This paper presents a case history of the ground investigation, design, construction and performance of a grade-separated interchange on the A38 trunk road at Plymouth, which was built over a low lying, marshy site, underlain by a complex distribution of soft organic silt and clay and landfill.

The geology of the site and the integration of the geotechnical study into the design process in an interactive manner are described. Earth and rockfill embankments of up to 10 m in height were built in stages, using vertical band drains to accelerate the rate of consolidation of the soft foundation soils. Soft ground beneath one of the bridge sites was pre-loaded by a temporary surcharge of fill, before installing the pile foundations. The ground treatment and the earthworks are described, the use of extensive instrumentation to control construction is discussed and the results of monitoring are presented.

The location, design and construction of the structural elements of the interchange, including two single span bridges, three culverts and a pedestrian underpass, were severely constrained by the geological conditions at the site and are described in the paper. The contribution which careful geotechnical study integrated with ground treatment and instrumentation can make to design and construction of such schemes is discussed.

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