Bridges

The Northern Corridor Improvements project is an accelerated highway development scheme that is being delivered under an alliance contract for Waka Kotahi NZ Transport Agency (Waka Kotahi) to improve travel time and reliability of inter-regional travel between Auckland and Northland. Located on Aucklandís North Shore, the project will relieve traffic congestion by adding more traffic lanes along the State Highway 1 Northern Motorway (SH1) as well as constructing an extension to an adjacent designated busway corridor. Additionally, the project will provide a direct motorway-to-motorway connection between State Highway 18 Upper Harbour Highway (SH18) and SH1. The project scope includes the construction of twelve bridge structures, 9km of retaining walls and over 7km of new walking and cycling paths.

A key structure on the project is the SH1-SH18 Underpass - a 115m-long underpass structure that provides connectivity between the SH1 southbound and SH18 westbound carriageways. This structure, comprising twin contiguous bored pile walls and a spanning reinforced concrete slab, has had to be built in a 'top down' fashion directly below the live SH1 motorway in five transverse stages, necessitating several traffic management switches and consideration of interface detailing and the practical means of connecting between the stages. Excavation below the structure provides the space for the link route, and safety and resilience in a potential vehicle fire situation on the link has been a significant consideration for the structure.

This paper will describe the design development and features of the structure with particular focus on overcoming the technical issues associated both with staged construction and with in-service functionality. It will demonstrate how a collaborative alliance culture has assisted in the development of constructible design details and in identifying modified approaches to suit changing construction methods and challenges.