Piling contract value of A$10.7m
Keller Australian company Vibropile believes close cooperation with structural and geotechnical consultants led to a 30% cost saving with its innovative foundation solution for a 92-storey apartment block in Melbourne
Melbourne’s Eureka Tower, a 300m high 92-storey tower, was the world’s tallest apartment tower at the time of its construction.
Located in the south Australian city’s Southbank area, an unusual feature of the building is its slenderness; it has a height to base ratio of 6 to 1. As a result, concrete in the columns is very high strength while the construction of the foundations also proved to be a challenging task.
Largely these stem from geological conditions at the site, which are complex and difficult - the complexity being primarily due to two layers of basalt above the Silurian Siltstone bedrock that occurs at a depth of about 35m.
The upper basalt was “sandwiched” between layers of clay that varied in consistency from soft to very stiff. The lower layer of basalt was generally overlain by sands and gravels, occasionally underlain by the same sand and gravel strata or occasionally sitting directly on the siltstone bedrock. The ground water level was high, just 2m below ground level.
The basalt was of very high strength, in excess of 200MPa, but was not continuous across the site. To add to the complexity, the loadings imposed on the foundations by the structure were high.
The builder, Grocon Constructors, in conjunction with consultant engineer Connell Mott MacDonald and geotechnical consultant Golder Associates, issued documentation for the tower foundations comprising bored piles socketed into the strong bedrock.
Initially the piling work was priced on the “conforming” solutions specified by the consultants, but the cost considerably exceeded budget.
Founding piles on the upper basalt was not an option because the high loads would result in excessive settlement in the compressible soils underlying the upper basalt.
But the lower basalt was a suitable founding medium, provided it was sufficiently thick. But because of its discontinuous nature and variable thickness, this was difficult to determine.
Golder undertook an extensive geotechnical investigation to establish where the basalt had a minimum thickness of the 5m required to ensure satisfactory performance of the foundation. Where 5m of rock was not present, piles would need to be socketed in the underlying siltstone, below the basalt.
A number of design aspects were discussed between Keller-company Vibropile and Grocon’s consulting engineers, with the intention of reducing foundation costs and also construction times, to enable the project to be completed within the allocated time frame.
The initial foundation scheme was for 76 large diameter bored piles; but these were expensive and time consuming. As an alternative, Vibropile proposed using Continuous Flight Auger (CFA) piles where conditions were suitable for founding on the lower basalt. Where conditions were not suitable for founding CFA piles on basalt, bored piles were socketed into the siltstone.
Based on Golder’s ground investigation, Vibropile developed a solution of 28 large diameter bored piles, of 1200mm nominal diameter, socketed deep into siltstone; and 243 CFA piles of 750mm diameter founded on the basalt.
This final solution reduced the foundation costs by more than 30% and enabled the programme to be achieved. Vibropile puts the success of the project down to co-operation between consulting structural and geotechnical engineers and piling contractor in what were extremely difficult geotechnical conditions.
And it wasn’t just Vibropile that considered the project innovative and novel. In 2003 it received the Victoria Master Builders Association's 'Most Innovative Construction Method' award for its involvement in the project. |
