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Artist’s impression of the completed Cannons Creek Bridge

Cannons Creek Bridge ready for ‘launching’

Transmission Gully’s Cannons Creek Bridge is one of the project’s most significant structures. In coming months this huge structure will reach a major milestone when the spans are ‘launched’ across the Cannons Creek Gorge to eventually sit on the bridge piers that are currently being completed.

WEB EXCLUSIVE

Standing 60 m above the stream below, the Cannons Creek Bridge is the largest structure in the entire Transmission Gully motorway project. The finished bridge will be four lanes wide, 226 m long and 24.5 m wide. A total of 44 steel girders, each 1.8 m wide and 3 m deep, make up the framework of the bridge deck which is supported by two abutments and two piers. 

According to NZ Transport Agency (NZTA) senior manager project delivery Chris Hunt, the bridge is a showcase of technical design and innovation. “The design and construction of the Cannons Creek Bridge is significant. The technical challenges associated with designing and building a long, high, curving bridge that is aesthetically pleasing, yet able to withstand a 1 in 2500-year seismic event in an area of high seismicity, require an innovative approach.”

Further challenges include:

  • • Minimising the environmental effects while working within a Department of Conservation (DoC) reserve and within the wider Belmont Regional Park environment
  • • Working directly under, and in close proximity to, multiple power lines, including the Benmore–Haywards inter-island DC link
  • • Relocation of the Kapuni high-pressure gas line, which ran under one of the bridge piers
  • • Excavation of the bridge pier foundations in a steep narrow gorge, adjacent to the Cannons Creek Stream
  • • Getting heavy cranes, machinery and materials to the site, given its remote location.

Launching the bridge

The challenging geographical features of the site, in particular the steep sides of the gorge and the need to span a large distance, mean that launching is the most effective process for installing the bridge deck structure. Launching is where a pre-assembled bridge frame, in the form of steel girders, is ‘launched’ across an open space to land on the bridge piers. This bridge launch is being undertaken in four phases and will eventually see the bridge reach the abutment on the other side of the gorge.

Phase 1 is the much-anticipated initial launch, after months of behind-the-scenes work to prepare the bridge for launching. Girders have been assembled in the launch yard, ready for the custom-made launch nose, pulling frame and precast panels to be attached. Prior to the brake being removed, the launch nose will be lifted, and the bridge will move from trestles onto rockers, allowing the back end of the girders to be lowered to lift the nose, and the entire structure will slide effortlessly across onto the first pier – only to be repeated three more times.

The entire four-phase launching process is expected to take around 10 months. Each phase will see tail assemblies removed and more girders added to the structure before the launching process continues, allowing the structure to safely reach the far side.

Watch an excellent animation of the launch sequence here. Want to see it in real life? A good public viewing spot is from Belmont Regional Park – follow a short (steep) walk from the regional park carpark located near the top of Takapu Road.


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