Extending the runway at the Isle of Man airport into the sea has required some of the biggest rock armour available to withstand the harsh weather conditions. Jessica Rowson reports.
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The Isle of Man airport is a lifeline for those living on the island stranded in the Irish Sea midway between Great Britain and Ireland. Airport operator the Isle of Man Department of Transport is understandably keen to ensure that services are uninterrupted.
So when the International Civil Aviation Organisation increased the recommended size for the runway end safety area (RESA), the department was anxious to comply quickly to make sure that the airport can continue at the same capacity.
But simply extending the runway safety area − the additional section each end of the runway built in the event of an aircraft undershooting or overshooting − at the Isle of Man was not going to be straightforward, with the existing runway abutting the sea. Over the last 18 months contractor Balfour Beatty has been building up a headland into the Irish Sea onto which the runway area can extend by 200m.
“The recommendations for the runway end safety area is increasing from 90m x 92m to 240m x 150m,” says Balfour Beatty contracts director Keith Bowman. “The extension means if the larger RESA becomes mandatory, they’re covered and the same aircraft can continue at the same capacity.”
The line of the promontory was created by a 700m long rubble mound seawall, extending 250m seaward, which is protected against waves by rock armour, weighing up to 42t − some of the biggest in the world.
“To move the rock armour took a 120t specialist base machine with extra ballast at the back and a strengthened jib and dipper arm,” says Bowman. “No one has built a breakwater with such big armours before.” Around 200,000t of armour stone was imported from Norway, which is the closest supplier for this size of rock − the other major supplier being Canada.
The supply of the stone was critical to the project − it had to all be delivered and placed during a period of calm weather. “If a storm comes and the stones are not all in, you could lose all of it,” says Bowman.
The supplier could not guarantee that all the armour stone could be delivered in one summer so the project was extended over two years, with a finish date of September 2010. Work is progressing well on site with all of the armour stone delivered and 90% of it placed, putting the project on track to complete ahead of schedule.
The armour was transported from Norway on a 20,000t capacity barge, then transferred to a smaller barge nearer the shore that could access the slipway where the specialist excavator was waiting to pick up the stones one at a time in its bucket.
The rock was placed by specialist Highland Quality Construction equipped with a GPS unit in its cab, programmed with information about design and layout. A sensor on the excavator’s bucket then determines the positioning.
Uncovering the past
Weather was not the only risk that Balfour Beatty had to contend with.
Significant archaeological finds can often put a project behind schedule. But despite some major discoveries during the project, it still remains on track.
Two human skulls and the remains of a Bronze Age settlement dating back 3,500 years were unearthed within hours of work starting on the runway extension project. And then this year there was an even more dramatic discovery − the remains of a Manx dwelling dating back 8,000 years.
“There have been significant Bronze Age finds,” says Bowman. “One of the most exciting was a skeleton with sword marks on the ribs. Also, the oldest habitat discovered in the UK was found and we got the archaeologists in at an early stage.”
Balfour Beatty has also had to be careful that it didn’t do anything to spoil the Isle of Man’s beautiful natural habitat. But it seems that the work provides more of an attraction to the local wildlife than a deterrent.
“We’ve spotted sharks and dolphins,” says Bowman. “We’re working in the sea and they come to see what’s going on.”
“By using GPS, we don’t need engineers to get involved with the heavy equipment,” says Bowman. “It reduces risk and makes the job more accurate.” The wall is designed to withstand a 1:100 year storm with no damage and a 1:1,000 year storm with less than 5% damage.
It will have to face a battering as the sea is particularly rough in this area due to the complex and irregular topography of the seabed. To make sure that the seawall could cope with the unusual wave loading, a 1:50 scale model was tested at HR Wallingford and initial designs had to be adjusted to suit. “It took 18 tests [to perfect the design], but it was money well spent,” says Bowman.
“We started with 33t armour stone. Around 2.5% to 5% of the first layer of armour stone is allowed to be lost in a 1 in 1,000 year storm, but when we tested it with 33t, there was hundreds of armour stone lying around. The force of the waves here is phenomenal. We’ve designed it for 6.5m high waves and since we’ve built it the area has experienced force 8-9 easterly winds with waves 4.5m high.”
The land enclosed by the seawall was reclaimed using marine dredged sand. In August, marine specialist Dredging International used its jumbo tracker suction hopper dredger to collect the sand from an underwater quarry in Morecambe Bay and transport it to site. This was then used to fill the area behind the seawall, which was placed in just 9.5 days.
“If we’d taken it from a local quarry by road, it would have taken much longer − maybe around 150 days − and would have clogged up local roads,” says Bowman.
The promontory works are just one part of a larger £37M airport upgrade contract which includes resurfacing the main runway and creating new taxiways. The airport had to be kept operational throughout the works. Balfour Beatty teamed up with local contractor Colas for the airside work and had specific time windows in which it could access the site.
“We took possession at 9.30pm but then had to be clear by 5.30am for the mail plane,” says Bowman. “We were resurfacing through the night and then had to reopen the next morning.”
The contractors removed the top 50mm of the existing runway’s concrete surfacing and replaced it with a new wearing course. The removed concrete was recycled − crushed up and used as sub-base for the promontory works and new taxiways.
Betons Bitumineux pour Chaussees Aeronautiques (BBA) asphalt was used for the runway wearing course. This provides the same durability and lifespan as Marshall asphalt but can be put down in a slightly thinner layer, hence saving on production time and material costs.
“It’s a thinner surface that does same job,” says Bowman. “It’s just as good, has been used for 20 years in France and was used at Charles de Gaulle airport. The runway here is 1,800m long and 5m wide. If you save 10mm over that area, that’s a lot of money saved.”
Isle of Man: Airports don’t get tougher than this