CASE HISTORY Ref: UK / CH / CP — Rev:03, February 17 COASTAL EROSION PROTECTION AT ISLE OF , , UK COASTAL PROTECTION Product: Polymeric Woven Gabions & Reno® Mattresses

Problem Located in the , the Isle of Benbecula is among the most westerly places in the British Isles. It is exposed to the constant erosion and storm force winds of the Atlantic Ocean. This coastal erosion was encroaching on the South Western perimeter of Benbecula airport, a vital link with the mainland for those living on the Island. Client, Highlands and Islands Ltd wanted to limit this erosion, thereby protecting their airport operations.

Solution Project Consulting Engineers, Pick Everard designed a robust revetment that could withstand the exposure conditions. The 500m long revetment structure would stabilise and protect the sand dune system. 0.5m thick Reno Mattresses were selected to protect the sloped revetment and a low height crest wall constructed from gabions would retain the and control wave Location of the Isle of Benbecula overtopping. A similar solution had been adopted successfully at a MOD Firing Range elsewhere on Benbecula

Due to the dynamic loading conditions and potential for the foundation conditions to change over the life of the structure, there was the possibility of differential settlement. Therefore, double twist woven steel wire mesh gabions were selected as their inherent flexibility would enable the structure to accommodate differential settlement without sustaining damage.

Preserving the natural shoreline, the box gabions and heavy Reno Mattresses were filled with more than 8,000 tonnes of locally quarried stone. These heavy revetment mattresses, provided a natural looking solution, which would soon accrete sands and silts in which vegetation can establish. Original construction of revetment at firing range Construction of the revetment was carried out by

Client: HIGHLANDS & ISLANDS AIRPORTS LTD Main contractor: AGGREGATE INDUSTRIES UK LTD Designer: PICK EVERARD, GLASGOW Products used: GABIONS & RENO® MATTRESSES

Date of construction Construction at airport January Phase 1 (2003) & additional works (2016)

Clean Stone Fill to Reno mattresses

Gabion crest wall

500mm Reno Mattress

Geotextile beneath Reno Mattress

Typical cross section through revetment contractor Aggregate Industries UK Ltd of Stornoway. Maccaferri assisted Pick Everard with technical advice on the project. Maccaferri’s 130 years of experience in hydraulic and coastal works led to specific details being incorporated into the project to enhance performance of the solution.

The expected hydraulic conditions influence the thickness of Reno Mattress selected; the thicker the mattress, the greater protection offered. The mattresses were installed upon a non-woven needle punched geotextile to act as a filter and limit the wash- out of fine soils from beneath the mattress under water flow. Allowed to continue, washout leads to voids The revetment after construction - note accretion of sand beneath a revetment and subsequent settlement of the erosion protection. Inflexible revetments can fail as they deflect into these voids.

Vegetation establishing in revetment © Maccaferri Ltd. All rights reserved. Maccaferri will enforce Copyright. enforce will rightsMaccaferri ©Maccaferri All reserved. Ltd.

Maccaferri Ltd: Specialist areas: Building 168, Harwell Campus, Didcot, OX11 0QT Sales: E: [email protected] T: (+44) 1865 770555 Technical: E: [email protected] Rep of Ireland: Construction: E: [email protected] T: (+353) 18851662 www.geostrong.net www.maccaferri.com/uk