Customer: Watercare Services Ltd
Contract Type: Design & Construct
Location: Auckland, New Zealand
This upgrade and remediation contract for Cosseys Dam, Auckland’s third largest dam, was awarded to McConnell Dowell and its JV partner after Watercare detected deterioration in the 1950s built earth fill structure. Located in the Hunua Ranges the dam holds 14% of the city’s water storage.
Works included excavation of the downstream dam core and shoulder to base rock. The excavated material was stockpiled for subsequent re-compaction of the dam up to a higher crest level following installation of a new in-dam filter and drainage system.
The project scope also included construction of a new access bridge over the Wairoa River, valve tower strengthening works, and mechanical refurbishment of the valve tower. The earthfill dam stands 49m high and 170m long at the crest, and a 230m long diversion tunnel links to the valve tower and scour systems.
The complex geometry of the dam foundations, Watercare’s operational needs (managing water supply) and the environmental considerations required by working within a regional park provided a complex range of design challenges. 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 innovative techniques on the project that provided flexibility to alter construction methodologies if necessary. 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 10,000m3 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 utilized 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 resource pool and a team experienced in delivering construction works in difficult terrain allowed us to vary and maneuver the plant, depending on the area available to achieve the right compaction. Contractor innovation helped to manage 60mm rain events and saved the client both time and money.