Civil Contractors/Hirepool Construction Excellence Awards 2017: Category One B’s Winner

Category 1B: projects with a value of less than $5 million ( company turnover is more than $10 million)

Winner: Waiotahi Contractor for Aniwhenua Power Station dam

Remediation of a serious leak in the dam forming the Aniwhenua Power Station head pond in the remote Kaingaroa Forest presented unprecedented challenge, complexity, urgency and risk.

In September 2015 the leak was discovered through the lining of the dam that needed to be repaired with urgency due to the associated risks of flooding and damage plus a daily revenue loss of $60,000.

Repairing the leak, deep under water amongst 30 years of debris, required a ‘design-on-the-go’ approach to drain the head pond and canal and construct significant and complex flood protection before the onset of high rainfall in late autumn and winter.

Nova Energy, reliant on the dam for hydro-electricity production, selected Waiotahi Contractors and consultants Tonkin + Taylor to design and undertake the complex works, with a value of $2.78 million, within an extremely tight timeline.

Waiotahi Contractors is a family owned business that started out in 1957 draining the boggy farmlands of Opotiki. Over the years the company has grown to a diversified civil construction, quarrying and trucking company with a turnover of around $20 million.

The company says the key to winning the tender was it initiative to work openly with Tonkin + Taylor during pre-tender to devise workable methodologies with many unknowns. Work started in February 2016 and was completed 58 days later.

The hydro-electric dam was decommissioned to allow the repair, with high loss of generation costs for the client. Works also needed to be completed before the high winter rainfall. Accordingly, Waiotahi, at one stage of the works, planned and resourced a seven-day-a-week, two-shifts-per-day schedule to ensure this project was completed according to the demanding timeline.

Suffice to say, this project was far from a run-of-the-mill project for many reasons, a lot of the technical detail was unknown at the outset as the problem was under water, so it involved complex and technical work under ‘almost impossible’ deadlines. The project was also high risk, requiring close collaboration to allow for shared innovation and IP with all works carefully taking into account health and safety needs.

A design-on-the-go approach had to be taken, as the problem area (the leak) was deep under water and amongst years of accumulated debris.

The high rainfall due in late autumn and winter would have made the project extremely difficulty to complete if project ‘milestones’ were not met.

Before pond remediation works could start, a decant pond system had to be built to enable sediment control. A stream also needed to be diverted temporarily.

It was found that the canal head pond sealing layer had failed. All overlying material including protective riprap had to be removed in stages to enable a repair in the dry. This task was exacerbated by the fact that a substantial stream entered the head pond from the west, so early works included the removal of material to be salvaged or otherwise carted to waste. A cut off bund and weir system was constructed. This required installation of pipe support pads for a large diameter glass fibre bypass pipe, necessary for remedial works to be carried out in the dry.

After accessing the extent of damaged embankment material it was replaced with compacted new, low permeability, ash fill and filter material.

Project challenges were overcome through a close working relationship with Tonkin + Taylor and the juggling of multiple subcontractors.

Despite technical challenges and rain events, Waiotahi Contractors completed the contract within budget and on time, which was essential to allow Nova to resume full production of hydro-electricity. Contract bonus awards for meeting construction and quality of work targets saw Waiotahi picking up two $10,000 bonus payments.

Due to the technical complexity, Nova Energy expected there would be a need for re-work of some sort. However, the repair was done, the leak was fixed, the liner in place, and the power station recommissioned on time and operational, no defects and all minor omissions sorted as per the Certificate of Practical Completion.

This excellent project performance was acknowledged by client, who praised the Waiotahi team, led by Shane Robertson (site manager) and Aaron McCormack (site supervisor), for the high quality of their work, and the management of their team and the subcontractors.

Additional work for Nova and Southern Energy continues to be awarded to Waiotahi, the most recent being in May 2017.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s