Steel tubes provided a solution when a piling operation hit problems, as Rodney Byles discovers.
It was always expected that some of the precast concrete piles for the new Olympicsized Wales National Swimming Pool in Swansea might be difficult. But reassessment of the operation by main contractor Shepherd Construction and its specialist piling contractor Aarsleff Piling has paid dividends in overcoming problems driving through the stiff made ground with obstructions.
Before Aarsleff started on the ú245,000 (-348,000) subcontract the company expected to drive just under 560 of its own 300mm square, single and jointed precast concrete piles from 11m to 18m long through about 6m of fill and underlying dense gravel.
But the Banut 500 piling rig, fitted with a 5t Uddcombe drop hammer, ran into difficulties with occasional piles refusing and breaking, according to Aarsleff Piling contracts engineer Gary Kime.
'We predicted about 10% of the precast concrete piles might hit obstructions and break, but in the end only about 7% were affected, ' said Shepherd Construction site manager Frank Lambert. 'Aarsleff Piling suggested replacing the concrete piles with steel tubes in those locations where the concrete piles failed. We thought it was a good idea, which solved the problem.'
'In driving replacement steel tubes next to the affected concrete piles and in particular trouble spots, we can put the same impact energy from the drop hammer into a much smaller area of steel. We had successfully used a similar technique on the grandstand foundations for the new Rockingham Motor Speedway at Corby, Northamptonshire.'
For the replacement steel tubular piles, Aarsleff used 244.5mm outside diameter oil well casing, which was collared and coupled together in 10m to 13m long sections.
The precast concrete and tubular steel piles, driven on a 3m grid pattern, were designed for compressive working loads up to 650kN. Precision Monitoring & Control carried out independent dynamic and static testing to verify capacity of the concrete piles and the replacement tubes.
Shepherd Construction is now well into its -13.9M main contract, starting in March 2001. The main 50m and 25m practice pool complex sits within Swansea University's existing sports centre ground and completion is scheduled for October.