Why Does the Leaning Tower of Pisa Lean?

(Last Updated On: June 25, 2019)
Why Does the Leaning Tower of Pisa Lean?

The Leaning Tower of Pisa is one of the most well-known buildings in the world, yet few people actually know why it leans so precariously or where it all began. The subject of thousands of photographs every single day, this iconic tower has one of the most interesting stories of any historical monument in Europe.

History of the Leaning Tower of Pisa

Situated in the Italian city of Pisa, near the shores of the Mediterranean Sea, construction of this famous tower began way back in 1172 AD. Deciding that the town was in dire need of a new bell tower for the cathedral next door, local widow Donna Berta di Bernardo donated sixty silver pieces to kickstart the build, a huge sum of money at the time. Ground was broken the following year and construction continued steadily through 1178, at which point two floors were completed and progress was being made.

Why Does the Leaning Tower of Pisa Lean?

Despite the fact that they were only two floors into this ambitious project, the soft ground below had already begun causing problems. Situated between two rivers (the Arno and Serchio), the base was made up mainly of clay, sand and ground-up seashells, a mixture that would not prove particularly adept at stabilizing large, heavy buildings. Considering that the word “pisa” comes from a Greek word meaning “marshy land”, this probably shouldn’t have been surprising.

However, in a strange way, the extreme conflicts taking place in the area at the time turned out to be the saving grace of Pisa’s largest construction project. Continual war with neighboring republics such as Genoa, Lucca and Florence meant survival had to be prioritized and construction of the tower was put on hold for most of the following century. Over this period the ground had time to dry out and harden, providing a somewhat more stable base than before.

When things on the battlefront eventually calmed down and Pisans were able to think about less violent pursuits once again, construction ramped back up on the neglected bell tower. By this point, the tower featured a relatively minor 0.2 degree list to the north. However, by the time they reached the seventh floor the entire structure had a fairly pronounced 1.6 degree lean to the south. Rather than start over, the workers attempted to counteract the lean by adding additional material on the south side to make the walls higher and create a level top. Needless to say, this did not have the desired effect. The additional weight actually caused that side to sink even further, making the lean even more noticeable by the time construction was completed in 1372.

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Mussolini to the Rescue?

Leaning Tower of Pisa c. 1890

In 1838, architect Alessandro Della Gherardesca came to the conclusion that despite the fact that the tower was still not exactly level, it had now stood unchanged for well over four centuries, therefore it must be stable enough to withstand a few revisions. He started by digging away much of the soil around the base of the tower in order to create a walkway for sightseers. Within days the tower had shifted another foot, and increased pressure on the foundation led them to immediately cease all further attempts at beautification.

In 1934, dictator Benito Mussolini decided this uncooperative tower was an affront to his supreme power and vowed to finally fix it for good. His architects drilled 360 holes into the foundation on the high side, filling them with concrete to counteract the angle. But… For complex reasons related to physics, the tower shifted and ended up leaning even further.

Modern Technology Steps In

In 1990, based on projections that, as it currently stood, further erosion would lead to a full collapse by 2040, a commission was set up to stabilize the tower. Utilizing methods not previously available – such as extracted soil, anchored cables, lead counterweights and steel reinforcements – the tower was straightened from a 5.5 degree lean back to the 4 degrees it stood at in 1838. Physicists today estimate this famous tower should now be able to stay upright for the enjoyment of traveling Instagrammers for at least another 200 years.

One aspect of the leaning tower that had never been fully resolved until recently was just how such a precarious building managed to stay standing through all four major earthquakes of its lifespan. Well, recent research by a group of engineers from the University of Bristol have now determined that its unique combination of height, stiffness and an extremely soft base has made it impervious to the normally destructive effects of earthquake-related ground motion. This phenomenon is referred to as Dynamic Soil-Structure Interaction and, as a world leader in this category, the Leaning Tower of Pisa is now being used as a model for quake-resistant architecture all over the world.

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