Case Studies

Hafod y Llan Hydro

The steep slopes of Mount Snowdon make it an iconic image of the famous National Park and a destination for serious climbers, casual walkers and sightseers alike.  But the past few months have seen more than the usual tourists, hikers and sheep at the famous peak as the National Trust’s Hafod y Llan hydro-electric power station has taken shape.

While the 640kW scheme will permanently add to the uses that Snowdon has been put to over the years, careful planning, design and project management means that, even before the installation is fully completed, it’s already effectively disguised  within the rugged landscape.  But this well-camouflaged scheme hides an extraordinary feat of engineering: once fully operational, the hydro-electric plant will play a key role in delivering the National Trust’s ‘Fit for the Future’ initiative to invest in renewable energy, with a generating capacity equivalent to the power requirements of all the Trust’s properties in Wales.

Constructing such a significant scheme within such a challenging landscape has been no simple undertaking, and doing so while delivering the National Trust’s remit to conserve and protect the environment has added to the project’s complexities.  However, that environmental responsibility was central to Hydropower Services’ design for the scheme and local contractor, GHJ Civil Engineering & Construction leveraged its local and hydro scheme knowledge to manage both the project’s logistical and environmental challenges.

Specifying the Penstock

The Hafod y Llan scheme will extract water from the Afon Cwm Llan river at the head of the valley which will be fed through a 1km Black PE100 penstock, supplied by PE pipe specialist, GPS PE Pipe Systems. At the foot of the penstock, GHJ will construct a power house, which has been design to look like a typical Snowdonia out house. Here the rapid flow of water will enter the turbine, generating electricity before being returned to the river downstream without any effect on the water quality.

PE pipe was chosen for the penstock installation thanks to its flexibility which has allowed the contractor to bend and shape the pipe to the contours of the landscape, routing the penstock as close as possible to the river.  The use of PE has also enabled the design team to match the wall thickness (SDR) of the pipe to the required water pressure at differing points along the route, providing cost and raw material savings.  The only exception to this is the final section of penstock where the water enters the turbine at very high pressure and the wall thickness that would have been needed for PE pipe made ductile iron a cheaper alternative.

Explains Alan Jones from GHJ: “Using PE pipe made the installation less invasive on the landscape because the pipe’s flexibility enables the penstock route to stay true to the contours of the hillside.  Using PE also made the installation more cost effective because it required less excavation and gave us some leeway for small diversions around clumps of heavy rock.  Indeed, the ductile iron’s maximum bend tolerance of two degrees meant that we had to painstakingly cut through rock to lay the final section of pipe at the base of the penstock and the increased excavation time and costs involved in this demonstrate the balancing act between material costs and installation requirements on projects like this.”

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 particularly arduous 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 license, 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 900 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 significant 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.

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 pipes sections. The company has also provided a bespoke flange to connect the final section of PE pipe to the ductile iron section at 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.”

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