A 2-km length of the North Wales Coast Road (A55) between Colwyn Bay and Conwy runs along the floor of the Afon Ganol Valley which once formed part of the River Conwy estuary (Fig. 1). The road embankment, which in this area varies in height from 2 to 7 m, is constructed over soft alluvial clays and peat, up to 15 m (Fig. 2). The varying ground conditions along the length of the road embankment resulted in two completely different forms of embankment design.

Geology and embankment design

At the western end where the embankment was highest and widest in order to accommodate two slip roads to a low level rotary interchange, the alluvial clay was thickest, but no peat layers were found. Continuous sampling of the alluvium proved it to be a soft grey silty clay in which three main layers were identified, each 4-5 m thick. The upper layer contained near vertical root stems, the middle layer contained fine sand partings and the lower layer was very similar to the upper layer, except it contained small pockets and thin layers of peat. Some partly decomposed wood fragments were also found in the lower layer. A typical plot of the Atterberg limits and bulk density against depth clearly shows the presence of the three layers (Fig. 3).

In this area it was decided to promote primary consolidation using band drains to keep construction time to a minimum and to reduce post-construction settlements. Laboratory consolidation and in situ permeability

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