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Laboritory conditions

While foundations for offshore wind farms may be a hot topic in geotechnics, the growing demand for testing of renewable energies is presenting the foundations market with a different challenge. Claire Symes reports.

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The piles used several different types of reinforcement to meet the loading demands

Many believe that systems for generating electricity from wind and waves have the potential to reduce reliance on burning fossil fuels to meet ever growing energy demand.

But before the multi-million pound green energy generators can be installed, complex testing is needed to ensure they can stand the test of time - as well as the wind and waves.

Testing these generators will soon be possible at a specially commissioned £42M facility being constructed by Shepherd Construction at the National Renewable Energy Centre (Narec) at the dockyard in Blyth, Northumberland. To withstand the high forces involved in analysing renewable energy systems, Shepherd is working with Van Elle to deliver the complex foundations needed.

The work is being carried out under a framework agreement between Shepherd and Narec and work on site to build the new facilities, which will house test rigs for full-scale wind and marine prototype devices, started in January 2011. Three schemes are currently underway - the blade testing facility, the marine drive train testing facility, and the nacelle wind turbine test facility. The first two will be completed this summer, while the nacelle wind turbine test facility is due for completion next summer.

Specialist consultants

The foundations design being used at the site is the result of collaboration between Shepherd and Van Elle, as well as several specialist consultants. Shepherd Construction project manager Neil Matthias says: “Van Elle works closely with us to find a solution for the site. It is quite a challenging one as it is a brownfield site with a number of infilled docks.”

Shepherd appointed Van Elle in December 2010 not based just on the value of the tender, but on Van Elle’s solutions for the project. “Van Elle was one of five companies bidding for the work and the presentation the company gave offered the best ideas for dealing with the issues at this site,” says Matthias.

One of the key elements of Van Elle’s solution was the company’s approach to dealing with the potential presence of buried timber and pockets of asbestos in the infilled docks. Van Elle proposed to prebore the pile positions with a narrower diameter probe to check ground conditions. Despite the risks of piling through such thick made ground, Van Elle has not needed to case the piles.

Work started first on foundations for the blade testing facility. The finished building may just look like a large warehouse, but the testing activities inside will place a high demand on supporting piles. The facility will be the largest of its type and capable of testing wind turbine blades up to 100m long - Narec’s existing facility only has capacity for 50m long blades.

Once complete the 6,115m2 steel framed building will house 40t and 50t gantry cranes to move the blades into position at a testing hub which will test the flexing and durability of the blades. A blade pull hall within the building will also test the horizontal flexing of the blades through application of static loads using winches fixed to the reinforced floor.

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Strength in numbers: C50 and C60 concrete was used for some of the piles

“The testing will exert high tension and lateral loads - with forces of between 750kN and 3,000kN designed for, but analysis for dynamic loading was also necessary and we worked with Arup on this element of the project,” says Van Elle contracts engineer Andy Waghorn.

“We worked with Spain-based consultant IDOM to develop the design for the piles to cope with the high compressive loads expected to be exerted on the test bed”

Andy Waghorn, Van Elle

Van Elle has installed a series of 450mm, 600mm and 750mm diameter continuous flight auger (CFA) piles - 500 in total - to depths of 12m to 18m.

“The depth of the piles varied because we had identified a competent stratum that could support the piles but the depth to that stratum varied and we had to analyse each pile as it was constructed,” says Waghorn. “The site lends itself well to the use of CFA piles. The strength is variable and there is a coal seam at 10m to 14m below ground level, so we needed to use high-powered rigs to ensure the piles passed through.”

Van Elle is using a Soilmec CM70, Soilmec SF50 and its brand new Llamada P140 drill rigs on the project. The new Llamada rig offers 185kN/m of torque and 20kN of pull down force with five gears.

The structure is supported by a combination of 66, 450mm diameter piles up to 14m long, which offer a safe working load of 750kN, and 397, 600mm diameter piles up to 18m long that are designed to have a safe working load of 3,000kN, as well as taking tension loads of 900kN and lateral loads of 150kN. The blade testing hub is supported by 84, Arup-designed 750mm diameter piles that are up to 15m long.

