Construction World September 2019

COVER STORY

S andton was officially established in July 1969, deriving its name from a combination of two of its suburbs, Sandown and Bryanston. At that time Sandton was primarily a residential area made up of a few suburbs comprising mainly smallholdings. The construction of Sandton City in the early 1970s was the first major development in the area and other landmark developments quickly followed as a result of the urban decay in downtown Johannesburg, historically the province’s main business hub. The ongoing decline in the Johannesburg CBD led to a veritable exodus of large corporates to Sandton in the 1990s, establishing it as the new financial district of South Africa and enabling it to take over the title of Johannesburg's premier business centre. Specialist geotechnical contractor Franki Africa has made a leading contribution to providing geotechnical solutions for decades of development within the Sandton Central Business District (CBD). “Our first job in the ‘the northern suburbs of Johannesburg’, an area that would later be called Sandton, in 1960 was for a private residence and involved installing 53 piles approximately 4,6 " in length that were hand dug,” recalls Mike Taitz, divisional director at Franki Africa (see top right) . Since then, Franki Africa has been

involved in landmark developments such as the Michelangelo Hotel, the Johannesburg Stock Exchange, Sandton Gautrain Station, Discovery and Sasol Offices. Challenging conditions The Sandton area is underlain by highly undulating bedrock granite with residual granite on top of it. As a result of irregular weathering patterns in this granite, the level of rock is neither constant nor continuous. In addition, the area is punctuated by numerous diabase dykes resulting in the formation of large boulders and hard rock spheroids within the residual soil. Due to the complex geology, a variety of solutions have been developed to counter these challenges and make the developments economically viable. “For the 100 Grayston project, we anticipated the presence of diabase boulders on the southern portion of the site,” says Brett Markides, senior design engineer at Franki Africa. “We had to work closely with the engineers and project team to develop solutions as the work progressed to ensure the project remained on budget and on programme.” The 100 Grayston project is an extension to the Investec Head Office in the heart of Sandton CBD. The development comprises a multi-level office block with a double

Daily record for pile construction for residential building, 1960.

basement level. Franki was awarded the tender for the design and supply of lateral support works and foundation piles to accommodate varying loads up to 12 000 kN. According to Dr Nicol Chang, Technical Director at Franki Africa, it is important that high quality geotechnical investigation be carried out during the planning phases of these projects. It is clearly evident from numerous publications that cost overruns (or construction cost) for geotechnical projects reduces significantly with increased site investigation. “Accurate predictions of design and cost can only be produced when the professional team has full confidence in the conditions of the ground,” says Dr Chang.

Lateral support for 34 m deep basement excavation for Discovery.

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CONSTRUCTION WORLD SEPTEMBER 2019