单选题
The tower of Pisa has been leaning so long—nearly 840 years—that it"s natural to assume it will
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gravity forever. But the famous structure has been in danger of collapsing almost since its first brick was
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.
It began leaning shortly after construction began in 1173. Builders had only reached the third of the tower"s
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eight stories when its foundation began to settle unevenly on soft soil composed
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mud, sand and clay. As a result, the structure leaned
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to the north. Laborers tried to
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it by making the columns and arches of the third story on the sinking northern side slightly taller. Then political unrest halted construction.
The tower sat
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for nearly 100 years, but it wasn"t done moving. By the time work restarted in 1272, the tower tilted to the south—the
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it still leans today. Engineers tried to make another
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, only to have their work interrupted once again in 1278 with just seven stories completed.
Unfortunately, the building continued to settle, sometimes at an
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rate. Finally, between 1360 and 1370, workers finished the project, once again trying to correct the lean
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angling the eighth story, with its bell room, northward.
In 1989, a similarly constructed bell tower in Pavia, Italy, collapsed suddenly. Officials became
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worried the tower of Pisa would suffer a similar fate that they closed the monument to the
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. A year later, they rallied together an international team to see
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the tower could be brought back from the brink.
By 2001, the team had decreased the tower"s lean by 44 centimeters, enough to make officials
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that they could reopen the monument. The actions taken by Burland and his team could,
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, stabilize the structure forever. The real threat now comes from the masonry itself, especially the material in the
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stories, where most of the forces caused by the centuries-long leaning have been directed. If any of this masonry crush, the tower could collapse. And even a
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earthquake in the region could have devastating consequences.
After 200 years, another intervention may be required, but the
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available to make improvements could be far more advanced and
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the tower for another 800 years.