Barrett Byrd Associates

Detailed planning and carefully choreographed movements of plant and materials have ensured runway surfacing success on Tiree for contractor Colas.

Colas is now bringing its Ermont TSM 225 mobile asphalt plant back from Tiree in the Inner Hebrides; performing the relatively simple task of reversing an operation well rehearsed earlier in the year. The island’s airport runway underwent major resurfacing in September after extensive preparations by Colas, including detailed planning of material movements and mock landings of its plant in Birmingham.

This was to prepare for the particular logistical and physical demands of working on Tiree. There is little room to manoeuvre through Tiree’s port and ‘B’ roads. Colas had to be sure it could get its mobile plant to the airport and then keep asphalt production supplied with aggregates and bitumen.

“Good planning was vital,” says Colas’ Business Manager for Airfields Carl Fergusson. “Precise timing was needed to maintain deliveries and asphalt production because no-one was holding the ferry for us and it only hangs around in each port for about 20 minutes. Missing the boat would mean a 24 hour delay, so good planning was the key to everything.”

Plant and materials were shipped over to Tiree because the asphalt had to be mixed on site to ensure high quality of the finished product in a relatively remote location, four hours by ferry from Oban. Aggregate was needed every third day from Oban on the mainland and Cashel in southern Ireland, while road tanker deliveries of bitumen would be arriving daily from Nynas’s Dundee refinery on the Caledonian MacBrayne ferry.

Cal Mac operates daily from Oban to Tiree up until mid October and intermittently thereafter. So the runway surfacing project had to coincide with Tiree’s summer holiday traffic and this year’s Wave Classic windsurfing world cup event. The Wave Classic, which is regularly held around Tiree due to the island’s ideal environment for windsurfers, doubles Tiree’s normal population of 800; an increase Colas had to take into consideration when planning its work.

“Local labour was used where possible, but at least 40 Colas operatives and staff had to be accommodated and the increased activity of the Wave Classic and our work was expected to have an impact on the island’s roads,” Fergusson says.

Colas’ Ermont asphalt plant comprising of material silos, mixing drum and generators is transported on eleven trailers that push the limits of length and width for transport on narrow roads. Before leaving for Tiree, Colas pre-measured Tiree’s port, road junctions and bridges on the route to the island’s airport and then mocked up the loading and landing of its plant at its Birmingham yard, with cones marking out the limits of manoeuvrability.

“A lot of planning went into transporting the plant,” Fergusson says. “A detailed storage plan was produced to show the plant could be stowed to maritime rules, with the trailers reversed onto the boat so they came off in the right direction at Tiree. Even then, we had only one day available for the trip in the month, because the plant could only be disembarked on the high tide,” Fergusson adds.

Colas shipped its plant to Tiree on a specially chartered ferry from Oban in August. The landings and 2km journey from port to airport benefited from the detailed preparations. Lessons learned in Birmingham showed a couple of junctions on Tiree would have to be modified to accommodate the long trailers before they arrived and that great care was needed when they did arrive. “There were literally centimetres to spare in places, but once on the island, we were up and running within a week,” says Fergusson. “The next challenge was keeping the supply operation going.”

That was throughout September. A total of 22,000t of asphalt was needed for resurfacing Tiree’s main runway; one of three at the airport on different alignments. Small aircraft could use the cross runways, but larger planes needed the main runway Colas was working on, which restricted time available for surfacing between scheduled flights. Production within each shift was boosted however by use of a type of asphalt new to UK airport surfacing (see box) and by Colas’ Ermont TSM 225 plant.

This is one of the UK’s largest with a production capacity of 225t per hour and enough bitumen and fuel storage to keep it running continuously through a 12 hour shift. Around 1000t and more than 300m of the 100mm deep asphalt overlay could be laid between flights and the surfacing finished during 21 working days in September.

In addition, an extra 20m by 50m aircraft parking apron has been built and a fair amount of ‘delethalisation’ work done. This last involves the building of ramps to eliminate below surface slab edges where the main runway meets taxiways, so as not to snag the undercarriage of any plane in the unlikely event that it skids off the runway. All has now been finished, leaving Colas with the final job of dismantling its plant (without the need to demolish temporary foundations because none were needed) and shipping it back on the next high tide.

New material is a boost for UK airports

Tiree’s runway has been resurfaced with a type of asphalt that is new to UK airports and offers a lot more value than materials traditionally used at UK airfields. The new material, developed by Colas from the French BBA (Beton Bitumeux Aeronautique) standard, was first introduced at Sumburgh Airport on the Shetlands in 2006. Sumburgh and Tiree operator Highlands & Islands Airports Ltd (HIAL) opted for a repeat this year, attracting interest from other UK airport authorities.

“Representatives from BAA have visited Tiree to inspect the results,” says Colas Business Manager for Airfields Carl Fergusson. “Marshall Asphalt has been commonly used for surfacing UK runways for the past 50 years and UK airport operators and their contractors rarely use any alternative. This is understandable, given that Marshall Asphalt is proven to perform well, but it is manufactured to a recipe based mixture design that allows very little opportunity for innovation or variation.

“In contrast, the BBA standard specifies asphalts in terms of their required performance in use. Mix design can be varied to withstand particular and often very heavy frequencies and weight of traffic and BBA materials also present greater production outputs and flexibility for project programming. They are substantially easier to mix and lay and BBA asphalt does not require grooving of the finished surface, although Tiree’s runway was grooved to give HIAL absolute confidence,” Fergusson adds.

Colas is marketing its BBA asphalt in the UK as Betoflex and commissioned Scott Wilson Pavement Engineering to carry out a technical study this year, of the feasibility of BBA materials for UK use in comparison to Marshall Asphalt. The findings, published in a full report, came out very much in favour of BBA, which can typically reduce required layer thickness by 15%, says the report.

“Crucially, BBA has superior physical and mechanical properties,” Fergusson says, “ leading to savings in whole life cost for airport operators due to less maintenance intervention and disruption.”

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