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Difficult to predict ground needed a flexible approach at a former US bomber airfield

What are claimed to be the only cruciform type piled foundations in the UK to support onshore wind turbines have been installed at the former Second World War North Pickenham airfield, 3km south east of Swaffham, Norfolk. They form an integral part of a £15.5M project for renewable energy provider Enertrag UK to supply power to over 10,000 homes in the area.

Local contractor RG Carter Civil Engineering is building eight foundation bases and access roads for the 1.8MW wind turbines under an approximately £1.1M design and build project for client Vestas - Celtic Wind Technology. RG Carter awarded Aarsleff Piling an about £150,000 contract for the 350mm 2 continuously reinforced precast concrete piles for the foundation bases. These were to the designs of German consulting engineer HCE Ingenieurgesellschaft, based in Hamburg.

The turbine bases are placed on the western side of the former airfi eld, which was the base for the 8 th USAAF 491 st and 492nd Bomb Group operating B24 Liberators.

Some of the crew huts remain and are now used as storage for agricultural equipment. Most of the runways and perimeter track are intact and partly occupied by the turkey houses of Bernard Matthews.

The SI had revealed substrata of weak to very weak chalk - a difficult material to predict - meaning the engineered solution needed to be flexible to deal with a variety of soil characteristics. Precast driven piling was the obvious choice since it provides continuous feedback from the ground.

To provide an indication of performance, site workers drove two probe piles at each turbine location. The length of the probe piles was calculated by applying empirical chalk values to the information available from the site investigation.

The probe piles were then subject to dynamic testing to prove the actual resistance values achieved compared to the empirical values used.

Aarsleff used a mixture of experience and insitu test data to rationalise the design lengths and mitigate the cost risks associated with the variable substrata. A calculated length for each turbine base was agreed and the piles ordered to provide this length, plus additional scope to accommodate the risk of piles still failing to reach the required performance, at depth. The cost of this risk factor was mitigated by being able to drive piles to a greater depth should they fail to achieve the criteria at the manufactured length, and provided a foundation that could resist substantial loads within a variable material.

Piling began in earnest with site workers placing working piles using a Banut 700 fixed leader rig equipped with a 6t drop hammer. The design called for 28 piles in each of the eight bases in four lines of seven. The four lines of piles were to be spaced 13m apart in a cruciform layout with piles in each line spaced at 700mm centres. These comprised a mixture of verticals, 1:4 and 1:8 rakers, with the outer rakers also rotated 15degrees.

But in several locations, the piling did not go to plan with the ground proving much harder than that experienced during the driving of the probe piles. The hard ground provided greater end bearing than that calculated, but also reduced the shaft length and therefore the tension capacity.

Initially, hard driving was attempted to reach the calculated length, but this resulted in several breakages. Following a team meeting, it was agreed to drive the piles to a set and then test to establish the tension values achieved. This proved very effective in reducing breakages and provided an opportunity to reengineer the design as and when variations in ground condition were encountered.

It also ensured the work carried out was no more than actually required and so was the most effi cient response to the problems posed by the chalk layers. 'Quite often a short pile would achieve the tension value required and therefore the number of remedial piles required was reduced, ' says Carter site manager, Chris Bradshaw.

'From the beginning we have had to work hard to engineer a solution for this project while at the same time leaving enough scope available to overcome the vagaries associated with the chalk, ' adds Carter general manager, Michael Turner.

As each base piling was finished, Carter followed on completing the heavily reinforced concrete foundation bases and other ancillary works and access roads. The Vestas V90 wind turbines, which have a hub height of 80m and 90m diameter blades, are expected to be fully operational two months ahead of programme by the end of this month.

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