) 600-diameter soldier piles, 240 (no) strand anchors and 250 (no) rock bolts, and a total gunite area of 2 900 m2.