In June 2021, flooding damaged a culvert on State Highway 73 near Craigieburn. Water had been flowing through cracks in the structure, washing away material beneath the traffic lane and creating a life-threatening risk of road failure. Waka Kotahi required an urgent, staged response.

Liam Cai was employed by Isaac Construction as the Project Manager to deliver the culvert replacement. Working alongside designer WSP, the team replaced the damaged DN1200 culvert with new concrete infrastructure — upstream headwall, downstream rock cascade, and full carriageway reinstatement — with SH73 open to traffic throughout.

Our Approach

SH73 is the primary route between Canterbury and the West Coast so keeping the road open was a key objective. The team designed and built a custom temporary traffic lane, 7m wide with 45m safety barriers, capable of accommodating B-trains and truck-and-trailer combinations.

Works were sequenced across eight stages — pipe laying, concrete bedding, downstream and upstream reinstatement, and final carriageway works — planned in close collaboration with the designer and client before any excavation began.

The remote location required practical ingenuity

With conventional survey equipment unavailable on site, the team used a drone to set and verify pipeline alignment — accurate installation without the tools that couldn't get there.

Environmental requirements were strictly met

Fish recovery was completed by an ecologist before any instream works. The downstream rock cascade was built to DOC specifications using locally approved rock, finished to blend naturally into the surrounding environment.

Daily progress reports and weekly summaries were provided throughout to keep all stakeholders informed on progress. The risk register was reviewed continuously as conditions evolved — ongoing storm events and flooding added complexity throughout the programme.

Project Outcomes

Waka Kotahi signed off two days after completion with zero snag list items.

SH73 remained open throughout with minimal disruption. All environmental controls were met and signed off. Liam delivered what the client needed: a technically complex job, managed cleanly, closed out fast.