Could the Leaning Tower of Pisa Ever Collapse?

The tower of Pisa has defied gravity for nearly 840 years, making it easy to believe it will never fall. However, the iconic structure has faced the threat of toppling almost from the moment its foundation was set.

Construction of the tower started in 1173, but it began to tilt almost immediately due to the unstable ground beneath it, made up of mud, sand, and clay. By the time workers reached the third story, the northern side had already started sinking. To counteract this, they made the columns and arches on that side slightly taller. However, political turmoil halted progress after the fourth story, leaving the tower unfinished for nearly a century.

During its 100-year pause, the tower continued to shift. By 1272, when construction resumed, it was leaning southward—the direction it still leans today. Engineers attempted to correct the tilt in the fifth story, but work was interrupted again in 1278, leaving the tower with only seven completed stories.

Despite efforts to stabilize it, the tower kept sinking, sometimes at a worrying pace. The most severe tilt occurred in the early 14th century, yet construction persisted. Between 1360 and 1370, the tower was finally completed, with workers attempting to balance the lean by angling the eighth story and bell chamber slightly northward.

By the late 16th century, when Galileo Galilei reportedly conducted his famous experiment by dropping a cannonball and a musket ball from the tower, it had already tilted approximately 3 degrees from vertical. However, systematic monitoring only began in 1911, revealing a surprising fact: the tower's top was shifting at a rate of about 1.2 millimeters (0.05 inches) annually.

In 1935, concerns arose that excessive groundwater beneath the foundation could weaken the structure and hasten its tilt. To address this, workers drilled a series of angled holes into the base and filled them with a cement grouting mixture. Unfortunately, this intervention backfired, causing the tower to lean even more dramatically. This misstep made future preservation teams more cautious, though numerous engineers and masons continued to study the tower, proposing solutions and attempting to stabilize it with various braces and reinforcements.

Despite these efforts, none proved successful, and over time, the tower's tilt increased to 5.5 degrees. Then, in 1989, a bell tower in Pavia, northern Italy, with a similar design, suddenly collapsed, raising alarms.

Leaning on a New Plan for Pisa

Fearing a similar fate for the Leaning Tower of Pisa, officials closed the monument to the public. A year later, they assembled an international team of experts to determine if the tower could be saved from collapse.

John Burland, a soil mechanics expert from Imperial College London, played a crucial role in the team. He proposed the idea of removing soil from beneath the tower's northern foundation to help straighten it. To test this theory, the team conducted computer simulations and models, which confirmed the plan's feasibility.

With a strategy in place, workers secured the tower's first level with steel bands to prevent stone damage. They then added 750 metric tons (827 tons) of lead weights to the northern side and constructed a concrete ring around the base, attaching cables anchored deep underground. Using a 200-millimeter (7.9-inch) drill, they carefully extracted small amounts of soil—15 to 20 liters (4 to 5 gallons) at a time—from beneath the foundation. This process, combined with the tension from the cables, gradually pulled the tower back toward stability. Over several years, they repeated this in 41 locations, meticulously monitoring progress.

By 2001, the team had reduced the tower's lean by 44 centimeters (17 inches), prompting officials to reopen it to the public. Remarkably, the tower continued to straighten even after drilling ceased, and by May 2008, sensors showed no further movement. By then, the lean had decreased by an additional 4 centimeters (2 inches), and the structure appeared stable.

Burland and his team's efforts could potentially stabilize the tower indefinitely. However, the masonry in the lower levels, which has borne the brunt of the tower's tilt for centuries, remains a concern. If this masonry were to crumble, the tower could collapse. Additionally, even a small earthquake in the area could pose a significant threat.

Despite these risks, engineers believe the tower will remain stable for at least 200 more years. By then, new technologies may offer even more advanced solutions, potentially preserving the iconic structure for another 800 years.