Why doesn’t the Leaning Tower of Pisa fall over? - Alex Gendler

Summary

In 1990, engineers worked urgently to stabilize the Leaning Tower of Pisa, which was at risk of toppling due to its dangerous tilt of 5.5 degrees. The tower's unstable foundation, caused by Pisa's marshy soil, had led to centuries of unsuccessful attempts to correct the tilt. However, with advanced technology, researchers finally succeeded in stabilizing the tower in 1992 by removing soil and counterbalancing the structure. The tower was straightened to a 4-degree tilt, ensuring its stability for the next 300 years while preserving its iconic imperfection.

Takeaways

• 😀 In 1990, the Italian government enlisted top engineers to stabilize the Leaning Tower of Pisa after it was determined to be at risk of toppling.
• 😀 The tower was leaning at 5.5 degrees in 1990, just over the critical 5.44-degree tipping point, prompting immediate action.
• 😀 The Leaning Tower of Pisa was built in the 12th century as part of a cathedral complex, but its foundation was inadequate due to unstable soil.
• 😀 The marshy land in Pisa, composed of clay, mud, and wet sand, led to the tower's foundation sinking shortly after construction began.
• 😀 Engineers in the 12th century used a 3-meter foundation for the tower, which was insufficient to handle the soft soil beneath it.
• 😀 Construction paused for nearly a century due to warfare, which allowed the soil to settle, providing a more stable base for the structure when work resumed.
• 😀 In an effort to counteract the tilt, architects made adjustments, including making floors taller on the southern side, but this worsened the lean.
• 😀 Numerous failed attempts over centuries included excavating the foundation, injecting mortar, and other strategies that exacerbated the problem.
• 😀 Advanced technology in the 1970s allowed engineers to accurately model the tower’s trajectory and the soil's stability, providing clearer insight into the problem.
• 😀 In 1992, engineers removed soil from beneath the tower’s north end, temporarily counterbalanced it with 600 tons of lead, and anchored the structure with steel cables, ultimately reducing the tilt to 4 degrees.
• 😀 The Leaning Tower of Pisa is now stable and expected to remain standing for at least another 300 years, symbolizing the balance between beauty and imperfection.

Q & A

What was the main challenge faced by the engineers in 1990 regarding the Leaning Tower of Pisa?

The main challenge was that the Leaning Tower of Pisa was leaning at 5.5 degrees, and the engineers projected that it would topple if it reached an angle of 5.44 degrees. They needed to stabilize the tower to prevent it from falling.

How did the engineers in 1990 determine the urgency of the situation?

The engineers used computer models to project that the tower would collapse if the tilt reached 5.44 degrees, and since it was already leaning at 5.5 degrees, they understood that immediate action was necessary.

Why did the Leaning Tower of Pisa tilt in the first place?

The tilt occurred because the foundation was built on unstable ground consisting of clay, mud, and wet sand. The architects underestimated the soil's instability and built a shallow foundation, which caused the southern side of the tower to sink.

What is the significance of Pisa's name in relation to the tower's tilt?

Pisa's name comes from a Greek word meaning 'marshy land,' which accurately describes the unstable soil beneath the city, contributing to the foundation's sinking and the tower's tilt.

What construction flaw contributed to the tower’s tilt during its early years?

The foundation was only three meters deep, which was insufficient for the weight of the tower. The unstable soil caused the southern side to sink, leading to the tower's tilt.

What happened to the construction of the tower during the 12th century?

Construction of the Leaning Tower of Pisa began in 1173 but was halted for nearly a century due to warfare in Pisa. This long pause allowed the soil to settle and provided a more stable foundation for further construction.

How did engineers in the 13th century try to compensate for the tilt of the tower?

In 1272, under the direction of architect Giovanni di Simone, workers built the next floors taller on the southern side to counterbalance the tilt. However, this added weight caused the southern side to sink even further.

What happened during the 1935 attempt to stabilize the Leaning Tower of Pisa?

In 1935, the Italian Corps of Engineers injected mortar into the foundation to stabilize it. However, the mortar wasn’t distributed evenly, leading to another sudden drop in the tower's lean.

What technological advancements helped engineers in the 1970s to understand the tower’s structure?

In the 1970s, engineers developed tests to identify the missing variables in the soil composition and used new computing technology to model the tower’s stability and the amount of excavation needed to prevent its collapse.

What steps did engineers take in 1992 to stabilize the Leaning Tower of Pisa?

In 1992, engineers drilled diagonal tunnels to remove 38 cubic meters of soil from under the tower’s north end, temporarily counterbalanced the structure with 600 tons of lead ingots, and anchored the base with steel cables to prevent further tilting.