In 1990, the Leaning Tower of Pisa was closed after its dangerous tilt raised fears of collapse; how engineers used a bizarre trick to move the 14,000-ton tower back 17 inches without making it straight

In 1990, the Leaning Tower of Pisa in Italy was closed for repairs. What was the problem? The tower, built over a period of 200 years, between 1173 and 1372 A.D., was leaning too much. The 1990 closure was the first time in the tower's long life that it was was closed for repair. Today, the tower is back where it belongs but it is leaning about 18 inches less than before.

By 1990, the Leaning Tower of Pisa had tilted more than 5.5 degrees, with the top leaning about 15 feet beyond its base. The pressure on the medieval stonework at the bottom was becoming dangerous, raising fears that the tower could collapse, according to a report in Science Blog.

Italy closed the tower to visitors and cleared the area beneath the leaning side as a safety measure. A team of 13 experts, led by Imperial College London soil-mechanics professor John Burland, was then asked to find a solution. But there was one major condition: they had to save the tower without making it completely straight.

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The tower's famous lean was what made it special. But that same lean was also putting the structure at risk. “While some architectural follies are the product of unforeseeable bouts of bad luck, the Leaning Tower of Pisa’s signature tilt could have been avoided with better planning,” writes Michael Arbeiter for Mental Floss. “A shallow foundation and the soft ground of Pisa—composed of sand, clay and deposits from the Tuscan rivers Arno and Serchio—were too unstable to support the building even in the early stages of its construction.”

The Leaning tower was slowly sinking

Engineers raised concerns that the Leaning Tower of Pisa could eventually collapse as it has been slowly sinking for centuries. By the year 1990, the tower had already tilted more than 5.5 degrees, with its top overhanging the base by about 15 feet.

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The 14,000-ton marble tower was built on soft ground, with a foundation only about three metres deep. The soil beneath one side was softer than the other, causing the tower to start sinking southward almost as soon as construction began in 1173.

The construction of the tower faced hurdles for almost 100 years owing to wars and global conflicts. During that time, the soil had time to settle and when work resumed, builders tried to compensate for the tilt by making some upper floors slightly taller on one side. This gave the tower its subtle curved, or “banana,” shape. Over the centuries, the lean continued to increase.

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Engineers faced a difficult challenge

Italy had no option but to shut the historic monument in the 1990s. A team of 13 experts, led by soil mechanics professor John Burland of Imperial College London, was asked to save the tower without making it straight. The tower's famous lean was both its biggest problem and its biggest attraction.

Engineers considered several solutions, including placing heavy lead blocks on the north side as a counterweight. One proposed anchoring method even caused the tower to move unexpectedly overnight.

Eventually, they chose a surprisingly simple approach and the solution was to remove the soil. Starting in 1999, engineers drilled 41 small holes beneath the northern side of the tower. They slowly removed tiny amounts of soil, sometimes only a few litres at a time.

As soil was removed, gravity gently pulled the tower back toward the north. The process was extremely slow and carefully monitored and engineers removed dozens of tons of soil while constantly checking the tower's movement.

They also installed steel cables as a precaution and worked on drainage because changes in the water table could affect the soft ground beneath the tower.

The tower moved back 17 inches

By 2001, the tower had shifted back by about 44 centimetres, or 17 inches. That reduced its tilt by roughly half a degree — enough to significantly reduce the stress on the medieval masonry, even though visitors could not see the difference.

After 11 years of closure, the tower reopened to visitors on June 16, 2001. Engineers estimated that the building was safe for at least another 300 years.

Interestingly, the tower continued moving slightly after reopening. Measurements showed that it gradually straightened by another few centimetres before eventually settling.

FAQs

Why was the Leaning Tower of Pisa closed in 1990?

The tower was closed because its tilt had become dangerous. By 1990, it had leaned more than 5.5 degrees, putting increasing pressure on the medieval structure.

How much was the Leaning Tower of Pisa leaning?

By 1990, the top of the tower was about 15 feet beyond its base, creating serious concerns about its stability.

Why does the Leaning Tower of Pisa lean?

The tower was built on soft, unstable ground with a shallow foundation. Uneven soil beneath the structure caused one side to sink more than the other.

How did engineers save the Leaning Tower of Pisa?

Beginning in 1999, engineers carefully removed small amounts of soil from beneath the northern side. This allowed gravity to gently pull the tower back.

How much did the tower move back?

By 2001, engineers had moved the tower back about 44 centimetres, or 17 inches.

Did engineers make the Leaning Tower of Pisa straight?

No. The goal was to reduce the dangerous tilt while preserving the tower's famous appearance.

How long was the Leaning Tower of Pisa closed?

The tower remained closed for about 11 years, from 1990 until it reopened in June 2001.

When did the Leaning Tower of Pisa reopen?

It reopened to visitors on June 16, 2001, after engineers completed the major stabilization work.