By far the largest engineering feature of the new road will be the $11m trench – 150m long, 11 to 13m wide and 7m deep – which skirts the hill by The Terrace tunnel. The trench follows the orientation of local fault lines, and has been designed with a 670-year return period (0.95g peak ground acceleration1) earthquake in mind. And to add to the fun, it sits atop a local aquifer, so preventing the trench from collapsing requires special measures.

For a start, the walls and floor of the trench will be made of reinforced concrete up to 1,200mm thick. Steel struts, 760mm in diameter, at 5m intervals will hold the top of the walls open, forming a grid above motorists as they head north out of the city. Because of the high water table, extensive use will be made of soil nails, in 200m of the largest soil-nailed wall in New Zealand. On their own, the soil nails will cost around $1.3m. Made of reinforcing steel, the nails are from 4 to 14 metres in length. The holes for the nails are drilled into the wall at a slight downward angle from the horizontal. The holes are then filled with grout and then re-drilled to take the nails, then grouted again. A total of 7km of soil nails will be used.

Groundwater pressures and potential subsidence are the biggest engineering risks of the project. An extensive network of sensors has been deployed to ensure that nearby buildings are not affected by the construction; the amount of rainfall will have a significant effect on the progress or otherwise of the project. So far, according to Ms Adams, there have been fewer problems than were planned for.

A $7m underground stormwater pipe, 2.1m in diameter, will follow the bypass route between Aro St and Taranaki St. Cutting through Palmer St and into Aro St, it will eventually replace a 130-year-old brick stormwater drain, but in the short term will have no connection to the sea. Wellington City Council needs the completed Inner City Bypass to reduce traffic sufficiently down Taranaki St before it can consider constructing a pipe down Taranaki St to the sea.

Altogether the Inner City Bypass project is less about transport than about urban renewal. The animated video simulations developed by Truescape for Transit show a leafy boulevard far removed from the concrete monstrosity first envisaged by De Leuw Cather. Given that Wellington's urban growth south and east of the Basin Reserve has largely stalled, it is difficult not to conclude that the design that has emerged after all these years is the right one, balancing the needs of Wellington's pedestrians and motorists in a sensitive and intelligent way. When it is completed in mid-2007, most Wellingtonians will wonder what all the fuss was about.