Challenging construction
The logistics of transporting the pipe to site, storing it during
construction, constructing the water intake in the river and
excavating the trench were all arduous thanks to the terrain
and the conditions that lasted well into spring.
GPS delivered the pipe in 12m lengths in two separate
consignments, and GHJ constructed a secure compound at
the bottom of the site where the pipe could be held safely until
required. The excavators and butt fusion welding machine
were painstakingly transported to the top of the site, with the
GHJ team having to take into account the risk of landslips and
silt entering the river during that process. It took six weeks to
transport the required 13-tonne excavators into position at the
top of the site and the exacting process involved breaking a
route through rock at the rate of just a metre a day in order to
create a temporary road.
Construction of the weir, carried out to meet the project’s
Environment Agency extraction licence, was designed to
leave sufficient water in the river to protect native moss
and vegetation and to maintain the dramatic character of a
waterfall along the route, which belies the new technology
being installed in the mountain. Bags of aggregate were used
to create a temporary diversion, providing a dry working area
so that the water intake could be constructed at the head of
the penstock, and GHJ had to monitor the weather carefully
throughout this process to ensure that the risk of flash floods
was managed effectively.
The open trench for the penstock was excavated before
the pipe was transported up the mountain to site, and the
demanding landscape once again made this a painstaking
process.
Alan Jones continues, “The working corridor was very narrow
with just a foot either side of the trench in some places with
vertical drops next to the working corridor and only limited
passing places. We also had to protect the river, particularly
where we were constructing culverts where crossings were
required. To do this we used stones to filter the water and
straw bales on the downstream side of the culverts to trap any
mud and sediment.”
Careful management of the river was not only important for
environmental reasons but was also necessary to maintain a
dry trench while the PE pipe installation took place.
GHJ created cut off drains in the sides of the trench at a
90° angle to enable water to exit the trench, so that the pipe
could be installed quickly once the complete trench had been
excavated and to ensure that there was no damage to the
trench from water erosion.
Flexible approach
The PE pipe was transported to numerous safe drop-off points
along the route of the trench using a helicopter and specially
adapted handling equipment to
protect the pipe from damage
during transit, with wind and
down drafts proving a signifi-
cant challenge on the exposed
site.
The pipe was laid out in sections
and GHJ then used an innovative
method of loading the welding
machine and tracking it forward
to weld the pipe in the trench in
a continuous downhill operation,
constructing a custom-made
steel-frame structure over the
welding equipment to protect the
joints from the adverse weather
conditions.
While the PE100 pipe is flexible,
the need to install it in both
vertical and horizontal alignment
with the trench due to the natural
contours of the land and the
incline of the mountainside made
the installation more complicated.
Cadair Idris
Cadair Idris
118
Tube Products International March 2014
