VII. Read the text above once again and choose the sentence summarizing it:

kinds of construction

materials

purposes

stages of construction

basic

clay

bridge

finishing

The undersea portion of the tunnel consists of volcanic rock, pyroclastic rock, and sedimentary rock.The area is folded into a nearly verticalanticline, 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 and the centre portion consists of sand-like mudstone. Igneous intrusions and faults caused crushing of the rock and complicated the tunnelling procedures.

In September 1971, the decision was made to commence work on the tunnel. Arduous construction in difficult geological conditions proceeded. 34 workers were killed during construction. On January 27,1983, Japanese Prime MinisterYasuhiro Nakasonepressed a switch that set off a blast that completed the pilot tunnel. The tunnel was opened onMarch 13,1988, at a cost of 538.4 billion yen (US$3.6 billion). Once the tunnel was completed, all railway transport between Honshū and Hokkaidō utilised the tunnel. However, for passenger transport, 90% of people use air due to the speed and cost. For example, to travel betweenTokyoandSapporoby train takes more than 10 hours and 30 minutes, with several transfers. By air, the journey is 3 hours and 30 minutes.

Beneath the Tsugaru Strait, the use of a tunnel boring machine (TBM)was abandoned after less than 2 km due 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.

A 2002 report by Michitsugu Ikuma described, for the undersea section, that "the tunnel structure appears to remain in a good condition". The amount of inflow has been decreasing with time, although it "increases right after a large earthquake".

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 km 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 m intervals. The pilot tunnel served as the service tunnel for the 5 km centre portion.

Surveying started in 1946. In 1971, 25 years later, construction began. In August 1982, less than 700 m remained to be excavated. First contact between the two sides was in 1983. TheTsugaru Straithas eastern and western necks, both approximately 20 km across. Initial surveys undertaken in 1946 indicated that the eastern neck was up to 200 m deep with volcanic geology. The western neck had a maximum depth of 140 m and geology consisting mostly of sedimentary rocks of theNeogene period. The western neck was selected, with its conditions considered favourable for tunnelling.