Whirokino and Manuwatu River Bridges © Fletchers (Brian Perry Civil); Whirokino and Manuwatu River Bridges © Fletchers (Brian Perry Civil);

Whirokino and Manawatu River Bridge Replacement Program, New Zealand

Keeping bridges standing throughout a crisis

Resilience has a particular meaning in the earthquake-prone areas of New Zealand. When disaster strikes, major travel routes are lifelines in the recovery effort. The New Zealand Transport Agency (NZTA) has a duty of care to ensure highways remain open – albeit at a reduced level – during and after an emergency. But when high earthquake activity meets weak ground, this can be easier said than done. The Whirokino and Manawatu River Bridge Replacement Project was designed to improve efficiency, resilience and safety in a challenging environment.

The 1930s Whirokino Trestle and 1940s Manawatu River Bridge, which span the Manawatu River and an adjacent floodway on the west coast of the lower North Island of New Zealand, are nearing the end of their economic lives, and need considerable ongoing maintenance for safety.

The New Zealand Transport Agency (NZTA) decided to replace them with stronger, wider bridges that will need far less maintenance. Importantly, the new bridges will be more resilient, safer, stronger, with wider traffic lanes and shoulders, improved cycling facilities and reduced delays for over-sized vehicles (by as much as 20 minutes).

Project Summary

NZ$170 million to build

20 minutesTravel time saving for heavy vehicles

2020Bridge opening

© Fletchers (Brian Perry Civil)

The project proved to be very complex from a geotechnical point of view, as the area is affected by high earthquake hazard and variable, weak ground conditions. The most critical elements were the bridge abutments and piled foundations for the bridges. The bridge piled foundations were designed for the effects of liquefiable ground and the bridge abutments for lateral spreading (landslide caused by earthquake shaking and ground liquefaction). The foundations were designed to be resilient across a range of earthquake hazards, from frequent small and moderate earthquakes up to major earthquakes.

The project is Arup NZ’s first major Design & Construct project for the New Zealand Transport Agency. We have developed great working relationships with our design and construction partners on the project. ”

Andy Dodds Andy Dodds Geotechnical Lead

Arup designed a ground improvement system comprising interconnected in-ground (cellular) walls of overlapping concrete columns to provide support at the bridge abutments and protect the piled bridge foundations from the effects of lateral spread. 

Work on site © Fletchers (Brian Perry Civil) Work on site © Fletchers (Brian Perry Civil)

Seismic actions also governed the design of an agricultural underpass, which provides a 4m x 4m clear opening through the new embankment between the replacement bridges.  Racking deformations caused by the shaking of this in-ground ‘concrete box’ were accommodated by design. Significant undercut of soft peat material was carried out to limit the longitudinal differential settlements to tolerable levels.

Arup’s geotechnical and seismic engineering expertise has been useful in achieving the NZTA’s required level of resilience at a favourable cost, ensuring the people and businesses of the Manawatu-Wanganui region will enjoy the benefits of a modern, functional, safe design that will perform well when major earthquakes occur.

© Fletchers (Brian Perry Civil)

Lots of good, clear communication and excellent scope definition in the first instance has made working with the Arup team straightforward. The team has been accessible and given a great level of service. ” Blair Mould Contract Manager, Brian Perry Civil (The Fletcher Construction Company Ltd)