At its completion in 1965, Lake Benmore was immediately hailed as a landmark piece of civil engineering and construction – for its height, capacity and contribution to the country’s electricity needs. These overshadowed a far more innovative feature – the concrete penstocks fitted to its power station. LAWRENCE SCHÄFFLER reports.
OWNED BY MERIDIAN ENERGY, Lake Benmore’s power station produces some 540MW and is our second biggest hydro-electric plant after Manapouri. When the project began in 1958 it was the biggest construction job ever attempted in the country and, filled to its 12.5 million cubic metre capacity, it became the Southern Hemisphere’s largest earth-fill dam.
It remains our largest earth dam and, at 118 metres to its crest, our highest. The electricity generated by the power station’s six 90MW turbines plays a major role in delivering power to the North Island via the high voltage direct current link.
Less well-known is that the new power station also notched up an international milestone: its penstocks were the first in the world to be built in prestressed concrete rather than the more traditional steel solution. This step was particularly noteworthy considering that prestressed concrete was a relatively unknown technology at the time – internationally – and in this country certainly, a fledging, untried concept.
Not for nothing was the Ministry of Works’ (MOW) design team’s courage in committing to the concrete penstocks often described as a “radical, blind leap of faith”.
Prior to Benmore, all of the previously built hydro power stations on the Waikato and Clutha rivers used conventional short, steel penstocks passing through a concrete dam into the power house below. The penstocks (prefabricated steel rings about 5.5 metres in diameter) were assembled on site, and lowered into positon from a “flying fox” cableway.
But Benmore presented unusual factors and, coupled with New Zealand’s delicate economy in the early 60s, these demanded a different solution.
For a start, the power station’s design required much longer penstocks (six of them, 5.3 metres in diameter and 130 metres in length) laid along a rock spur on a 35o slope. The static head would be double that of the existing hydro stations.
While awkward topography eliminated the possibility of erecting an aerial cableway over the penstock area, accessing large amounts of high-tensile steel would also be difficult. It would have to be imported and, with the government nursing a volatile foreign exchange equation, the cost would leave a substantial dent in the Overseas Fund.
Furthermore, the cost of welding large quantities of mild plate (up to 65mm thick) or 35mm high-tensile plate would be significant, and it came with an additional risk: the possibility of the project being delayed through industrial action by the then-active Boilermakers Union.
Collectively, these considerations prompted the MOW to explore a novel alternative – build the penstocks in prestressed concrete.
Having decided on the concrete approach, the Ministry decided (in August 1959) to invite a French prestressing expert – Yves Guyon of the highly-regarded STUP – to New Zealand to help with the design. STUP – the Société Technique pour l’Utilisation de la Précontrainte (technical corporation for the utilisation of prestressed concrete) – was established in Paris at the end of WWII to promote prestressed concrete and other inventions of the renowned French engineer, Eugene Freyssinet.
The design provided for a normal operating pressure of 900kPa and a maximum working pressure (water-hammer) of 1200kPa. The six penstocks would be created from 318 precast units, each 2.4 metres long, 5.3 metres in diameter and weighing 57 tonnes. Their wall thickness would be 450mm. A special mortar for the joints between the units was developed.
With typical Kiwi resourcefulness they acquired a third-hand concrete batching plant sourced from the US Navy. It had been used for the rebuilding of Pearl Harbor following the 1941 “day of infamy” Japanese bombing attack.
Given the project’s uncharted nature, the design team prudently decided to first carry out extensive tests on a section of full-size units. This saw two precast units joined with the new mortar and tested under pressure over a two-month period. Under pressures varying from 2000kPa to normal the tests continued for several weeks – with no signs of failure. Green for go.
Casting the units
Because of the awkward terrain, the construction crews opted to mix the aggregate on site rather than bring it in by truck from further afield. With typical Kiwi resourcefulness they acquired a third-hand concrete batching plant sourced from the US Navy. It had been used for the rebuilding of Pearl Harbor following the 1941 “day of infamy” Japanese bombing attack.
A 37.8 metre-high concrete spillway, 119 metres long, was constructed to hold the penstocks which were cast in place – in itself a fairly unusual technique for the time. Water flows over the spillway at some 50km/h and reaches 120km/h by the time it arrives at the deflector plates at the bottom.
At the foot of a rock spur stood the 152 metre-long turbine building. It was equipped with six Canadian-manufactured 90MW generators – and on January 14, 1965 they rumbled into action for the first time.
The vision and courage of the MOW design team in opting for the concrete penstocks was recognised by the international engineering community. The project featured in the American Concrete Institute publication – Concrete: A Pictorial Celebration – published in 2004 to commemorate the Institute’s Centennial.
It remains one of only two New Zealand entries ever featured in this august publication.