Design and Construction of Tunnels for Northern Gateway Project, New Zealand

This paper describes the design and construction of twin 400 m long road tunnels as part of the Northern Motorway Extension Project in Auckland, New Zealand. Construction of the tunnel civil works is now completed and installation of mechanical and electrical systems is well underway. The tunnels are expected to be open for traffic at the end of 2008. The paper will address the design aspects of the project including: tunnel alignment and cross-section, geotechnical conditions and rock support, waterproofing and concrete lining and fire and life safety. The paper will cover the bored tunnel section, the cut-and-cover tunnel section, cross passages and portal structures. The design brief included a requirement for the tunnels to be a gateway to Auckland and this required particular attention to be given to the visual appearance of the finished tunnel structure. The final arrangement of the portal structures and a requirement for recesses in the concrete lining for emergency cabinets, deluge pipework and cable runs, reflects these requirements. The tunnels were excavated by a roadheader and road planer using a heading and bench. Initial support was provided by a combination of lattice girders, cement grouted rock bolts and steel fibre reinforced shotcrete. The permanent lining consisted of installation of a drainage geotextile and waterproof membrane, followed by cast in situ unreinforced concrete. Each portal consists of a cut-and-cover section. This paper describes the tunnel construction methods, including the particular requirements for safety, the environment and community relations. The tunnel is fitted with a compartmentalised deluge system for fire fighting purposes. The tunnel control centre is incorporated into the control centre for Auckland Harbour Bridge, some 30 km from the tunnel location.