Problems of engineering in alluviated drowned valleys of Devon and Cornwall are typified by the proposed 2.7km Barnstaple Western bypass, which crosses the tidal estuary of the River Taw on the north Devon coast.
Preliminary site investigation by geotechnical consultant Frederick Sherrell identified a deep buried channel in the rockhead beneath the Taw Estuary, infilled by alluvial and estuarine sediments.
The northern abutment and pier of the new bridge will be founded on the bedrock at shallow depth, explains Frederick Sherrell director John Harris. Bridge piers located in the main navigation channel are designed for ship impact and will be constructed on deep caisson or spread foundations taken down to bedrock. Finally the south pier and abutment will be constructed on piles driven through the alluvium and into the bedrock at depths of 12m to 13m below flood plain level.
South of the bridge the road will cross the alluviated flood plain on a 13m high shallow sided embankment, constructed in stages over 15 weeks with geosynthetic basal reinforcement.
Client Devon County Council's engineering design group is responsible for the overall design of the £22M project, which has a provisional construction start date of 2002.