By Lisa van Beurden
When a storm caused catastrophic damage to State HighwaY 25A, a joint venture stepped in to deliver a solution that combined speed, simplicity and precision.
In January 2023, the Coromandel Peninsula was battered by a series of storms. Cracks in State Highway 25A (SH25A) between Kopu and Hikuai, combined with an increasingly unstable slope, eventually gave way to a massive slip that destroyed more than 100 metres of the highway.
The impact was far-reaching: from increased travel time and cost; to restricted access to health care and education; and major disruptions for freight, tourism and local businesses.
Engineering firm Beca was engaged early in the recovery. When cracks began to appear in the highway, it launched a geotechnical investigation to assess the damage. After the slip, Beca worked with NZ Transport Agency Waka Kotahi (NZTA) to evaluate three main options: a northern bypass, a bridge to replace the lost section or a rebuild using retaining walls.
A multi-span bridge was deemed the most reliable, resilient and safe solution – reducing dangerous groundworks and wintertime earth-compacting challenges.
NZTA sought an integrated team that could deliver a rapid design-and-construct solution. The McConnell Dowell and Fulton Hogan joint venture (JV) partnered with Beca and Tonkin + Taylor and, with support from structural steel contractor Eastbridge, won the contract to deliver a bridge.
Design sprints
The required span, site constraints and tight programme demanded a ‘bread and butter’ solution, and the team settled on a multi-girder steel composite bridge design.
‘Steel is the most versatile material for longer spans,’ says Liam Edwards, Technical Director – Bridge Engineering, Beca. ‘A precast concrete super-T wouldn’t be suitable for the proposed span lengths. Concrete structures are also heavy; they place more load on the foundations, increase the seismic mass and bigger cranes are required to erect them.’
The JV began with a concept design for a 126-metre-long, four-span, steel girder bridge. A steep eastern slope complicated the site, prompting a move away from that face and an increased end span of about 40 metres. A three-span structure was developed to balance the loading.
Opportunely, Beca had recently designed a series of bridges for the Takitimu North Link Minden Gully. Stage 1 will connect Tauranga and Te Puna with a new four-lane expressway and shared path. Of the project’s eight bridges, one closely matched the Taparahi Bridge requirements and became the baseline for a steel option – a three-span, two-lane structure over a gully.
Using the steel composite design as a template, the team completed the tender in two weeks – an exercise that would typically take months – and won the contract.
While some design tweaks were expected, the team could confidently press ahead, knowing the numbers would validate the design. It wasn’t a typical process – instead of completing and reviewing the full design before construction, most design and build work had to progress concurrently to meet the Taparahi programme.
‘It required a series of “design sprints” whereby two engineering teams worked in parallel to develop and confirm the design,’ says Edwards.
Before locking in the main procurement items, the team had to confirm the foundation design, overall bridge structure and seismic considerations. NZTA also engaged a peer reviewer early on to assist.
Pile sizes and steel casings were ordered in week one, then they progressed to the superstructure design. The JV worked closely with Eastbridge, holding weekly design and fabrication reviews. Beca released steel details in staged increments for fabrication.
‘At week four, we confirmed all the details for the steelwork sizes so the fabricator could start cutting plate and welding the main girders,’ says Edwards.
In the meantime, Beca worked to validate the design details for the stiffeners and bracing.
Typically, design is completed before fabrication begins; but the tight programme required design and shop drawings to progress concurrently – a risky approach mitigated by the team’s technical expertise, deep experience with steel composite bridges, and clear, frequent communication. The team made calculated, risk-based decisions in real time, knowing that changes might arise, but confident they could be resolved.
Once off the critical path, the design process returned to normal for elements such as the reinforced concrete abutment beams, which were designedand checked, then built.
‘The key challenge was to deliver a bridge as fast as we possibly could. To do that, steel was absolutely critical,’ says Edwards.
Rapid steel fabrication
With structural steel for the bridge’s superstructure on the critical path, Eastbridge was engaged early in the design and procurement process. Armed with the shop drawings, the steel detailer worked directly with the design and construction team as the girders were developed. The bridge design was deliberately uncomplicated, but Eastbridge ensured no features were introduced that would add time and complexity to the steelwork.
