Cairnmuir Slide

Several large scale landslides in the Cromwell Gorge were stabilised over the period of constructing the Clyde Dam. The Cairnmuir Slide was one of them.

The Clyde Dam, a 100 metre high concrete gravity dam was constructed across the Mata-Au Clutha River at the southern end of the Cromwell Gorge between 1977 and 1992.

Over that time, several large scale landslides which would be affected by the filling of the dam were stabilised. The Cairnmuir Slide was different - its failure surface was above the maximum lake level. However, monitoring indicated there was a risk of failure of the landslide into the lake, which would create waves large enough to go over the dam, with catastrophic effects for the townships of Clyde and Alexandra.

The main problem was found to be rainwater seeping into the base of the landslide. The solution produced the ususual terraced landscape you see today.

The lower area had a general slope of about 45 degrees. This enabled it to be waterproofed with a geo membrane, weighed down with a continuous series of wire baskets filled with stone.

The upper area was then waterproofed by constructing vertical reinforced earth and retaining walls following the contour, then sealing the resulting flat surfaces by spraying on a bitumen/stone surface.

Rainwater was concentrated in lined gullys on either side, taking it safely into the lake.