Building a billion-dollar airport runway on waterlogged-ground described as the “consistency of toothpaste” understandably calls for cutting-edge engineering solutions.
As it turns out, it also requires extensive risk mitigation, one of the longest dredging pump-outs in the world and the longest linear length of wicks drains ever installed in Australia.
The Brisbane Airport Corporation’s (BAC) $1.3 billion, 360 ha New Parallel Runway (NPR) project is an ambitious undertaking, which will see a new 3.3 km runway and 12 km of taxiway pavements join the Brisbane Airport’s existing main runway and smaller cross-runway. Completion is forecast for mid-2020.
Phase 1 preparatory works commenced in 2012, with the site cleared of vegetation. However, arguably the most complex and time-consuming phase of the project began in 2014, with the start of the site reclamation, expected to take until mid-2017 to complete.
On shaky ground (for now)
Geotechnical testing of the site, which included cone penetrometer tests and boreholes, confirmed the unpredictability of the ground and the challenge to achieve uniform ongoing settlement after the new runway opens, Project Director Paul Coughlan explains.
“We’re on an old estuarine delta of the Brisbane River,” Coughlan says.
“We have these very deep, soft, alluvial soils down to about 35m in places running across where the runway would be built. They had what is described as the strength of toothpaste.
“It has a California Bearing Ratio (CBR, AS 1289 6.1.1) of between 1 and 1.5. That’s incredibly soft soil. You would normally see runway projects built on CBR’s of 10 or better.
“Put in perspective, the Federal Aviation Authority in the US have developed their runway pavement software modelling. Their software packages don’t assume a CBR of anything less than 3. We’re not even on that package!”
BAC is using New York-based engineering designers Jacobs Engineering, which purchased Australian-based SKM in 2013, to now complete the engineering design of the new runway and its associated infrastructure.
“A couple of their very experienced people have relocated from the US, and their Senior Director said that, in 35 years of airport experience in the US, he has never had to build a runway on CBR of 1 to 1.5,” Coughlan says.
To reclaim the site, the project needed a lot of sand – about 11 million m³.
A process known as surcharging is being used to achieve the reclamation, whereby a heavy load is applied to the soil, causing the ground level to sink as the earth is consolidated to form a stable foundation. The level of the NPR will be raised 3m, to protect it against rises in sea levels. The settlement will take about four years, and is expected to be completed in mid-2017.
“Sand provides three benefits for us,” Coughlan says. “You’ve got to build your land up above projected flood and storm-tide levels. The sand itself is a very good product to provide weight to squeeze all that underlying water out. Third, it becomes part of a very stable foundation on which to build your runway.
“In the worst parts, the height of the sand above original ground level is 8m,” Coughlan continues.“That weight of sand will go down 3m in some areas. The runway will be built at a minimal level of RL 5.2 on airport datum, and that’s to get it above flood levels, storm-tide levels, and we have factored in a sea-level rise.”
Wicking away the moisture
To expedite the process, 330,000 wick drains were installed across about 40% of the site, to depths of up to 35 m. The purpose of the wick drains is to provide an uninhibited vertical pathway for soil moisture to be squeezed from the ground under the downward pressure of the heavy sand and the effects of gravity.
“What we found following the installation of the wick drains was that at more than 8 million m, it’s the largest linear length of wick drains ever installed in a single project in Australia,” Coughlan says
The mammoth project was a joint effort from contractors Menard Bachy and SoilWicks Australia under the dredge contractor Jan De Nul.
A challenging project
The entire project – which will double the 24-hour airport’s arrival and departure capacity – hinged on finding a suitable, environmentally-sound and economical source of sand, with a grain size of 250um or less.
“Basically, if we couldn’t locate a suitable source of sand and get the necessary Federal Government and Queensland Government approvals then the project couldn’t commence,” Coughlan says. “It was all dependent on getting a good, economical source of sand.”
During the project’s Environmental Impact Study, conducted in 2006/07, that source of sand was found in Moreton Bay. However, moving the sand from the dredge to the reclamation site was never going to be easy.
“The challenge for us was to economically extract and transfer that sand onto the NPR site, and that required a very large dredge,” Coughlan says. “The closest to the site the dredge could comfortably berth for pump-out was at the mouth of the Brisbane River. This meant the dredge had to pump the sand upwards of 8 km from its mooring to the runway site.
“That’s one of the longest pump-outs in the world. We couldn’t berth out the front of the airport because at low tide the mudflats extend for many kilometres before water is of a suitable depth to accommodate a large dredge.”
“The only location with enough water depth to suitably berth the dredge was what’s called the swing basin for the Port of Brisbane at the mouth of the Brisbane River which is directly adjacent to the Luggage Point Sewage Treatment Plant, the main treatment plant for greater Brisbane River.
