Leaning Tower of Pisa | The Incredible Physics behind its Lean

Summary

The Leaning Tower of Pisa, built in 1173, faced a dangerous tilt due to its shallow foundation on marshy soil. Attempts to fix the tilt included using lead counterweights and injecting liquid nitrogen, but these methods initially worsened the problem. In 1999, engineers successfully used under-excavation to straighten the tower, reducing its tilt to 3.97 degrees. The tower's unique position was preserved to maintain its famous lean. Despite misconceptions about differential settlement, the tower's survival through multiple earthquakes and its ongoing stabilization efforts have made it a remarkable feat of civil engineering.

Takeaways

• 😀 Engineers attempted to stabilize the Leaning Tower of Pisa in 1995 by injecting liquid nitrogen to strengthen the soil, but this backfired, worsening the tower's tilt.
• 😀 The Leaning Tower of Pisa's construction began in 1173, with engineers using a shallow foundation on marshy soil, which contributed to its eventual tilt.
• 😀 The phenomenon of 'leaning instability' caused the tower to lean progressively, despite various attempts to correct it during construction.
• 😀 Attempts to fix the tilt included using lighter materials for the upper floors and creating visual illusions, but these only resulted in a banana-shaped tower.
• 😀 In 1934, Mussolini's solution to fix the tilt—injecting concrete into the foundation—caused the south side to compress further, increasing the lean to 5°.
• 😀 By 1990, the tower's lean reached 5.5°, forcing it to close to the public. Emergency measures like adding lead counterweights on the north side slightly reduced the tilt.
• 😀 A more effective solution involved 'under-excavation,' where soil was removed from the north side to slowly straighten the tower.
• 😀 Engineers initially tested under-excavation on a small scale before performing it at the actual site, successfully reducing the tower's tilt by 1° in 1999.
• 😀 After further excavation and removing 38 cubic meters of soil, the lean was reduced to 3.97° by 2001, a stable angle that preserved the tower's iconic tilt.
• 😀 The tower's marshy foundation, though problematic, acted as a buffer during earthquakes, saving the tower from collapse during events like the 1846 earthquake.
• 😀 A common misconception about the tower’s tilt is that the south side’s soil was weaker than the north's, when in fact the lean was caused by leaning instability and water content differences under the foundation.

Q & A

What was the initial problem with the Leaning Tower of Pisa during construction?

The main issue during construction was that the tower's foundation was too shallow for the marshy soil beneath it, which led to the tower's tilt. The soft ground caused the tower to lean as it settled over time.

How did the construction pause during the 12th century affect the Leaning Tower of Pisa?

During the construction halt due to war, the tower developed a tilt because the soil beneath it settled unevenly, causing the tower to lean further.

What is 'Leaning Instability' and how did it affect the tower?

Leaning instability refers to the gradual tilt of a structure due to uneven settling of the ground beneath it. In the case of the Leaning Tower of Pisa, the phenomenon caused the tower to lean more as it grew taller.

How did Mussolini try to fix the tilt of the tower in the 1930s?

Mussolini attempted to fix the tower's tilt by drilling holes into the foundation and injecting concrete, but this actually worsened the tilt, making the tower lean even more.

What was the solution attempted in 1995 to stabilize the tower?

In 1995, engineers attempted to inject liquid nitrogen into the soil beneath the tower to strengthen it. However, this method backfired because freezing the water in the soil caused it to expand, making the tower lean more.

What method ultimately succeeded in stabilizing the Leaning Tower of Pisa?

The successful method was under-excavation, where soil was removed from the north side of the foundation to straighten the tower. This technique was tested and refined before being applied at the actual site.

How did the Cino ring contribute to the tower's stability?

The Cino ring, a concrete ring placed in 1838, provided additional structural support and helped reduce water infiltration around the tower's foundation. This greatly contributed to the tower's stability.

What role did dynamic soil-structure interaction play in saving the tower from earthquakes?

The marshy soil around the tower acted as a buffer during seismic activity, preventing the tower from collapsing during earthquakes. This phenomenon, called dynamic soil-structure interaction, helped protect the tower during high-intensity quakes.

What is the misconception about the soil under the Leaning Tower of Pisa?

A common misconception is that the soil beneath the tower was uneven, causing differential settlement. In reality, the cause of the lean was 'Leaning Instability,' not uneven soil strength.

Why did engineers stop trying to completely straighten the Leaning Tower of Pisa?

Engineers stopped trying to straighten the tower fully to preserve its unique leaning feature, which is a key attraction for tourists. Instead, they left the tower with a tilt of 3.97°, which is considered stable.