State Highway 16 Causeway Upgrade – Motorway Widening Design Over Soft Ground and Post Construction Monitoring Verification

Ching Dai, Velautham Jeyakanthan and Shanmuganathan Raathiv


The SH16 Causeway Upgrade Project in Auckland, New Zealand forms part of the overall Western Ring Route Project, an alternative to Auckland’s State Highway 1, linking Manukau, Auckland, Waitakere and the North Shore, improving network resilience, and travel time reliability. The Causeway Upgrade was procured as an alliance due to the complex technical and environmental nature of the project. The project route is constructed over soft marine sediments. The Causeway Alliance team compared ground improvement alternatives during the tender and design phases and selected, designed and constructed an innovative wick drain with preload option. Monitoring of a New Zealand Transport Agency (NZTA) trial embankment was also carried out from the end of its construction and the ground model and material parameters were further explored. A wick drain and surcharge program was designed to achieve settlement requirements, and consolidation settlement back analysis was carried out to compare with the ongoing performance to allow refinement of the surcharge and wick drain design. Comprehensive numerical analyses were also carried out to assess road embankment stability and consolidation deformations. Post construction performance assessment and monitoring have also been carried out. The design and construction have been shown to be a cost effective and technically achievable solution, which is best situated to the existing ground conditions. This paper summaries the key components in the analysis development and outlines innovative soft ground improvement design and construction carried out at the Causeway.