Many travelers visit the Leaning Tower of Pisa every year, attracted by its gravity-defying tilt that has endured through the centuries. But how much longer is this iconic Italian attraction going to last?
Understanding the structural integrity of the towers of the future will require research into their past.
Construction on the leaning tower of Pisa began on the bell tower within the Piazza del Duomo, or Cathedral Square, in 1173, marking the start of two centuries of construction interrupted by war.
Even from the first floor, the tower builders noticed a southward slope. Main suspect? Soft soil beneath, softened by the area’s high water levels.
Instead of canceling the effort and starting over again, the builders got creative ideas. They built each floor at an angle to try to correct the slope — only to make the tower tilt even more. This results in a slightly “banana shape.”
Upon completion around 1370, the tower tilted at 1.6 degrees. The finished structure is a hollow cylinder that rises eight stories, reaching approximately 196 feet in height. The stone skeleton — made up of crushed stone and mortar — is lined with marble, columns and domes.
As the slope of the tower gradually grew to 5.5 degrees, the Italian government took action to protect the landmark.
In 1990, a committee of experts was appointed to reduce lean — but without losing it and its tourist appeal.
Organizers first attached 600 tons of lead to the base of the tower’s north side in 1993, hoping to compensate for the descending south side. But this did not stop the level of slope, even after they added an additional 300 tonnes to the north side, along with the ground anchor.
After further brainstorming, the committee attempted “bottom digging” — that is, using a long tube and drill to remove soil beneath the north side of the tower foundation non-invasively.
As the soil was removed, the structure slowly began to rotate northward. This effort reduces the tower’s tilt by 10%, making it a 5-degree angle.
This was only a temporary fix, and it was impossible to estimate how long the tower would stand. In the next 300 years, it could tilt back to the 5.5-degree slope of the 1990s, shifting over soft ground again. But for now, the tower is safe for several reasons.
First, long interruptions in tower construction allow time for the structure to settle in malleable soil, reinforcing the structure to the next building.
In addition, the base of the tower is thicker than its column-covered top, its center of mass is lower than the ground, making it more stable.
Although no physical intervention is planned, the tower is monitored continuously with instruments that measure factors such as its level of inclination, and the water level.
