Te Ara a Toa (Bridge 20)

Te Ara a Toa (Bridge 20) over Cannons Creek is one of the Transmission Gully motorway’s most significant structures.

Artist’s impression

Artist’s impression of the completed Te Ara a Toa (Bridge 20) over Cannons Creek.

According to Waka Kotahi NZ Transport Agency Senior Manager Project Delivery Chris Hunt, Te Ara a Toa is a showcase of technical design and innovation.

“The design and construction of Te Ara a Toa 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”.

Chris adds further challenges include:

Minimising the environmental effects while working within a 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 and machinery and materials to the site given its remote location.

The facts and figures

Standing 60m above the stream below, Te Ara a Toa is the largest structure in the entire Transmission Gully motorway project. The finished bridge will be four lanes wide, 226 metres long and 24.5 metres wide. A total of 44 steel girders, each 1.8m wide and 3m deep make up the framework of the bridge deck which is supported by two abutments and two piers. Over 10,000 cubic metres of concrete will be used to build the bridge and all of its components – equivalent to more than four 2m deep Olympic-sized swimming pools.

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.

Steel girders at the launch yard ready for the launch nose to be attached.

Aerial view from above showing where the bridge will be launched from (off page on right) across to pier 2 on the far left.

The entire four-phase launching process is expected to take around ten 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.