2010 – Pukaki Canal Intake structure repairs – November 2009 – 62 hours on the slab

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Jim Walker, Jamie Macgregor

The Pukaki Canal Inlet structure is a large gated culvert and stilling basin structure, it is a High PIC appurtenant structure to the Pukaki Dam, located in the Mackenzie Basin area of New Zealand’s South Island.

The 560m3/s capacity inlet structure is founded on glacial moraines. It controls flow from the178 km2 Lake Pukaki storage into the 80m wide, 22km long Pukaki/Ohau canal. It is the owner’s (Meridian Energy) most important valve, as it feeds 1550MW of hydro generation on the Waitaki River.

A risk assessment in late 2009 identified a previously unrecognised trigger for a potential failure mode for the stilling basin. Principally, ongoing erosion of the reinforced concrete base slab could lead to failure of water stops in the slab joints potentially leading to slab uplift, foundation erosion, and ultimately, catastrophic failure of the Pukaki Dam. To better define the risk to the structure, further inspection of the stilling basin was recommended.

A dewatered inspection of the stilling basin was required, as further dive inspections would not improve our understanding of structure condition. Because the stilling basin cannot be isolated from the canal, this requires dewatering the entire Pukaki/Ohau canal, presenting significant risks of damage to the canals from slumping and lining failure. A dewatered outage also has major business revenue impacts.

This paper describes how Meridian were able to take advantage of a transmission network outage, scheduled for just six days after the risk was identified, to plan, safely dewater, inspect, and rewater 22km of hydro canal, and not just to inspect the Pukaki Canal Inlet structure, but also to implement repairs to the stilling basin slab which have successfully mitigated the structure safety and operational risks. This huge undertaking involved mobilising an army of people, plant and materials, and cost over NZ$1.8m. From identifying the risk to the structure, to completing repairs took just 13 (very busy) days.

Lessons learned in the areas of dam safety and asset management are presented. As well as those contributing to the success of the project in seizing an opportunity to mitigate the identified dam safety and operational risks.

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