Torrens Road to River Torrens, Australia

As a pivotal part of the corridor, T2T will deliver economic and social benefits by busting congestion for motorists and helping to keep Adelaide moving. Mott MacDonald provided design services for T2T, along with Aurecon and local firm Wallbridge Gilbert Aztec, working closely with the South Australian Department of Planning, Transport and Infrastructure (DPTI) and contractor CPB via the T2T Alliance. The project was funded by the state and national governments.

Opportunity

Once completed, the North-South Corridor will create a 78km link across Adelaide, from Gawler in the north to Old Noarlunga in the south. Projected vehicle numbers and freight traffic data showed this vital transport spine would not be able to deal with future growth. It has now been upgraded to provide a non-stop motorway along its full length. The 4km T2T section was a complex project with a high-risk profile. Major utility services were involved, included high-voltage cables and a substation. DPTI decided the project would be best delivered using the alliance delivery model, which inspired the project team to develop sustainable, game-changing technical solutions.

Solution

A 3km section of lowered, non-stop motorway forming the spine of T2T required 6km of retaining walls. The traditional solution would have been to build cantilevered contiguous piled walls using a significant amount of concrete. But by applying an unconventional unsaturated soil mechanics approach to Adelaide’s stiff clay soils, an optimised design was produced using 3m-7m-long steel nails drilled into the face of the excavated stiff clay walls and encased in grout, with a thin shotcrete surface layer providing stability. The design was tailored to suit ground conditions. Seven typical profiles were developed considering likely geological solutions and in-ground services layouts.

Observational construction methodology complemented these design profiles. The profiles removed design iteration and cut time requirements, and the lightweight, low-carbon soil-nail retaining wall solution provided huge financial savings, while also ensuring structural integrity of the wall and safety for motorists.

Outcome

The innovative retaining wall design has been verified by the University of New South Wales. The T2T motorway opened in September 2018. More than 50,000 vehicles are using this section of the North-South Corridor daily, and it will be able to cater for anticipated growth of up to 115,000 vehicles per day. T2T achieved the DPTI’s project aspirations to create a non-stop motorway while maintaining the amenity of the local rail and road network.

An Australia-leading, and probably world-leading, example of innovative engineering design.

Professor Adrian Russell

University of New South Wales