"Our best solution was to moor the trailer hopper suction dredge, the Charles Darwin, in the Brisbane River at Luggage Point and run the pipeline from the dredge, across the treatment plant land, through the operational airfield to the NPR site.
“That was a real challenge, and we worked very cooperatively with the Brisbane City Council and Queensland Urban Utilities who run that plant,” Coughlan says. “Even before we went to tender, we had agreements in place where we knew exactly what the corridor for that plant would be. We had to go up and over the plant’s outfall channels, which were over 60-years-old. We had a settlement tolerance in the order of 10mm."
The next obstacle in the pipeline’s path was Brisbane Airport’s existing airfield.
“We had to find a way to go around the existing runway, and then we had to go under the cross runway,” Coughlan says.
“We placed large concrete culverts 250m in length under the cross runway which the dredge contractor then used to thread the pipeline through. Three 12m length, 1m diameter steel pipes would be welded together at a time and fed through the culvert along internal roller devices to provide the underground linkage which allowed the cross runway to continue operating without interruption. From an engineering point of view, just installing that pipeline was quite a challenge.”
Risk mitigation was a vital part of the pipeline planning, Coughlan adds, with the sandpaper-like water and sand slurry capable of eroding steel pipes at a rate of 1mm per 1 million m³ of sand pumped.
“When you pump this sand out, you’re pumping it out under a lot of pressure,” he says. “You had to factor in what if the pipe where it’s bolted together breaks. There were certain parts like under the cross runway where we couldn’t bolt it together, it had to be welded. Because we couldn’t tolerate a failure there, we actually stipulated that the thickness of the steel pipe under that cross runway had to be 32mm. At the sewage treatment plant, Queensland Urban Utilities couldn’t tolerate a failure, so we were able to say the steel had to be thicker there as well.
“We had to factor in that the dredge contractors may have to turn the pipe, because most of the sand is in the bottom third of a circumference.
“When you pump over that distance, you have to factor in that you can get build up of sand in the pipe. As a risk-mitigation under the cross runway, we put in two developer pipes under the cross-runway."
Careful planning and risk mitigation saw the challenging dredging project completed with only minor hiccups.
“As it turned out, we didn’t get any blockages under the cross runway, and we had one blockage in the whole job, which was on a piece of pipe that was on the ground, so was easily accessible,” Coughlan says.
Dredge contractor Jan de Nul were awarded the dredging tender. The Charles Darwin trailing suction hopper pumped sand from a carefully chosen, environmentally-sound site to the NPR in a water and sand slurry at a ratio of about 5:1. As part of a range of environmental safeguards, a 24/7 real-time monitoring program of the dredge works was put in place, to ensure water quality limits were met throughout the process.
A waiting game
The last cubic metre of sand was pumped on 7 December 2014. Now, it’s a case of waiting until mid-2017, when the ground will be sufficiently compacted, and construction of the runway and taxiway pavements can begin.
Coughlan says parts of the runway have already dropped 2.5m, and will continue to drop even after the reclamation is completed.
“About a third of the sand we’ve placed comes of the site when we begin to build the actual runway,” he says. “Two-thirds of it stays there and gets shaped, and that becomes the sub-base on which we build the taxiway and runway pavements.
“The ground will continue to settle for decades,” he continues. “What we want it to do is settle uniformly and not too much. Based on our design, over the next 40 years on average it will continue to settle 200mm.”
In comparison, the existing main runway has settled about 350 mm since it was opened in 1988.
“Geotechnical engineering is not an exact science,” Coughlan adds. “The good news is that under all that sand are about 700 settlement plates which are continuously monitoring the settlement rate across the site. Our geotechnical consultants Golder Associates analyse the regular measurements and compare the extent of settlement to their modelling predictions. So far, the consolidation of the sand platform is performing as predicted.”
“Predictable” seems to be the watchword for the NPR project.
“We’re on schedule, we’re on budget,” Coughlan says. “There hasn’t been, that I can think of, one surprise. The key really is to actually give yourself time at that front end of the project to really understand all those challenges, and what all your design and construction options are.
“We don’t want the learning to occur during the construction.”
Images – courtesy of Brisbane Airport Corporation, Jan De Nul
1. Bulldozers at the NPR site pump-out.
2. 330,000 wick drains were inserted in about 40% of the NPR site to expedite the consolidation process.
3. An aqua-coloured stabalising agent was applied to the site, to minimise windblown sand.
4. The Charles Darwin's suction head.
5. The Charles Darwin trailing suction hopper.
6. A map of the NPR site.