Man-made islands grow in Tokyo Bay.

They do not yet appear on maps, but two new man-made islands are being formed in Tokyo Bay. The concrete and steel islands will support the Trans-Tokyo Bay Highway, a 15-kilometer-long four-lane toll road that will connect Kawasaki City in the west to Kisarazu City in the east when it is completed in 1996. The highway is expected to reduce traffic congestion within the metropolitan Tokyo area.

Only a third of the highway will run above the water, in the form of a bridge. The remainder will consist of two parallel 10-kilometer undersea tunnels. Kawasaki Man-made Island will be located at the midpoint of the two tunnels and Kisarazu Man-made Island will connect the tunnels to the bridge. While the highway is being built, the islands are serving as launching platforms for the drilling equipment that is boring the tunnels in the ocean floor. When the tunnels are complete, the islands will provide ventilation to the tunnel and facilities for maintenance and management personnel.

Kajima Corp. (Tokyo) is heading the joint venture, which includes Nippon Steel Corp. (Tokyo) and Toyo Contruction Co. (Tokyo), that is building the western half of the island that will bisect the tunnels. The entire doughnut-shaped structure of Kawasaki Man-made Island will measure 198 meters in diameter when it is finished and will consist of two concentric rings of interconnected jacket-type steel trestles with a slurry wall between them.

The construction site for the Kawasaki Man-made Island lies five kilometers offshore of that city at an average depth of 28 meters. The soft seabed had to be improved considerably to provide a solid foundation for the structure. Beginning in April 1989, sand compaction piles and deep mixing method cement slurry injection were used to stabilize an area 220 meters in diameter and 30 meters deep at the site.

While the foundation was prepared, Nippon Steel began casting the steel trestles that would serve as the Kawasaki Man-made Island's skeleton. Each of the trestles for the island's outer ring measured 35 meters wide and 35 meters high, about as tall as a nine-story building, and each weighed about 2300 metric tons. After they were cast, the units were loaded on barges and towed to the Tokyo Bay construction site 885 kilometers away.

A 4100-ton floating crane lifted the huge trestles and dropped them into place on the ocean floor. The trestle sections were secured by means of 116 steel tube piles driven 70 to 80 meters into the now-stable foundation below and connected to each other. Steel-reinforced concrete slabs served as decking so construction crews could make use of the trestle ring for later work.

Outside of the trestle ring a series of precast concrete panels have been placed to provide a wave barrier for the site. At present, 257 steel pipe sheet piles measuring 45 meters in length are being driven into the foundation between the dual rings of the island. They will form a waterproof containing wall for the earth that will be filled in the space provided.