Soil causes the Pisa tower to lean — and protects it

The Leaning Tower of Pisa, with its 5.5-degree lean, has vexed engineers for centuries.

Partly built on unexpectedly soft soil, the ancient bell tower began to lean before it was even finished, a historical goof that went on to become one of the world’s historical oddities.

How can something so obviously structurally unsound endure in an earthquake-prone region for hundreds of years? Professor George Mylonakis wanted to know why.

Mylonakis, an engineering professor who studies geotechnics and soil-structure interaction, and more than a dozen researchers came up with an answer that involves that famous soft soil.

According to Phys.org, the engineers determined that the tower’s height and stiffness, “combined with the softness of the foundation soil, causes the vibrational characteristics of the structure to be modified substantially, in such a way that the Tower does not resonate with earthquake ground motion.”

So during a quake, the tower doesn’t shake as much as the earth beneath it, in further defiance of gravity.

“Ironically, the very same soil that caused the leaning instability and brought the tower to the verge of collapse can be credited for helping it survive these seismic events,” Mylonakis told the website.

The tower’s insulation from earthquakes does not mean it can metaphorically thumb its nose at nearby, earthquakeravaged buildings.

It has continued to settle throughout its history and by the early part of the 20th century, was in real danger of falling.

In 1990, the Italian government closed the tower to visitors and began a restoration project, according to Smithsonian Magazine. Restorers put 900 tons of lead counterweights to offset the lean.