Forming the southern section of the Western Ring Route, a new 4.5 kilometre motorway replaces Wiri Station Road as the main link from Manukau city centre to the Southwestern Motorway.
- Location: Auckland
- Started: 2006
- Cost: $210,000,000
- Completed: 2011
- Product: CTB, basecourse, drainage aggregates, bulk fill
- Contractor: Leighton Works
Leighton Works Makes the Connection
The NZTA motorway project has been constructed by Leighton Works, a joint venture between Leighton Contractors and DownerEDIWorks.
Valued at an estimated $210 million, the new motorway link is made up of two lanes in each direction. The design also allows for a third lane to be added to each side in the future, giving six lanes in total. Within this section of motorway, two interchanges at Cavendish Drive and Lambie Drive are included — as well as motorway-to-motorway connections toSH20 and SH1 at Manukau City. The work includes five motorway crossings over local roads and streams, as well as the main North Island rail line. The project also included preparatory work towards the construction of a rail link to the Manukau city centre from the main North Island rail line. A Manukau City Council and KiwiRail funded project, it forms a 1.8 kilometre connection between the existing main trunk line at Wiri and a new station at Manukau City Centre.
Largest plant-mixed CTB project
The SH20-1 project saw the largest plant mixed, cement treated basecourse (CTB) application in New Zealand’s infrastructure history. While often used in Australian projects, plant mixed CTB is a relatively new concept in New Zealand.
Developed by Australian engineers Golder Associates, and based on the specification published by VicRoads Section 815 Cementitously Treated Crushed Rock for Subbase Pavement, the CTB pavement design utilised a GAP20, typically more common in Australian road design. A GAP20 product from Hunua Quarry was specified with a 3% cement blend. Optimum moisture content (OMC) was targeted with a tolerance of ±1%, tested using a nuclear density meter (NDM) at each load.
A high level of quality assurance for the project encompassed a comprehensive management plan, developed by Winstone Aggregates, to document the production, testing, reporting and delivery procedures. In total, more than 55,000m³ of CTB has been placed in the project.
The numbers
- Over 220,000 square metres of road pavement
- 344 super T beams
- 12 bridges
- 7 sediment ponds to collect and treat storm water
- 16.8km of drainage
- Over 1,000,000m³ of material moved
- Over 110,000m³ of GAP65
- Almost 240,000m³ of brown rock
Intricate patterns reflect the unique Pacifica identity
of Manukau
Although not quite as obvious as the large concrete structures on the SH1-SH20 flyover, other changes to the landscape form part of the project. These include intricate, inlaid patterns on bridge barriers and retaining walls.
The designs have been developed to reflect the Pacific influence on Manukau. The bridge barriers were made off-site at the project’s precast yard in Takanini before being brought to the site and installed onto the bridges.
Last but not least, a challenging task to complete the motorway project
To complete the motorway project, the Leighton Works team demolished a stretch of the old SH20 route and realigned the Puhinui Stream to allow for the construction of the Roscommon-Puhinui connecting road, a task that has involved a series of challenging tasks.
Excavation work began by carving out a new path for the Puhinui stream alongside the existing stream. At each end, a steel sheet pile wall was installed, performing as a dam, to keep the existing stream out of the new channel until required. Following completion of the excavation, the sheet piling was removed allowing the new stream to fill with water while the old stream drained. As the old stream drained, the fish were rescued and relocated under the supervision of fresh water ecologists. During the project, over 1,000 specimens of aquatic life have been relocated. The new 150 metre stream has been specially constructed to imitate a natural stream with rocks and bends. The flow has been slowed and more shelter has been provided for the fish.
Once the stream was been diverted, construction began on the final connecting road linking the Cavendish and Puhinui Interchanges. The soft ground beneath the old stream was removed before placing over 20,000m³ of material, starting with a layer of rock to provide a solid foundation. Once the correct height was reached, the road pavement was constructed with the connecting road opening at the start of 2011.
