Benmore Hydro—II 1000-Yard Dam Higher Than Cathedral Spire

Press, Volume XCVII, Issue 28490, 21 January 1958, Page 9

Benmore Hydro—II 1000-Yard Dam Higher Than Cathedral Spire

[By a Staff Reporter of “The Press”)

The Benmore hydro-electric station on the Waitaki river, where preliminary construction is now under way, will be one of the biggest projects of its kind in the Southern Hemisphere and by far the biggest ever undertaken in New Zealand—a country with a reputation for extensive and advanced achievements in the highly-technical field of water development.

Dominating the work at Benmore will be the huge earth dam, with the massive power intake block, penstocks and powerhouse on the right bank, and the spillway structure on the left bank. These structures will require more than half a million cubic yards of concrete —the equivalent of three solid blocks each the size of Parliament buildings. The earth dam will be the biggest single feature of the scheme, the length of its crest, on' which will be a two-lane roadway, being 1000 yards. It will soar above river level half as high again as the Christchurch Cathedral spire and will be 350 ft above the foundation rock. At the bottom it will be 600 yards thick and the volume of the whole dam, which will link two hillsides, will be about 18 million cubic yards. Tremendous Lake The water that backs up behind the dam when the culvert is sealed will be New Zealand’s largest man-made lake, covering 3000 acres of tussock terraces and providing power storage equal to that of Lakes Pukaki and Tekapo combined. Its volume will be 72,000 million cubic feet. The area of water in the lake will be 30 square miles and the volume one and a half times that of Wellington harbour. It will take 12 weeks to fill at the average flow of the Waitaki. (The average flow of the Waikato at Karapiro would take four and a half months to do it.) Why a collossal earth dam? The main reason for an earth dam at Benmore is that the physical size of the project would make construction in concrete exceedingly difficult if not impossible. Another is the high price of concrete (an estimated £5O million for Benmore if it was possible; compared with £36 million for earth). Previous Experience Earth dam construction is no new development for New Zealand or for its engineer projects. The Ministry of Works has already had considerable experience in working out and applying the best techniques. Earth dams are already in operation at Cobb and Lake Pukaki. There is another at Whakaramaru and one is under construction at Lake Hawea. The penstocks leading the water from the lake to the turbines will be set on a trimmed rock face at a 30 degree slope and with a length of 600 feet. They will probably be built of reinforced concrete lined with steel, and have a diameter of 17ft 6in. Situated at the foot of the penstock slope will be the powerhouse, comparable with that at Roxburgh. It will be 500 ft long, with about 70ft showing above ground level and its main bulk below. It is likely to be a building having a steel frame sheathed with precast concrete slabs. The machines will not be much bigger than those at Roxburgh, but they will produce twice the power because of the greater hydraulic head—3osft compared with 150 ft.

The intake structure built of reinforced concrete will be sited on the base of a convenient hillside spur and will embody a short wing dam to keep the end slopes of the earth dam from encroaching on either the intake or the penstocks. Here there will be control gates for shutting off the flow to the power-house when required. The concrete structures are designed along conventional lines, following Roxburgh as closely as possible, so that design time will be reduced. Present Progress

Construction began three months ago with the stripping of overburden on the site of the river diversion cut, and the carriage of huts from Roxburgh for No. 1 camp to house 200 workmen. This camp should be ready for occupation next week. Other work which has been going on for some time includes investigations for foundations and procuring and testing gravels for concrete and soils and gravels for the earth dam. Bore holes are being drilled to recover rock cores to determine the nature of foundations. This will go on for the rest of this year. In the immediate future heavy plant will be moved into the river diversion channel and work there undertaken in earnest.

It is proposed to install six ’BO,OOO kilowatt generators in the power-house. (To be Continued)