Design, monitoring and analysis of a trial embankment over soft alluvial deposits for the Tauranga Northern Link: a Case Study

The Tauranga Northern Link (TNL) is a designated 6.9km long, 4-lane road alignment for State Highway 2. The ground model is complex and across the valley floors up to 20m of geologically recent, soft alluvial sediments were encountered. Analysis of laboratory oedometer testing results was undertaken to determine suitable preliminary consolidation parameters for design. Primary and secondary settlement calculations were completed in accordance with Terzaghi 1-dimensional consolidation theory. Preliminary calculations indicated that without mitigation measures, post-construction consolidation and secondary creep settlements would exceed the NZTA Bridge Manual serviceability limit state performance criteria. To refine the design, a trial embankment was constructed adjacent to the Wairoa River where significant soft alluvial deposits were encountered, to enable back-calculation of actual consolidation parameters for the soil units using the Asaoka method and Terzaghi’s theory of consolidation. Construction and ongoing monitoring results are discussed as a case study within this paper, along with comparison of the predicted versus actual design parameters and settlement results.