Australia’s longest road tunnel on track for 2020 completion

Driving from Newcastle to Melbourne without having to encounter a set of traffic lights is just one benefit of the NorthConnex tunnel. Roads & Infrastructure Magazine explains.

It had been mooted since 1977. Put a direct link between the M1 and M2 motorways in Sydney. Not only would it ease the congestion on one of the busiest arterial corridors in the city – Pennant Hills Road – but would save trucking companies a fortune in maintenance. No longer would they have to change gears when stopping at any of the 21 traffic lights, nor will those gear changes belch diesel into the atmosphere of the surrounding suburbs. Finally, 38 years later, in 2015, it was announced the link would go ahead after a lot of consultation between the community, as well as local and state authorities.

With an estimated 50,000-80,000 daily users along Pennant Hills Road, it is hoped that the daily usage will be reduced by at least half. All trucks must use the tunnel, with two exceptions; those making deliveries in the local suburbs, and those not permitted in the tunnel in line with road regulations such as dangerous goods.

The $3 billion dual carriageway is almost near completion and is a successful example of an unsolicited bid by Transurban, delivered by Lend Lease and Bouygues Construction joint venture (LLBJV). Government contributions include $573 million from the NSW State Government and $412 million from the Australian Federal Government.

LLBJV Construction Director, Vince Newton, has worked on many tunnel projects over the past 20 years. He’s enjoying his time on this one. He was attracted to the project, mainly because of the challenges it created.

“I’ve worked in a number of tunnelling jobs and this is probably longer than the Cross-city, Eastern Distributor and M5 East tunnels combined,” he says. “The scale of this job makes it unique among other projects.”

“It’s unique as far as most tunnelling jobs go in that three-quarters to 80 per cent of the excavation has been done from shafts,” he says. “At our Wilson Rd site, the shaft was 90 metres deep, which was quite a challenge – the sinking of it, and then the excavation out of the shaft. Then there was the Trelawney Shaft, which was 56 metres deep.”

They did shafts mainly due to the length of the tunnel. There is a mainline connection at the north end of the tunnel in the suburb of Wahroonga. Another is at the southern end in the suburb of Pennant Hills. Then there are ramps at the southern end, which are very close to the mainline connections. The other ramps are up at Wahroonga on Pearces Corner. With no ramps in the majority of the tunnel, the only real option was to excavate via the shafts in the middle.

“The issue is that those parts of the tunnel are too deep for temporary ramps,” says Mr. Newton. “With most tunnel jobs, at the portals, you’d have ramps you could operate from. It’s the length and the depth of this project that made is necessary to tunnel from the mid tunnel shafts as well.”

They also sunk a temporary shaft near Pearces Corner and excavated both north and south from there. The tunnel is dual carriageways separated by about 10m of sandstone. Thinking of the future, the designers have made room for an extra lane in each thoroughfare that can be added at a later date.

But how do you remove all that tunnel dirt through shafts?

“NorthConnex devised a creative solution using vertical conveyors. Traditionally shaft excavation would use a large bucket known as a kibble to remove spoil. This can be quite time consuming, so instead we looked for new and innovative technologies. Inspired by a tunnelling project in London, the project used vertical conveyors which sandwich the dirt between two belts and then transport it 90m vertical to the surface,” Newton says.

“The tunnel spoil was then transported to Hornsby Quarry, just a few kilometres away from the nearest excavation site. Yet another conveyor system – this one about 160 metres long – was used to transport the dirt into the quarry void. We placed more than 1 million cubic metres of dirt. The site is nearly ready to hand over to Hornsby Council, who will turn the Quarry into a recreational precinct.”

All the different sections of tunnel have now been connected. Modern technology was key making sure the connections were perfect. With 70 surveyors being used across the tunnel and civil sites – along with new technology – there were no mismatches in the middle.

“The roadheaders used to excavate the tunnel also have a very sophisticated guidance system which tells the roadheader operator exactly where to cut. This ensures we have the right tunnel shape from the very outset,” says Newton.

“We also used machines called surface miners to assist with excavation. Surface miners are traditionally used in open cut iron ore mines in Western Australia. They have a large cutting drum which removes about 50cm of rock at every pass. NorthConnex used two surface miners to cut the lower portion of the tunnel known as the bench. In order to bring these machines underground, we fitted them out with dust hoods to ensure we maintained good air quality. The machines were effective and I wouldn’t be surprised if they become part of mainstream tunnelling equipment in the future.”

