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Wembley comeback

PILING & DEEP FOUNDATIONS

Wembley is the scene of many schemes aimed at reinvigorating the rundown area. Gareth Beazant reports on piling operations at a new housing development.

Wembley might be internationally known as the home of English football but the north-west London suburb has for years suffered from industrial and social decline.

Now things are looking up, thanks to the National Stadium finally getting the go-ahead, as well as separate injections of government funds to regenerate the area.

Tower blocks are disappearing to make way for neater housing, mixing affordable and key-worker accommodation with more exclusive homes.

One example of the revival is a £25M Bellway Homes development in North Wembley that will provide 344 properties supporting 1139 residents. The highest buildings will be four storeys, with driven piled foundations installed by Aarsleff Piling on a £210,000 contract.

In total 2100 piles will be driven on the graveyard of a former Wembley industry giant - the GEC Hirst Research Centre.

Aarsleff started on phase one of the project in January and phase two in August. Before work began, testing was done to ensure that surrounding properties would not be affected by the driving operations.

'We carried out dynamic, vibration and noise monitoring tests, ' explains Aarsleff piling manager Pat Deighan. 'Towards the back of site is a large housing area and the clay soil has meant that there is very little vibration.

All tests proved that surrounding buildings would not be harmed.' Along with the need to monitor the thump factor, speed has played a crucial role in the project with builders following close behind the foundation work, explaining why a driven method was chosen over CFA.

'CFA piles here would probably have had diameters of 450mm so there would have been more concrete, ' says Deighan. 'Driven piles can prove quicker as there is no spoil removal, it's clean and there's less traffic to carry the spoil out and get the concrete in.

To keep ahead of the builders, teams of two men are working two Banut rigs.

Each rig weighs 45t, is 16.2m long and 3.35m wide. They have a maximum pitched pile length of 17m and a minimum working height of 13.7m to the top of the Cat head. The hammer is a 4-5 tonne Banut/Uddcomb hydraulic free fall drop and each rig can be modified to become job specific.

The self-erecting rigs can be ready an hour after arrival and each self-loads piles from a lorry.

At Wembley, this means up to 30, 250mm square piles a day can be installed. Pile lengths range from 9m to 14m with load capacities from 10t to 54t.

Housing accounts for 40% of Aarsleff's business and it has established good connections with the major players. The 4ha site in Wembley is accompanied by a similar Bellway development in nearby Hendon where one rig is being used.

'We have a good relationship with Bellway Homes, ' says Deighan. 'Typically on a project we will receive the calculations and then we'll design the piles.

This will then be sent for approval and we can start.' The precast piles are made in Aarsleff's factory in Newark, Nottinghamshire. It has targeted precast concrete piling since it began casting piles in 1993. Since then it has manufactured 2M linear metres and has been steadily increasing production - in summer last year it peaked at 16,500 linear metres a week.

Over the past nine years factory methods have evolved. At first the firm adopted handmade reinforcement cages by using a preformed helical stirrup with cutto-length straight bars, which were hand-tied together. However in Denmark, Centrum Paele (part of parent company Per Aarsleff) was using a fully automatic welding and fabrication robot machine to produce cages direct from coiled bar.

As production in the UK grew, the firm introduced the same welding technology to increase the length of a single precast concrete element.

Until then the bulk of standard length, single element precast piles in the UK was 13m. Aarsleff can now make and install 18m long, 250mm square section precast concrete piles. The welding robot enables it to produce a very rigid concrete pile.

Aarsleff worked with its parent company to design and develop a self-compacting concrete mix and eliminate the need for vibrating the concrete in the moulds.

Concrete firm RMC was involved in the trials and helped develop a mix for the UK. It is an Ordinary Portland PFA mix with an additive resulting in a Class 4a sulphates mix, which achieves higher striking strengths at 16 hours and driving strengths at 10 days than conventional sulphate resistant mix.

It flows into the moulds like water and can be stripped within 16 hours and installed with a hammer within 10 days. Fullscale production casting started in the middle of June 2000. The uses of this concrete mix led Aarsleff to design and build a casting machine that was commissioned in spring of 2001 (GE March 2001).

Perhaps the biggest change at the factory came this summer when Aarsleff and Stent Foundations agreed to merge their concrete pile manufacturing operations in England and Wales into a single jointly owned precast concrete pile manufacturing company. Based in Newark, the new company is named Centrum Pile, and its operation allows Aarsleff to focus on contracting.

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