The Leaning Tower of Pisa stands as one of the world’s most recognizable architectural landmarks, not only for its elegant Romanesque design but also-and especially-for its famous unintended tilt. From a civil engineering perspective, the tower is a fascinating case study in foundation failures, structural adaptation, and modern stabilization techniques.

The Size of the Tower of Pisa

Construction of the tower began in 1173 in Pisa, Italy, as a freestanding bell tower for the nearby cathedral. The project was ambitious for its time, with the tower intended to reach a height of 56 meters and constructed almost entirely from white marble. However, just five years into construction, as workers began building the second floor, the structure started to lean noticeably to the south. The cause was quickly traced to the tower’s shallow foundation-only about three meters deep-laid atop soft, unstable subsoil rich in clay and water. This foundation was inadequate for supporting the immense weight of the tower, which totals approximately 14.500 metric tons.

Construction of the tower began in 1173 in Pisa, Italy, as a freestanding bell tower for the nearby cathedral. The project was ambitious for its time, with the tower intended to reach a height of 56 meters and constructed almost entirely from white marble. However, just five years into construction, as workers began building the second floor, the structure started to lean noticeably to the south. The cause was quickly traced to the tower’s shallow foundation-only about three meters deep-laid atop soft, unstable subsoil rich in clay and water. This foundation was inadequate for supporting the immense weight of the tower, which totals approximately 14.500 metric tons.

The initial response by medieval builders was to attempt to compensate for the tilt by constructing upper floors with one side taller than the other, resulting in a subtle curve in the structure rather than a simple straight lean. This adaptation, while ingenious for the time, was ultimately insufficient to halt the progression of the tilt. Construction was further delayed for decades due to wars and political strife, which, ironically, allowed the soil to settle and likely prevented the tower from collapsing during its early years.

Despite these challenges, construction resumed and was finally completed in 1372. Several engineers contributed to its completion, including Giovanni di Simone and Tommaso Pisano, who both faced the daunting task of building atop a moving foundation. The finished tower consists of eight stories, including the bell chamber, and features a hollow masonry cylinder surrounded by six colonnades with columns and vaults. The walls are faced with tightly jointed San Giuliano marble, and a spiral staircase winds up within the walls, with a slight difference in the number of steps on the north and south sides due to the tilt.

Over the centuries, the tilt continued to increase, reaching a dangerous angle of 5.5 degrees by the late 20th century. This posed a significant risk of collapse, prompting the Italian government to close the tower to the public in 1990 and commission a team of international experts, led by Professor John Burland, to devise a stabilization plan. The chosen method was soil extraction: engineers drilled inclined holes beneath the north side of the foundation (opposite the lean) and carefully removed small amounts of soil. This allowed the tower to settle back slightly, reducing the tilt by about 38 centimeters and bringing the angle to a safer 3.99 degrees. Additional stabilization measures included the installation of lead counterweights and steel tendons to further secure the structure.

One of the most remarkable aspects of the Leaning Tower of Pisa is its resilience. Despite its precarious appearance, the tower has survived numerous earthquakes and storms over the centuries. Civil engineers attribute this to its unique structural characteristics: the tower’s center of gravity, though shifted by the lean, remains within its base, preventing toppling. The “banana-shaped” curve, a result of construction adaptations, helps distribute weight more favorably, while the tower’s hollow design reduces overall mass.

Today, the Leaning Tower of Pisa is stable and open to visitors, with engineers predicting its safety for at least another 200 years. The tower’s story offers invaluable lessons in soil mechanics, foundation engineering, and the importance of continuous monitoring and adaptation. It stands not only as a marvel of medieval architecture but also as a living laboratory for civil engineers worldwide.

References
Institution of Civil Engineers. (n.d.). Stabilising the Leaning Tower of Pisa. Retrieved May 11, 2025, from https://www.ice.org.uk/what-is-civil-engineering/infrastructure-projects/stabilising-the-leaning-tower-of-pisa

EBSCO. (n.d.). Leaning Tower of Pisa. EBSCO Research Starters. Retrieved May 11, 2025, from https://www.ebsco.com/research-starters/history/leaning-tower-pisa

LeaningTowerPisa.com. (n.d.). Who built the Leaning Tower of Pisa? Retrieved May 11, 2025, from https://leaningtowerpisa.com/facts/who-built-pisa-leaning-tower

The Structural Engineer. (2024, June 19). The Leaning Tower of Pisa: Ingenious design and ongoing stability efforts. Retrieved May 11, 2025, from https://www.thestructuralengineer.info/news/the-leaning-tower-of-pisa-ingenious-design-and-ongoing-stability-efforts