An artist’s impression of Brisbane Airport’s new runway.

It may look like an ordinary underpass, but it’s a complex engineering feat that required hundreds of workers and is a lynchpin to Brisbane Airport’s $1.3 billion runway development.

The $120 million Dryandra Underpass is up to five metres below sea level, and required the removal of 750,000 cubic metres of sand and the installation of 35km of conduit.

About 20,000 cubic metres of concrete and 4000 tonnes of reinforced steel has been used to build the underpass floor and walls as well the taxiway and road bridge decks.

The project by contractor McConnell Dowell includes parts of taxiways that will connect the new runway system with the existing system and terminals and partitioned four-lane roadway with two airside and two landside lanes.

The roadway provides access to the airport’s general aviation precinct, aeromedical services and the Acacia Street plane-spotters’ loop.

The underpass is a significant milestone because it marks the start of paving works after the three years it has taken for the “surcharge” —  the 11 million cubic metres of dredged sand used to reinforce the soft soils and clays on which the runway is built —  to settle.

“It’s a milestone to allow the surcharge to be completed but more importantly it’s actually the first delivery of the concrete infrastructure for the new runway, which is two taxiway bridges as well as two airside road bridges,’’ the runway’s project director Paul Coughlan says.

Waiting for the surcharge to settle was a marathon event with the last cubic metre of sand pumped through on 7 December 2014.

“People describe watching the grass grow, well we watched the ground settle,’’ Coughlan jokes.

But it’s been worth the wait and the engineer describes the result as “quite phenomenal”.

Settlement at the northern end of the runway has been about 2.8 metres, at the southern end it’s been about 2.5 metres and around the underpass about 2 metres. In the runway’s centre section, the ground has settled about a metre in keeping with expert predictions.

“So you have a large amount of ground settlement, all of which was predicted,’’ Coughlan says. “The reclamation design predicted with 95 per cent confidence that the worse ground settlement would max out at 3 metres, so it was pleasing that the geotechnical consultants had been thorough and we’ve maxed it out at about 2.8 metres.

“It’s gone well and we’re reaping the benefits now.’’

The Dryandra Road project itself posed a big engineering challenge for several reasons.

Not only was it built on soft soils beneath groundwater level, but it was a confined site.

“If you look at aerials for the site, you see this massive area for the actual works and then when you look carefully you see this small area which is the underpass,’’ Coughlan says.

“At its peak I think we had what we call ‘down the hole’ probably about 200 workers. And it’s a real credit to McConnell Dowell that this intense construction in a small area has been done with an outstanding safety record and FOD (foreign object debris) management. I can’t recall one report of FOD getting airside and it’s fairly close to the domestic apron.”

The deepest point of the project, a big pump station well, is about five metres below sea level and the average excavation through the underpass is about 4m to 4.5m below. The project also involved the installation of 5km of water and sewer pipes.

Coughlan jokes that the people driving through the underpass when it opens in a few weeks won’t appreciate that “they’re actually driving underwater”.

“It is a fair way below sea level, so the whole design had to really deal with high water pressures,’’ he says.  “The base of the underpass is about 4 metres below ground water so you have a really high hydrostatic pressure under the underpass.”

The dewatering system was required to remove the equivalent of two Olympic swimming pools within a single 24-hour period to enable the underpass to be built in the dry.

Helping to address this are 701 piles driven down about 30 metres, something many people assume is to allow the taxiway bridges to cope with the weight of the A380.

“Actually most of the piles are there to stop it lifting out of the ground with this high uplift hydrostatic pressure,’’ Coughlan says.

The floor of the underpass is 800mm thick with special expansion joints and water treatment barriers that will also help withstand higher groundwater levels experienced after very heavy rainfall events.

Coughlan says groundwater wells established around the site a decade ago helped provide a good understanding of how heavy localised storms dumping up to 300m of rain in a day could lift groundwater levels quickly.

“Even though it’s about four metres below groundwater, we’ve actually designed it to withstand a bit over five metres,’’ he says.

