Waterview Connection Southern Tunnel portal design and construction

The Waterview Connection project comprises 5 km of new motorway in urban Auckland, New Zealand (NZ). The new motorway connects the existing south-western and north-western motorways to complete the Western Ring Route and provide a direct connection between the central city and Auckland International Airport. The project includes twin 2.4km long, three lane tunnels up to 45m deep and retained portal approaches up to 29m deep. This paper focusses on the Southern Approach Trench (SAT) geotechnical design and performance during construction. The SAT was constructed in challenging geological and hydrogeological conditions, with these conditions dictating the design solution. A significant amount of temporary works requirements were built into the permanent structures, such as cement stabilised blocks behind the headwall to facilitate a safe TBM launch and retrieval and allowance for TBM loading on the headwall. Detailed geotechnical investigations, in-situ testing and construction observations and analysis of monitoring data during TBM launch and breakthrough at the Northern Portal facilitated improvements and optimisation of the permanent and temporary works design and will also allow design optimisation of future designs of this nature.