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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.
The penstock is already completely obscured by the backfill
operation and, where possible, GHJ has removed all the
temporary culverts required during the installation, with the
exception of those where the pipe rests on the culvert.
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.
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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