Another consideration, and also a reason the tunnel’s lowest point goes to down 90 metres, is that during the excavation process they crossed under the new Northwest rail link which is due to open in the near future. “We actually passed reasonably close to it,” says Mr. Newton.

There were not too many hiccups in the process.

“There were a couple of unexpected fault zones that we found, but we didn’t find any coal,” says Newton, tongue planted firmly in cheek.

“It was challenging in some parts. There were fault zones around the Wilson shaft and also up at Wahroonga. Crossing the fault zones required installing additional heavy supports, particularly in the vicinity of the North Shore Rail Line overbridge. We had to put a lot of additional support in the fault zone in the tunnel to make sure there was no movement of the rail infrastructure. We had to do a lot of detailed monitoring of the rail as we tunnelled underneath it. A lot of real-time monitoring was needed.”

Trucking companies can expect to pay about $21 a trip through the tunnel. The builders believe that there are some added benefits to the newer construction. For a start, it connects to main interstate highways. And they have made it higher, which will allow some transport specialists to add more cargo to their loads.

“This tunnel will have a clearance height of 5.3 metres, while most tunnels in Sydney have a clearance of 4.4 metres to 4.8 metres,” says Mr. Newton. “This is mainly because NorthConnex has been designed as a heavy vehicle tunnel. It really is to get trucks off [suburban] roads. I’m sure there will be some trucks that can’t use the tunnel due to height issues, but we think most will be okay.”

There will be height detectors leading to the approaches of the tunnel, so there will be no excuses for those with loads that are too high entering the tunnel.

The Hornsby area, where the northern part of the tunnel is situated, has one of the highest rainfall rates in Sydney. What about the amount of water that might accumulate in the tunnel? And if it pours a lot, where will it go?

“The actual portals and ramps are designed to minimise the amount of rain run-off entering the tunnel. We don’t get a lot of water coming in,” he says. “However, at the low point of the tunnel we have a low-point sump, which is quite large. Within the tunnel we have stormwater lines, as you would expect on any road. These lines go from one end to the other of the tunnel and are between 400mm to 750mm in diameter. There’s an inlet pit every 40 metres to collect the water off the road and takes it down to the low-point sump, where we have some fairly big holding tanks that can hold the water prior to discharging it to a water treatment plant. The tunnel itself has a full waterproof membrane. All these things are more in place to cater for the fire deluge system – rain water as well as ground water.”

The tunnel’s driving surface will be made of concrete, not asphalt. Why not use the traditional road material?

“Why not use asphalt?” says Mr. Newton. “That’s a good technical question. Concrete pavements require less maintenance, which means less disruption to motorists.

“Other tunnels use asphalt as a wearing surface on top of concrete pavements. This is the first tunnel I’ve done that doesn’t have an asphalt wearing surface. You get similar friction coefficients with concrete as with asphalt. If you drive up the North Coast, you can see that significant amounts of the new Pacific Highway roads are concrete roads.”

Another technology that is being used is a robot that drills all the bolt fixings in the roof. These help hold up all the lights, deluge systems, cable trays and all the other systems on the roof.

“I think there is something like 100,000 fixtures that have to go into the shotcrete,” says Mr. Newton. “The machine has a robot arm similar to what you would see in a process factory. It is mounted on a wheeled carriage and travels along drilling the roof.”

The project also designed a special crane mechanism to install fire rated emergency access passages along the tunnel.

“To ensure people can evacuate the tunnel safely, we are building Longitudinal Egress Passages which direct people to the nearest tunnel exit. Installing the passages could have been time consuming; however, our engineering team designed a pre-cast concrete solution similar to Lego. The passages were built off-site in sections and then lifted into place using a crane inside the tunnel.

Normally, this would require several lifts and multiple cranes, however the project innovated and came up with a lifting mechanism which allows each section to be rotated mid-air and then installed. The installation is now as smooth as a production line.”

One of the interesting aspects of the construction – some parking issues aside – is that there seems to have been minimal disruption to the traffic flows in the immediate area where the build is taking place. Mr. Newton has an interesting take on that.

“I think what it shows is just how busy Pennant Hills Road is,” says Mr. Newton. “If you look at the truck volumes we are adding to the existing traffic flows as a percentage of that traffic, we are quite a small impact. We take particular care not to schedule deliveries during peak travel times as much as possible. So, for example, our concrete deliveries for paving works all occur at night – specifically to ensure we don’t impact motorists on Pennant Hills Rd.” The tunnel was due to finish in 2019, but is due to be completed sometime in 2020.