Ask Smithsonian: Will the Leaning Tower of Pisa Ever Topple?

Imperceptible changes are occurring, but no worries, a collapse is not in the forecast

Millions of tourists flock to Pisa, Italy, each year to see the monumental eight-story tower that seems to teeter precariously—and miraculously, without falling—in the Piazza del Duomo.

But it’s no miracle that the bell tower—built as a companion to a cathedral—is still standing. It’s due to multiple feats of engineering that may preserve the Tower of Pisa’s lean at a precise angle for potentially centuries to come.

Construction began in 1173, but by the time the third floor was complete, the foundation started settling and the tower began leaning to the north. The Tower builders hadn’t exactly picked the most auspicious site for a heavy marble-laden monument—the ground was primarily made up of mud, sand and clay. Builders tried to compensate by making the columns and arches on that north side a bit longer.

Soon after, there was the first of several work stoppages. Construction did not resume until 1272, and by that time, the tower was listing south—the opposite direction. Work was interrupted again just six years later, with seven stories completed. After yet another very long hiatus, the tower was finally completed in 1370 with the eighth story.

It took some 200 years to complete the Tower, but there were only about 20 years of actual work. Talk about a nightmare construction project!

Meanwhile, the Tower had continued to settle over those two centuries, sometimes at a pace that seemed to surely threaten its ability to stay upright. At its completion, the builders angled the eighth story to the north, as a kind of counterbalance to the southern drift.

In 1911, engineers began more precise measurements of the Tower’s movement. Additional measures of the movement of the Tower’s various levels were begun in the late 1920s. Engineers took a crack at propping up the Tower in the 1930s, and again in the 1960s. But it was clear by the late 1980s that the southward lean was taking an inexorable path towards failure in the 20th century.

By the 1990s, the top of the Tower was documented to be moving about 1.5 millimeters (.05 inches) per year. That may sound small, but it was moving at a far greater pace than had been observed in previous centuries.

Italian authorities became increasingly concerned that the famed Tower might fall over. Thus began a massive 10-year restoration project that closed the Tower to tourism starting in 1990. The job specifications were tough: the monument’s character could not be changed in any way, which meant that engineers could not add any visible supports, and they couldn’t do any rebuilding, no matter how minor, said John B. Burland, one of the leaders of the restoration project, and emeritus professor of civil and environmental engineering at the Imperial College of London.

Initially, engineers used almost 900 tons of lead counterweights that were affixed to the north side of the Tower to control the southward lean while they pondered how best to achieve the end goal. The weights would not be allowed to stay. They determined that controlled extraction of soil from under the north side—called underexcavation—held promise. The extraction began in early 2000 and was completed just over a year later, moving the Tower back towards the north. 

“As it turns out, we straightened the Tower by about 48 centimeters,” said Burland. That 19 inches of straightening stabilized the Tower, but was small enough to not be noticeable to all the selfie-taking tourists.

“Over the last few years it has continued to move northwards, but by only a very small amount—fractions of a millimeter—and at a decreasing rate,” Burland said.  That’s because engineers put equipment in place that allows them to make small adjustments to the water pressure beneath the foundation, which helps to stabilize the water table under the Tower, he said.

Burland predicts that the northward movement will stop in a few years, but that the tower will then begin to move south again, albeit at a very slow rate.

Despite the small movements, “it is extremely unlikely that the foundations of the tower will fail,” said Burland.  If anything causes the tower to collapse “it is much more likely that it would be due to a very large earthquake,” he said. But he gauges that risk as fairly low.

The Leaning Tower of Pisa is likely to continue to amaze for centuries to come.

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