Mott MacDonald: the Channel Tunnel
The world's longest underwater tunnel set new benchmarks in every facet of civil engineering, especially underground excavation (including the largest undersea cavern), lining design (meeting higher fabrication, quality and dimensional standards than ever attempted on this scale) and design endeavour (Mott MacDonald alone contributed 650 man years)
Costain: the Channel Tunnel
Costain would like to nominate the Channel Tunnel as the project which had the most impact on civil engineering this century. Costain was involved from design concept and takes great pride in the achievements and that the project will go down in history as one of the most significant civil engineering feats of all time.
Carillion: the Channel Tunnel
The Channel Tunnel, once dreamed of by Napoleon, finally became a reality when the first commercial trains began running in 1994. The project, a triumph of combined Anglo-French engineering and construction expertise, is actually three tunnels. It has an overall length of over 49km with 38km under the sea.