Normally, the designer produces ‘issued for construction’ drawings, which the steel fabricator uses to generate shop drawings for review and approval – a process that takes one or two iterations. But with no time for back and forth, the steel detailer issued fabrication drawings in real time as design decisions were made.
Eastbridge was well-positioned to deliver the superstructure quickly, with ample stock and the right materials ready to begin fabrication as soon as it received the design details. It didn’t hurt that Eastbridge had delivered the ‘template’ bridge for Takitimu North Link Minden Gully.
Eastbridge also had the capacity and resources to deliver, thanks to a recent immigration recruitment drive. Like much of the industry, it faced labour shortages in 2022 and welcomed the new immigration channels introduced by the government of the day.
‘We brought in a lot of fresh fabricators and welders in early 2023. By the time fabrication started on the bridge, they were well and truly embedded in the business, and we had sufficient resource to hit the ground running,’ says Jayden Mellsop, General Manager, Eastbridge.
Clear communication – especially around timing – was key. The designer had to meet several critical dates to issue fabrication details, while the fabricator kept the site team informed so each steel section could be received and staged – right down to the last piece of steel, which signalled when the builder could start laying the concrete deck on top.
Prefabricating the steel girder bridge elements offsite reduced vulnerability to weather. Once complete, Eastbridge transported the beams to site and they were lifted into place – including 30-metre girders delivered in sets of three.
The biggest challenge was time, says Mellsop. To accelerate the programme, Eastbridge secured staff buy-in and introduced a night shift. Still feeling the raw impact of Cyclone Gabrielle, its Napier-based team was ‘itching to contribute’ – eager to make a difference, even though the project supported Coromandel, not its own local community.
Trust in each party’s expertise was important. Eastbridge having been independently audited to satisfy the requirements of the Steel Fabrication Certification scheme gave the wider team confidence in its steelwork processes.
‘We reduced the risk profile for the client and gave them confidence that we could deliver the steelwork when we said we would,’ says Mellsop. ‘That was arguably the most significant value we brought to the team.’
Collective determination
While the superstructure was under construction in the steel contractor’s workshop, the substructure was being built onsite amid an active slip and had to be carefully staged to limit the site’s vulnerability to the weather.
‘It drove a lot of the decision-making in that early phase,’ says Aimee Pene, Construction Manager, McConnell Dowell/Fulton Hogan JV.
First, the site was cleared and the ground prepared. The slip floor was raised nearly 12 metres using lime, cement and fill, creating a stable base for crane lifts, rain or shine.
It was close. ‘Because the programme was so tight and the steel fabricators were working double shifts, the site crew finished the working platform in its entirety just days before the first lift,’ says Pene.
Construction required 250-tonne cranes to lift up to 30-tonne steel girders. The steel solution reduced the onsite loading – an equivalent concrete beam would be nearer to 60 tonnes and require a bigger, heavier crane.
Steel allowed a deeper-sized girder that, in turn, reduced the design from a four- to a three-span bridge. The impact on groundworks was substantial: three piers in the slip zone became two. ‘It was massive for us – it reduced the number of piles and the amount of in-slip work,’ says Pene.
The offsite fabrication philosophy for steel was extended to the precast concrete deck. Instead of pouring the top deck in situ, full-depth precast panels were designed to sit on the steel girders, allowing construction to continue even in wet weather. Coordinating steel and precast elements required meticulous planning; precision was critical.
‘We brought Eastbridge into conversations early,’ says Pene. ‘Steel is a huge portion of the bridge and it’s on the critical path; it was important to have their buy-in to ensure we could do what we wanted to do.’
Eastbridge was heavily involved in detailing the precast deck reinforcement. Its draftsman worked with the precast team to ensure millimetre-perfect alignment between the steel reinforcement and the stud placement on the girders. Eastbridge developed a clever detail: tapering the top flange of each girder created a slight crossfall, easing precast deck placement and reducing onsite tolerance issues.
Driven by a collective determination to deliver, SH25A’s new Taparahi Bridge opened to traffic less than 12 months after the slip occurred – and just four months after the first pile was driven. It was a remarkable achievement, given that a bridge of this scale would typically take 18–24 months to design and construct.