Precast Concrete
Fast track for Aarsleff
Close co-operation between Morrison Construction, the main design and build contractor for the Rockingham Motor Speedway at Corby, Northamptonshire and its specialist piling sub-contractor Aarsleff Piling, was a major factor in solving unexpected piling problems on the multi-million pound project’s main spectator grandstand. The 27m long precast concrete piles took an unforeseen exceptionally long time to drive through the extremely stiff made ground, that combined with numerous underground obstructions, prevented many piles reaching the underlying limestone and prompted a total reappraisal of the piling operation.
Such a high rate of refusal had not been expected and Aarsleff Piling and the Morrison Construction design team worked together to find an alternative solution. Aarsleff Piling suggested swapping the concrete compression piles with steel tubes. Morrison’s thought it was a good idea and the design team verified the changes, which they felt then solved the problem as the tubes were being driven through the fill and into the underlying limestone.
Aarsleff started piling on the approximate £200,000 first phase grandstand and was expected to drive about 345 of their 250mm and 300mm square precast piles through 26m of fill and toe into the underlying limestone. But the company’s own Banut 700 piling rig, with 5t drop hammer, soon ran into difficulties with piles taking up to 2 hours to drive and some refusing at 20m. The material was described as boulder clay with SPT values in the low 20s and so no major driving problems were expected. It was thought the piles might refuse at the limestone, which was perhaps higher than expected, so a borehole was sunk to confirm the depth of the limestone layer, which was at about 26m below ground level as originally expected.
With many of the piles stopping in the fill Aarsleff initially tried to reduce skin friction by lubricating the pile shaft with water during driving in the very stiff fill. Aarsleff placed a shoe on the pile toe, which was slightly larger than the pile cross section to create a void round the shaft. The annulus was filled with water during driving, which, combined with changing to a 6t hammer on the rig. This cut driving time in half, but piles were still refusing. An alternative was needed so Aarsleff suggested replacing the smaller concrete piles with steel tubes. The hammer could put the same impact energy into a much smaller area of steel and this method got through the very stiff fill and dealt with the obstructions. Aarsleff had already driven about 50 out of the approximate 180 of the 250mm square piles and replaced the remainder with 244.5mm outside diameter oil well casing, that were collared and coupled together in 10m to 13m long sections. The tubes were much more successful and driving time was considerably reduced but we then continued with the larger 300mm square concrete piles to accommodate tension loads.
The piles, driven in clusters of 2, 4, 6 and 7 piles, were designed for working loads up to 465kN for the smaller size piles, while the larger 300mm square concrete piles had to accommodate 650kN compression and some up to 110kN tension. Negative skin friction had been independently assessed at up to 700kN per pile. Independent dynamic testing of the piles by Precision Monitoring & Control verified capacity of the concrete piles and the replacement tubes on the first phase of the grandstand piling.
Aarsleff also won the second and larger £350,000 phase of the grandstand piling which required 424 piles. From experience gained during the first piling contract, Morrison, in consultation with Aarsleff, changed over as much as possible to steel tubes and only retained 63 of the 300mm precast concrete square tension piles from the original scheme. As with the first phase the 250mm square piles were replaced with 244.4mm diameter steel tubes, while 153 of the originally planned 216 precast, 300mm square piles were swapped for larger 273mm outside diameter tubes. In addition Aarsleff also installed an 11m square steel sheet piled cofferdam to provide support for an access shaft to a pedestrian subway under the racetrack.
Despite the severity of the ground conditions Aarsleff completed the project on schedule for Morrison to follow on casting the pile caps and placing the columns for the 11,000-seat grandstand.
The multi-purpose motor sport complex contains a 2.4km long banked high-speed oval track, designed to host major US style oval racing. There was also a 3.9km inner circuit designed to full international standards. Rockingham Motor Speedway is Britain’s first purpose designed motor racing venue since Brooklands opened in 1907.
- Design and build contractor:
- Morrison Construction Ltd.
- Specialist Piling contractor:
- Aarsleff Piling Ltd.
- Design team:
- Edge Consulants and Pick Everard
