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At the time of its construction Benmore was the largest earth-fill dam in the Southern Hemisphere and certainly the biggest construction job ever attempted in New Zealand. RICHARD CAMPBELL looks back on the making of this great hydro-generating icon.
A large workforce was assembled to undertake the project which was primarily carried out by the (then) New Zealand Ministry of Works and hired private contractors. Many of the workforce had prior experience with dam construction having just come off the recently completed Roxburgh dam in Otago, the first really large dam to be constructed in the country, and a new town, Otematata, was created to house the workforce and their families. At the peak of its construction in 1964 Benmore employed a workforce of over 1400. Earlier investigations not only revealed that the natural geology of the Kurow area provided a perfect spot for a dam, but a good proportion of the materials to be used in its construction were all available in reasonably close proximity to the site. A second hand ex-United States Navy Johnston concrete batching plant, which had been used in the reconstruction of Pearl Harbour facilities following the infamous Japanese bombing attack in 1941 that drew the US into WWII, was bought to mix aggregate on site rather than truck concrete in great distances.
Diversion of the Waitaki river took place in August 1960 after a 30.48 metre coffer dam was built and twin channels cut in the left bank of the river, with the flow being directed through a twin-barreled culvert designed for a maximum flow of 70,000 cubic feet per second. Once the river was diverted work on the main dam began at a rapid pace It took only four years to construct the core of the dam, which, considering the scale of the project, was very respectable. The final core material was placed in April 1964. A 37.8 metre high concrete spillway, 119 metres long, was constructed to hold the 4 x 5.5 metre diameter penstocks which were cast in place – quite unusual for the time. Water flows over the spillway at approximately 50 kph and reaches 120 kph by the time it arrives at the deflector plates of the bottom. The 152 metre-long powerhouse building was built at the foot of a rock spur which also anchored the penstocks. It was outfitted with six Canadian manufactured 90,000 kW generators and the first power was generated on January 14, 1965. This was purely a test to see that all systems were in operational order. The dam was officially commissioned on May 15, 1965 by then Prime Minister, Sir Keith Holyoake.
Lake Benmore, with its shoreline of 116 kilometers and covering an area of 75 square kilometers, began to form on November 30, 1964 after the diversion channels were closed to allow the dam to fill. It took just over a month for the water building up behind the dam to reach mean operating level, which was achieved on January 14, 1965. Only then could the turbines and other machinery be tested. After the dust settled, so to speak, Lake Benmore has become a popular tourist attraction and is favoured for boating and recreational use.
Cost below budget
It also provided a very skilled workforce in dam construction that was put to good use at the next major hydroelectric development – Aviemore. Benmore is currently the second largest hydroelectric power generator in New Zealand. As a fitting tribute to the men and machines who worked on the project, a 1950s vintage Caterpillar D8 has been placed permanently on display at the viewing area adjacent to the dam. Earthmoving equipment
A very large contingent of private contractors also worked under contract to the ministry to complete the job. The result was a huge spread of earthmoving gear, including scrapers, bulldozers, rear dumps, shovels and grader, some of which had never been seen here before.
Contractor Vol.34 No.5 June 2010 |
Planning had begun to dam the Waitaki river for hydroelectric power as early as 1952 but initial work did not start until 1959 on the Benmore dam design with its initial output of 540,000kW, fed directly into the national grid.
This had first been used at the Roxburgh dam and was found to be quite suitable so was transported to Benmore and used to mix aggregate for the penstocks, spillways and other concrete structures. This batching plant was also destined to see service on the Aviemore dam and the Pukaki dam and must surely rank as a shining example of value for money for the Kiwi taxpayer.
Behind the dam lies its artificial lake, Lake Benmore, with a capacity of 12.5 million cubic metres of water.
During the planning stages it was estimated at the time that Benmore would cost $72.5 million dollars to construct. However the job came in on time and under budget at around $65 million.
The Ministry of Works, which was allocated to the job, already had a substantial inventory for the massive job but a lot of it was specially imported by the MOW for the job.