Client: Avalon Industrial Services

Location: Arapuni, Waikato

Arapuni Hydro Power Station is situated in a gorge of the Waikato River, surrounded on both sides by steep banks and cliffs. Several years ago, a slip on the riverbank released a significant amount of material into the tail race. Allied Concrete’s Cambridge plant has been working at the site to protect the lower toe of the Arapuni Dam Bank.

After the slip occurred, it presented two significant issues. Dam owners are required to maintain unimpeded flow through the tail race; and the slip threatened, in time, to undercut the only access road to the powerhouse. Considering these risks, a multi-year effort has been underway to both reduce the deposit of material obstructing the tail race, and to stabilise and protect the bank and road from further erosion.

Allied Concrete undertook work to protect the lower toe of the slope with a concrete “seawall”. This wall had to be built on the riverbed in the approximate location of the old riverbank.

Building a wall of concrete in a riverbed comes with many risks and restrictions, but fortunately, there was a dam upstream to control the flow into the tail race.

Mercury Energy had a scheduled station shut lasting up to 34 hours, meaning the river was diverted and water level dropped below the worksite. As such windows only occur once every 3-5 years, time was of the essence. Using two teams, Allied Concrete and others made quick work. This involved supplying the first pour after-hours, starting at 6pm onsite, followed by a second pour at 1pm the following day. With one hour to spare, 44.4m3 of concrete was placed between the front rows of the wall proper, and the backfilling of the lower level to seal ready for future backfilling of drainage material.