Bridges
- Publication no: ABC2022-004-22
- Published: 18 November 2022
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In late 2019, a concerned driver reported significant settlement at the Alice River Bridge on the Landsborough Highway (Blackall ñ Barcaldine) in Queensland, Australia. This ten span 140 m long bridge was constructed circa 1976, with each pier founded on four reinforced concrete square driven piles. Pier 4 was the only pier found to be settling, with no settlement recorded during past bridge inspections. Total settlement was later recorded to be over 120 mm. Inspection revealed that the settlement had caused significant damage to the supporting headstock including; concrete cracking and spalling and drummy concrete at all bearing positions. Immediate management measures were introduced to further limit settlement and structural damage to the headstock. Reduced speed and single lane traffic restrictions were implemented while a side-track was constructed.
A project was initiated to:
- investigate the cause of the settlement
- design and construct an underpinning solution
- design and install temporary supports to allow unrestricted access during the wet season, and
- design and implement a procedure for correcting the bridge deck level and repairing the headstock.
Geotechnical assessment of the bridge found Pier 4 to have insufficient pile geotechnical capacity due to shallow pile founding depth, localised scour, and reactive clay soils. Increases in vehicle loadings over time resulted in geotechnical failure and settlement at Pier 4. Once initiated, settlement was potentially accelerated by increased dynamic action due to the change of grade at the pier. Through close collaboration between the Department of Transport and Main Roads, RoadTek, Barcaldine Regional Council and AECOM, the bridge was successfully underpinned and repaired. Underpinning was achieved through casting a pad footing tied into the existing piles. The footing was then used to support a steel jacking frame, which doubled as a temporary support to the superstructure during the wet season. The girders were lifted to correct the bridge deck level and the headstock hydro-demolished and recast.