Work is completing to extend the capacity of the Loch Coire nan Arr dam, to increase the capcity of a salmon farm.
Faber Maunsell | AECOM developed a solution to control seepage in the foundation by forming a geosynthetic barrier membrane blanket over much of the foundation. The main fill for the embankment is rockfill, which is sealed by an upstream geosynthetic barrier membrane linked to the foundation membrane. on the construction of to a design by Faber Maunsell | AECOM.
Ken Turnbull, Regional Director at Faber Maunsell | AECOM said: “This project presented many difficult and unique design challenges for us. The main challenge on the project was the difficult geotechnical conditions. The site is on deep glacial tilts which lack any significant clay content, and although they are dense or very dense in their natural state, they are sensitive to water when disturbed.
“The permeability is generally moderately low but is also variable. These factors make it very difficult to excavate a cut-off down to bedrock, also to use the material for embankment fill given the expected wet weather conditions over the winter construction period,” he said.
The downstream face of the embankment is formed by landscaping fill partly for disposal of excavated material and partly to help blend the dam into the surrounding scenic mountain landscape, which is also an environmental Special Protection Area.
Another challenge was the reservoir sizing and yield. The client’s intention to expand their fishfarm production necessitated close cooperation between Faber Maunsell | AECOM and the client to determine the seasonal water demand required to support such expansion.
A hydrological simulation was carried out to ensure that the client’s plans for expansion could be supported by a reliable water supply in all seasons throughout the year. This hydrological water resource simulation enabled the height of the dam to be fixed at a level to store sufficient water for the planned production increase. It was decided to construct a new dam immediately downstream of the existing structure to provide the required storage.
The spillway arrangement for passing floods was also quite challenging and unique, partly because of environmental opposition to a conventional concrete spillway and partly because of lack of easy access to a rock foundation. A conventional spillway was discounted in favour of an overflow rock armour spillway.
Using results from a published example as well as established guidelines for the design of overtopping rock armour protection, Faber Maunsell | AECOM developed an overflow spillway comprising large rock sizes orientated by selective placing to resist the drag forces induced by the passage of large floods which would otherwise lead to progressive erosion of the underlying compacted rockfill.
Design details were also developed for tying in the upstream membrane into the spillway rock armour at the weir section in such a manner that the full supply capacity of the dam would be retained even after the occurrence of a large flood event.
The site is located within an environmentally sensitive area which meant a number of planning conditions had to be met. The loch and catchment are situated within the Beinn Bhan Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC), which has been designated principally for botanical reasons. In addition, the site lies within the Wester Ross National Scenic Area (NSA).
Faber Maunsell | AECOM worked in close collaboration with the Scottish Environment Protection Agency (SEPA) and Scottish Natural Heritage (SNH) to address any issues prior to permission being awarded in September 2004. This included all works being undertaken in accordance with SEPA’s requirements in respect of works in or near waterways.
In order to address all of these issues Faber Maunsell prepared a Construction Management Statement (CMS) that clearly set out how these conditions would be met. As required by the planning conditions Faber Maunsell consulted with SNH and SEPA to ensure that their concerns were addressed and that appropriate mitigation and safeguards were incorporated into the CMS and would be implemented during construction.
Construction started on 24th November 2008 and completion is due in May 2009. RJ McLeod was the main contractor.