The Kingsford Smith Drive (KSD) Upgrade project widened the road from four to six lanes and created significantly improved pedestrian and cycle facilities around the Brett’s Wharf precinct. The road widening into the Brisbane River is built on deep and very soft to soft Holocene clay soils.
The main cantilevered cycleway structure relies on large diameter rock socketed bored piles, whilst various forms of ground improvement such as Controlled Stiffness Columns and driven timber piles are required to reduce long-term consolidation settlements to within the specified limits. The permanent work's design required the large diameter bored piles to be installed after the completion of ground improvement.
Brett’s Wharf, originally constructed in the late 1920s, was used for commercial shipping activities until the early 1990s. The timber wharf foundation comprised driven timber piles that had been left in place following the demolition of the superstructure. To accelerate the construction programme, it was suggested that the 90-year-old existing Brett’s Wharf timber piles be utilised to support a temporary piling platform. Previous assessments suggested that the rigid inclusions were too widely spaced with too many unknowns regarding their installation to be relied upon to support the heavy load.
Scope
CMW undertook detailed three-dimensional numerical modelling, combined with onsite testing and observations of the existing timber piles, to justify that the 10ft x 12 ft grid of 90-year old rigid inclusions provided sufficient stability for the temporary piling platform.
CMW have worked closely with the construction team on the temporary works to ensure the platform was constructible and would perform satisfactorily. CMW provided full-time site supervision during construction.
Findings/Challenges
Given the deep, soft soils traditional geotechnical stability calculations suggested that the 10 ft x 12 ft grid of existing timber piles were too far spaced to provide sufficient support for the 120t piling rig. CMW interrogated the geotechnical data and found there was a significant strength difference between Holocene soils within the intertidal zone and those offshore (most of the site investigation was ‘offshore’). The more representative geotechnical parameters, combined with 3D numerical modelling, showed that a geogrid reinforced platform would work.
By using the existing timber piles and enabling the bored piling works to occur ahead of the permanent works ground improvement, significant project time and cost savings have been realised.
“The team at CMW provided excellent temporary geotechnical works designs and ongoing support for the Kingsford Smith Drive Brett’s Wharf works. The designs developed were cost-effective and successfully mitigated the challenging geotechnical conditions to provide an adequate factor of safety and peace of mind for all involved” – Lend Lease Engineering, Senior Project Engineer