800FT ATIAMURI EARTH WING DAM NEARING COMPLETION

Press, Volume XCVII, Issue 28627, 2 July 1958, Page 19

800FT ATIAMURI EARTH WING DAM NEARING COMPLETION

“The Press*' Special Service

WELLINGTON, June 1. The earth wing dam at Atiamuri is now complete, except for grassing the crest and downstream slopes, the Minister of Works and Minister in Charge of the State Electricity Department, Mr Watt, announced today. More than 85 per cent, of the work on the concrete dam has been done, and with this task being pressed ahead night and day, the first electricity should become available early next year, several months ahead ,of the original programme.

Atiamuri’s dam has a total length of 1300 feet between the rocky sides of the Waikato river valley, of which 500 feet is accounted for by the concrete structure in the present river bed and the remainder by an earth dam on the left bank. The concrete dam is a massive gravity type requiring some 154.000 cubic yards of concrete and with a height of 120 feet from the lowest point of its foundations to the crest. Both dam and powerhouse are founded on a dome-like formation of extremely hard volcanic rock. The earth dam extends across the remaining 800 feet of valley floor on the left bank, where the river ran. back in geological history. “It is a straight, neatly symmetrical embankment which appears quite insignificant by comparison with the rest of the work.” the Minister commented. “There is much more to it than meets the eye. however. It is a most interesting piece of engineering and a unique achievement for New Zealand.” Prevtnwißlverßeds

In its previous courses the river laid down a deposit of boulders, gravels and fine sands varying in denth from 50 to 120 feet and overlying an impervious rock. These materials were no barrier to water, and the old river beds had to be sealed off right down to the rock for the full width of the valley. The method adopted was to excavate a trench almost down to normal ground water level to hold the watertight core of the earth dam, and below this to dig another much narrower trench to allow the pouring of a five-feet thick reinforced concrete cut-off wall keyed a minimum of five feet into the rock. Below this again and 30 feet into the rock, holes were drilled for a curtain of grouting—a thin cement and water mixture pumped in under pressure to seal all interstices. Some 300.000 cubic y<irds of river deposits were excavated to make the main trench. This was

the full 800 feet long; its width at the bottom varied from 30 feet in relatively firm material to 300 feet in sands; and at the top its width varied from 150 to 400 feet. The depth to normal ground water level averaged 60 feet. Drainage Problem Below this level there was a ‘hen unknown but obviously very considerable volume of water, and drainage was essential to permit the final excavation and concrete work. This was achieved by cutting a special drainage gallery in the rock below the deepest area of river deposits. First a horizontal drive at original ground level, eight feet high and six feet wide, was extended for about 70 feet into the rock of the right bank. From it a vertical shaft was sunk for some 125 feet, and from this again th? drainage gallery was carried through the rock beneath the earth dam site for about 330 feet. Vertical bores from the open cut into this gallery were fitted with control valves and a pump chamber was installed. Pumping from the drainage gallery back to' the river began at a rate of 1800 gallons a minute, but after two days the ground water level was depressed along the length of the cut for a sufficient width to permit excavation and concreting of the cut-off wall. Pumping at 250 gallons a minute maintained this level. Grouting Gallery On top of the concrete cut-off wall a gallery eight feet high and four feet wide was built to extend into the core of the earth dam. This was to allow grouting, a long, time-consuming process, by drilling down through the cut-off wall while the dam materials were being placed and compacted around and on top of the grouting crews. About 460.000 cubic yards of material were used to build the earth dam, 100,000 cubic yards being needed' for the core. This consisted of unsilicified pumice breccia, a soft rock which could be easily excavated and then recompacted to form a watertight mass. It was taken from a borrow area about half a mile from the site, transported, placed and compacted at an average rate of between 3000 and 4000 cubic yards a day.

Compacted sand formed the earth dam shoulder materials, while the upstream face was finished with boulder rip-rap as a protection against wave action and rapid drawdown of water for the generators. Soil and gravel were used to finish the crest and

downstream face, and these areas will be sown down in grass.

Summarising the progress of other work on the project, Mr Watt said' just over 80 per cent, of the concrete for the powerhouse had been poured and about 60 per cent, of the precast concrete slab cladding for walls and roof is in place; the structural steel work is about 80 per cent, completed; and the installation of the first turbine and generator unit is well under way. Atiamuri will have an installed capacity of 63,000 kilowatts, with provision for a fourth unit which would make peak capacity 84.000 kw. It is the fifth of the chain of hydro-electric stations on the Waikato, and will be followed by Ohakuri (112,000 kw) and Waiapapa (50,000 kw) both due to provide their first electricity in 1961. /