Civil engineering & public works
Japan building its longest submerged tube tunnel
One of the six steel shell tunnel elements. It is 80 metres long, 13 metres wide and 7.95 metres high
THE technical and economic advan- tages of building underwater tunnels by the submerged tube method have come to be widely recognised since the method was first developed in the early part of this century.
Hong Kong's mile-long cross-har- bour road tunnel will be a submerged twin-tube and in several other parts of the world projects of this type are now underway. The longest will be the 5,800-metre railroad being laid 30 metres under the bed of the San Francisco Bay.
In Japan, a number of small-scale projects have been built in Tokyo and Osaka and work is now in progress on the country's longest submerged tube
tunnel
a rail tunnel of 570 metres below the River Tama,
Being constructed jointly by Ohbayashi-Gumi Ltd. and the Kajima Construction Co. Ltd., the tunnel will form part of the Keiyo Line of the Japan Railway Construction Corp. The 100-kilometre railroad will act as a vital cargo transportation route linking Shiohama in Kawasaki City with Kisarazu City, through the reclaimed land beside Tokyo Bay.
Of the 570 metres of the Tama River underwater tunnel, 480 metres will be covered by six tunnel tube elements, each having a length of 80 metres. The remaining 90 metres will be two pneumatic caissons. Deepest
point across the river will be 3.5 metres to the bed, the tide speed of which is about 0.5 metres a second (Fig. 1 and 2).
With the tunneling method being employed, a shaft is first built and the first tunnel element is placed. The last tunnel element is arranged so that it can be connected with a caisson, which has already been laid.
The river bed must be excavated to a depth of 15 to 17 metres below the water surface. The course is then laid with a 70 centimetre thickness of crushed stone, upon which the tube elements are to be placed.
The steel shells of the tunnel ele- ments are prefabricated at a shipyard,
Far East BUILDER, December 1969
35