Seikan Undersea Tunnel

The 5.5m diameter Seikan Tunnel in Japan will extend 54 kilometers, crossing under the Tsugaru Straits. When completed the −2 billion tunnel will be the longest in the world, and will carry both standard-gauge rail traffic as well as the 260 km/hr Shinkansen passenger express trains. To construct this tunnel, however, it has been necessary to go through extremely difficult geologic conditions, including about a dozen major faults, volcanic strata, and areas with water bearing formations transmitting up to the full 250-meter hydrostatic head from the sea above. The main construction procedures are outlined, including excavation of a pilot bore and service bore as well as the main tunnel. Long horizontal exploratory drilling has been employed, and excavation itself is preceded by extensive chemical grouting.