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Smooth operators

Bangkok

Out on the airfield a truly immense paving operation is under way. Contractor IOT, a second but separate JV with the same members as the terminal contractor, has eight concrete batching plants and five large asphalt plants working continuously to provide the material for 1M. m 2 of pavement quality concrete and asphalt runways and taxiways totalling 2M. m 2. Work on the £203M contract also includes construction of an 820m long, 54m wide concrete cut and cover tunnel for a people-mover. This will also link to a future satellite terminal, as the airport will expand from an initial 45m passenger annual capacity to 100m.

Much of the ground clearance for the project was done in the 1990s, transforming a one time 'cobra swamp' into a usable space. To stabilise the water table classic Dutch techniques were used to create an embankment around the airfield and the network of 40km of canals within it, says Scott Wilson's appropriately Dutch Lauri Van Run. The firm is leading a second supervision group of five consultants which includes Norway's Norconsult and three local firms.

'The drainage also holds up to seven days of tropical rainfall inside the airport to protect surrounding villages from flooding' he adds.

On Bangkok's notorious 'toothpaste', 12m-20m thick marine clay, settlement is a huge problem. Piling was an option but finally a major surcharging operation was carried out using thousands of vertical preformed drains to help an estimated 1.5m-1.8m settlement.

Paving sets major quality challenges and a central supervision issue has been to convince the contractor that 'a runway is not simply a giant road' says van Run. 'We got them to establish proper testing laboratories and so forth. Everything is about control - smoothness, density and level.' 'Aircraft don't like bumps and humps and you have to achieve tolerances of not more than 3mm in any 3m.' The supervisors have taken a non-confrontational approach, he says - 'the only one that works in Asia' - and have instead offered technical advice and expertise.

An example he points to on the airfield is the use of an extended straight edge level marker on the paving machines, replacing the shorter 'sledge' that the machine usually uses to follow the subbase or previous asphalt layer. It reduces 'waviness'.

High quality is also important on the concrete hardstanding areas where 450mm thick concrete is laid in long strips.

After 12 hours this is saw cut into 4.5m by 5m bays.

The bays serve to reduce both shrinkage cracking, and expansion and contraction effects with consequent induced spalling of the concrete.

'Spalled concrete is responsible for a large part of the flying object debris within airports, ' says van Run.

Once again quality is crucial and the water/cement ratio is critical. And to maintain consistency the logistics are also important, he adds, when placing 2,000m 3 of low 20mm slump concrete.

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