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WIND IN THE MOORLANDS

PILING & FOUNDATIONS

A new onshore wind farm faced challenges posed by moorlands in north west England.

Alexandra Wynne FInds out more.

'We wanted to get a handle on what all the risks were, so while we were looking for near surface mine workings, we also looked for rock head that was either too shallow or too deep and high groundwater, ' says consultant Scott Wilson geotechnical engineer, Mark Bruton.

He is describing the tricky ground on a large wind farm project near Rochdale, Greater Manchester. 'We needed to prove the ground conditions for each turbine, ' he says.

The company is working on the Scout Moor wind farm project that also needs a system of oating access roads and grout curtain foundations to negotiate the boggy peat moor and mine workings.

Scott Wilson is responsible for the conceptual and detailed design of the project and faced the challenge of providing stable foundations for 26 turbines, each 100m tall including blade, on a site with variable ground, as well as providing 12km of roads for construction trafc.

Initial desk study information using geological maps, memoirs and local mining records showed evidence of six about 150mm to 300mm thick coal seams and recorded mine workings on site. But designers knew there would be a risk that other unrecorded workings could exist.

The team had to come up with a way of making sure that foundations had suf cient stability because of the possibility that voids would make the ground unsuitable for the turbines. Additional geotechnical investigation work involved digging a trial pit and drilling a 100mm rotary percussive core borehole at each of the turbine locations. The team devised a trafc light system to classify risks posed by mine workings at each site.

Low risk sites, where preliminary boreholes and trial pits revealed no evidence of mines or voids, were classi d green and require no additional strengthening work before foundations are built.

Boreholes at higher risk amber and red sites showed between 50% to 100% voids in the ground caused by near surface mines. The team decided to treat these sites with a drill and grout technique to create stable ground for the turbines.

Site workers begin by proof grouting amber and red sites and then drilling 75mm boreholes at the centre and each corner of the 15.5m 2 foundation footprint. They drill down to a minimum of 2m below the mine workings (typically the maximum depth has been 20m) before pressure pumping grout in to ll voids.

Bruton says in most cases it is clear cut whether mine workings are under the surface because holes continue to take grout. But in some areas the workings have collapsed and grout sufciently lls voids to create strong enough ground for contractors to begin work on foundations.

Bruton adds that it is not possible to strengthen all ground by endlessly pumping grout into boreholes that continue to take it, so the team designed a grout curtain to form a contained box beneath the surface for sites with many voids.

Rig operators drill holes 3m away from the perimeter all around the foundation footprint at 1.5m centres.

A stronger mix grout is pumped into each borehole to create the curtain wall before primary and secondary in ll holes (on 3m and 1.5m grids respectively) are drilled and lled with weaker grout to create solid and stable ground for foundations.

The same ground improvement work is being done to strengthen pads sitting adjacent to each turbine to take cranes that will lift and erect turbines.

The site's geology comprises mudstone and sandstone. Bruton says that 23 of the turbines will sit on sandstone, which at its weakest will cope with loads in excess of 2000kN/m 2. The remaining three found on mudstone, which raised concerns that it might cause excessive settlement. However, plate load tests established the strata could bear twice the 300kN/m 2 working load required.

A 100mm thick concrete blinding forms a level basis for the foundation work. Reinforcing bars with holding down bolts from turbine supplier, Nordex are built up to form the foundation structure before the concrete base is cast. The fi rst turbine towers are due to arrive from November and will connect to the supplier bolt system.

Engineers looked at the possibility of using anchors to reduce the 3m deep excavation needed to form gravity foundations. However, bolts for anchors need constant monitoring for tension and corrosion and it was ruled out as too high maintenance in the longer term.

Bruton says piling was considered for turbine foundations at sites with suitable ground, but logistically it became too diffi ult to negotiate access for the large piling rigs needed to do the work.

McNicholas Construction Services is the main contractor and won an about £11.5M contract from client Scout Moor Wind Farm. McNicholas contract manager, Colin Lawrenson says the team was keen to minimise logistical issues on site.

'We wanted to simplify the project and reduce the need for additional subcontractors. In the end, we thought we'd be better off digging down to create the foundations.'

The team decided on gravity foundations designed for buoyancy that would include drainage channels to cope with high groundwater on site. Water seepage in trial pits and groundwater monitoring standpipes have revealed that the groundwater table ranges from near surface to 5m depths at turbine locations.

Although this choice of design removed concerns about mobilising piling rigs, the project team still had to come up with a suitable design to provide over 12km of access roads to cut through the previously undeveloped moorland.

In some locations bedrock and clays lie at surface level, but the majority of the site is covered in up to 3m of soft peat. Bruton says Scott Wilson offered two designs to enable construction traffic to move about the site.

Two layers of CMS Secugrid 40/40, supplied by Naue, are rolled by hand along the length of the road where bedrock appears at surface level or peat is less than 600mm thick. The two geogrids are separated by 300mm of compacted 6F2 graded stone and finished with 700mm on top.

Where peat thickness exceeds 600mm Bruton says the design becomes more of an art form than a science because the sheer strength of peat is very diffi cult to quantify. In addition, some stretches of the road sit above mine workings.

Although designers were less concerned about the risk of collapse than at turbine sites, they wanted to be confi dent that the road could cope with 16t axle loads. In these more vulnerable locations, the lower geogrid was substituted with a stronger layer of Secugrid 400/40.

Lawrenson says that although this fl ating road system will require maintenance during enabling works, there is little point in creating more permanent roads serving the wind farm until construction traffic is reduced.

Each of the Nordex N80, 60m tall turbine towers have three 40m long blades and can generate 2.5MW of electricity. It is anticipated that the combined power of 65MW will be enough to supply the average needs of 40,000 homes each year. Its 25- year operational period is due to begin in mid-2008.

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