FLOATING OUT A DAM

Otago Daily Times, Issue 23831, 10 June 1939, Page 11

FLOATING OUT A DAM

HEADWORKS FOR IRRIGATION NOVEL SCHEME FOR INTAKE REVOLUTIONARY ENGINEERING METHODS (Special to Daily Times) CHRISTCHURCH, June 9. Concrete sections (each weighing 100 tons) of the intake dam for the irrigation diversion race will, according to present proposals, be floated out into the Rangitata River and sunk Nothing of a similar nature has been attempted before in New Zealand and Mr T. G. Beck, the engineer in charge, promises to add another engineering feat to a scheme which for its revolutionary mechanisation, is one of the most spectacular undertaken in the Dominion.

Nature has, at the mouth of the Rangitata Gorge, provided the engineers with a dam. For a half-mile or so below the site of the intake the Rangitata River is a turbulent rapid, swirling unevenly over the piles of immense boulders deep down in its waters. Above the rapids, the water is naturally banked up into a placid, smooth-flowing stream. In consequence, no big dam is needed, but a structure has to be built across the river to divert the water into the race which will carry a quantity equalling the normal winter flow of the Rangitata across 40 miles of country to the Rakaia. In summer draw-offs will be made for irrigation and in the winter the full flow will be discharged through the trubines of the Highbank hydroelectric station. Excavation of Long Race Rebuilt since it carved out the immense Hawkswood cutting on the South Island Main Trunk railway the Ruston steam shovel, now 15 years old. is*digging the first section of the long race. No other machine could have handled the rough country below the gorge. Boulders which have defied handling by the capacious shovel, and all the power behind it are encountered with great frequency, and blasting has had to be resorted to. Working along the terrace, the shovel has dug a canal in places 40 feet deep, with a width of 70 feet. So far the bottom is down to the mean level of the river flowing parallel with it, and when the intake building is over another 14 feet will be scraped off the bottom level.

With the seepage from the ftiver, the first section of the race is now a pond It will be Dumped out and a gate erected at the river entrance end. On the dry floor of the race, the sections for the intake dam will be built. About 10 sections are proposed. Of concrete, each will weigh 100 tons. The engineers believe that with the bulkheading of the ends, the sections will float—but with little freeboard—when the gate is opened and the river flows into the canal.

On the floating of these concrete blocks, or pontoons, depends the success of the scheme. The foundation for them in the bed of the Rangitata will be dredged by the steam shovel, which will be shifted along the great bank it has made above the river. The bank of spoil now has the irregular appearance of-a dredge tailings deposit; but it will be levelled off by bulldozers to give a safe and wide track for the steam shovel. Method to be Used Already plans are advanced for placing the concrete sections in position on the eventful day when they are floated m the race. Sittings have been 'made for numerous winches which will warp the sections across the wide river and hold them till the hawsers are unshackled and the'block drops into its mathematically correct position on the river-bed.

If the floating-out of the dam in sections is carried out, the work is likelv to create as wide interest among engineers as did the dammins oi the Kawarau for the Cromwell Development Company's irrigation scheme 21 years ago. An Otago engineer con ceived the idea of placing reinforced concrete pillars perpendicularly on the banks and blasting them into the river m a manner that would effectively block the flow to allow for a diversion into the irrigation race. The pillars weighed 715 tons and 513 tons each being 50 feet high and 16 feet square.