COP Brief development

By far the most difficult engineering problem the project has faced is the soil conditions encountered on the three routes. Route K, which will be the last to be completed, crosses an area of estuarine swamp bordered by higher ground consisting primarily of complex multi-layered volcanic deposits. The swampy soil in places is up to 18 metres deep. Routes P and J are through predominantly complex volcanic deposits that were laid down over a long period, undergoing a prolonged weathering process. The challenge therefore was two-fold: to create a suitably stable roadway through the swamp, and to stabilise the volcanic structures to support stable roadway, while forming and securing the various cuttings required.

The complexities of the volcanic deposits and the swamp were further complicated by the boundaries between the various sub-strata interacting on a micro scale. On all three routes it was found that within a few tens of metres the underlying soil type and structure could run through several significant variations, with markedly different settlement and deformation characteristics. The challenge therefore was to develop a scheme to stabilise these different soils, to ensure that the roadway would maintain a stable profile. Previous roads constructed in these conditions have typically undergone uneven settlement, becoming “wavy".