Dec. 15, 2001: Leaning Tower of Pisa Gets New Angle

Mercredi, 15 Décembre 2010 13:00

Dec. 15, 2001: Leaning Tower of Pisa Gets New Angle

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2001: Italy’s Leaning Tower of Pisa reopens its doors to tourists after a $27 million effort to keep it from tilting so much it might fall over.

Construction began on the tower in 1173. It’s the freestanding

bell tower, or campanile, of the cathedral next door. The structures, along with a separate baptistery, make a sublime three-part architectural composition that would have become world-famous even if the tower had not developed its well-known tilt.

But tilt it did, as the clay soil subsided beneath one side of the foundation. Construction was halted in 1185, but resumed a half-century later.

Further delays caused by war and civic upheavals probably kept the tower from collapsing before it was finished. The interruptions allowed the underlying soil — with the “consistency of jelly or foam rubber” — to compact a little. As the work continued sporadically, masons corrected a little for the tilt, resulting in a tower that not only leans, but is actually curved.

The tower was completed around 1370, and there it stood, without further problems for nearly half a millennium. (In the 17th century, Galileo may or may not have dropped a musket ball and a cannonball from the tower to refute Aristotle’s theory that heavier objects fall faster than lighter ones. That account is found nowhere in Galileo’s extensive writings, appearing first in an effusive, posthumous biography.)

Excavation work nearby in the late 1830s destabilized the tower’s base, and it began to lean a little more every year. After a tower collapsed in Pavia in 1989, the Italian government decided to close the Leaning Tower to tourists and do something to prevent its collapse.

Someone proposed a scheme to drill 10,000 holes in the tower to reduce its weight. (That could have been disastrous, because centuries of standing out of kilter had already stressed some stone structural elements to the point of crumbling.) Another proposal was to build an exact replica of the tower leaning against it from the opposite direction to prop it up. (Yeah, right.)

Also considered was a plan to take the tower apart stone by stone and then rebuild it in a way that would keep it from toppling over. Photographers documented every detail of the structure, and 6,400 of those shots are now available in a remarkable photo tour now archived at the Internet Archive’s Wayback Machine.

In the end, authorities decided to try straightening the tower a bit by getting the high side of the foundation to sink a little. Construction workers placed 100 tons of lead weights on the lip of the north-side foundation, and 340-foot steel cables were installed to hold the tower from any further leaning. Then, 41 corkscrew drills angled under the north side to remove soil underneath the 800-year-old foundation.

It took three years, but it worked. The tower settled toward the high side, reducing its 6-degree tilt by half a degree, and its 13-foot overhang by about 17 inches. The temporary safety cables were removed.

Tourists now enter the tower only in guided groups of 40. The 35-minute tour costs 15 euros (about $20).

The cyclically varying pitch and roll of the 284-step spiral staircase make for a dizzying experience, especially walking down.

The tower’s tilt now approximates what it had three centuries ago, and engineers say it won’t need another overhaul for a few hundred more years.

Source: Various

Top Photo: Pisa’s tourist-magnet Leaning Tower was in danger of leaning all the way over, until some clever engineering straightened it up a little.
Richard G. Pacheco/Courtesy Randy Alfred

Bottom Photo: The Leaning Tower of Pisa stands at one end of a grassy plaza, near the east end of the cathedral for which it serves as the bell tower. The cathedral, in Romanesque style with Moorish influences, was built between 1063 and 1350. The tower went up (and a bit sideways) between 1173 and 1370.
Richard G. Pacheco/Courtesy Randy Alfred

This article first appeared on Wired.com Dec. 15, 2008.

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