World’s Longest Under sea Tunnel – Seikan tunnel

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The Seikan Tunnel is a 53.85 km (33.46 mi) railway tunnel from Japan, with a 23.3 kilometer (14.5 mi) long portion under the seabed. Track level is about 140 meter (460 ft) below seabed and 240 meter (790 ft) below sea level. It is the longest undersea tunnel in the world, although the Channel Tunnel between the United Kingdom and France has a longer under-sea portion. It travels beneath the Tsugaru Strait – connecting the Aomori Prefecture on the the Japanese island of Honshu and the island of Hokkaido – as part of the Kaiko Line of the Hokkaido Railway Company. Although it is the longest traffic (railway or road) tunnel in the world, faster and cheaper air travel has left the Seikan Tunnel comparatively underused. Its claim to the record for the longest tunnel will be taken when the Gotthard Base Tunnel, a European railway tunnel, is completed in around 2018. It is also the deepest rail in the world.

Surveying started in 1946 and in 1971, twenty-five years later, construction began. By August 1982, less than 700 meters of the tunnel remained to be excavated. First contact between the two sides was in 1983.

The Tsugaru Strait has eastern and western necks, both approximately 20 kilometers across. Initial surveys taken in 1946 indicated that the western neck was up to 200 meters deep with volcanic geology. The western neck had a maximum of 140 meters and geology consisting mostly of sedimentary rocks of the Neogene period. The western neck was selected, with its conditions considered favorable for tunneling.

Geology of undersea portion consists of volcanic rock, pyroclastic rock, and sedimentary rock of the late Tertiary era. The area is folded neatly into a nearly vertical anticline, which means that the youngest rock is in the is in the center of the Strait, and encountered last. Divided roughly into thirds, the Honshu side consists of volcanic rocks (andesite, basalt, etc.); the Hokkaido side consists of Sedimentary rocks ( tertiary period tuff , mud stone, etc.) ; and the center portion consist of Kuromatsunai Strata ( Tertiary period sand-like mudstone). Igneous intrusions and faults caused crushing of rock and complicated the tunneling procedures.

Initial geological investigation occurred from 1946-1963 which involved drilling the seabed, sonic surveys, submarine boring, observations using a mini submarine, and seismic and magnetic surveys. To establish a greater understanding, a horizontal pilot boring was undertaken along the line of both the service and pilot tunnels.

Tunneling occurred both simultaneously on both the northern and southern ends. The dry land portions were tackled with traditional mountain tunneling techniques, with a single main tunnel. The service tunnel was periodically connected to the main tunnel for the central five-kilometer portion. Beneath the Tsugaru Strait, the use of tunnel boring machine (TBM) was abandoned after less than two kilometers owning to the variable nature of the rock and difficulty in accessing the face for advanced grouting. Blasting with dynamite and mechanical picking where then used to excavate.

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Originally posted 2010-07-15 12:00:49. Republished by Blog Post Promoter

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