This cache more or less adjacent to the the bridge variously known as Cannons Creek Bridge, Cannons Creek Viaduct, Bridge 20 (it's construction reference name) and now formally Te Ara a Toa. It is the most significant structure (and, one assumes, the 20th bridge) along the new Transmission Gully motorway due to open in 2020 (but actually opened in 2022). At the time of placing the cache, the span was generally complete and almost ready for traffic.
This is not necessarily the best place to view the bridge (head up Takapu Ridge if you want to look down on it) but it's relatively close, for whatever that's worth.
State of the bridge in late 2019
The Gap
The motorway route follows a reasonably "natural" route for the most part and indeed this section follows the same general route as the existing Duck Creek Track behind the suburb of Whitby. But Cannons Creek has cut a bit of a gorge which means the only way for a motorway to get across it is by bridge. Takapu Road achieves the same result by descending into the valley and climbing steeply up the other side.
Takapu Road climbing out of the valley prior to construction starting.
The creek cuts across the image where the road bottoms out.
The future bridge will be off the right hand side.
The bridge
Although not officially known as a viaduct it's construction meets the definition of one - i.e. a bridge of multiple short spans mounted on piers. The spans are classic box truss which, while not having the elegance of an arch support, is perfectly adequate for the span lengths and gives the bridge a surprisingly low profile which helps it fit into the landscape.
If you've ever watched a documentary about the Tacoma Narrows bridge and its famously fatal oscillations you might know that collapse was caused by a phenomenon called aeroelastic flutter induced partly by bridge's slender deck profile. It's replacement has a thicker profile which allows the wind to pass through the truss instead of being forced over and under. The Cannons Creek spans are too short to have the same problem but it's something the engineers will have considered by having an enclosed truss.
Notable was the method of placing the bridge deck which used a method called "incremental launch" to allow assembling the bridge sections on the southern bank and pushing the bridge out over the span from one end. Clearly some precise engineering is required to balance the cantilever and land the "nose" on each pier before pushing out to the next one.
Perhaps surprisingly, the bridge components were built in Napier and transported to the Wellington site over 44 trips for assembly.
The 3 spans are 86m, 74m and 68m for total span of around 230m. The bridge is also slightly curved with a radius of 1100m. The deck is 60m above the valley floor.
Box truss skeleton still visible earlier in construction.
At first glance the bridge looks highly susceptible to collapse in an earthquake because of the slender piers and rectangular profile. But, like many modern bridges and buildings, the bridge deck is not rigidly attached to the foundation. The deck sits on large base isolators (just like Te Papa) which allows it to move with some independence from the piers and removing any stress where they meet.
Close-up of base isolators.
A Note on the Name
In November 2020 the bridge received it's official name from Ngati Toa: Te Ara a Toa - meaning "Pathway of Toa". Chosen because Toa symbolises "strength and might" - so say its namers.
The cache
The cache is a rather uneventful affair and getting you here is more about the location than the cache. The cache is located just beyond the corner where the track turns and heads up the hill. It only has a log and pencil.
Getting here
Parking and trailhead coords are provided but additional information regarding the terrain and creek and can be found in the description for Below the Viaduct which is slightly down the hill from here. It's a slightly tricky track in parts and you will need to cross the creek. I've never seen it more than a few centimetres deep but probably best not to try soon after rain.