Japan Bridges Its Inland Sea

By Awake! correspondent in Japan

CROSSING Japan’s Seto Inland Sea by ferry used to take an hour. But on April 10 of last year, the Seto Ohashi Bridge was opened, linking two of Japan’s main islands, Honshu and Shikoku. Driving across the Inland Sea now takes less than ten minutes.

This convenience, however, does not come cheap. The toll is 5,500 yen (about $45) each way. But that is little when compared to the total cost: 1,130 billion yen (8.7 billion dollars) and 17 lives. Also, its construction required about ten years, or nine million man-days, of work. There had to be good reasons for such a costly feat.

For one thing, transportation between the two islands is no longer at the mercy of unpredictable weather. In 1955 a ferry sank, and 168 lives were lost. Furthermore, the bridge is deemed an economic boon to agricultural Shikoku Island now that it is linked to Honshu, the principal island of Japan. At 380 yen (about $3) per person, the train is decidedly an economical way to cross.

Although referred to as one bridge, it is actually a 5.8-mile-long [9.4 km] series of bridges and skyways over five islands across the Seto Inland Sea. It is made up of three suspension bridges, two cable-stayed bridges, a truss bridge, and the viaducts linking them. One of the suspension bridges, the Minami Bisan-Seto Ohashi, is the world’s longest double-decked suspension bridge that carries both railway and highway traffic.

Mr. Tetsuo Yamane of the Honshu-Shikoku Bridge Authority in Tokyo provided some interesting insights into the construction. He worked on the bridge project for 13 years and was a construction overseer for the substructure of the bridge.

“Most difficult of all,” explained Mr. Yamane, “was laying the undersea foundations. By undersea blasting we shattered the rock bed and excavated the bed with a grab dredger. Then, caissons, or frame forms, as big as ten-story buildings were built at a shipyard, towed to the construction site, and sunk into the water. We stuffed stones into the caissons and poured mortar into them, using a newly developed mortar-plant barge called Century.”

Workers had to labor under the most adverse conditions. “The foundations were laid deep under the water, about 50 meters [160 ft] down,” continued Mr. Yamane. “In addition to that, the tide around the construction site was very strong, at a speed of five knots. It is equivalent to working in a 250 kilometers-per-hour [160 mi/​hr] wind. The excavations and the sunken caissons had to be inspected while the tide was still. But visibility in the water was virtually nil. Just 10 or 20 meters [30-70 ft] underwater, you can see practically nothing. With lights taken underwater to spotlight places requiring inspection, we took pictures and videos from a distance of 50 centimeters [20 in.].”

Since the bridge is in the Seto-Inland Sea National Park, the overall harmony of the bridge with the surrounding scenery had to be considered. The motif of “the whole landscaping is a Japanese garden with an arrangement of stepping-stones,” said Professor Toshiaki Ohta, who worked on designing the bridge.

Harmony of another sort was also achieved. Last March the undersea Seikan Tunnel was opened, connecting Hokkaido, Japan’s northernmost island, to Honshu. Now, with the opening of the Seto Ohashi Bridge, the last link in joining together Japan’s four main islands, Hokkaido, Honshu, Shikoku, and Kyushu, has been forged. Thus is fulfilled a long cherished dream of the Japanese people.