Llyn Teifi Reservoir Spillway Rehabilitation

Llyn Teifi Reservoir Spillway Rehabilitation

Our requirement was to carry out rehabilitation works to the existing Llyn Teifi impounding reservoir dam wall structure.

Llyn Teifi reservoir is situated approximately 4 miles East of Pontrhydfendigaid, about 1500ft up the western slopes of the Cambrian Mountain, which can be reached by footpath from the south, or by a small road from Ffair Rhos, close to Pontrhydfendigaid. The lake was famed even in medieval times for the quality of trout and eel fishing to be had, the lake is still fished and now provides a drinking water supply for the surrounding area and the coastal belt of Ceredigion to the West.

The Teifi dam spillway is under the Reservoirs Act 1975, this Act requires reservoirs to be under supervision at all times and periodically inspected by specialist qualified civil engineers.  As a result of one of these inspections concerns were highlighted about the stability and deterioration of the concrete due to freeze/thaw action over many years. Therefore the need for refurbishment work was identified.

Kaymac Marine & Civil Engineering Ltd were commissioned by Dŵr Cymru Welsh Water to refurbish the structure. Ahead of mobilisation, Dŵr Cymru Welsh Water carried out upgrades to the existing access roads to facilitate the delivery of the required welfare, plant and materials.

The rehabilitation works themselves consisted of diving activities below the water line and traditional civil engineering works on the spillway face.  Isolation measures were put in place including the installation of a temporary blanking plate to an outlet situated within the proximity of the diving working area.

To facilitate the 175mm Ø core drilling through the spillway face, a limpet dam was designed and installed by a Kaymac dive team, which when dewatered, allowed gravity drain pipes to be installed through the structure in dry conditions.  These feeder pipes were then grouted into place, effectively re-sealing the structure and removing the risk of the reservoir level rising above the limpet dam crest and water escaping through the core hole.  The existing concrete invert on the downstream side of the spillway was then reduced to allow the encapsulation of these pipes in concrete.

The rehabilitation works to the front spillway face included breaking out sections of the existing concrete, drilling and fixing steel reinforcement ahead of installing a bespoke formwork system to allow the new concrete to be placed to the required design profile.  The concrete was placed in 3 vertical lifts to the required design level through utilizing a concrete pump positioned approximately 40m away.  Access for these works was achieved through the use of scaffold and mobile elevated platforms.  A new manhole chamber was constructed downstream of the structure to access the gravity drain feeder pipes installed through the structure to drain away any overspill water between the membrane and the original dam.

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