TPi March 2014

temporary culverts required during the installation, with the exception of those where the pipe rests on the culvert.

Alan Jones continues, “We were able to bend the pipe but it would try to spring back to its natural shape so in some places we had to position stones in the trench to help the pipe keep to the required contours. We then backfilled as we went along, using special rotation buckets fixed to the excavators that could rotate to the angle of the mountain. This speeded up the installation and helped to return the mountainside to its natural state as quickly as possible, using the temporary road we’d constructed as part of the project as the backfill material for the trench.” The majority of the penstock is GPS black PE 100 pipe in SDR 26 with a relatively thin pipe wall for the 560mm diameter, but as the water pressure increases within the sections of penstock closer to the turbines, the wall thickness was increased to SDR 17 and then SDR 13.6, while the diameter remains the same to maximise flow rates. To ensure that the changes in pipe dimensions were accommodated seamlessly along the route of the penstock, GPS provided custom-made change pieces to make the connections between the different SDR pipe sections. The company has also provided a bespoke flange to connect the final section of PE pipe to the ductile iron section at an entrance to the power house.

Alan Jones adds, “Returning the site to nature has been an important part of the brief throughout the project and there are already few signs that a major civil engineering scheme has taken place on the mountainside. The turf transplantation and seeding, along with natural growth and the infamous Snowdonia rainfall, will help accelerate this restoration of the landscape so that there will be no evidence of the PE penstock snaking down the hillside until you enter the power house.” Cadair Idris Cadair Idris is located in Gwynedd, at the southern edge of the National Park, near Dolgellau. The idea of a hydroelectric power station was first mooted by one of the farmers that occupy the steep hillside several years ago and the current scheme first began when hydro development company Dragon Hydro identified the farmland as a potential hydro site in 2008.

The penstock is already completely obscured by the backfill operation and, where possible, GHJ has removed all the

Explains David Roberts, Dragon Hydro’s consultant on the scheme, “The site we identified borders two farms but the

Hafod y Llan

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March 2014 Tube Products International