Why road bridges are proposed instead of underpasses

The Takaanini Level Crossings project aims to remove the level crossings in the area and replace them with better and safer ways to cross the rail lines.

Train frequencies are planned to increase over the next decade, resulting in level crossing barrier arms being down for more than half an hour of every hour which will further delay people and vehicles from crossing the tracks.

In September, the AT Board approved the bridge option at Walters Road crossing for route protection. More information about why a bridge is recommended instead of an underpass at Walters Road, including a summary of the peer review findings, can be found here.

Why did we recommend a bridge instead of an underpass?

Based on expert assessment across multiple disciplines, the bridge consistently outperformed the underpass.

The criteria used for the further assessment included investment objectives, property impacts, urban design, visual landscape, stormwater and natural hazards (including flooding) outcomes, construction disruption, impacts on utilities, stakeholder and community feedback, and cost and risk implications.

In summary, the assessment found an underpass does not deliver a resilient outcome:

    • It’s likely to flood in significant rainfall events
    • even with a pump system installed an underpass is likely to be closed during flooding due to power failure or stormwater systems becoming overloaded

    In addition:

    • Construction of an underpass is more complex to construct over the rail line
    • An underpass requires more land and land acquisition adding to cost, impact and complexity

    What are we proposing as a result of these assessments:

    • To create a new bridge over the rail line at Manuia Road, accommodating all transport modes, with a particular focus on providing for heavy vehicles accessing the industrial area
    • Replace current level crossings at Taka Street and Walters Road with bridges accommodating all transport modes
    • Close the existing Spartan Road and Manuroa Road level crossings and replace them with bridges for active modes (walking, cycling, scooting).

    How did we decide on a proposed network?

    We evaluated options using a formal assessment process which identified:

    1. A proposed network – how many crossings are needed, where they should be located and what transport modes need to be accommodated at each crossing
    2. The physical form of crossingswhether the road should go over or under the rail line or whether the rail line should go over or under the road

    The key considerations in reaching the proposed network were:

    • The need to physically separate trains from the road for safety reasons – this is known as ‘grade separation’
    • High number of crossings in the Takaanini area
    • Whether all modes (driving, cycling, scooting, walking) needed to be accommodated at each crossing.

    Initial assessment - what types of crossings did we consider?

    Analysis from engineering and rail experts, input from Auckland Transport, Waka Kotahi, Manawhenua and KiwiRail, other technical assessments and feedback from the community and stakeholders were all considered in this assessment process.

    Creating a rail viaduct (bridge for the rail line to run over Takaanini roads) and rail tunnel options were found to have the highest potential for disruption, negative visual impact and high cost – because trains require greater distances than vehicles to climb up and down. This means that:

    • A rail viaduct or tunnel would need at least 1km of ramping to achieve the required clearance to work, and that a 3.7km structure would be needed to clear level crossings at Manuroa Road, Taka Street, and Walters Road
    • It is not possible to construct a rail viaduct or tunnel at Spartan Road given its proximity to the State Highway 1 motorway interchange. It’s also made more complex by the nearby Papakura Stream.

    For these reasons, the rail viaduct and tunnel options were discarded prior to shortlisting.

    This left the road bridge and road underpass as the shortlisted options. The initial assessment of these options is summarised below.

    Road over rail (bridge)

    Key considerations Preference

    Key roading design requirements including slope (gradients), clearance over the rail line, the width of the bridge and ability to accommodate for rail four-tracking in the future.

    A need to maintain access to properties adjacent to the rail line via access lanes.

    Preferred in initial assessment.

    Retested in relation to Walters Road

    Road under rail (underpass)

    Key considerationsPreference
    Broadly similar roading design requirements are required for an underpass as a bridge, but more land is required for an underpass to prevent soil and ground water impacts affecting private property. This is because there is potential for the ground to lower (ground settlement) with an underpass especially where ground water is encountered, and this could have impacts on surrounding private properties like soil or building instability.

    Construction of an underpass may also require temporary diversion of the rail line which could also have greater property impacts than a bridge.

    With an underpass there are higher costs, complexity, and risk. As well as flooding, safety, flooding and resilience concerns.

    Not preferred in initial assessment. Retested in relation to Walters Road

    Visualisations Disclaimer

    Visual simulations are indicative only and have been generated using civil engineering 3D renders over photographs for the purposes of showing potential location, form and scale of the project.

    The reference design is subject to further development and planting/screening indicatively shows some but not all visual and urban design mitigation proposed through the project.

    Simulated future urban built form (i.e, grey buildings have been shown based on building form anticipated by operative planning provisions as at June 2023 on sites where current buildings are included within the proposed designation boundaries.

    Detailed design and levels to be confirmed.