A team of engineers has finally solved the mystery of how the seemingly unstable Leaning Tower of Pisa in Italy has managed to stay standing for more than six hundred years, even in a seismically active region. A team led by Roma Tre University concluded that the tower’s height of 183 feet, the soft soil in which it stands, and the structural strength of the its marble all contribute to its remarkable resilience. This phenomenon is known as dynamic soil-structure interaction (DSSI). “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 University of Bristol researcher George Mylonakis in a statement.
Construction on Pisa’s bell tower began in 1173, and the tower reportedly started to lean when builders reached the third story. Even then, engineers understood that the site’s unique soil mix was responsible for the leaning. After religious wars and conflict interrupted construction, the tower was finally completed in 1370. Though the tower’s lean appears to be stable, efforts throughout the 20th and 21st century have decreased its severity over time.
The research team expanded on previous studies by examining structural and seismic data records over time. They also engaged with a deep analysis of the physical, mechanical, and chemical properties of the materials used to build the tower, as well as the rock and soil in which it was anchored. Because of DDSI, the ground in which the Tower stands is insulated from seismic shocks, protecting it from the frequent and powerful earthquakes that have historically affected Pisa. These findings will be presented at the 16th European Conference in Earthquake Engineering in June.