Have you ever wondered why the Leaning Tower of Pisa is tilted?
Today, you’re going to learn the reason.
The Leaning Tower of Pisa is the bell tower of Pisa Cathedral, built in the Field of Miracles. A bit of history:
Construction began in 1173 AD with a 3-meter-deep slab, and on top of it, they started building the walls. When the building reached the third floor, it was noticed that it was starting to lean.
We’re talking about a 14,500-ton tower built on 3-meter-deep foundations resting on sand and silt soil. Imagine the geotechnical studies before construction at the time—just enough.
After detecting the slope, the engineers of the time built the walls of the remaining floors so that the floor height on the sinking side was greater than the height on the other side, thus compensating for the angle of inclination.
Smart move! But this actually caused the tower to lean even more due to the greater subsidence of the foundations on the already sinking side compared to the other side due to the added weight.
Construction lasted 199 years, and the various pauses during that time were one of the reasons that allowed the soil to compact.
The compacted clay soil was the reason the tower didn’t collapse and withstood four earthquakes without falling. However, if it stayed like that on this type of ground, this gem would eventually collapse. So it was time to “play” with modern engineering.
Modern engineers calculated the center of gravity of the tower, and the calculations concluded that the tower would collapse if it reached an inclination of more than 5.44 degrees.
The tower was closed in 1990 for repairs. They dug 361 holes, 40 meters deep, and injected 90 tons of concrete into the soil through these holes. (This is equivalent to 361 columns, 13 stories high, “driven” into the ground.)
In foundation terms, pilings were made under the slab. It’s like placing columns that penetrate through the unstable soil layer and “anchor” into the deeper layer of soil to keep the slab equally stable.
Next, soil was removed from the less sunken side of the tower so that it would sink on that side and reduce its tilt to the original four degrees. Finally, steel fixings were used to secure the tower’s base to the stabilized ground.
They could have made the tower vertical, but they didn’t want it to lose its fame and tourist value due to its tilt. And after confirming it could withstand 300 years without tilting further, the tower was reopened to the public.
Source: Jorge Bucay