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Apron analysis

The Greiner-Maunsell masterplan for Chek Lap Kok island created two runways, which flank the terminal building. The south runway was finished in 1997 and earlier than planned expansion means a second runway (HK$4.3bn/ £335M) to the north of the terminal building is under construction, due for completion late next year.

Orientated east-west to take account of prevailing winds, the strips are 3.8km long, with three parallel taxiways each. There is 1.25km2 of apron for passenger, cargo and commercial flights, and aircraft maintenance. This expanse of hard surface straddles solid granite platform and new landfill. As a result, pavement design has required careful analysis.

A joint venture led by WS Atkins with MMBP and Burns & McDonnell carried out a masterplan review and detailed the £1.65M south runway, taxiways and aprons in 1995. Because subsidence of up to 1.1m was expected, Atkins project director Bob Haywood says that at tender stage the Airport Authority asked each consortium to submit its own settlement calculations.

'We had no control at all over the materials that went into the landfill', Haywood says. 'Therefore we looked at the whole site, and instead of using fairly straightforward textbook parameters, chose our figures from a test reclamation on the same site.' As well as variations in ground composition, designs had to account for hard areas above service and vehicle tunnels.

Runways allow little margin for gradient or inconsistencies of surface: 'The vertical profile of a runway has to be very controlled. It has to be the same on opening day, after the initial settlement period, and in 20 years time,' Haywood says. Consequently the team designed in extra depth of pavement allowing protrusions caused by subsidence to be milled off. Slabs were favoured because they can be easily moved and replaced; reworkable and liftable water-bound sub-layers were used in place of concrete.

Linear elastic calculation was employed to study the material properties and forces exerted on the runways to optimise kinetic performance where hard surfaces lay on soft ground. Atkins senior group engineer Peter Tindall says: 'Historically calculation and specification has always been done on an empirical basis where what worked last time is the model for new projects'. In terms of pavement design linear elastic calculation is new 'and as traffic gets heavier, it is clearly the way forward. You can't risk failure'. Meanwhile, computers are now powerful enough to perform calculations as an integral feature of the design process.

As part of the design and construction Atkins brought in UK firm Dynatest to carry out heavy weight Deflectometer testing. 'What we try to do is simulate the passing wheel load of a plane as closely as possible,' says Tindall. Pavement conditions are traditionally assessed by eye, giving only surface information. However, the tests measure the degree and area of distortion, providing information on conditions inside the pavement.

Tindall points out, though, that 'Chek Lap Kok is unique. The runway has never had a plane on it. We are able to set a baseline standard which allows you to measure exactly how the surface deteriorates over time.

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