Mercer to Longswamp Expressway

  • Whangamarino River Bridge
  • Mercer Interchange looking North
  • Mercer Interchange looking South

BBO were engaged by Transit New Zealand in 1998 as consultant for the completion of investigation, design and construction administration/surveillance for the Mercer to Long Swamp Expressway project. The four laning of 12km of SH1 included two interchanges, six bridges, four intersections, 1km of rail deviation, and 1,700,000m³ of earthworks. BBO’s role included survey, stakeholder consultation, designation and resource consent procurement, detailed design, and construction contract administration.

The project was characterised by various complexities and constraints throughout the process that presented challenges. The project required BBO to take a very innovative approach due to the problems and constraints of the project and this contributed to the positive results obtained.

The first major constraint to the project was identified by BBO during the preliminary geotechnical investigation that identified large-scale potential hillside stability risks. Further investigations confirmed the presence of slide planes at depth with low factor of safety against massive block slide movements of the hills into the Waikato River. Such an event would jeopardise the security of SH1 and the NIMT railway. BBO undertook an assessment of the risks of proceeding and developed an alternative solution involving relocating the Mercer Interchange south onto firmer ground and constructing two new southbound SH1 lanes skirting around the back of the block slide features. The alternative solution was adopted by Transit and was constructed.

The majority of the project’s earthworks cut was from north of the Whangamarino River, which then required haulage over the river and the NIMT railway to fill embankments to the south. The constrained linear nature of the site required truck haulage along and across SH1 presenting challenges in terms of minimising the impact on state highway traffic. BBO saw the opportunity to construct a permanent bridge over the NIMT railway and Whangamarino River to facilitate earthworks haulage once it commenced. The bridge was designed and constructed to carry fully laden motorscraper and dump truck loading allowing 1,000,000m³ to be hauled over the bridge. The bridge was an innovation by BBO and was built four years before it was scheduled to be open to public motor traffic.

Also during the project a previously unidentified issue was raised in relation to the presence of a taniwha in an area adjacent to the Waikato River that would be affected by construction. BBO identified a low cost solution to the unique risk that would enhance the environment, mitigate local hapu concerns and avoided delays to the construction program.

Other complexities presented to BBO also included:

  • Unprecedented geotechnical challenges and risks that came to light throughout the design process and construction phase;
  • Minimising the impact on SH1 traffic while engaging in required truck haulage across the state highway;
  • Maintenance of all road, rail and pedestrian traffic movements throughout the construction of the Mercer interchange;

BBO received positive public feedback from Ngati Naho iwi and kaumatua in recognition of the extensive consultation process and the consideration of cultural issues. BBO’s achievements on this project were recognised with an Award of Merit at the ACENZ Awards of Excellence 2007.

“Our stakeholders and customers are delighted with the finished product and I have received many compliments not only about the safety improvement but also about how the improved driving experience is changing people’s trip behaviour in choosing this route whereas beforehand they wouldn’t”.

- Transit’s Regional Manager