Country: New Zealand
Key Area: Transportation
Services: Civil Engineering,Geotechnical Engineering,Transportation Planning & Engineering

Transmission Gully has long been viewed as the best solution to Wellington’s northern highway accessibility needs. The idea for an inland, alternative route to bypass the existing congested coastal highway and the suburbs north of Wellington was first raised in 1940 and has been under consideration by various parties ever since. The New Zealand Transport Agency (NZTA) recently published a Draft Scheme Assessment Report for the 27 km long route, which was compiled by a project team led by Opus.

The report brings together findings from the investigative and preliminary design work of Phase 1 of the project. It is a ‘draft’ report because feedback is still being sought from the community and general public on the preferred route and this may result in changes to the final report and highway route. Opus’ involvement with the Transmission Gully project dates back to 2007, when we were awarded the preliminary geotechnical assessment contract.

A consortium, was selected for their knowledge gained from previous involvement with the proposed highway,was and comprised of: Opus; Holmes Consulting (who provided the preliminary structural design work for the proposed viaducts, bridges and walls); Boffa Miskell (who provided the ecological and landscape/urban design advice) and GHD (who provided the planning services). In addition, specialists from Opus’ Central Laboratories provided assessments on the noise levels expected along the route. The NZTA co-ordinated the various contracts in consultation with Opus throughout the contract period.

A lot of the preliminary work was done by helicopter and the same method was often deployed in getting teams and equipment to difficult sites for drilling and geotechnical work. Our engineering geological team walked and examined the entire route following office based data assessment in the preliminary mapping phase. In all, 88 bore holes were drilled by 10 drilling rigs and 177 trial pits excavated. Extensive laboratory testing of soil and rock samples were also undertaken by Opus.

The preferred design provides greater security against natural hazards such as earthquakes and it also reduces impacts on ecologically important streams and a nearby wetland and harbour inlet which is of significance in terms of indigenous flora and fauna. Extensive measures have been incorporated in the proposal to minimise the impact on this sensitive environment. Consideration has also been given to the road safety aspects, with crawler lanes provided for slower moving vehicles over the steeper sections of the route.

Over part of its length the route runs adjacent to and crosses active faults, however the route has been designed to minimise any damage in the event of a fault rupturing. A number of cost effective solutions were developed as the project progressed. One innovation that significantly improved the way stakeholders were able to understand the complexity and the information we were providing was developed using Google Earth technology.

The proposed ‘new’ route for the highway runs roughly south from Mackays Crossing at the northern end to reconnect with the existing highway about 15 kms north of Wellington city. The cost of the preferred route developed by the project team was calculated to be NZ$1 billion. This represents a saving of NZ$200 million over the original route. The next phase of the project is for the procurement of professional services to take the project to a stage when resource consents and amendments to the designation can be lodged.