Strategic planning and collaboration for transport infrastructure in Milldale, Auckland

A vital link supporting Milldale’s growth

The vision for the Milldale Infrastructure Projects began in 2016 as part of Auckland Council's strategic plan to enable growth in the area west of State Highway 1 (SH1) near Wainui.

Initially involved in providing strategic advice for funding agreements that would enable the infrastructure development to proceed, we were trusted to take the design and consenting forward for the extension of Argent Lane and development of Milldale Crossing.

Developing Argent Lane to support growth

The Argent Lane extension project realigned and upgraded an existing rural road connection to an urbanised four-lane arterial connection between Old Pine Valley Road to Dairy Flat Highway and the Silverdale Interchange. The project included intersection upgrades, property acquisition, stormwater treatment and restoration.

Auckland Transport’s objective was to provide a safe and resilient road corridor that would enable a suitable transport connection to Milldale that is future proofed for future development in the wider area. Public transport, cycle and pedestrian pathways were also key to enable connection and accessibility to social and economic opportunities within the wider Milldale and Silverdale area.

The extension was identified as a key mitigation measure for the additional traffic movements associated with anticipated growth of the area. Extensive consultations with stakeholders and future development considerations influenced road design parameters and utility service requirements, leading to adjustments in road platform widths. In response, our team introduced design enhancements that balanced affordability with the evolving needs of the project.

Due to the high complexity of the project involving land take, wetland removal and relocation, and legislative requirements, careful management of the resource consent process was required to enable the extension. The area's prime housing status meant that acquiring additional land proved contentious. We therefore minimised land acquisition by slightly widening the existing road reserve and using low-level retaining walls, effectively reducing land take.

Our planning team fostered a close working partnership with the client and Mana Whenua to deliver and obtain regulatory approvals for a future-proofed, efficient and safe road design that also provided environmental benefits.

Milldale Crossing

A key enabler for future-proofing the area has been the development of the Milldale Crossing, a 94m bridge that serves as a vital connection across SH1, linking the Highgate Business Park with a new housing subdivision in the suburb of Milldale. As well as providing safe access for vehicles, cyclists and pedestrians, the Milldale Crossing also needed to accommodate the 'Orewa 3' 726mm diameter watermain which once complete will provide another point of supply for the Hibiscus Coast area giving the community a more resilient water supply.

“Milldale Crossing faced various challenges including stakeholder engagement through changing design philosophies, challenging geotechnical considerations and COVID-19 lockdowns disrupting design and construction,” said Damian Smith, Mott MacDonald project manager for the project and technical director of project strategy and delivery. “Despite these challenges, the team worked collaboratively to successfully deliver a high-quality outcome that meets the community’s needs.”

Reducing operational maintenance

To reduce operational maintenance, the bridge was constructed using a low-maintenance, marine-grade weathering steel and concrete composite, supported by ten substantial girders. Using marine-grade weathering steel for the bridge construction – a first in New Zealand – increased durability and significantly decreased operational maintenance requirements.

Challenging ground conditions

In the early days, it was clear that there would be significant complications arising from the challenging ground conditions. On one side, there was an existing 14-metre-high reinforced mechanically stablished earth (MSE) with a 70 degree slope, and on the other side, there was “Onerahi Chaos” or “Northland Allochthon” – geological formation typical in the Northland region of New Zealand and as complex as the names suggest. Various foundation options were considered. In the end, we opted for driven casings with end plugs to improve control over pile capacities and address the challenge of lengthy piles, resulting in an effective solution despite the ground conditions.

Futureproofing for population growth

To ensure the project was future proofed for the growing population in the area, the bridge had to be designed to allow for various scenarios. This included accommodating a possible future busway which would run alongside the current SH1 corridor as an extension of the existing Northern Busway route. A new MSE retaining wall on the west side and running parallel to the highway was designed to allow opening of the surrounding land for future residential housing while also being positioned to allow for the future busway.

Climate change impacts

Incorporating carbon metrics into the multi-criteria analysis (MCA) process helped to inform optimal road alignment and facilitate the selection of low-carbon options, aligning the project with sustainable development goals.

Incorporating Māori design

A key part of the Milldale Infrastructure Projects was collaboration with Mana Whenua, who provided cultural design oversight of the project. A Māori narrative was developed and mahi toi (Māori design) used in the throw screen, pavement approach designs and guard rail design. The designs were developed to work with the structural elements of the bridge and avoid additional add-ons.

To foster effective stakeholder engagement, 3D models of the bridge were used to help visualise the project and alleviate any concerns.

An artwork installation by Reuben Kirkwood, from Tika Creative, provides the final touch to the project. The aurei (cloak pin) carving at the Milldale approach symbolises the deep cultural ties between the bridge and the land, while the pathway patterns reflect the area's industrial heritage, incorporating colours inspired by local Rewarewa trees.

Close collaboration with both Auckland Transport and the New Zealand Transport Agency were key throughout the design and ongoing construction phase, with the two organisations responsible for the asset upon completion.

With a total investment exceeding NZ$40M, the crossing greatly enhances residents' access to a variety of amenities along the Hibiscus Coast, fostering connectivity and convenience. Now open, the Milldale Crossing marks a significant advancement for the local community.