As part of the Causeway Alliance, Tetra Tech provided embankment design, ground improvement strategies, and geotechnical engineering for the State Highway 16 (SH16) Causeway Upgrade in New Zealand. Safety, innovative engineering, and reduction of construction costs were priorities for the Causeway Alliance, which included AECOM, Tetra Tech, CPB Contractors, Fulton Hogan, Jacobs, and the Transport Agency.

The SH16 Causeway Upgrade is a key part of the Western Ring Route, one of the New Zealand Government's seven roads of national significance. The Causeway Alliance was brought together to provide innovative solutions for the roadway’s ground improvements.

The new Western Ring Route will provide a strategic alternative to SH1, central Auckland, and the Auckland Harbour Bridge as well as provide greater capacity around the city.

Tetra Tech has provided innovative and cost-effective ground improvement solutions, while minimizing environmental impacts. Their knowledge of technical issues associated with the Causeway Upgrade Project have given me confidence that our engineering risk will be reduced and that we will stay on target for our completion date.

Mark Evans, Alliance Project Manager

The project’s challenges included deep, very soft soils on which this 4.8-kilometer (3-mile) stretch of SH16 is constructed. If left untreated, this section would have required ongoing repairs and experience further settlement and flooding.

The project also ran through the Motu Manawa Pollen Island Marine Reserve, which required measures to ensure that all project works had minimal impact on the reserve and surrounding environment.

The Causeway Alliance provided value for money with innovative solutions for ground improvements. The team reduced the need for potential future reclamation and safeguarded against coastal erosion and storm tides while maintaining full access to traffic and cyclists.

This innovation will ultimately mean a more efficient network system, as well as improved public transport connections and cycleway access.

To prevent flooding, the Causeway Alliance raised the motorway by 1.5–2.4 meters (4.9–7.8 feet). The design incorporated a cost-effective solution of wick drains, preloading, and the use of lightweight pumice sand to mitigate both total and differential settlement risk.

By optimizing the final embankment width and adopting an asymmetric widening option, the Causeway Alliance significantly reduced the project’s overall footprint.

The work was completed in stages so that disruption to the community, the environment, and traffic was kept to a minimum.


The SH16 Causeway Upgrade received an Award of Merit at the ACENZ 2017 Innovate Awards in recognition of an outstanding project.