Benmore 75p.c. Complete With Two Years To Go

Press, Volume CII, Issue 30152, 8 June 1963, Page 13

Benmore 75p.c. Complete With Two Years To Go

The £36,400,000 Benmore hydro scheme, designed to produce as much power out of the Waitaki river as all the existing South Island power stations put together, is now threequarters of the way to completion. The biggest construction group ever assembled 4n New Zealand for one job has been working on the giant earth dam since early 1958. First power from its turbines is confidently predicted for April, 1965, by Mr G. A. Tait, the acting project engineer.

Already, Benmore has changed the whole dun-coloured face of the countryside at Otematata, 18 miles up-river from Kurow and 60 miles from Oamaru. The arid, snow-dusted hills look down now on a new hill rising at their feet, growing day by day and swarming with vehicles and men—industrious as ants.

Where nothing was before, a village of 4300 has spilled across the tussock,, with its modern hotel, shops, cinema, maternity hospital and school. Benmore is only part of an ambitious plan to harness the power potential of the Waitaki waterway and the lakes and river that feed it. Already there are the Waitaki Station producing 105,000 kilowatts just above Kurow, and the Tekapo station producing 25.000 kilowatts at the outlet of Lake Tekapo in the Mackenzie Country. Benmore will have a peak capacity of 540.000 kilowatts —twice that of Roxburgh—and Aviemore. between Benmore and Waitaki, will produce another 200,000 kilowatts when it is finished in 1969 at a cost of £22,000.000. Other proposed hydro scnemes for the river and lake complex will result from a plan to make use of the 700 ft fall between Lake Tekapo and Lake Pukaki. These are still in the investigation stage, but the scheme is to diver, the Tekapo river near the lake outlet into a canal and across the plains to Lake Pukaki. The combined flow from the two lakes would then be led tn another canal across country to join the Ohau river These changes would be aimed at three more hydro stations —one of 50,000 kilowatts between <he lakes at Maryhum; another of 135,000 kilowatts at the end of he canal at Lake Pukaki; and the third, of 500,000 kilowatts, at Ohau, above Benmore.

Together, the seven hydro stations could produce more t ian 1.500,000 kilowatts in the Waitaki Basin. Benmore. Aviemore. Waitaki and Tekapo alone will produce 870.000. and the proposed Ohau dam will be nearly as big as Benmore. Changed Landscape

Mackenzie, the sheep thief, would not know his secret s ock route if all these plans became realities. Each dam on the Waitaki means a huge lake behind it, flooding the tussock land and filling the deep valleys. Waitaki has had its lake since 1934, but Benmore’s lake will dwarf it It will extend 18 miles up the river from the 360 ft high dam, and cover an area of 34 square miles. At the river's average flow of 12.000 cusecs (12.000 cubic feet of water passing through in a second), the lake is expected to take three months to fill when the gates are finallylowered on the diversion tunnels. The Waitaki power station will be starved of water for about two months so that Benmore can get started. Waterproof It seems hard to credit that a dam built of clay and lumps of rock—with no waterproof concrete skin—will be able to hold the rushing Waitaki’s water in such an enormous quantity. But a sample of clay which has been subjected in tests to the sort of conditions for which the dam is designed shows how completely impervious the dam core will be. The sample is as dense and as hard as rock. All materials for the earth dam. its silty gravel core and its bulk of gravel and rock are gathered within a fewmiles of the site from what are euphemistically called “borrow" areas. The spoil is carried to the dam by mammoth haulers travelling at speed along perfect road surfaces designed and maintained just for them. Other vehicles may not use these roads, and the ordinary sealed road which crosses one of them is controlled by traffic lights, forcing its traffic to give way Penstock Progress Three-quarters of the 27million ton earth dam has been built. The mobile equipment for this job has a total horsepower calculated by the engineers at 32,450. The whole diversion culvert has been built, and the intake above the penstocks is complete. The six penstocks are more than half finished, although

the penstock tubes themselves are not so far advanced. But every day, one Bft. 58-ton penstock section is being made on the site and placed in position. These sections, with a 17ft 6in internal diameter, are made of concrete, prestressed both lengthwise and clockwise. There are 500 of them to be made on the assembly line, and well over 100 have been made and placed already. Each section is slid down the penstock slope and mortared to the one below.

Water from the lake will pour down the penstock tubes, which provide a vertical head of water of 300 ft. and into the powerhouse’s scroll cases —snail-shell whorls designed to project the water with even more force at the six turbines. From the powerhouse, now- half-completed, the power will go through transformers to the alternating current and direct current stations. The direct current station will be for the power which is to be channelled into the North Island grid through the Cook Strait cable. Safety Valve

Half a mile away on the other side of the curved dam is the spillway, the safety valve which will carry flood waters through its four control gates set 10ft below the top of the dam. The spillway

is designed to carry a “1000 year flood" —10 times the average flow of the Waitaki river. The spillway is now 70 per cent, complete. Benmore’s working day starts at 7.30 a.m. when a whistle wakes the whole town to its work of serving the project—a Ministry of Works job in every respect, except for some ancillary work not directly associated with work on the site. The 1200 men or. the job (800 of them single men* work a lot of overtime, even under floodlights strung along the top of the dam at night. They earn an average of £4O a week, and some up to £6O with shift work and danger-money. Work is going ahead at such a pace that earth roads disappear overnight, target marks on the dam abutments are quickly overtaken, and the project publicity brochure is out of date almost before the print is dry. Unpleasant Job One of the least pleasant jobs is driving the vibrating rollers harnessed in threes to compact the brown clay core. A man is said to be able to do only one (day’s work in that job. Where the earth dam adjoins the rock abutments on each side of the valley the bond is formed by hand-held pneumatic thumpers. Every week-end about 2000 visitors come to Benmore from all over the country to see their tax money being snent—loo.ooo visitors a year. Mr Tait thinks there may even be some money saved out of the estimated cost if work continues to keep so well up to schedule,