Design and installation of new Rock Anchors to ensure the long term security of Seathwaite Tarn IR

Seathwaite Tarn IR is situated 5km west of Coniston, Cumbria. The dam is of unusual hybrid construction; part curved concrete gravity and part earth/rock fill with concrete cut‐off wall. A concrete cut‐off extends into the bedrock under the entire length of the dam. The dam was constructed in stages between 1906 and 1907 and has suffered a history of leakage and modifications.

A post tensioned system comprising 35 single‐bar rock anchors with a single layer of corrosion protection were designed and installed in 1974 to improve the stability of the gravity section of the dam. The anchors have been subject to “lift checks”, with stress adjusted by “shimming” where appropriate, at approximately 10 yearly intervals. The anchor design is out of current code requirements, but their performance has generally been satisfactory; however, they have suffered from under‐head corrosion. The extent of the corrosion could not be assessed and its effects have prevented performance testing. United Utilities took the decision to replace the anchors in order to maintain the long term security of the dam.