Design Of A Piled Raft Foundation For Twin Gold Coast Towers

Golder Associates Pty Ltd (Golder Associates) Melbourne office was commissioned by Grocon Pty Ltd (Grocon) to assess the feasibility of engineering a piled raft foundation for the twin 50 storey and 40 storey towers of the Oracle.

The twin towers are to be joined by a multilevel basement and a podium structure. A three level basement excavation is to be up to 10 m deep and is to be supported by a driven and anchored sheet pile wall.

The 3 level podium structure and the towers were initially proposed to be supported either on bored or continuous flight auger (CFA) piles some of which were required to resist permanent uplift loads. The site investigation revealed the presence of a stiff clay layer up to 10 m thick which was present above the weathered siltstone which underlies the site at depth. Up to 15 m thickness of dense to very dense sand extends from basement excavation level to the top of the stiff clay.

Initial geotechnical calculations, carried out by others, indicated that a raft founded on the dense sands would settle between 280 mm and 550 mm under the centre of the 50 storey tower. It was therefore recommended that the towers be supported on bored piles founded into the lower siltstone.

A later analysis of the performance of a piled raft carried out by Golder Associates, indicated that acceptable piled raft settlement performance and bearing capacity could be obtained without the need to extend piles to the rock. A more rigorous analysis carried out for final design, including additional geotechnical investigations and laboratory testing of the clay material, confirmed that a piled raft would provide acceptable settlement performance providing the piles were extended to below the clay layer.

The piled raft analysis provided the structural engineers with estimated pile and ground stiffness values that were incorporated into the structural design models used to predict design actions for the raft and the piles.

The design performance of the piles was confirmed by high strain dynamic pile testing and initial on site monitoring of the CFA piles to confirm that the piles were being constructed to the design depth and with full penetration of the clay layer.