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Usk Dam
River Usk, west of Trecastle, Sennybridge, Powys, Wales, UK
Usk Dam
associated engineer
Binnie, Deacon & Gourlay
date  1950 - 6th August 1955
era  Modern  |  category  Dam/Reservoir  |  reference  SN831289
ICE reference number  HEW 1240
photo  © Nigel Davies and licensed for reuse under this Creative Commons Licence
The dam over the River Usk and the reservoir it impounds were constructed to improve the water supply to Swansea in South Wales. The earthen dam is the first use of horizontal drainage blankets in Britain. They facilitate the dissipation of pore pressures and their installation is now standard practice for dams with clay fill.
Usk Dam is situated close to the source of the River Usk. It was designed by engineers Binnie, Deacon & Gourlay for Swansea Corporation. Its crest length is 480m, and it is up to 33.2m high and 134m wide at the base. The embankment consists of boulder clay fill with a puddled clay core, varying in thickness from 1.8m at the top to 4.9m at the base. The dam straddles a 1.8m thick concrete cut-off wall, which extends a maximum depth of 23.5m below ground level.
The upstream (west) face of the dam is sloped, varying from 1 in 3 at the top to 1 in 4 at the toe. It is protected by concrete slabs laid on quarry spoil and restrained by a concrete toe block.
The downstream face of the dam is grassed, and features two horizontal berms, at 289m and 298.7m above sea level, respectively. The upper part of the face slopes at 1 in 2, while between the berms it is 1 in 2.5, with the lower slope at 1 in 3. A stone 'mattress' was placed between the ground and the fill, beneath the two lower slopes, and stone blocks retain the toe of the embankment.
A spillway channel is located at the south end of the dam. It has a concrete base and masonry walls and curves northwards to discharge into the river.
During construction, a thin layer of water-bearing silt was identified beneath part of the site, about 3m below ground level, sandwiched between deposits of boulder clay. To avoid having to remove the silt, vertical sand drains were installed in the berms. These extend through the silt and connect with the stone mattress. Pore pressure monitoring was undertaken to ensure that this method work satisfactorily.
After the first season of earthworks (1952), filling had been completed up to the level of the lower berm. However, dissipation of pore water in the embankment was taking place too slowly. It meant that further placement of material could trigger slip failures. To prevent this, a drainage blanket was placed over the upper surface of the boulder clay, avoiding the puddled clay core. It slopes slightly downwards towards the faces of the embankment and emerges just above the lower berm.
The blanket consists of 300mm of river gravel, overlaid with about 200mm of crushed stone, topped with 450mm of river gravel. A second similar blanket (950mm thick) was placed at the upper berm level during 1953 and 1954. The blankets improved pore drainage rates and soil placement was able to proceed.
The technique had been suggested by Dr Alec Westley Skempton (1914-2001, professor from 1955 and knighted in 2000) and Dr Alan Wilfred Bishop (1920-88), both of Imperial College London, and this was its first British use. It worked so well that it has now become industry standard practice for earth dams with clay cores.
On 6th August 1955, Queen Elizabeth II and HRH Duke of Edinburgh opened the Usk Dam. The reservoir it impounds has a water area of 113 hectares and a capacity of 12.3 million cu m. Its top water level is 307m above sea level.
The River Usk flows east, away from Swansea, at the reservoir site. Water extraction takes place some distance to the east of the dam. The water passes through a tunnel some 2.4km long before discharging into a pipeline conveying it the remaining 53km to Swansea.
With the subsequent construction of other reservoirs and philosophy of creating a ‘water grid’ network across South Wales, the reservoir now functions largely to regulate flow in the River Usk. It is also a supply point for areas more local to the site.
Contractor: Richard Costain Ltd
Research: ECPK
"Soil Mechanics: Principles and Practice" by Graham Barnes, Palgrave Macmillan, Basingstoke, 2010
"Lessons from historical dam incidents", Flood and Coastal Erosion Risk Management Research and Development Programme, Environment Agency, Bristol, August 2011
reference sources   CEH Wales

Usk Dam