“So in the event you have a really massive storm and you get a temporary spike in your groundwater levels, we’ve actually designed it for that.”

The taxiway bridges are designed to cater for not just fully-laden Airbus A380 superjumbos but anything bigger that may come along in the future.

This includes aircraft weighing up to 710 tonnes, significantly above the 578-tonne maximum take-off weight of even the proposed A380plus.

The runway team analysed the growth of aircraft from the Boeing 747-100 onwards to determine the strength of the taxiway bridges, which are 1.6m thick at their thickest point near the centre walls.

It also looked closely at jet blast because the outer engine of an A380 is slightly over the edge.

“It’s been designed to ensure that jet blasts are deflected away from vehicles travelling in the underpass,’’ Coughlan says.

“These jet blast screens  also stop anyone being able to throw anything up into the wing or engine.”

As Brisbane Airport Corporation chief executive Gert-Jan de Graff recently pointed out, however, this is just the tip of the iceberg of what will happen in coming months as Australia’s biggest aviation project heads towards completion.

The initial layer of crushed rock has already been placed on portions of the new runway and taxiway system, and the contractor will connect the rest of the taxiway system into the bridges.

Reaching the finish line will mean about 15 continuous months of paving, according to Coughlan.

He says work on the 3300m runway and 12km of taxiways is progressing well and is on track to be ready by the forecast date of mid-2020.

Work on the piles and walkways for the High Intensity Approach Lighting (HIAL) system, which extends about 400m into Moreton Bay, is complete after a five-week closure of the airport’s cross-runway.

The closure also allowed the piles to be driven for the northern airfield lighting equipment room and the shaping of the northern runway end safety area (RESA). Other work has included the building of an airside perimeter road, security fences and landscaping.

The Airfield Contractor has moved most of 5 million cubic metres of sand which is excess from the surcharging.

Much of this has been placed in preparation for a western terminal earmarked for construction somewhere around 2030, with the balance placed in the northern precinct of the airport.

Also ticked off is factory testing in Europe of the airfield ground lighting (AGL) system and control monitoring system.

Coughlan has been told the AGL will be the first fully LED runway system in the southern hemisphere to use a state-of-the art control monitoring system where every individual light can be managed and monitored.

Another milestone will occur on 8 November when the existing runway will be officially named 01 Right and 19 Left.

New area movement guidance signs have been delivered and pilot documentation and air traffic control changes are well in hand.

“And that’s from a safety point of view,’’ Coughlan says.

“Our investigations showed that pretty much once the new runway starts to look a bit like a runway the safety move is to ensure you rename your existing runway.

“Certainly, by the end of this year there will be significant progress on the paving of the entire new runway so it will start to look like a runway.”

Brisbane’s runway is the first major capital city runway since Sydney’s third runway was opened in 1994 (Toowoomba’s Wellcamp airport has been built since) and it won’t be the last.

Melbourne, Perth and Western Sydney all have runways planned and Coughlan says Brisbane will be sharing what it has learned with the industry.

Some of those lessons came from the existing runway and have prompted the airport to make changes such as burying its drainage system and introducing wider paving to stop jet blasts damaging vegetation and creating FOD.

Coughlan is also proud of the economic contribution the runway project has made to South-East Queensland. He says more than 90 per cent of the spend is going into the region and another 4 per cent goes to other parts of Queensland and Australia.

BAC estimates the project will add a $5 billion in annual economic benefit and generate 7800 jobs by 2035 in addition to the almost 700 people working on the job during the peak construction phase.

“Obviously things like the airfield ground lighting, which is very specialised lighting, is imported and the steel for the approach structure is imported,’’ Coughlan says.

“But pretty much everything else is true blue Aussie.”

Steve Creedy

Author

An award-winning journalist, Steve began covering aviation in the United States in the early nineties before returning to Australia later that decade and editing The Australian’s aviation section for 17 years. He is editor of Airline Ratings and has co-authored books on industry initiatives aimed at reducing greenhouse emissions.

Steve has joined the AAA to write interesting and informative editorial on the aviation industry.