Bridges

The $400 million Tonkin Gap and Associated Works Project is part of an infrastructure package delievered for Main Roads WA and PTAWA. The contract was awarded to the Tonkin Gap Alliance - BMD, Georgiou Group, WA Limestone, BG&E and GHD. Connecting the previous major upgrades of Gateway WA and NorthLink WA, this section of Tonkin Highway forms part of a vital freight and commuter access route known as the Perth to Darwin National Highway.

A package of ìAssociated Worksî to construct rail-enabling works for the METRONET Morley-Ellenbrook Line (MEL) was included in the project. The package included the construction of road and rail bridges and dive structures to enable trains to enter the median strip of Tonkin Highway at Bayswater and then exit in Malaga, approximately 8 km to the north.

While both dives share a similar concept and size, the design and construction of each required focused attention to account for different ground conditions, groundwater table, future loading requirements and the resource availability during the COVID 19 pandemic. The Southern Dive adopted cased continuous flight auger piles as deep foundation elements. The Northern Dive adopted diaphragm walls with interlocking panels to create a watertight structure in the temporary condition. A WA-first internal watertight 'cassette' structure, consisting of a base slab and internal lining walls integrated with the bridge structure and permanent props.

The paper will discuss the design process, including staged soil-structure interaction dealing with high groundwater levels through geometry, articulation and detailing, the integral bridge structures, rationalisation of design features to optimise structural efficiency, construction challenges at each of the two dives and facilitation of the MEL railway installation. Both dives are currently under construction and are expected to be completed by late 2022.