Thames-Paeroa to close as first NZ state highway timber bridge in 50 years is built

The replacement of Onetai Bridge, halfway between Paeroa and Kopu on State Highway 26, marks a significant change in bridge design. Photo / NZTA

A stretch of road connecting Thames and Paeroa will be closed for up to a month from mid-February as construction of the first state highway bridge built from timber in 50 years gets under way.

NZ Transport Agency Waka Kotahi (NZTA) acting regional manager of infrastructure Bruce Waugh said Onetai Bridge, about halfway between Paeroa and Kopu on State Highway 26 is to be replaced.

“As SH26 is the only through route between Paeroa and Thames on the eastern side of the Waihou River, the detour route will be via State Highway 2, Hauraki Rd and State Highway 25 for northbound traffic and the reverse for southbound traffic.

“We’ll be able to provide more information and timings in the new year, once we finalise our construction programme; we recognise the impact this closure will have on road users, particularly those from local communities and are ensuring they are given early advice of these works.”

Given the narrow width of the existing bridge and its approaches, it could not be replaced in stages, so the highway would need to be closed at the worksite for up to four weeks while the old bridge was demolished, and its replacement installed, Waugh said.

“We expect this to be from mid-February.”

The replacement project was announced in early 2023.

Onetai Bridge was constructed in 1976.

The existing bridge was designed and constructed in 1976 and has reached the end of its economic life.

Its replacement would be three metres wider, giving drivers more space, and with additional safety features including a new type of side barrier.

“Although bridges in New Zealand are generally built with concrete, this bridge will be built with a timber deck and bridge beams,” Waugh said.

“Timber deck bridges are built successfully overseas, and we are now piloting this approach in New Zealand.”

The off-site prefabrication of key bridge components allowed for rapid on-site construction, he said.

The detour route will be in place for up to a month.

It would be the first state highway bridge built from timber in 50 years, representing a move towards using climate-friendly materials.

On announcing the project in 2023, NZTA regional manager maintenance and operations Rob Campbell said timber elements of the new design made the project more than just a run-of-the-mill replacement.

Road bridges in New Zealand were generally built with concrete or steel and Onetai would be the first to mark a return to more sustainable design, with timber beams and deck.

“We’re hoping to make this the new norm, so while it might seem like a minor project, it marks a groundbreaking shift in design for bridge replacements on state highways.”

The new bridge will be constructed from timber.

It came down to layering – using an engineered timber product called glulam, made from layers (laminates) of wood glued together.

Campbell said glulam had double the strength per kilogram compared with steel and was specially designed to be hard-wearing and durable.

“The new Onetai Bridge is designed to last at least 100 years, linking the greater Waikato region with the Coromandel Peninsula, and providing a small but vital part of our state highway network.

“The replacement will add to the resilience and safety of this route, as the new design will be stronger and wider than the old bridge.”

Some of the timber beams to be used in construction of the bridge.

Glulam is being used increasingly around the world for bridges, both on highways and on local roads.

“The first and most obvious benefit is that glulam’s main component, pine wood, is grown right here in Aotearoa and therefore does not need to be mined or go through the high-energy manufacturing processes needed to make steel and concrete,” Campbell said.

“New Zealand glulam is made from responsibly sourced radiata pine and actually has a negative carbon footprint during the production phase. This is because the trees have absorbed carbon dioxide from the atmosphere during the 30 to 40 years they take to reach maturity, and new growth starts that process all over again.”

The bridge would have concrete foundations, and new scour protection both upstream and downstream. Scour, which happens when water flows change or rivers flood, is a common cause of bridge failure.

NZTA undertook a business case in 2020 for the replacement of the two-lane bridge.

Construction funding was approved under the 2021–24 National Land Transport Programme.

During the business case, NZTA spoke with iwi partners, key stakeholders including the Hauraki District Council, and affected landowners to gain information for the initial concept design.