Bypassing 21 sets of traffic lights, Sydney’s new NorthConnex tunnel is set to save commuters up to 15 minutes of travel time, and allows a traffic light-free journey between Newcastle and Melbourne.
But few of the drivers passing through the sleek, futuristic nine kilometre tunnel, which links the M1 Pacific Motorway at Wahroonga and the Hills M2 Motorway at West Pennant Hills, know what goes into creating such infrastructure.
“It’s a world of its own,” Christian Bodner, Technical Director at Aurecon and Tunnel Design Team Leader for the Aurecon-SMEC Joint Venture (JV), told create.
“Many people wouldn’t be aware of all the things that go into it — the drainage, the services, the equipment rooms, the substations… The longer a tunnel gets, the more that needs to go underground to support it.”
Unique from the start
Bodner, who worked on the project for the past seven years, said the NorthConnex project was unique, right from the very beginning.
“The project didn’t have what typically occurs — a documented reference design,” he said.
“It was more an open tender process in which the proponents or shortlisted parties were invited to engineer optimum alignments that connected the M1 and the M2… Our successful bid actually resulted in a longer tunnel, but it had certain advantages.”
Charlie Shackell, Major Projects Director at Aurecon and Project Director for the Aurecon-SMEC JV, was also involved in NorthConnex from the start. He said the JV’s successful bid took significant traffic staging and construction away from the Pacific Highway interchange.
“There’s the Pearces Corner on- and off-ramp, but otherwise it’s a straight motorway-to-motorway connection, which significantly reduces the impact to the community in that area,” he said.
Once they won the project, one of the major challenges for the team came in dealing with Sydney sandstone.
“That sedimentary rock is overconsolidated,” Bodner said.
“[So] what happens at greater depths is that in certain places, you can get what we refer to as ‘stress spalling’ in the tunnel roof.”
An issue that materialises locally, stress spalling can mean the rock fails in shear due to high compressive strengths that develop in and around the tunnel excavation profile.
While engineering and modelling can be used to try to predict the responses, Bodner said it ultimately comes down to repair and re-support during excavation.
“You have to potentially rebuild because there’s limits on movements on rock walls,” he said.
“There’s also a need to repair the installed support system because it may get damaged in the process as well. So there’s a bit of rework to be done retrospectively, which obviously slows down progress and has implications on the overall program.”
Another major consideration was the interface with railway lines, with the project requiring tunnelling under three railway lines, including the Sydney Metro.
“The third [rail interface] was the interesting one — the North Shore Railway Line that crosses over the M1 at Wahroonga,” Bodner said.
“Our southbound tunnel needed to go under the railway line, and the issue there was that we were much shallower, something like 10 or 12 metres separation from the ground surface.”
The same location also interfaced with a Sydney Water Pumping Station, and coincided with a major fault zone in the sandstone.
“There was a geological feature 10 metres wide that came into the tunnel horizon then unfortunately turned and travelled with us for the last 300 metres,” explained Bodner.
“So we had to change the support drastically, from rock bolts to a more passive support system involving structural arch linings and some forward reinforcement.”
Waterproofing was another consideration in building NorthConnex; an owner requirement specified by the operator, Transurban.
“It’s the only recent tunnel that has a continuous water-proofing membrane over its entire 21 kilometre-length,” Bodner said.
“This meant there was a whole suite of activities in and around waterproofing.”
To achieve this waterproofing membrane, which was required to protect the mechanical and electric equipment mounted in the tunnel, a spray-on product was used.
“Any groundwater that penetrates down towards the tunnel then hits that lining and is directed to the sides and drained out of the tunnel,” Shackell said.
“It’s like a big umbrella that shields all the equipment and ensures no drips on the highway or on the equipment in the roof.”
This added an extra challenge to the design and construction.
“You have the primary support, smoothing layer, then the water-proofing membrane,” Bodner said.
“After that, there’s another lining that goes inside that confines the waterproofing layer and is also the layer providing support for all the suspended equipment, such as the cable trays.
“These are all separate activities that happen behind the actual excavation. Due to the long lengths and areas and volumes involved here, it takes some time to install.”
But the challenges didn’t end with the sandstone, rail interfaces and waterproofing. The construction of the tunnel, led by Lendlease Bouygues Construction Joint Venture, also involved some innovation.
“The depth of the tunnel meant … to have [an] inclined access to drive in and out would be extremely long and difficult,” Shackell said.
“This meant we only had the exits at either end, at grade, for taking out spoil. The rest was extracted vertically through conveyors, through shafts at three locations.”
These vertical conveyors proved effective for the job, transporting 2.3 million cubic metres of spoil to the surface, over a vertical distance of 90 metres. All of this spoil was reused, either by the nearby Hornsby Quarry or at other project sites across Sydney.
The construction also involved the use of surface miners for the first time in civil infrastructure in Australia. Typically seen in the mining industry, these machines were used in benching throughout the tunnel construction.
“You tend to build these tunnels in stages,” Shackell said.
“You do a heading at the top, taken out by road headers, but they’re not very good at working beneath their feet… [so the construction contractor] used surface miners, which can do much thicker benches.”
A huge project that involved 122,000 truckloads of concrete and 18.5 million hours of work, Bodner and Shackell said NorthConnex generally had good community support.
“The traffic situation at Pennant Hills Road was really critical,” Bodner said.
“People sometimes get upset about impact during construction, but overall the community was right behind it. I have friends who live around that area, and they kept asking me for the past six years, ‘when is this going to open?’
“It was definitely a missing link and we hope it will serve the community well for many, many years to come.”
Shackell added that he took the opportunity to drive through the new tunnel when it opened on 30 October.
“I was very proud to pay the $16 toll [for a return trip] to drive my kids through,” he said.
“It’s a great link.”
Interested in the design, construction and utilisation of underground space? Learn more with the Australian Tunnelling Society.