On December 18, 1997, Tokyo Bay Aqua-line opened. It is the motorway where Ukishima in Kawasaki City (Kanagawa Prefecture) connects with Nakashima in Kisarazu City (Chiba prefecture). The total length of this line is 15.1km, with a tunnel length of 9.576m ascending and 9.583m descending, making it the best undersea tunnel in the world constructed by the shield tunneling.
Tokyo Bay can be crossed by bridge and tunnel, and takes 15 minutes to go from Kisarazu to Kawasaki. Prior to construction, commuting between these two cities took one hour or more. The sea route of the Aqua-line measures 5.1km, while the undersea route length is about 10km. “Umi Hotaru”, an artificial island, is 4.4km off the coast of Kisarazu and separates these two sections.
The upper and lower part of the bridge were manufactured on land and transported by sea, to be set up with a crane and bogie ship. The tunnel part was dug with a sealed shield machine in order to overcome the difficult conditions set by high water pressure and a soft foundation. The machine is cylindrical, 14.14m in diameter, 13.5m in body length, and weighs 3.2t, making it fall into the world’s largest class. Using eight shield machines, the tunnel was constructed and finished at the depth of 60m below sea level. There is an upper tunnel and lower one, with a width of two traffic lanes on each side.
In considering environmental preservation, a film was placed to prevent contamination in the sea and soundproofing measures were established. This received praise worldwide.