Laiventures

Since the Leaning Tower of Pisa's gravity-defying tilt has lasted for centuries, millions of tourists visit each year. The question is, for how much longer will this venerable Italian landmark remain? Looking at the tower's past will help us determine whether or not it will maintain its structural integrity in the future.

It was in 1173 that work began on the bell tower in the Cathedral Square, or Piazza del Duomo, that construction was interrupted by wars for the next two centuries. Builders of the tower noticed a southward lean even from the first few floors. Do you know who's to blame here? The area's high water table softens the soil beneath.

Instead of giving up and starting over, the builders came up with a novel solution. They tried to fix the tower's tilt by building each floor at an angle, only to have it lean even more. An Italian researcher at the University of Bristol in the United Kingdom has dubbed the result a "banana shape" because of the way it was constructed.

The tower had a 1.6-degree tilt when it was completed around the year 1370. The final product was an eight-story hollow cylinder that reached 196 feet (60 meters) in height. Stone fragments and mortar were used to build a masonry skeleton that was covered in marble, columns, and vaults.

According to Fiorentino, the Italian government took action as the tower's slant gradually increased to 5.5 degrees. During the 1990s, a committee of experts was formed to mitigate the lean, but not eliminate it and its tourist attraction.

For Fiorentino, it's an iconic image of Italy. "The extent to which we can alter the monument is hotly debated.... It's ingrained in the ethos."

First, the committee attached 600 tons of lead to the north side of the tower in 1993, hoping to compensate for the sinking of the southern side, which had already begun to fall. Even after adding an additional 300 tons (272 metric tons) to the north side and installing ground anchors, the tilt continued, according to Fiorentino. "Underexcavation" — using long tubes and drills to non-invasively remove the ground beneath the north side of the tower's foundation — was attempted by the committee after more brainstorming.

Northward movement occurred gradually as more soil was removed. Because of these actions, it is now only leaning 5 degrees, down from its previous 10% lean. It was claimed that the action had "turned back the clock by 200 years," according to Fiorentino at the time.

He explained that this was only a short-term fix, and he couldn't predict how long the tower would last. Over the next three hundred years, it may tilt back to its 1990s 5.5-degree lean, shifting once more on soft soil. Fiorentino, on the other hand, asserted that the tower is safe for the time being.

First, the tower's construction was interrupted for long periods of time, allowing the structure to settle into the malleable soil and strengthen its structure until the next building bout. In addition, the tower's center of mass is lower to the ground because its base is thicker than its column-coated upper half.

For Fiorentino, it's a mystery why the tower has held up during earthquakes, despite its precarious position. The soft soil beneath the tower's foundation protects the structure from the region's earthquakes, as he and his colleagues discovered, and this provides the tower with a longer, less destructive natural vibration period.