Impact from the construction of a working platform and girder lifting operations on pile/column displacements

To facilitate the initial construction of the bridge pier piles on the bank of Nambucca River in NSW Australia, a working platform was constructed over 25 m thick soft alluvial deposits. Subsequent to the pile installation and the construction of columns and headstock, it was determined that the platform needed to be raised by 1.5 m in order to allow for adequate clearance between the boom and the headstock. The raising of the working platform generated sub-soil flow that caused the piles to displace horizontally of up to 50 mm towards the river. Further, the erection of each 160 tonne girder was carried out by a super-lift operation at the raised platform. This exerted enormous bearing pressure on the platform. Surveying indicated that during the first two girder lifts, the tops of the bridge columns had displaced a further 39 mm towards the river, then recovered by about 15 mm after the lifting operation. To mitigate potential further lateral displacement for the remaining two girder lifts, a 5 m wide berm was constructed on the riverside of the pier. Surveying indicated that the placement of this counterweight had effectively limited further movement of the columns to less than 10 mm. One of the concerns with the lateral displacement of the pier was that the bending moment induced in the piles may exceed the moment capacity. Numerical analysis was carried out to back-analyse the measured lateral displacement and to assess the likely future performance of the piles.

This paper focuses on the geotechnical design of the working platform and the predictions of the lateral displacements of the pier piles, with or without the counter weights at the riverside of the pier. The comparison of the predictions and measurements, as well as the permanent effects on the pier piles are also outlined.