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Customer: Watercare Services Ltd Contract Type: Design & Construct Location: Auckland, New Zealand
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Fast Facts
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We developed some innovative design and construction solutions to upgrade 1950s-built Cosseys Dam, after Watercare detected significant deterioration. Located in the environmentally sensitive Hunua Ranges, the earth fill structure is Auckland's third largest dam and holds 14% of the city’s water storage.
The earthfill dam stands 49 m high and 170 m long at the crest. It has a 230 m long diversion tunnel linking to a valve tower and scour systems.
Our scope included excavation of the downstream dam core and shoulder to base rock; placement of 300,000 cubic metres of new fill; construction of a new access bridge over the Wairoa River; and strengthening and mechanical refurbishment of the valve tower.
The project was complex, with deep variable dam foundations, Watercare’s operational needs to be considered, and the environmental contraints of working within a regional park, all contributing to our challenge.
Challenges and innovations
A key challenge was placing the fill material in a storage area, maintaining separation of the individual filling types, and conditioning the wet materials excavated from the perched water tables. Extremely soft underlying sediments, significant rainfall, plus the constrained nature of the site added to the challenge.
We developed a number of innovatiins on the project that provided flexibility to alter construction methodologies when necessary. For example, the decision to draw the dam down to 3% full and stockpile the excavated material within the boundary of the reservoir, was a major factor in the successful implementation of the project and the mitigation of wet weather effects.
To create the storage area we displaced some 10,000 m3 of very wet silt and piped the tributary. The majority of the excavation was removed using CAT 631 motor-scrapers and the material was carted up a 1:7 haul road to the storage area. A special trolley was designed to transport the guard valve up the tunnel and utilised rock bolts and chain block, eliminating the need to remove platforms and other items of permanent work in the tunnel. Pontoon and work boats were used to transport plant across the dam rather than cutting access roads in steep banks.
Our extensive resources and personnel experienced in delivering construction works in difficult terrain allowed us to vary and manoeuvre the plant, depending on the area available, to achieve the right compaction. Innovations also helped use manage 60 mm rain events and saved the our customer both time and money.
