STIFF MADE ground at the site for a new swimming pool and diving centre in Leeds, West Yorkshire saw piling subcontractor Aarsleff Piling doing precontract trials to get tubular steel piles into the ground.
The company liaised with main contractor Sir Robert McAlpine as it expected it would be difficult to drive the required tubular steel piles into the underlying mudstone. The trials were to determine the best drop hammer to cope with the anticipated hard driving.
'The pool and diving centre are being built on the car park of the multi-sport South Leeds Stadium, which is on the site of a former opencast coal mine, ' says Aarsleff contracts engineer Matthew King.
'We anticipated the mining backfill and underlying mudstone could pose a problem driving the piles, which were up to 26m long.' It first tested its 5t Banut hammer but because of the very hard driving it was working beyond its recommended limits and unable to achieve the desired penetration. From the drive information gained Aarsleff decided on a bigger 7t Junttan HHK-5/7A hammer that could work safely within its capabilities. This switch meant the rig crews were able to achieve the specified penetration and working pile loads.
The company claims the hydraulically accelerated ram boosted impact energy and increased efficiency by up to 20% at full stroke compared to a conventional free fall 7t drop hammer. It produces maximum impact energy of 84kNm at a full stroke of 1.2m. Impact energy, ram stroke and blow rate can be adjusted to suit ground conditions and pile type.
Aarsleff began its £673,000 contract at the end of July to install about 13,000 linear metres of piling. The 530 steel piles consisted of various sizes of recycled oil well drill casing, ranging from 178mm to 273mm diameter, which was collared and coupled together in 10m to 13m long sections.
The tubular steel piles were designed predominantly for compressive working loads up to 1,100kN with some tension piles catering for 200kN. Precision Monitoring & Control carried out independent dynamic and static testing to verify capacity of the driven tubes. Pull out tests were also done to check the integrity of the interference fit of the piles' collared joints. The subcontractor finished its 10 week contract at the end of September.
Sir Robert McAlpine, working for client Leeds City Council, is building the swimming pool complex adjacent to the existing South Leeds Stadium, to a design by architect and structural engineer William Saunders Partnership. The company started on its £15M contract in April 2005 and expects to complete in time for the complex to open in February 2007.