A team of engineers has finally unlocked the puzzle of why the seemingly unstable Leaning Tower of Pisa is still standing despite the central Italian region’s long history of powerful earthquakes and the passage of more than 600 years.
Led by Roma Tre University, the investigation went beyond previous studies of the tower’s unexpected stability by analyzing the available structural and seismic data records and examining the physical, mechanical, and chemical properties of the construction materials and the rock and soil underneath.
Their results, slated to be presented at the 16th European Conference in Earthquake Engineering next month, revealed that the Tower’s ability to withstand strong seismic vibration comes from a phenomenon called dynamic soil-structure interaction (DSSI).
Essentially, due to a remarkably serendipitous interplay between the tower's height (55 meters or 183 feet), the soft soil it is anchored in, and the rigidity of the marble constituting most of its structure, the tower has vibrational characteristics that prevent it from resonating - aka shaking - when seismic waves pass through the ground.
"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 George Mylonakis, from Bristol's Department of Civil Engineering, in a statement.
Construction of the bell tower, located on the grounds of the Piazza dei Miracoli cathedral, began in 1173. The structure is said to have started leaning by the time workers had crafted the third of the eight planned stories, and the soft soil underneath – composed of sand, mud, and clay – was quickly identified as the cause. At the time, the tower leaned north, so the architect (whose identity remains up for debate) tried to compensate by crafting smaller columns and arches on the north side of the third level. But before the builders could complete the fourth story, construction was halted by the outbreak of a series of religious wars.
Attempts were made to complete the bell tower when the dust settled in 1272. Then leaning south due to soil movement over the previous century, the tower's new engineers also tried to compensate for the angle but had to stop building – after completing the seventh story – when political drama erupted once again in 1278. The north-tilting eighth story was finally completed around 1370, giving the south-leaning tower its characteristic curved appearance.
Following a series of stabilization efforts throughout the 20th century and early 2000s, the tower’s lean has been lessened. As of today, the top of the tower is offset from the center of the base by 3.9 meters (12 feet 10 inches).