Foundation Design and Osterberg Cell Pile Load Testing in Brisbane, Australia-Paper

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The Waterfront Brisbane project comprises two high-rise towers, a three-level basement beside the Brisbane River, and a deep foundation system of barrettes and bored piles. The site is underlain by soft Quaternary alluvium over weathered Neranleigh-Fernvale Beds, a rock formation whose strength and quality generally increase with depth. Because the soils could not support the development’s heavy loads, major foundations were socketed into rock, with critical tower foundations targeting rock mass unit N2 or better.<br /><br />The design integrated an engineering geological model, seepage assessment, pile and barrette analyses, and staged three-dimensional modelling of interactions among foundations, diaphragm walls and basement slabs. The north tower used barrettes to enable construction to advance while the basement was excavated; other tower and basement loads were supported by bored piles. Predicted tower-column settlements were generally 20–30 mm.<br /><br />An instrumented 1,200 mm bored test pile was evaluated using an Osterberg cell. The test applied up to 32.37 MN in both upward and downward directions. Measurements indicated mobilized shaft resistance of up to about 1.2 MPa and peak end bearing of around 27 MPa. Extrapolation of the test data produced ultimate shaft resistance estimates broadly consistent with empirical methods. Back-analysis estimated ultimate end-bearing resistance at approximately 65 MPa and rock mass stiffness at 5 GPa.<br /><br />Overall, the test results supported the adopted foundation design parameters. However, the test did not mobilize ultimate shaft resistance at every instrument level, and interpreting the measurements required judgement. The authors recommend conducting Osterberg cell tests well before production, particularly in complex ground conditions, to allow sufficient time for data interpretation and design refinement.

Keywords

Waterfront Brisbane

high-rise towers

deep foundations

barrettes

bored piles

Brisbane River

Quaternary alluvium

Neranleigh-Fernvale Beds

Osterberg cell test

rock socketing