Building new infrastructure at the base of an earth dam is anything but straightforward; add in working below ground level on the edge of an active spillway with 10 cubic metres of water per second pushing through the site, now we’re having fun!

The Opuha Low Level Outlet project was a resilience upgrade to critical water infrastructure serving both irrigation and hydro generation operations. The work centred on installing a new bypass system to enhance active water level control through spilling downstream, requiring a new in-situ concrete valve structure, erosion protection slab, and mechanical pipeline, valve and spray cone extension, all bifurcated off the existing bypass.

Works had to be undertaken while keeping water moving through the system; the solution required a high level of engineering expertise, precise planning and a strong dose of practical ingenuity. The Breen team constructed a temporary cofferdam in the tailrace, carefully engineered to divert water around the active worksite and maintain flow through the system. Behind the cofferdam, continuous 24/7 dewatering operations were essential to keep the excavation stable and the work area dry, as water passed by at 10m3 per second.

The scale of the construction work was significant. More than 150 rock anchors were installed in the tailrace, followed by a 400mm concrete slab. The project also involved 4m-high in-situ wall pours, and the management of heavy crane lifts for high-value components.

Work had to be carefully coordinated around operational outages, bringing together diving, confined-space welding and mechanical installation. Significant concrete demolition was also required, including wire sawing 12-tonne blocks.

Of course, Mother Nature created some interruptions.

Lake levels had to stay below 90%, otherwise drawdown had to happen. Significant rain events triggered two drawdowns, closing the site, flooding the work area and partially destroying the cofferdam.

The project also required detailed excavation, screening and backfilling of the dam toe with different fill zones.

“The job was full of unique challenges that required timely solutions, with collaboration, communication and a positive attitude,” said Richard Ireland, Divisional Manager – Civil. “We worked through options, defined preferences and delivered time and time again, a real team effort.”

The technical project across two weather-enforced shutdowns and a site that kept trying to fill with water, the job got done. The Breen team kept working to keep the job on track throughout by adapting to the circumstances and challenges presented daily, it showed Breen’s resourcefulness, commitment and drive by all those involved.

 

Client: Opuha Water
Breen team: Andrew Kerr (Project Manager), Tyler Martin (Foreman)
Scope of works: Site Preparation | Excavation | Structures | Environmental Management | Coffer Dams | Working at Heights Shutdown Coordination