Landslides affect dam

Press, 26 July 1989, Page 6

Landslides affect dam

PA Alexandra Landslides affected an area of more than 1115 ha in the Cromwell Gorge, site of the Clyde dam, Electricorp Production officials announced yesterday.

Electricorp yesterday outlined the main work being done to stabilise the hillsides which will border Lake Dunstan, behind the Clyde dam. It had detected a total of 10 landslides in the Cromwell Gorge. Work to stabilise the land had delayed the filling of Lake Dunstan from next month over the last few years had revealed “downslope” movements generally less than 30mm a year but up to 100 mm a year. Electricorp rated those movements as “very low.” Cromwell’s Mayor, Mr Duncan Butcher, said there was little local concern that earthquakes or landslides could be dangerous when the Clyde power station opened. The Cromwell Borough Council and the business community regretted the delay in filling the lake, he said.

Investors might be persuaded to put money into other projects. Mr Butcher said Electricorp’s decision to delay the filling would reassure many local people that all necessary steps were being taken on lakeside stabilisation. On a tour of the remedial work yesterday, geol-

ogists and engineers, including Professor David Stapledon, of the South Australian Institution of Technology, outlined the work required. The largest of the landslides is at Nine Mile Creek, the Cromwell end of the gorge, covering an area of 700 ha with a volume of 1000 million cubic metres. Electricorp said that before the lake was filled there would be stringent tests to make sure that all the work was completed and systems set up to monitor any movements. Each landslide would be assessed for stability during and after the lakefilling. Where necessary, preventative works including drainage systems and toe buttresses had been done. The Electricorp officials said monitoring of the landslides was a big task, with surface survey points, inclinometer holes and observation wells installed on all main landslides to monitor groundwater levels. Electricorp said an experienced Canadian engineer from British Columbia had advised on the drainage works at one of the landslides because that method had been successfully used to stabil-

ise landslides in similar situations in Canada. Another remedy was toe buttresses, which offset the fall in resistance when water inundates the toe of the slide. Several of the Cromwell Gorge slides had had substantial toe buttresses built, a special press conference was told. Professor Stapledon said he was impressed by the stabilisation work. He has been advising the Geological Survey on remedial work, and said yesterday the'slides were slow and creeping. Professor Stapledon spent a month at the site earlier this year reviewing work and making recommendations which have been adopted. It was important to do the work now because failure to do so could result in the road from Cromwell to Clyde being gradually undermined, he said. Local geologists had a high level of knowledge on slides, he said. This was not matched in many countries. The corporation said the total cost of the Clyde project was expected to be $916 million, allowing for inflation — about $2B million above the 1980 estimate.