The Waitaki Bridges Replacement project is an example of both construction excellence, innovation, and outstanding community engagement.
BY MARY SEARLE BELL.
THE WAITAKI RIVER runs through the Mackenzie Basin in the lower South Island. The riverbed is the political divider between the Canterbury and Otago regions. Its braided beauty is popular for both fishing and jet boating.
Fed by three glacial lakes – Benmore, Aviemore and Waitaki – it is cold year round. A fast flowing river, the water volume is normally low in winter, but as the snow on the Southern Alps melts in spring, this increases significantly.
On its 110 kilometre journey from the mountains to the sea, it passes the rural communities of Kurow and Glenavy before reaching the Pacific Ocean on the coast between Timaru and Oamaru.
At Kurow, a pair of bridges were built across the river in 1881. The first from the tiny town of Kurow to Kurow Island mid-stream and the second from the island to connect to the community of Hakataramea on the northern side. The narrow wooden bridges were originally built for rail, but later opened to a single lane of traffic.
A vital link between the two communities, the bridges served for 132 years, enabling the area to grow and develop. In recent times the aging structures were increasingly closed during high river flows and for repairs and maintenance work. As the route is important for both freight and tourism, and the alternative route a 160 kilometre detour, the NZ Transport Agency decided to replace them.
Opting for an ECI (early contractor involvement) model, the agency awarded the contract to McConnell Dowell in 2011. Because the ECI model means the contract is given without any formal design, it allows for greater flexibility and innovation.
McConnell Dowell, and its design partners URS, worked with the NZTA and its technical advisors, Opus, to develop the design. In doing so, they were all able to fully understand the project constraints and explore various solutions together.
As a result, significant savings were found through reconsideration of the construction method, sequencing and the types of materials used.
McConnell Dowell construction manager Joe Edwards says the team were able to do “a lot of very good planning ahead of time”.
“Very little, if anything, changed once we got to site.”
With a price tag of $20.1 million, the new Waitaki Bridges (one 206 metres long, the second 92 metres) are of composite construction. The beams and deck are built from weathering steel, topped with precast concrete planks and an in situ concrete deck.
They boast two lanes of traffic as well as a barrier separated cycle and pedestrian lane on the downstream side. They also have increased capacity for oversized agricultural machinery and freight trucks.
A number of construction innovations helped the project to net first place in Category 3 – Projects with a value between $5 million and $30 million at this year’s Hirepool Construction Excellence Awards. In particular:
• The use of a high-strength, low alloy weathering steel allows longer spans, meaning fewer piles and less work in the river. But particularly, the weathering steel forms a protective rust patina which almost eliminates corrosion. The rusting process starts the same way as conventional steel but the specific alloying elements in weathering steel produce a stable rust layer that adheres to the base metal.
Over time (two to five years) the orange-brown rust changes to a dark-brown colour. The earthy tones of the weathering steel will allow the bridges to blend into their beautiful rural setting.
The weathering steel also meant the bridges did not require painting at the time of construction, nor are there any ongoing painting maintenance costs.
• A temporary trestle alongside the longer of the two original bridges helped facilitate construction of the new bridge and subsequent deconstruction of the old one. The use of the trestle saw the bridges completed ahead of schedule and deconstruction work finished six months early.
• The concrete finishing of the bridge deck was to such a high standard that the usual layer of levelling asphalt was not required prior to the final sealing. This too provided time and cost savings, and Joe says the team is very proud of the good quality achieved.
It is also worth noting the challenges of the site itself – work over water is always complex, the fast flow of the Waitaki River added to this. There were also extremes in temperatures – from the blistering heat of summer to winter’s snow. The site was also prone to flooding. On top of this, Joe says the Waitaki Valley acted as a wind tunnel, which had a significant impact on crane work.
Construction innovations and challenges aside, there was much more to this project than simply replacing the old bridges. For starters, these original structures were of historic significance and Heritage New Zealand, along with the locals, wanted a legacy to remain. To this end, two spans from the original bridges have been incorporated into an historic display on Kurow Island and the timber components from the old structures have been used in other projects in the lower half of the South Island.
Heritage New Zealand also required photographic and video records of the deconstruction work be provided to the local museum. McConnell Dowell also created a future legacy by providing the same of the construction work to the museum too.
Perhaps, though, what made this project an outstanding success was the high level of community involvement from day one. A partnering charter was signed by NZTA, Opus, McConnell Dowell, and Waitaki and Waimate district councils, with support from a number of community groups. Its mission was to ‘provide a strategic link between two regions and local communities to create a legacy that respects the past and inspires the future’.
“We set pretty high goals with the community in mind,” says Joe. “We actively measured our performance against the charter throughout the project – we hit or bettered every target.
“It’s McConnell Dowell policy – when we go to a community, we become part of that community. Our team lived in Kurow, or nearby, and we employed locals where possible.”
From Kurow (population: 400) McConnell Dowell recruited eight locals in the field workforce of about 15.
“We actively targeted those with little or no construction experience – we were happy to train them both formally and on site,” explains Joe. “Most of our local hires are still with us.”
Community engagement was also enhanced by a number of added-value side projects. A new double-land jet boat ramp on the island is very popular with the locals.
Joe says the community also approached the team and asked them to help with constructing a cycle track around the island, which they did happily. So too when the local preschool asked for a little wooden bridge to be built in their playground.
“This kind of thing is really easy for us to do but meant a lot to the community,” says Joe.