Economic Design of Piling Platforms on Coarse Grained Subgrade-Paper

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The paper presents an economical, performance-based method for designing piling platforms on coarse-grained subgrades. Conventional guidance may produce overly conservative designs when soil friction angles are underestimated or made-ground variability is uncertain.<br /><br />At Barangaroo South in Sydney, a 600 mm imported granular platform was initially proposed for a 2 ha area, at an estimated cost of AUD 1 million. The site already had an asphalt-and-road-base hardstand over heterogeneous reclaimed granular fill. Rather than replace it, the project team tested whether the existing pavement could safely support heavy tracked plant.<br /><br />Seven plate bearing tests were used to assess ground response and develop a non-linear load–settlement model based on small-strain stiffness theory. Because proof rolling at the plant’s ultimate design pressure would have been impractical, the model was used to set acceptance criteria for a lower, practical proof load of 160 kPa. Site-wide proof rolling was monitored with modified Benkelman Beam tests and level surveys.<br /><br />A maximum deflection limit of 5 mm was set for proof rolling. Measured deflections met this criterion, and most permanent settlements were under 6 mm, within the 8 mm limit. One localized area over a backfilled trench exceeded the limit and was remedied with steel road plates.<br /><br />The case study shows that combining targeted plate tests, non-linear stiffness analysis, and comprehensive proof rolling can address uncertainty in variable granular fill without relying on conservative assumptions or testing at ultimate loads. The approach enabled the existing hardstand to serve as the working platform, avoiding construction of a new platform across the site and delivering substantial time and cost savings. The author suggests the method may be transferable to other projects involving granular subgrades.

Keywords

piling platform

coarse-grained subgrade

performance-based design

plate bearing tests

non-linear load-settlement model

small-strain stiffness

proof rolling

granular fill

Benkelman Beam

Barangaroo South