Sinking City

For eight years the West German city of Duisburg has been sinking steadily--and Duisburg's anxious citizens have cheered every lost inch. In an engineering project of constantly increasing complexity, the city is being lowered on purpose, for Duisburg must sink to live.

High & Dry. Duisburg's troubles began with the river Rhine. The city's commerce flows through its Rhine harbor, which is ringed with steel mills and swarms with barge traffic. Years ago, the river started falling. Dredging and straightening of the channel downstream had made the water flow faster, and the quickened flow lowered the river's level. It also eroded the river bed, which lowered the water level still more. Duisburg's vital harbor got shallower and shallower. Dredging the harbor to keep pace with the fall of the river would have narrowed its sloping sides and left its crowded wharves and docks high and dry.

Most cities in a similar fix would have settled for moving their costly harbor works, but Duisburg found an ingenious way out. Under the city--harbor and all--lie three rich seams of coal. Engineers figured that if this coal was extracted properly, the ground above would settle evenly, and the whole harbor region could be lowered by as much as 7.5 ft., permitting the lowered Rhine to fill the harbor once more. There was $150 million worth of coal below the city, and it could be sold to pay for most of the surface damage caused by the settling.

Shaky Bridge. The daring operation has been under way since 1955, and it is working well. One and a half square miles of Duisburg, including streets, wharves, shipyards, steel mills, railroad yards and parks, is sinking on schedule But as the city prospered, traffic congestion on Duisburg streets got worse and worse, and the obvious solution, a mile-long highway bridge to carry traffic over the tangle of factories, railroads and waterways, seemed impossible because of the sinking ground.

Once more engineers came to the rescue. The recently opened Berlin Bridge has both its ends on reasonably solid ground. The six sections in between rest on piers that sink continually. On top of each pier are hydraulic jacks capable of raising the bridge as much as 30 inches. Massive roller bearings backed up by other jacks stand ready to compensate for horizontal motion. Fulltime crews patrol the bridge day and night, and when their measurements tell them the structure needs adjustment, they pump oil into the appropriate hydraulic cylinders to raise a section or shove it sideways. So far they have kept the roadway in fairly good shape. The streams of motorists who use the bridge have noticed only slight, occasional waviness. The waviness will probably continue until 1970, when the coal miners burrowing under Duisburg will have finished lowering their city.

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