Customer: Watercare Services Ltd Location: Glen Eden, Auckland, New Zealand Contract type:  Design and Construct

Fast Facts 2,000,000L storage tank 800m of 375 dia pipework 800m of 600 dia pipework The tank is four stories high from the floor to the roof or 11.5m tall It would take 25,000 bathtubs of water to fill the tank

McConnell Dowell was awarded this D&C contract for the upgrade of existing wastewater infrastructure in the Glen Eden area to reduce the number of overflows that occur following periods of prolonged heavy rain. The physical works included:

• Construction of a 2,000 m3 storage tank under an existing carpark and playing fields in Harold Moody Reserve
• Construction of a new tank overflow to the adjacent Waikumete Stream
• Construction of a new network sewer in Harold Moody Reserve
• Construction of 800 m of a new transmission sewer, 10 m deep, along Glendale Road from Ceramco Park to Harold Moody Reserve
• Construction of a new transmission sewer through Sherrybrooke Esplanade and Parrs Park, including a new box culvert, associated connections, and chambers
• Construction of a new network sewer from Rangeview Road to Sherrybrooke Esplanade, including drilling underneath the NIMT railway and construction of a pipe bridge over Waikumete Stream.

The Challenge

Minimising disruption to residents and the general public was a key client driver. Space was severely restricted at Harold Moody Reserve due to the adjacent sports fields, children’s playground and underlying transmission main.
Access to homes and parkland at Sherrybrooke Place and Harold Moody Reserve needed to be maintained safely throughout construction.

The new transmission sewer running along Glendale Road is 10 m deep, and installing a pipeline at that depth had the potential to cause major disruption to local residents and the travelling public.

Another challenge was protecting and maintaining the integrity of the live 910 mm North Harbour Water Main which runs directly alongside the new tank.

The Solution

The original tender design for the storage tank was for an open excavation, constructing the tank from the bottom upwards and backfilling the completed structure. The project team provided an alternative design for the storage tank - the ‘caisson’ - significantly reducing the working space required and minimising risk and cost for the client.

The ‘caisson’ design solution is rarely used in New Zealand but more common in large European cities where space is at a premium. The in-house design team’s lateral thinking came up with a construction method that was cost effective and suited the location.

The construction method involved the installation of a steel cutting ring at the base of the walls, followed by the subsequent casting of three 400mm, 18.5 m in dia, 4 m high concrete lifts, one on top of the other.

After each wall section was cast the earth within the ring was excavated, and with the aid of the cutting shoe, the caisson walls sank under their own weight. The annulus between the outside of the walls and the cut profile was filled with bentonite assisting the caisson’s even rate of descent and prevent collapse of the external ground.

Keys to success

The project was run in a collaborative manner, with all parties contributing to its success through innovative design and construct solutions. A minimal site footprint on a main local road which reduced the impact to local residents was achieved through detailed planning of access shaft locations, their construction and the pipe installation method.