Independent geotechnical contractor Keller subsidiary Franki Africa last month completed the construction of basement parking at wealth manager Citadel’s premises in Claremont, Cape Town.
The contract was awarded to Franki in July last year and included lateral support, bulk excavation, dewatering and foundation piling.
The type of basement construction on this project posed a number of challenges in both the general design and the design of the lateral support requirements.
“One of the major difficulties on this project was the restricted access to the site, with only two access points for materials and plant both exiting onto the very busy Cavendish and Warwick roads, in the Claremont central business district.
“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 Franki senior contracts manager Jim Oldknow.
He adds that, from a geotechnical perspective, the central challenge was that the very low shear strength of the kaolin soil, in which ground anchors had to be installed on this project, had a limiting effect on the maximum possible anchor loads.
“This, combined with the disintegrating qualities of the kaolin when exposed to groundwater, resulted in nominal maximum anchor loads of 450 kN.”
It is well known in the industry that lateral support movements can be substantial in these soils. The perched water table did not make working easier as 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, he notes.
Further, as the lateral support progressed, the perched water table was caught behind the gunite arches and transported to excavation levels by means of drains; constant dewatering was required to prevent the disintegration of the kaolin in the slurry/clay.
“Key to the success of the project was the creation of a suitably hard standing area and dewatering system at the final level in order to provide a safe working platform for the installation of the foundation piles,” says Oldknow.
According to Franki the choice of piles in any geotechnical project is critical. As a result of the depth of the kaolin, Franki Driven Cast-in-Situ piles were preferred to Continuous Flight Auger (CFA) piles as the more efficient solution. This was mainly because the Franki pile’s founding could be achieved at a shallower depth by forming an enlarged base at the toe of the pile.
He adds that the CFA piles, which were used for the soldier piling, would have to have been installed at a considerable depth given the weak shear strength of the kaolin and would have required the removal of the spoil material from the final basement excavation level.