Scientists unravel mystery behind Leaning Tower of Pisa

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Ironically, the soil that caused the tower to lean can also be credited for stopping it from falling

The Leaning Tower of Pisa has been an iconic landmark of Italy for hundreds of years. The 58-metre-high tower, that leans at a five degree angle, has been puzzling engineers worldwide because it has managed to survive at least four major earthquakes that have hit the area since 1280.

But now, a research group of 16 engineers at the University of Bristol has figured out what is keeping the structure from falling despite the seismic activity.

The mystery behind the Leaning Tower of Pisa has been hidden in the soil — Radoslaw Maciejewski / Shutterstock Scientists unveil mystery behind Leaning Tower of PisaThe mystery behind the Leaning Tower of Pisa’s survival has been hidden in the soil — Radoslaw Maciejewski / Shutterstock

“Given the vulnerability of the structure, which barely manages to stand vertically, it was expected to sustain serious damage or even collapse because of moderate seismic activity. Surprisingly this hasn’t happened and until now this has mystified engineers for a long time,” the researchers said in a press release.

Apparently, the solution is hidden in the ground. The team has revealed that the considerable height and stiffness of the Tower is combined with unusually soft soil which ensures the structure doesn’t resonate with earthquake ground motion. “This has been the key to its survival,” the researchers said. 

“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,” said Professor Mylonakis, head of the Earthquake and Geotechnical Engineering Research Group at the University of Bristol.

The phenomenon has already been known among scientists as the soil-structure interaction (DSSI). The revelation makes the Tower a world record holder in DSSI effects.

The Leaning Tower of Pisa has been attracting visitors for centuries. To maintain the historic structure, many measures have been implemented. In the 1920s, the city placed cement grouting into the foundations to stabilise it. In 1990, the site was completely closed to ensure the safety of tourists. It was reopened again in 2001.