The project required a durable intake and pump chamber solution in a highly constrained riverside location. With the chamber sitting four metres below the water table, the build presented significant challenges around constructability, groundwater pressure, and the 70-ton buoyancy force that the sealed structure would experience.

High river levels periodically flooded the lower part of the site and elevated the water table, requiring careful management of water pressure to prevent the sealed structures from floating.

Breen worked closely with the design consultants from an early stage to assess buildability risks and identify the most practical construction approach. Working alongside the structural and mechanical designers, Breen adopted a caisson construction technique, a method rarely used in New Zealand. The team constructed a four-metre tall in-situ concrete chamber above ground before progressively sinking it into its final position.

This approach required thorough front-end planning, detailed risk mitigation, and precise execution, but it reduced temporary works, removed mass excavation, removed continuous water pumping and minimised uncertainty on site, helping control both programme and cost. The site base of glacial Moraine introduced additional complexity throughout construction. Sheet piling was driven into the ground to create working zones, while an earth cofferdam positioned in the existing river channel diverted flows, allowing active dewatering management throughout the final pipe and floor sealing process.

The team placed concrete underwater using a tremie pipe, a specialist method that demanded a high level of precision to achieve a sound structural result. Oversized components were also precasted on-site due to access restrictions.

Once the caisson was in its final position, the intake and pump chamber were connected via 45 metres of 800-millimetre diameter HDPE pipeline – all of it installed underwater and requiring watertight sealing of each connection below the river and water table.

Close collaboration between Breen, the design team, and the client was central to the project’s success. Early contractor involvement allowed key risks to be addressed through design rather than during construction, resulting in a more efficient build process and a robust long-term solution.

Richard Ireland, Divisional Manager – Civil, praised his team: “It was a unique approach to a unique solution, thinking beyond the standard playbook and applying a solutions mindset to the project. It took a lot of upfront work to test, prepare, mitigate and design the structure. Well done to everyone involved in delivering a technically robust outcome under demanding conditions.”

The completed intake and pump chamber now provide Terraces Irrigation Limited with critical infrastructure designed to perform reliably in a demanding riverside environment.

 

Client: Terraces Irrigation Limited
Breen team: Andrew Kerr (Project Manager), Steve Lester (Foreman)
Scope of works: Site Preparation | Structural Works | Pipeline Installation | Underwater Excavation | Environmental Management