Construction World August 2015

PROJECTS AND CONTRACTS

CHALLENGING Cavendish project Over the years Keller’s Franki Africa (Franki) has performed some landmark geotechnical work in Southern Africa and beyond, providing interesting, innovative and often brilliant

The project In the words of Franki senior contracts manager Jim Oldknow: “Having carried out numerous basements in the Claremont area, we are able to anticipate the condi- tions and create the most cost-effective solutions. Because of this we are often the recommended tenderers or we are requested to negotiate the contract to the financial budget, as was the case with the Citadel contract,” he says. Challenges Oldknow adds that the type of basement construction on this project posed a number of challenges both in the general design and in the design of the lateral support require- ments. “The soils on this site consist of a 3 m – 4 m layer of transported silty sand beneath which is high-weathered decomposing granite consisting of weak clay in the form of kaolin. This extends to a considerable depth of 20 m. A perched water table rests on top of these kaolin clays and is visible within the first 2 m from natural ground level. “The basement, consisting of four levels, is approximately 50 m x 50 m with a depth of 12 m at the deepest wall height. One of the major difficulties on this project was the restricted access to the site with only two access points for materials and plant in

and out and both exiting onto the very busy Cavendish and Warwick roads in the Clare- mont CBD. This was exacerbated by the small footprint of the site combined with the depth of the basement. All this severely limited our ability to provide suitable access ramps for the heavy plant,” says Oldknow. “From a geotechnical perspective the central challenge was that the very low shear strength of the kaolin, in which ground anchors had to be installed on this project, has a limiting effect on the maximum possible anchor loads. This, combined with the disintegrating qualities of the kaolin, when exposed to ground water, results in nominal maximum anchor loads of 450 kN,” he says. It is well-known that lateral support movements can be considerable in these soils. And the perched water table certainly didn’t make things any easier. This had to be constantly managed to ensure that the in-situ soils did not become saturated and make the working platform inaccessible or unstable. As the lateral support progressed the perched water table was caught behind the gunite arches and transported to excavation levels by drains. This required constant de-watering in order to prevent the disintegration of the kaolin in slurry/clay. “Key to the success of the project was the creation of a suitably hard standing area and de-watering system

solutions for a vast range of challenging geological conditions. Such conditions contributed to Franki being awarded the Citadel Basement Parking Project in Claremont, Cape Town.

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The four level basement measures approximately 50 x 50 m with a depth of 12 m at the deepest wall height.

Quantity summary • 1 870 m²of lateral support

• 147 No. 500 mm Ø Soldier piles • Foundation Piling 210 No. 600mm Ø Franki DCIS Piles • 21 000m³ Bulk Earthworks

CONSTRUCTION WORLD AUGUST 2015