COP Outcome development and evaluation

To measure the actual settlement profile along the routes a Profileometer Pipe was laid across numerous key sections of roadway. A pressure altitude transducer was regularly pulled through the pipe to develop a settlement profile for particular areas. The basic settlement profile for estuarine soils follows a logarithmic progression, with a rapid initial settlement progressing over time to a much-reduced rate of settlement, suitable for roading. Compressing the soil involved overlaying the swamp with around 180,000 m3 of hard fill. On much of the swamp area the ground was overloaded to accelerate the consolidation process. This results in a slight spring-back when the additional load is removed but not enough to cause problems.

To assess the soil structure in the volcanic deposits Cone Penetration Testing was used extensively. While it yielded no great surprises the information gathered was vital for finding ways of stabilising the soil within the cuttings and under the roadway and bridge embankments. Again the complex nature of the soil boundaries and the different volcanic deposits complicated the design of several of the bridge embankments; marked changes in the underground topology required careful attention.

The unstable and varied volcanic deposits saw the extensive use of soil nails on several of the deeper cuttings. At up to 20 metres deep, the bigger cuttings transect multiple layers of volcanic deposits, all with the potential to collapse. One deep cuttings on Route K was further complicated by proximity to a significant archaeological site, which mandated the use of a steeper cutting face than might have been ideal. Here more than 200 soil nails up to 12 metres long, all post-tensioned and grouted in place, were used to stabilise the cutting faces.

Pumice is used as a lightweight fill to construct bridge embankments. However, the swampy conditions meant that the ground couldn't sustain the weight of earth embankments higher than seven metres. As some of the bridges needed 11-metre embankments, an innovative solution was to use polystyrene as a lightweight fill. While pumice is heavier than the plastic alternative, pumice at 1.3 t/ m3 is considerably lighter than either rock (1.9 t / m3) or brown soil (1.6 t / m3). Under an alternative design however, polystyrene would be used rather than pumice.