“Before we started on the project we carried out some trial bores to correlate drilling results with the logs and refusal. Our rigs are fully instrumented so we cancompare the trial results with drilling resistance to ensure the piles are bored to the right depth. But it does need continuous assessment,” says Waghorn.

Static compression

Pile tests were also carried out with static compression and tension testing to prove the pile working load. Integrity testing and cross-hole seismic testing was also carried out. “The results were what we were expecting,” says Waghorn.

Work on the foundations for the blade testing facility finished in June last year and while construction of the above ground structures got underway, Shepherd’s focus shifted to starting work on the marine testing facility (see box).

Marine development

Construction of the new facilities for Narec at Blyth is making best use of the dockyard and the centre for testing marine drive trains will retain part of one of the docks to create a pool in which some of the testing will be carried out. The facilities for testing marine drive trains up to 3MW in capacity will be housed in a 15m high, 600m2 steel frame facility, which, although smaller than the other two buildings currently under construction, is no less complex.

The facility is being built over a former dry dock, which was once used to build the Ark Royal, but is now being infilled in three 11,000m3 concrete pours over a period of eight weeks to bring the construction level up to the base of the slab level.

Shepherd has topped the backfilled dock with a 3.2m thick, 45m long and 15m wide reinforced concrete slab on which the steel test frame will be mounted. The slab was poured in three stages and given a power float finish to meet the 0.3mm tolerance demanded by Narec.

Van Elle’s next phase of work started this year with piling for the nacelle testing facility. The nacelle is the central motor, gearbox and aerodynamic cover on a wind turbine.

The 3,000m2 building will have facilities for testing nacelles of up to 15MW and will house four overhead cranes, two of which will be capable of lifting up to 250t each.

The company is currently installing 233 bearing piles to support the building, a further 60 piles for the foundations of the test bed and a secant piled wall.

“The secant piled wall was part of the solution we offered to Shepherd to help the site team cope with the perched groundwater which was found in pockets at 2.5m below ground level during the ground investigation,” says Waghorn.

“The wall is 70m long and the 278, 450mm diameter piles extend to 8m below ground to enable excavations to be carried out to 4m at a later stage of construction.”

In total 142, 600mm diameter piles up to 20m long and 91, 450mm diameter piles up to 18m long have been installed for the building. The 600mm piles are designed to take loads of 3,250kN in compression, 1,200kN in tension and 150kN of lateral loading, while the 450mm diameter piles have a capacity of 2,000kN and are designed to take 150kN and 60kN, in tension and lateral loading, respectively.

Work on the secant piled wall and foundation piles has been completed but work on the floor slab piles is still underway. “We worked with Spain based consultant IDOM to develop the design for the piles to cope with the high compressive loads that are expected to be exerted on the test bed,” says Waghorn.

“The forces from the nacelle testing are at the upper end of the working loads that can be achieved. We are using C60 concrete with an S4 slump for the piles to meet the high loading demands.”

The 900mm diameter piles - 60 in total - that are being installed for the test bed are up to 20m long and have a safe working load of up to 4,000kN, as well as high tensile and lateral loads. We are using centrally placed 305x305x97 UC section reinforcement cages from Ivanhoe Forge on these piles,” says Waghorn.

“We initially had concerns about the workability of the concrete and the ability to install the reinforcing cage into the mix but it has not proved to be an issue on site.”

Van Elle has worked closely with Tarmac over the supply of the concrete as the piling work has called for seven different mix designs. “We have used both C50 and C60 concrete for some of the more highly loaded piles, as well C35 and C40 for other piles,” says Waghorn. “There was also a soft and hard mix for the contiguous piled wall.”

Work so far is going well and Matthias is happy with progress, particularly with Van Elle’s approach to deliveries and safety.

“Logistics have been an ongoing issue due to lack of room on site, so deliveries have had to be carefully planned,” he says. This has included the steel reinforcement from Romtech. Van Elle organised for the pre-formed cages to be delivered just ahead of installation and for them to be pre-slung to improve safety and speed up delivery.

Work on the blade and marine testing facilities is currently on track to be completed this summer, while piling work on the nacelle facility will finish soon with the rest of the building scheduled for completion next summer.

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