Its builders and designers have struggled with earth riven by a faultline, weakened by a large underground cavity, and short on good, solid rock. David Loughrey has a look at how engineering solutions overcame a challenging site.
Above the roar of engines straining towards the summit of Lookout Point in Dunedin, and the rumble of low-geared trucks thundering downhill towards Caversham, the new Riselaw Rd bridge crossing the Southern Motorway is beginning to rise like a spartan concrete cathedral.
Twelve concrete beams now cross part of the span of about 35m from one side of the recently built four-lane highway to the other.
The work is part of an about $44 million, two-stage project to develop a wider, smoother approach through Dunedin's southern gateway.
That will provide a four-lane road between Andersons Bay Rd and Lookout Point, and the bridge over the highway just below the top of the hill.
The bridge has been one of the more difficult aspects of the New Zealand Transport Agency project, resulting in an evolving design as the challenges of the site became clear.
The bridge started with a simple concept.
''When we started it was going to be a simple earth embankment bridge like the Fairfield bridge, with the bridge spanning the embankments,'' NZTA project manager Simon Underwood said.
The 35m span was the limit of what could be built without supports in the middle, but a central pier was added to the design, allowing a more slender deck.
The original design Mr Underwood described as a ''specimen design''.
''There were things we didn't know about underground.
''It wasn't a surprise the design had to change.''
And change it did.
The simple structure became more complex as the earth over which it was built was better understood.
''We started to get in trouble because we could no longer sit the beams on the embankment,'' Mr Underwood said.
It was decided the embankments on each side of the bridge, and the bridge span itself, had to be separate entities.
That was both because the massive weight in the embankments needed to be allowed to settle, and to allow for movement from any seismic activity.
More issues became apparent the more NZTA found out about the site.
What was found underground, through digging and geotechnical work, meant more changes to the design of the bridge, as well as a $3.5 million cost increase.
When excavating to reach the sandstone bedrock, a substantial faultline was discovered, which Downer project manager Tony Clough described as a ''V-shaped'' line of soft earth.
The faultline was considered inactive, but another issue - a cavity under the Mornington side of the bridge the size of a double garage, which required 21 truck-loads of cement to fill - again changed the situation.
The Mornington side was originally designed to have two columns, but that was changed to three. In a seismic event, the Mornington side could slump, geotechnical investigations showed, which resulted in an innovative design feature for the piles in the earth underneath the columns.
Before the piles were inserted, a steel casing to surround them was driven into the earth, then the earth dug out from inside.
The pile was then dropped in, but offset on one side of the casing, allowing the earth and the casing to move without touching the pile.
''The whole embankment can move about 330mm without touching the pile,'' Mr Underwood said.
As well as that, it was decided some of the bridge piles had to reach further into the earth.
''Proof bores'' were drilled to find rock strong enough.
''We thought we were out of the woods, drilling on the Riselaw Rd side, but the proof bore showed weaker material below,'' Mr Underwood said.
Contractors kept drilling until they found the rock that was strong enough, with the result the piles ended up between 16m and 22m deep before they hit rock.
''The more we dig, the more we know, and the more questions we have,'' Mr Underwood said.
He likened the process to medical tests, or getting an X-ray.
''Below ground you're dealing with natural materials - there's lots of uncertainty.
''It was an incredibly challenging site.''
The bridge is expected to open in October or November.
