Hollow steel tubular piles have proved to be the best foundation solution on a contaminated site in Cardiff.
After several failed attempts to reclaim the contaminated 3.6ha Grove Park site on the northern outskirts of Cardiff, it is finally being transformed into a residential development.
The former clay pit, brickworks and disused landfill site has stood derelict for over 20 years but now property developer Deanminster, working closely with its consulting engineer Arup and foundation contractor Aarsleff Piling has a cost effective foundation solution using up to 30m long tubular steel piles which finally made the project viable.
The end result will be 177 houses, 44 sheltered homes and a medical centre with doctors' surgery.
The site was last used in the early 1980s by a demolition contractor dumping waste generated in the reshaping of Cardiff City centre, although a wide range of other materials was also deposited at the site. The landfill was subsequently capped.
Arup was no stranger to the site, having worked with other developers on four previous occasions. The nature and thickness of the fill effectively ruled out any form of ground improvement. It had therefore considered using bored concrete and precast concrete driven piling.
There were problems with both.'Bored piling would have required the disposal of the contaminated arisings from the pile drilling operation, with the added risk of possibly contaminating the groundwater table, ' says Arup head of geotechnics (Wales) Ian Statham.
'And driven precast concrete piles could not have dealt with obstructions without the considerable risk of pile shaft breakages.'
Statham explains that the options were considered to be environmentally and technically unacceptable and extremely costly, given the depth of the fill, which combined to make the site uneconomic to develop at the time.
When Deanminster subsequently requested Arup to reappraise the site in 2003, it looked instead at using hollow steel tubes.
Deanminster funded a site trial with Aarsleff installing 24 tubular steel test piles in various groups around the site. The tubes were lengths of recycled oil well drill casing, collared and coupled together in 10m to 13m long sections.
Aarsleff was confident that this method could get through the very stiff fill and cope with the obstructions. It was able to put the same impact energy required for a larger crosssection precast concrete pile, into a much smaller area of steel.
The steel piles did prove robust enough and there were no arisings from the hollow tubes.
From core samples taken from inside the tubes and in the surrounding areas and Aarsleff was confident the piles could reach and penetrate the tip's underlying mudstone and take the required loadings. It priced the piling operation accordingly, allowing Deanminster to proceed.
Deanminster appointed Sir Robert McAlpine as its principal contractor, which included supervising Aarsleff 's £450,000 contract.
Aarsleff ordered the steel tubes in March 2004 before starting on site for the first of two visits in June. 'We persuaded Deanminster to allow us to pre-order the steel tubes and stockpile them before our scheduled site start date, ' says Aarsleff piling manager Philip Chippindale.
'If we hadn't done that the project wouldn't have gone ahead, because since then the price has increased significantly and availability dried up.'
Aarsleff, using one of its own Banut 700 piling rigs with 5t drop hammer, is driving about 670 tubular steel piles. The 244mm outside diameter tubes and the larger 273mm tubes range in length from around 11m up to a vast 30m long in the central, deepest, area of the pit.
The two types are designed for compressive working loads of up to 425kN and 625kN respectively. All the piles are driven through the landfill to socket up to 2m into the mudstone bedrock.
The piles are installed in line with the structures load bearing walls. They are cut off to level prior to Sir Robert McAlpine capping them with cast insitu concrete ground beams.
When groundworks are complete house builder Westbury Homes (Holdings) will then follow on with the one, two and three bedroom houses, while other contractors will erect the sheltered accommodation and medical centre.