ROCK-FILLED DAM

Evening Star, Issue 20989, 31 December 1931, Page 1

ROCK-FILLED DAM

FIRST IN NEW ZEALAND INTERESTING engineering features OTAGO CENTRAL IRRIGATION [From Odr Correspondent.] WELLINGTON, December 31. In connection with the development of the Omakau-Matakanui-Lauder irrigation scheme in Central Otago the Public Works Department is damming the Upper Mauuherikia River by means, of a structure unique in New Zealand engineering. Large dams associated with reservoirs and hydro-electric; schemes in the dominion have all been constructed of concrete, but the Upper Mamiherikia dam is to. be rock-filled. This type of construction is familiar in many parts of the world, and has been considered at various times in connection with dominion schemes of water storage, but the conditions have always favoured concrete. However, this undertaking is eminently suited for the alternative, as there is all the material on the site excepting the steel rods for reinforcement. Practically speakings all the material and labour is to be New Zealand. Two hundred inen have already made a start, and there is work for two years on this structure. The dam in cross section will resemble a huge broad-based pyramid 160 ft high and holding water up to 150 ft. Its total length will be GOOlt across the valley, and its base will be 500 ft in thickness. Rock will be quarried out of the adjacent hillsides and simply tumbled into the site, all fine material being excluded and a certain amount of washing down being done to enable the rocks to be firmly settled.: A rock-filled dam is not intended to be absolutely impervious to water under the 150 ft head which this structure will carry, but there will be some important work on its upstream face which 1 will make it almost watertight. Gravel will be spread on the rocks of the face, and ou this will be bedded a reinforced concrete skin Min thick at the bottom of the dam, and running out to about 9in towards the top. The concrete is to be placed in position in slabs and spaces between filled with plastic bitumen. As a further measure of stability in case of settlement of the rock filling the concrete facing will be backed by reinforced concrete girders providing a strong support. In. the flattened “toe” of the upstream side the whole of the rock will be grouted with mortar as it is-placed,; and cut-off walls sft thick will be carried as far as necessary into the hillsides, the rock below being pressuregrouted. UNIQUE FEATURES. Rock-filled dams provide a fresh set of problems for the designing staff of the Public Works Department, which is responsible for this structure. It is impossible to permit of flood or surplus water spilling over the top, which narrows to a few feet, and it is therefore proposed to utilise a tunnel spillway which has not hitherto been included in any New Zealand dam. The tunnel will be 16ft inside diameter, and during construction will take the whole flow of the river. When the dam is complete the tunnel will, be fitted with regulating valves, and will become the spillway by the construction of a largo funnel-shaped opening on the upstream’ side of the dam, into which water will flow when the flood height is reached. There is more , than sufficient water in the river for normal irrigation requirements. and it is intended to keep the level of the dam at a fairly high point at all times. A further development planned for the futufe is the generation of electrical power, utilising the 150 ft head of the dam, plus the drop below—there is a waterfall just below the site. With this power some of the water of the river will be pumped over the saddle between the Manuherikia and Ida Valley in the vicinity of Hill’s Creek, and turned to good account for irrigation. The scheme on its present lines will provide irrigation for about 15,000 acres, but the flow of the river is capable of providing for considerable extension of the project when conditions are favourable.