Surveying, construction and geology
Surveying started in 1946 and in 1971, twenty-five years later, construction began. By August 1982, less than 700 metres 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 kilometres across. Initial surveys undertaken in 1946 indicated that the eastern neck was up to 200 metres deep with volcanic geology. The western neck had a maximum depth of 140 metres and geology consisting mostly of sedimentary rocks of the Neogene period. The western neck was selected, with its conditions considered favourable for tunnelling.
Geology of the undersea portion of the tunnel consists of volcanic rock, pyroclastic rock, and sedimentary rock of the late Tertiary era. The area is folded into a nearly vertical anticline, which means that the youngest rock is in the centre of the Strait, and encountered last. Divided roughly into thirds, the Honshū side consists of volcanic rocks (andesite, basalt etc); the Hokkaidō side consists of sedimentary rocks (Tertiary period tuff, mudstone, etc); and the centre portion consists of Kuromatsunai strata (Tertiary period sand-like mudstone). Igneous intrusions and faults caused crushing of the rock and complicated the tunnelling procedures.
Initial geological investigation occurred from 1946–1963 which involved drilling the sea-bed, 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.
Tunnelling occurred simultaneously from both the northern and southern ends. The dry land portions were tackled with traditional mountain tunnelling techniques, with a single main tunnel. However, for the 23.3-kilometre undersea portion, three bores were excavated with increasing diameters respectively: an initial pilot tunnel, a service tunnel, and finally the main tunnel. The service tunnel was periodically connected to the main tunnel with a series of connecting shafts, at 600- to 1,000-metre intervals. The pilot tunnel serves as the service tunnel for the central five-kilometre portion.
Beneath the Tsugaru Strait, the use of a tunnel boring machine (TBM) was abandoned after less than two kilometres owing to the variable nature of the rock and difficulty in accessing the face for advanced grouting. Blasting with dynamite and mechanical picking were then used to excavate.
Structure
Originally only narrow gauge track was laid through the tunnel, but in 2005 the Hokkaidō Shinkansen project started construction has included laying dual-gauge track and linking the tunnel into the Shinkansen network. Shinkansen trains will be able to traverse the tunnel to Hakodate from 2015 and eventually Sapporo. The tunnel has 52 kilometres of continuous welded rail. Two stations are located within the tunnel itself: Tappi-Kaitei Station and Yoshioka-Kaitei Station. The stations serve as emergency escape points. In the event of a fire or other disaster, both stations provide the equivalent safety of a much shorter tunnel. The effectiveness of the escape shafts located at the emergency stations is enhanced by having exhaust fans to extract smoke, television cameras to help route passengers to safety, thermal (infrared) fire alarm systems, and water spray nozzles.
Previously, both the stations contained museums detailing the history and function of the tunnel and which could be visited on special sightseeing tours. Only Tappi-Kaitei remains as a museum, the museum function at Yoshioka-Kaitei was closed on 16 March 2006 to provide a storage room for work on the upgrades related to the Hokkaidō Shinkansen service.
The two stations were the first railway stations in the world built under the sea.
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