Why Do Leaning Towers Like the Pisa Tower Not Collapse?
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The ‘Leaning Tower of Pisa,’ Italy’s famous tilted tower, is a well-known tourist attraction.
However, it is not the only renowned leaning structure. Across the world, from the Dancing House in the Netherlands to the Tiger Hill Pagoda in China, many tilted buildings captivate and surprise people.
But do you know why these structures lean? And why don’t such leaning buildings collapse?
Why Do Some Buildings Lean?
Professor Mandy Korf from Delft University in the Netherlands explains that there are various reasons behind structures leaning to one side.
In some cases, buildings such as the famous Dancing House in the Netherlands lean because of weak foundations or soil conditions.
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“Many buildings in central Amsterdam are based on wooden piles,” Korf explains.
According to her, wooden posts connected beneath the walls are driven into the ground.
These piles can be as deep as 12 meters underground, but the soil tends to be soft, soluble, or sandy in nature.
“As long as the wooden foundation remains in good condition, the buildings remain stable,” she said.
However, even without wood decay, weight imbalances can cause buildings to lean over time.
The Condition of Pisa Tower
The Leaning Tower of Pisa leans due to soil conditions beneath it.
Nugiante Squeglia, a member of Pisa Tower’s monitoring team and a professor at the University of Pisa, states that the tower began leaning right from the start of construction.
“The soil was very soft, so it started to lean as construction began, sinking by three to four meters,” Squeglia explained.
Human activities altering the soil can also cause buildings to lean. The Old Church in Delft is an example.
“Its cause is not like the Pisa Tower,” Korf said.
“Because it is near a canal, digging for the canal caused it to lean to one side.”
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Changes in the underground water table can also contribute to building tilts. Korf notes that sometimes the design of a building itself can create leaning conditions.
“Many houses in Amsterdam tilt forward because commercial buildings were originally constructed this way,” she says.
“Warehouses and buildings for easy transport were built next to canals and were constructed with a forward tilt. This forward tilt is common, but leaning side to side is unusual.”
Solutions for Leaning Buildings
So, with so many leaning structures, why aren’t we overly concerned?
Dr. Korf states that not all leaning buildings are structurally unsafe.
“To be structurally unstable, a building has to lean significantly,” she explained.
In some cases, leaning buildings need to be reinforced. The Pisa Tower itself has been partially straightened.
By the 20th century, the tower had tilted dangerously and was at risk of collapsing.
“At that time, the situation was very concerning,” Professor Squeglia said.
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After the collapse of Italy’s Civic Tower in 1989, the Pisa Tower was closed to visitors starting the next year.
Subsequently, measures were undertaken to stabilize and secure the structure.
“Soil removal of approximately 37 cubic meters was done from the northern side of the tower,” Squeglia reported.
Eleven years later, the tower was reopened to tourists.
A Unique Situation
Korf emphasizes that the method used to reinforce the Pisa Tower is not common but specific to this case.
“This was unique to Pisa; such measures are not typical,” she says.
In cases where a leaning building has wooden piles like those in Amsterdam, replacing these can halt the leaning process, she believes.
Image source, Antonello NUSCA/Gamma-Rapho via Getty Images
“Like adjusting a car’s wheel alignment, a leaning building can be made upright,” Korf explains. But sometimes this can cause more damage.
“If it has leaned too far, straightening it is very risky because the structure stabilizes in its leaned position,” she notes.
“Extreme caution is essential to avoid further damage.”
Although it is sometimes possible to halt building lean, there are challenges involved.
“Anything is achievable; everything is possible,” says Korf.
“But it comes with significant cost and complex processes.”
The Impact of Climate Change
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According to Korf’s research, over 75,000 houses in the Netherlands built on wooden piles are at risk.
This problem could grow larger over time.
“Climate change is affecting underground water levels as well. Sometimes these changes happen rapidly,” Korf said.
When underground water levels fall, the wooden piles are exposed to air, accelerating their decay. Changes in the water table also alter soil layers, which impacts buildings founded on different types of soil.
Generally, these processes occur slowly.
The Pisa Tower’s tilt was reduced by 40 centimeters over 11 years. This stabilization was completed in 2001. Engineers have stated that the structure is secure for the next 200 years.
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