Quinn’s Bridge Repair

Client: Watercare

Restoring a critical link in Auckland’s water supply

High in the Waitākere Ranges, the Upper Nihotupu Dam plays a vital role in Auckland’s water network. As the city’s highest dam, it’s one of Watercare’s most sustainable and cost-effective water sources, thanks to its gravity-fed delivery.

In January 2023, the Anniversary Weekend floods triggered a slip that undermined the foundations of Quinn’s Bridge, a 40-metre structure supporting the dam’s main outlet pipe. With the bridge out of action, the essential water supply from this major catchment was cut off.

Rapid repair in a remote location

Located deep in the bush, Quinn’s Bridge has no road access, so materials and equipment - from steel beams to small excavators - had to be flown in by helicopter. The only access for personnel was a 45-minute walk through five small tunnels.

Strict weather protocols added further constraints, with work stopping whenever wind or rainfall exceeded safety thresholds - not uncommon in the Waitākere Ranges.

Tackling constraints with creative solutions

With Cassidy Construction as lead contractor, GRIT was brought in as design engineer to collaborate on a safe, practical repair to restore the water supply fast, before a full foundation rebuild could be carried out.

To reduce the need for visits to the remote site, and inform robust decision-making, GRIT used 3D modelling to build a precise digital model of the bridge. This enabled reverse engineering of what was originally a tram bridge, to calculate its remaining capacity and minimum requirements to return it safely to service.

Working closely with Cassidy, a strategic repair solution was developed to meet timing and access constraints. Using a series of spliced steel members to shore up the timber structure, the original load path down into the existing foundations was restored. This avoided excavation and concrete works, by using steel components flown in and fixed on site. With some creative thinking, backed up by input from our mechanical engineers, the pipeline itself was also incorporated into the load-bearing system, further reducing the need for additional supports.

Buying time for a permanent fix

Within a tight schedule, the bridge was stabilised and the pipeline re-supported, enabling the dam to resume supplying water to Auckland before the increased demands of summer. 

This innovative temporary fix bought the project team time to design a permanent solution - with anchored rock foundations to ensure post-disaster resilience. That permanent repair is now underway, again in close collaboration with the client and contractor to ensure constructability and minimal impact on supply.

A complex problem in a challenging location, solved through pragmatic engineering and close partnership - the result proved that even urgent, high-stakes infrastructure issues can be resolved rapidly when everyone works towards a common goal.