Leaning Tower of Pisa: Why It Leans

The tilt of the Leaning Tower of Pisa is immediately striking. It manages to stand at a 4-degree angle despite being built on soft, unstable soil. The lean worsened each time a floor was added during its 12th-century construction. It could have succumbed to its faulty foundation, becoming another forgotten ruin of time. But the builders came up with an ingenious solution: adjust the structure to accommodate the lean.

The Genesis of a Fault-Lines

The Tower of Pisa was initially planned as a campanile, or freestanding bell tower. The project began as early as 1174, with the building intended to stand tall and straight. However, the ground beneath the structure was composed of soft, unstable clay and sand. It destabilized the foundation, causing the tower to lean to one side. This uneven settling resulted in a tilt that became dramatically more pronounced as the tower grew in height. The builders didn't abandon the project, but instead, they adapted to the issue. The upper floors were designed with intentional offsets to correct the lean. The tower would remain asymmetrical but also stable, creating an iconic curved shape. Architectural expert Matt Bawn believes that the master builders not only compensated for the tilt but also incorporated it in their designs.

The Cultural Miasma

The structure offers lessons about engineering and persistence, but it is not simply a technological marvel. Despite its unplanned curve, the tower has become a cultural icon; something that people associate with both Pisa and Italy. The tower was immortalized by artists and writers, from renowned Renaissance painter Andrea del Sarto to French novelist Honoré de Balzac. The tower's image has permeated popular culture, appearing in movies and cartoons. It has become a favorite destination for tourists, who flock to pose for photos alongside the tilted tower in the Piazza dei Miracoli. Today, the Tower's unique lean has become a symbol of resilience and ingenuity in the face of adversity. The site has received about 1.7 million visitors a year on average, making it one of the most visited sites in Italy. The number of visitors fluctuates considerably: in 2019, about 2.4 million people visited, whereas in 2020, the number collapsed because of the pandemic. The Tower's famous lean has given it a unique identity, and it remains a symbol of the city of Pisa.

Engineering with Gravity

Stabilizing the Tower of Pisa involves managing gravity, a force that affects structural integrity and a fundamental consideration in construction. According to Matt Bawn, the tower survived and stands today because of the builders' ingenuity. The builders adjusted for the tilt without attempting to correct it. "The leaning of Pisa is caused by its building over a foundation of unstable soil and land reclaimed from swamps," said Matt Bawn, adding: "The supposition that the builders knew that the tower was meant to lean as a design feature is wrong. "By incorporating this intentional counter-curve, they turned a design flaw into an architectural masterpiece." Builders also used Italian engineering techniques like the floating shaft approach, which didn't align the vertical elements with the centerline but allowed each new layer of stonework to adjust horizontally without disrupting the structural integrity. Every new course of the building didn't align with the middle axis, but it achieved a balance because of the tower's lean, and it would add up rather than fall apart.

The Ground Beneath the Tower

Unfortunately, the soil beneath the tower continues to be a challenge. In 1990, engineers intervened to stabilize the soil in order to prevent further damage. Engineers worked on hundreds of soil extraction wells and waterproofing, aiming to precisely control the soil pressure. Groundwater extraction and other soil treatment techniques were applied. This method stabilizes the soil and stops the tower from tilting further. It involves inserting steel rods to reinforce the clay and sand foundation.

Ever Widening Curves

In addition to the tilt, the tower is curved. Its eight stories are fitted with a gradually increasing curve to adjust for the lean. The curve increases with each floor, aligning the centerline of each floor with the center of the tower base. The first floor is curved very slightly, with only about 1.44 meters of displacement at the corners. In contrast, the top floor of the Tower is curved more sharply, with a perpendicular displacement on one side of the tower of 3.96 meters. The curved upper floors also helped distribute the building's weight more evenly, preventing the structure from toppling over.

Seeing It for Yourself

Visitors who want to experience the curved structure can enter the Tower in two ways. The first method is to climb 296 steps to the top. The second way is to visit the museum. The museum includes a multimedia presentation that covers the history and construction of the Tower, along with a detailed explanation of the technical challenges that led to the tilt. While visitors can't take photos at the top, they can purchase discounted tickets for online bookings and take photos at the base.