The UK energy utility said the rockfall was ‘very substantial’ but is confined to near the top of the power tunnel. No details were given for the cause of the tunnel failure.

SSE said planning is underway to establish the best method of repairing the power tunnel but added that Glendoe power plant would not generate for many months – ‘well into 2010 at the earliest’.

The rockfall was discovered in August after the power tunnel was partly blocked. No damage was caused to the equipment in the underground power plant.

Glendoe only began operations at the end of 2008. It is the UK’s biggest conventional hydro plant built in a half a century.

The project involved construction of more than 16km of tunnels, mostly by drill and blast but the 6.2km long headrace between the powerhouse and reservoir area was bored by TBM. It is still unclear if the area affected by the rockfall was excavated by TBM or of some drill and blast was used locally.

Geology along the tunnels is hard rock – schist and quartzite, with minor faults. Lining support comprised bolts, steel ribs, mesh and shotcrete, used as required.