Watercare’s Eastern Interceptor is a large reinforced concrete cast in-situ tunnel that follows a 19km long path from Orakei to the Mangere wastewater treatment plant. It delivers two-thirds of the total wastewater flow processed at Mangere. Sections of the tunnel have been affected by corrosion, significantly reducing the durability and performance of the sewer at these locations.

We were approached by Watercare’s head contractor, BBR Contech, in 2013 to undertake blasting inside the tunnel to remove debris and degraded concrete in the preparation of application of a new protective coating.

The work inside the sewer involves a logistically complex exercise where cleaning, preparation, application of high-performance concrete and curing all take place within very short operational windows.

Working in a live sewer also presents a number of health and safety issues, including biological risks and gases. In response to this challenging environment, we designed and built an automated trolley-mounted unit equipped with ultra-high pressure (UHP) water blasters to undertake the blasting work. UHP water blasting is very efficient and results in considerable time-savings. The technology is also very effective at removing the concrete without disturbing the sewage.

Winches attached to the trolley allow the unit to be pulled through the tunnel at a controlled speed. The trolley is capable of blasting 500m of tunnel at a time although we generally operate it over stretches of 150m.

Operators stationed outside the tunnel operate the ultra-high pressure blasters mounted on the trolley via remote control. The operators can monitor progress and work quality via CCTV cameras mounted on the trolley.

Using the remote controlled trolley and the CCTV equipment substantially reduces the amount of time that operators need to spend inside the tunnel, minimising health and safety risks.

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