Bigger-than-expected sinkhole job running ahead of schedule

Work to fill a cavity in the Oamaru Creek bed is progressing well. Inset: District Council...
Work to fill a cavity in the Oamaru Creek bed is progressing well. Inset: District Council network operations engineer Mark Renalson (left) and roading network engineers Josey Wallace and Kedar Dhakal stand in a shelter to protect divers constructing a coffer dam in the Oamaru Creek from the elements. PHOTOS: SUPPLIED
Work to fill a cavity in the Oamaru Creek bed and prevent more scouring of the bridge abutment is progressing ahead of schedule.

However, it has been a much bigger job than expected, Waitaki District Council network operations engineer Mark Renalson said.

When a sinkhole was discovered under the Humber St bridge, the speed limit was reduced to 30kmh and judder bars were installed while the council investigated the cause.

On Monday, the bridge was closed to all traffic and a coffer dam was built to divert the creek flow and allow access to the bridge abutment, reduce the risk of contaminants entering the waterway and allow wildlife and fish to continue their passage.

"Once the site was cleared, we actually found a really large hole in the creek bed.

"We only expected to find a cavity of about 500mm, but it’s averaging between 1.2m and 1.5m. It’s significantly larger than we expected.

"Effectively, it means the water can get underneath the abutment and pull out all the road aggregates."

The cavity must have formed since bridge repairs in 2017, when a sinkhole, big enough to fit a small car, developed after flooding, Mr Renalson said.

Yesterday, contractors poured a new toe in front of the abutment to fill the cavity.

"The next step will be to bring in the roading contractor and bring the top off the road and recompact everything, and then just monitor any settlement that goes on after that."

The council had used local contractors and suppliers for the project. Recreation Construction completed the concrete works and Banks Building Services constructed the coffer dam.

"It’s a good example of how much depth of skills and materials we [have here]."

The project was estimated to cost $60,000.

Work was progressing ahead of schedule, Mr Renalson said.

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