Civil Engineering August 2022 | Vol 30 No 7

30 August 2022 Civil Engineering Keller Geotechnics SA recently completed the Enablement Works Contract for the Cape Town Station Student Accommodation Project for the Eris Property Group. The site is located on the corners of Strand, Adderley and Old Marine Drive in Cape Town. T he R1.3 billion development will include 3 085-bed purpose-built student accommodation, 6 700 m 2 of modern retail space and a new world- class public square. This new retail precinct will be known as Cape Station. The development will also include a new world-class public square to serve as a landscaped urban garden with artwork and seating to encourage tourist engage- ment and dwell time. INNOVATIVE DESIGN AND VARIOUS PILE TYPES The piling and lateral support works made up the bulk of the contract and was designed in-house by the Keller design team. The lateral support and foundation scheme had to cater for a variety of load requirements, and the optimised design made use of four different pile types and seven different pile sizes. DRIVEN CAST IN-SITU PILES The majority of the piles were of the driven cast in-situ type (known as Franki piles). This pile type is typically very cost effective, has good load bearing capacity, and is well suited to the tricky ground conditions encountered in Cape Town. The piles were installed in two variants: compression only piles and tension/compression piles. The tension capacity was improved by retrofitting a rock anchor which is installed into the bedrock below the toe of the pile. This anchor forms an integral part of the pile. The anchor also enables the pile to transfer tension loads to the bedrock during certain seismic occurrences. ODEX PILES On the northern site boundary, a new culvert was constructed, requiring a relatively deep excavation adjacent to existing services and trees. The pres- ence of the trees precluded the use of a typical batter slope, anchors or a crane requiring high headroom. To satisfy all three constraints, Keller provided an alternative design to the initially planned sheet pile solution. The alternative design used reinforced ODEX piles installed in a row and socketed into the bedrock. This system provides a cantilever to support the embankment. The cantilevered design assures slope stability without the need of anchors, avoiding potential clashes with existing public services. TEMPORARY CASED AUGER PILES In certain areas of the building footprint, the structural loads warranted the ap- plication of temporary cased auger piles. Again, Keller designed a number of these piles to work in compression as well as tension. Tension capacity was achieved in two manners: retrofitted rock anchors and, in some instances, piles socketed into bedrock. Where rock sockets were chosen, the resistance to friction gained from the interface between the pile’s concrete and the rock socket enables tension loads to be transferred into the bedrock. MICROPILES The last type of piles installed were micro piles. These piles use smaller rigs and are particularly useful to install piles to support new structural elements in and around existing buildings. This is achieved by using a miniature drill rig that can fit through narrow door openings and requires very little headroom, making it suitable to manoeuvre and install piles inside buildings and below existing floors and ceilings. This prestigious project highlights the versatility and capability of the Keller team to deliver a complex turnkey design and construct geotechnical project.  Geotechn i cal Eng i neer i ng Strong foundation works for Cape Station Arnold van Taak Project Manager Keller Geotechnics SA Driven cast in-situ piling rigs