Steel features prominently in West Auckland’s award-winning new shared pathway.

West Auckland’s award-winning New Lynn to Avondale shared pathway is a walking and cycling route that links the two town centres and their train stations. It follows the gentle gradient of the rail corridor, and is part of a wider network of shared paths. It connects to the Waterview and south-eastern routes, enabling people to cycle safely all the way from New Lynn to the city centre.

The pathway comprises elevated helical boardwalks, balustrades and handrails, and two new pedestrian bridges. Situated over rough terrain and waterways, the project team collaborated to make the 2.9-kilometre pathway buildable and practicable. The success of the shared pathway’s design lies in its simplicity, making it easier to build and install.

Boardwalks were chosen over at-grade paths because they could achieve the required vertical gradients without the need for retaining walls and earthworks. The boardwalks presented a simpler and cheaper solution.

Lead engineer Aurecon chose to establish a steel structure because it offered a higher design life and larger spans than a timber alternative.

‘The lightweight solution also meant the section sizes were smaller compared with other options, which reduced the loads,’ says Dave Mockett, Technical Director at Aurecon.

The structure’s weight was reduced further with the use of a lightweight fiber-reinforced plastic (FRP) decking. The FRP provides a permeable surface for rainwater, and avoids an additional load adding to the existing stormwater system.

Auckland Transport specified that the boardwalks be at least three metres wide and able to accommodate lightweight maintenance vehicles. The pathway has two key boardwalk sections: one is 218 metres long and 4.5 metres above the ground at its highest point, and the second is 163 metres long and reaches almost 2.4 metres high.

The sheer scale of the boardwalks drove the modular design, which could be easily replicated.

‘Structural steel is well-suited to modular construction, and it was a relatively simple exercise for the team to adapt the design for the high piers,’ says Mockett. ‘Extra bracing was added to achieve the required deflection limit, and the same lightweight materials were able to be adopted.’

The boardwalk’s vertical and horizontal geometry varies considerably along its length. Steel offered a flexible solution to achieve the gentle gradients required for walking and cycling. The lightweight spans easily accommodated changes in the vertical and horizontal alignment, even with the modular system.

The position of the boardwalks in relation to the operational railway was important. The design aimed to keep the boardwalk as far as possible from the tracks to maximise work that could be undertaken while the railway was operational.

The railway corridor is also the backbone for KiwiRail’s critical underground services. Minimising the number of piles required directly reduced the risk of hitting the services – this was a core focus for the project team.

‘Steel was the ideal material for this purpose thanks to its high strength-to-weight ratio and ability to deliver long spans,’ says Mockett. 

Timber, on the other hand, would have required an estimated 80 per cent increase in the number of piles.

‘Buildability was a strong focus for the team,’ says Anthony Manuela, Contract Manager at Dempsey Wood. 

The use of steel allowed an efficient substructure, and the lightweight modular construction maximised the spans and reduced the number of piles. The effect was to limit the number of railway closures required to install the boardwalks and minimise disruption to passenger rail services.

Screw piles were the first option considered to support the boardwalks; however, the large volume of underground services meant that this approach, which takes time and precision, would have made it difficult to install these piles within the tolerances required for the structural steel connections.

Instead, the team proposed a shallow concrete foundation using short-bored micropiles – small-diameter steel rods installed in concrete-filled drill holes. This meant the superstructure units could be positioned with minimal adjustment.

Builder Dempsey Wood engaged RoadLab to develop the substructure design, which was more accurate, sped up construction and led to cost savings. The flat, capped‑off piles provided a level reference point to build off and enabled the team to work to practicable tolerances.

D&H Steel Construction spent time up-front, refining the details during the shop drawing stage to optimise steel usage, and ensure faster, smoother construction on site. Its focus on value engineering eliminated labour‑intensive welding of the boardwalks by replacing all of the proposed connections with bolted assemblies. It meant that all of the steel fabrication was completed offsite in a controlled workshop environment.

‘D&H salvaged the 600-millimetre “offcuts” and advised a change to the boardwalk design so the material could be used for the bracing,’ says Colin Ross, Project Manager at D&H Steel. ‘It meant no steel was wasted.’

The consistency applied to the section sizes and spans allowed the details to be repeated across the boardwalks.

One of the construction challenges of the rail corridor was access, which was limited. The team was faced with working in tight and difficult spaces, and alongside the rail track contours with its electrified overhead lines. Fortunately, the lightweight steel solution meant that the machinery was smaller, including cranes and trucks. It offered more flexibility to access constricted areas.

Arguably, the St Jude Street rail crossing was the most constrained part of the shared path. The remote site, which was extremely close to the rail powerlines and had very little space to manoeuvre, was the location for a new steel footbridge.

Installation of the 26-metre-long, fully welded bridge section started with a ‘block of line’, where the rail corridor was only closed for the duration of Labour Day weekend. If the team hadn’t made this deadline, the program would have been delayed for more than three months until the next rail closure.

Sandwiched between an apartment block and housing, the 18-tonne single-span bridge was flawlessly lifted over a main road, and electrified rail and powerlines, and lowered into place.

‘After three weeks of meticulous planning, a 450-tonne MegaWing crane, operating at its full 70-metre reach and with 160 tonnes of kentledge weights, took just 22 minutes to “land” the bridge from the truck to its final position,’ says Manuela.

Works were carried out in close proximity to the electrified rail tracks and overhead powerlines. The team was careful to prevent arcing between the overhead lines and the new steel structure. It meant working within strict clearance requirements, and ensuring that none of the equipment was capable of stretching further than the envelope that KiwiRail had defined.

‘An all-round team effort was key to the success of this project,’ says Ross. ‘All challenges were confronted head on and resolved on the fly. This could only be achieved with total commitment from everyone involved.’

The result is an impressive new infrastructure asset for the community to enjoy.