The final piece of the multi-decade North-South Corridor, the 10.5 km, $15.4 billion River Torrens to Darlington project – better known as T2D – is the state’s most complex.
Scheduled to open in 2031, the T2D will see three lanes of non-stop motorway operating in each direction between Gawler and Old Noarlunga. Completing 78 km of non-stop, traffic light-free motorway, it will enable drivers to bypass 21 sets of traffic lights as they travel through two separate tunnels connected by an open motorway.
As Malcolm Short, the DIT North-South Corridor Program Delivery Office Engineering Director explained, laying the groundwork for SA’s transportation future is a monumental undertaking with its own unique challenges.
A pressing need
Adelaide remains the only Australian capital city without a motorway-standard road providing easy access to and from the CBD.
The T2D project will be instrumental in reducing commuting times for frustrated motorists – it is forecast to shave up to 40 minutes off the commute in peak hour – and boosting the state’s economy.
“It will also be a safer corridor where the incidence of vehicle crashes on a motorway will be much less than what South Road currently experiences,” Short told create. “It will reduce traffic on parallel routes and improve the performance of the overall network.”
Around 65 per cent of the project will consist of two sets of parallel three-lane tunnels – 4.5 km southern tunnels between Clovelly Park and Glandore, and 2.2 km northern tunnels between Richmond and Hindmarsh.
“The two tunnels will be connected by 2.5 km of lowered motorway that enables the essential ramp connections to and from the surface road network to be made in that section,” Short said. “This will provide … key connections to important destinations, such as the CBD and the airport.”
The challenges of place
Extensive geotechnical investigation guided the construction methodology – “there was nothing surprising”, Short explained – but it’s the location rather than what lies underground that presents another ongoing challenge for the project’s duration.
“A major challenge is the T2D project also involves the construction of major works which involve a lot of plant and equipment in an existing corridor with existing traffic in urban Adelaide. It’s important to ensure the impacts to that existing traffic are minimised.”
In response, two lines in each direction on South Road will be maintained throughout construction to ensure traffic flow.
Engineering innovation
The project will see the first use of large-diameter tunnel boring machines (TBMs) to construct road tunnels in Adelaide.
Three 15 m diameter TBMs, each around 100 m long and weighing about 4500 t, will be fabricated overseas and assembled onsite. Key innovations around the use of these TBMs have been adopted on the project to reduce risk.
“The final design of the TBM launch sites has adopted a shallower alignment compared with the original reference design,” Short explained. “This means that the retention and excavation of the TBM launch sites is reduced in scope, and the height of the retaining walls to the sections of live motorway adjacent to the launch site is also reduced.”
This reduced scope has benefits in terms of enabling an earlier launch of the TBMs than originally scheduled, as well as reducing interaction with groundwater: “There are a number of aquifers in the in the various geological units under Adelaide, so the shallower the works are, the better off we are.”
The project will also minimise the risks associated with tunnel cross-passage construction by using mini TBMs rather than open face excavation. In an Australian first, the cross passages will be constructed using two smaller TBMs of 4.1 m in diameter, ensuring the ground is never exposed.
“It’s a completely sealed process, and there’s no risk of inflow of material into the tunnels or collapse of the cross passage,” said Short, who expects this approach to be used on other projects in Australia in the future.
Further reducing risk, the sump that collects water runoff at the tunnel’s low point will be located directly beneath the tunnels’ road deck rather than requiring additional excavation outside the tunnel cross section, as is commonly the case. This will be enabled by the use of a precast system supporting the road deck.
Construction is underway on the massive concrete manufacturing facility, 20 km north of Adelaide, that will supply more than 55,000 of these tunnel-lining segments for the T2D.
A motorway for the future
Between the two tunnel sections, a 2.5 km stretch of South Road will undergo a complete overhaul with new overpasses, cycling paths and urban green space, including a 20 per cent boost to the existing tree canopy.
Funded by state and federal governments, the T2D is mooted to create 5500 jobs a year across its life span and, like all major road projects, its effects will be felt further than its 10.5 km length.
“[It will] provide better freight access to the port, for example,” Short said. “It will reduce traffic on parallel routes and improve the performance of the overall network and unlock those economic benefits.”
On small-scale and larger projects, engineering innovations are speeding up construction. Learn how it’s impacting project earthworks at this Engineers Australia webinar.
