Overcoming airport underpass engineering challenges

by Chris Frame

Brisbane Airport has been in the midst of a dramatic transformation for the past six years. Its new runway system, on schedule to open in 2020, will enable the airport to maintain its position as the leading aviation hub for Queensland.

When finished, a new western airfield with a 3.3km long runway and up to 12kms of taxiways will open, doubling the airport’s capacity.

At the core of the project is the vision to grow Brisbane and Queensland’s connectivity to Australia and ultimately the world, providing capacity to allow new flights to new destinations. The project creates an immeasurable number of opportunities now and into the future through commerce, tourism and jobs.

Delivering such a vision on time and on budget requires teamwork, planning and a synergy of vision that is not easy to cultivate. But Brisbane Airport’s approach of forward planning, combined with an inclusive corporate culture, has allowed this project to meet or exceed each of its delivery milestones.

“The aim is to be operational by September 2020 and we’re tracking a few months ahead of that, so while there’s a long way to go it’s good to know we are in a very good position with the project,” said Paul Coughlan, Brisbane Airport Corporation’s New Runway Project Director.

Crossing the road
A key aspect of the project is the Dryandra Road underpass. Dryandra Road has been the only access to the northern part of the airport since it opened in 1988.

This important link connects critical aeromedical services from the Royal Flying Doctor with Brisbane’s motorway network. It also provides public access to Brisbane’s growing general aviation facilities.

However, Dryandra Road presented designers with an engineering challenge. The existing road ran through the planned link taxiways, which will connect the new runway with the existing taxiway system.

“Without an engineering solution to maintain Dryandra Road connectivity, you would literally have to shut down the northern aprons,” Mr Coughlan said.

“Our challenge was to find a cost-effective way to ensure access was available throughout the airport for the future, but just as importantly, during construction.”

Rising to the challenge, the team at Brisbane Airport Corporation (BAC) examined a variety of solutions. Settling on an underpass, the design meets the connectivity needs for vehicular traffic, while providing optimal operational efficiencies for aircraft passing overhead.

“We elected to go with an underpass design because we couldn’t take the taxiways up and over a ground-level road; the incline was just too great for aircraft to manage,” Mr Coughlan said.

“Dryandra Road links the airport to its northern facilities, so it could not be closed off until the new underground road was operational. Diversion works to construct and realign a new road were scheduled to be complete ahead of the runway and airfield works so that access to the north could be maintained during construction of the link taxiways.”

While to the untrained eye, the underpass resembles a tunnel, it is in fact a road under/taxiway over engineering structure that supports four bridges. Two are vehicular road bridges while the other two are large taxiway bridges, capable of accommodating the world’s largest existing aircraft, the A380, as well as any future heavier aircraft up to 710 tonnes.

The works also see the diversion of essential new utilities within the underpass alignment including water, sewage, power, telecommunications and fibre optics; the first such upgrade in 30 years in this area.

Thinking ahead
Building the underpass presented the BAC team with a number of challenges. The mangrove region where the airport is located is notorious for its soft soil conditions which presented engineering challenges throughout the New Runway Project.

Such challenges have included an engineered solution to expedite ground settlement during the land reclamation phase. Over 330,000 wick drains were installed across the new runway precinct, making it the largest wick drain project in Australia.

However, while the new runway sits on reclaimed land, the floor of the Dryandra Road underpass is situated below ground, at four and a half metres below mean sea level. The waterlogged soil effectively pushes up on the underpass structure, requiring a dewatering system to keep the site dry during the construction phase.

Ant Halapua is BAC’s Delivery Manager for the Dryandra Road works and his long-term involvement in the project has allowed him to develop a simple and effective analogy to explain the water issue.

“Essentially we are building a ship-like structure, so think of the ground and groundwater as your water which is pushing the whole ship up. There are a number of piles that will anchor it down to bedrock once the structure is commissioned, but while we are building this structure, for access, the contractor is using big pumps and pipes to keep the water under control — like a dry dock,” Mr Halapua said.

It fell to lead contractor McConnell Dowell to create a system to keep the water at bay.

“To reduce the size of the footprint, which helps reduce the scale of the dewatering required, the contractor opted to use a sheet piling system which made a big difference,” Mr Halapua said.

“The sheet piling system gave them better access to the work front and a framework to establish their dewatering system around. To ensure the system would work, they ran a series of deep bores down the centreline of the alignment to manage the water around their worksite.”

The power of planning
Exhaustive pre planning has been the hallmark of the New Runway Project. BAC prioritised the front end development work, driving the project’s success to date.

Forward planning and exhaustive testing allowed BAC to create a robust tender document, aiding contractors to innovate and overcome challenges well before they become problematic.

This included a detailed ground survey period where BAC thoroughly mapped ground conditions, supporting the selection of construction materials and construction methodology.

Furthermore, BAC’s vision for the airport has driven many decisions throughout the project which has led to the underpass being future-proofed from the start.

“We could have designed the underpass to hold the weight of an A380, but we took it further than that. We looked at detailed analysis of aircraft design evolution to future-proof the underpass design,” Mr Coughlan said.

The analysis reviewed the growth of existing airliners over time, such as the size and weight difference between a 747-100 and 747-8I. This analysis suggested a possible future A380 variant could be around 20 per cent heavier than the current model.

“As a result, the underpass and taxiway bridges can handle a greater weight than today’s A380 to enable the airport flexibility well into the future.”

Safety first
Safety is the number one requirement across BAC’s New Runway Project, and the Dryandra Road underpass is no exception.

Being below the water table and near an operational airfield, it presents a unique challenge. The need for efficiency is balanced with managing FOD (Foreign Object Debris), crane height restrictions and vehicular access near an active runway system.

“All of our contractors have to provide us with a method of work plan. Each of these plans needs to identify safety aspects. It is looking at every construction element and process, and identifying what could go wrong and then create procedures to ensure that doesn’t occur,” Mr Coughlan said.

Coughlan believes that safety starts in design, commenting that “a safe design is only as good as its constructability”.

As such, BAC designed an underpass that could be built safely, ensuring that the design did not add unnecessary safety risks to the project.

“We did a lot of risk assessment and analysis at that early stage. We didn’t rush the front end. We didn’t cut costs in the geo- tech and we didn’t rely on assumptions. We did the investigation that was needed,” Mr Coughlan said.

In addition, BAC requires documentation from each of their contractors to demonstrate their safety record, as well as how they will address safety requirements at the airfield.

This approach, in addition to the airport’s preference to work with mature contractors with a proven safety record, has seen the New Runway Project and Dryandra Road underpass progress with minimal safety concerns.

Future outlook
As opening day in 2020 comes closer into view, BAC’s meticulous planning and design means the project is not only ahead of schedule but also on budget.

The new runway and its associated taxiways will transform Brisbane Airport’s capacity, further connecting Queensland’s ever-growing population to the regions, the nation and the world.

At the heart of the project, the Dryandra Road underpass will become a permanent fixture; linking the essential northern precinct with the rest of the airfield. The underpass’ success will be measured not in its noticeability, but in its hidden efficacy.

“When people are driving through the underpass, they won’t realise that they are driving under sea level or the engineering that went into making this a reality,” Mr Coughlan said.

And that’s the way it should be; a seamless design, providing its function almost unnoticed. But one thing is for sure; Dryandra Road has left a lasting impression on the team involved in its construction as well as Brisbane Airport’s ability to connect Queensland – and Australia – with the world.

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