Sydney Gateway

Services Provided

Geotechnical design of piled foundations for 16 bridges, Plaxis 3D modelling and validation of abutment movements, Temporary works design for launch pads over soft soil and landfill for incrementally launched bridges over Alexandra Canal.

Project Brief

FSG provided geotechnical support to the bridge design team (BG&E) for this project, which included 16 bridges linking the WestConnex motorway network into Sydney Airport.  These included the 850 m long Viaduct now referred to as the Deborah Lawrie Flyover and the iconic twin network arch bridges which traverse the heritage listed Alexandra Canal. For each bridge we assessed ground conditions and optimised pile lengths based on axial and lateral geotechnical analyses, providing inputs into the structural design of the foundations for each structure.  At the abutments of various bridges, we undertook Plaxis 3D modelling to assess the impacts of the settlement of fill from approach embankments on the abutment piles. Works were completed against the backdrop of COVID-19, with our engineers splitting their time between project-office and remote working.

Site Challenges

The project geology consists of a thick horizon of soft and loose quaternary sediments deposited within the Botany Basin, overlying Ashfield Shale and Hawksbury Sandstone.  The foundation design for the bridges therefore had to consider up to 20 m of soft and loose soils, overlying variably weathered founding rock.  Liquefaction was a consideration through the thick deposits of loose sand at some locations.

The Alexandra Canal which traverses the site is a heritage listed brick lined channel which had to be protected from damage, and FSG had to demonstrate that our design solution had no material impact locally on the canal walls.

North of the Canal, two areas of landfill, including the former Tempe Tip had to be considered in the design of foundations for some of the bridges. Whilst ground improvement and design of approach embankments was by the main design joint venture, FSG had to complete our own modelling (including Plaxis 3D assessments) of the impacts of settlement and induced stresses of this fill on the bridge abutment piles.

Significant interaction with other assets and utilities also had to be considered, for example:

• Considerable congestion of underground utilities around piers for the Deborah Lawrie Flyover, in the airport precinct;
• Interaction with Port Botany freight railway lines
• Aviation gas and Sydney Desalination plant main lines