Undersea Tunnel Has 2 Safety Stations

Tappi station’s waiting room is huge, but no one starts a trip there. It provides a way out if something goes wrong under the Tsugaru Straits in the world’s longest tunnel.

Five trains stop at Tappi each day and let hundreds of people off for a look, by guided tour, at conditions 450 feet beneath the surface. The passengers then board the next train to finish the journey between Hokkaido, Japan’s northernmost main island, and the central island of Honshu.

The purpose of Tappi and Yoshioka, a similar station near the other end of the Seikan Tunnel, is to provide shelter or an exit in case of earthquakes or other trouble.

At 50 m.p.h., a train takes 40 minutes to complete the trip through the tunnel, an $8.5-billion tube 34 miles long that runs under the Tsugaru Straits for nearly 15 miles.

“Down that way, you can walk to the other side,” a guide said, pointing down a side tunnel. “It would take about six hours.”

Before the Seikan opened in March, 1988, rail passengers transferred to a ferry for a 4-hour voyage across the straits between Aomori, on Honshu, and Hakodate, on Hokkaido.

Sometimes the service was halted by storms. A typhoon sank a ferry in 1954, killing more than 1,100 people, and support for a tunnel grew.

Guides from Japan Railways show visitors to Tappi and Yoshioka some of the ways of coping with emergencies.

Each station is under land. The tunnel does not emerge from the ground for 8 1/2 miles beyond Tappi, and 10 1/2 miles past Yoshioka, but stranded passengers can take a cable car or stairway to the surface. It is 1,316 steps to the top from Tappi, about a 20-minute climb.

Both stations are in the service tunnels, which are big enough for a truck to drive through and run along both sides of the 36-foot-wide main railway tube.

Air pressure in the service tunnels is higher than in the main tunnel so smoke from a fire cannot enter them. The main tunnel has sprinkler and drainage systems.

Tappi and Yoshioka have restrooms, first-aid stations and benches for hundreds of people.

Sensors and television cameras monitor conditions inside the tunnel. A television monitor in Tappi, using remote-controlled cameras, gives a sweeping view of conditions on the surface similar to that of a submarine periscope.

When there is no emergency, passengers can stop at Tappi to walk in the service tunnels, buy soft drinks from a machine or purchase a small souvenir tunnel “passport” whose price of 1,000 yen ($6.70) includes a card good for that value in travel on Japan Railways.

At its center, the tunnel is 785 feet below the water’s surface and 330 feet under the seabed. To keep water from seeping in under high pressure, the earth around the tunnel has been reinforced for a distance of up to 65 feet.

Excavation began in 1971, with the cost estimated at about one-third the final total.

The tunnel required 13.8 million man-days of work and took the lives of 34 workers. Four serious cases of flooding delayed construction.

Techniques developed in the project are expected to help in construction of the 31-mile tunnel under the English Channel that will link England and France.

Seikan completes the ground linkage of Japan’s four main islands. The 2.3-mile Kanmon Tunnel between Honshu and the southern island of Kyushu was completed in 1942, and in April, 1988 the 7.7-mile Seto Ohashi Bridge opened between Honshu and Shikoku, east of Kyushu.