Tipping The Balance

In 1989 almost a million visitors clambered up the 294 steps and peered out over its vertiginous tilt--about 13 ft. off plumb and growing by an alarming .04 in. to .08 in. a year. Pisa's historic Leaning Tower, experts warned, had leaned too far and could topple at any time in the next decade or two. So the 800-year-old white marble structure, one of Italy's most famous monuments, was closed for nearly 12 years and for $25 million of ingenious engineering work to set it a little straighter. Not, of course, entirely straight--Dio mio!--that would have destroyed its tourist appeal. But straight enough to keep it stable for an estimated 300 more years.

Over last weekend, timed to the Feast of San Ranieri, Pisa's patron saint, the plaza beneath the tower was reopened to the public. The tower itself will be reopened in the fall--but only to 30 visitors at a time, accompanied by a guide. And no leaning out too far.

WHY DOES IT LEAN?

The tower is built directly on an ancient riverbed of soft, sandy soil, and the foundation is shallow for a structure that weighs 32 million lbs. (14,500 metric tons).

Various efforts over the centuries to stabilize the 192-ft. (58.4-m) tower had the opposite effect--increasing the tilt until the tower was on the verge of collapse.

MAKING THE TOWER LEAN BACK In recent decades, forces pulling the tower askew began to compound each other. Soil continued to give way underneath, while stress increased on the stones on the downward side at the base of the second level. A panel studied several ideas before selecting a low-tech but effective solution.

1 Giant weights stop the tower's tilt...

Beginning in 1993, nearly 2 million lbs. (870 metric tons) of lead weights were placed on the north side of the tower. Not only did the tower stop tilting, it moved--very slightly--back toward straight.

2 ...belts and cables keep it from collapsing...

Fearing that restoration work would topple the tower, a giant belt was looped around the building and connected to large weights a block away. Thinner steel bands were wrapped around the first level for added support.

3 ...and an array of drills slowly removes soil, allowing the structure to settle

Engineers then sank 41 parallel tubes diagonally under the foundation. A specialty designed auger--a giant drill bit--was inserted into each tube. A machine turned the augers one at a time to remove small amounts of soil over several months.

The drilling created small cavities all along the high side of the foundation, allowing engineers to "steer" the settling of the tower.

THE LEAN YEARS The tower began leaning almost from the beginning, in 1173. Over the course of the tower's 200-year construction, builders tried to compensate for the list, resulting in a slightly banana-shaped structure.

LEVELS 1 to 3 1173-1178

Very little documentation on the building exists, but the date construction began--Aug. 9, 1173--is part of a carving to the right of the entrance.

The building actually leaned slightly in the other direction--to the north--until more levels were added during the 13th century.

4 to 7 c. 1230s-1278

In an attempt to straighten the tower, builders try using thicker stones on the downward side.

War with Genoa--and Pisa's defeat in 1284--suspends building for nearly a century.

About 1198, documents show, a bell ringer worked at the tower, which was about this high at the time.

BELFRY 1360-1370

Galileo is said to have performed his famous falling-object experiment from this level, though no proof exists.

Sources: Coordination Committee for the Safeguard of the Tower of Pisa; University of Pisa; CS Informatica, Pisa: Eyewitness Guide to Italy; Associated Press