Keeping the lights on in Invergarry, Scotland

A winter storm in 2018 set off a landslide near Quoich Dam, Scotland, which brought down a steel lattice tower on the transmission line between Fort Augustus and Skye, causing significant disruption to the power supply. TGS, via NorPower, supported an on-site team, which included geological advisers and designers, to assess the damage and the site conditions to form a solution for repairing the line – the main electricity supply to Skye and the Western Isles.

The Project

A site assessment indicated that there was a risk of further rock/landfall. Moreover, five of the towers were vulnerable to future slips and had to be replaced with three new NeSTS 132 kV double circuit poles with increased spans. This was the first time this new tower design was to be used in the UK. The tower was to be sited next to the live line and on a sloping hillside.

The Solutions

We had to overcome several challenges to apply our solutions. Logistically, the terrain and location were arduous. We had to attend to planning access for equipment and materials. Working close to a live overhead electric line also needed attention. The tower bases required either 12 or 16 piles to carry a combination of compression, tension, shear and moment loads. Because of the variable depths of overburden soil, comprising peat, dense sands and gravels with cobbles, we installed drilled minipiles using an overburden drilling system and permanent casings. The piles were founded in the underlying psammite formation. A sacrificial pile was installed and tested at each tower location for design verification purposes. Throughout the project, and owing to difficult ground conditions and site logistics, we used neat cement grout combined with full-depth reinforcement. Careful consideration was given to the site restrictions needed to ensure the work had no adverse impact on the adjacent watercourse and single-track road.