What Caused the Tower of Pisa to Lean?
The famous Leaning Tower of Pisa in Italy is the campanile (bell tower) of the cathedral in Pisa. The tower tips so far to one side – 17 feet off perpendicular – that it looks as though it might fall over. It has been leaning for over 600 years. Of course, it wasn’t supposed to lean, but the foundation was not built on firm enough ground, and the heavy marble tower sank unevenly in the soft soil. The tower has leaned more with the years.
Not long ago the foundations were strengthened, but engineers are still not certain that the tower will not fall one day. Over the years, many experts have searched to find the cause of the lean of this world wonder. It has been found that about 10,000 years ago, the location of the Tower was a river estuary. Therefore, the water and the tide were constantly flowing in and out of the site, depositing layers of soft sand and silt.
The reason the Tower has sunk into the ground is due to the soft, sand-like texture of the soil. The Tower tilts southward because the soil under the south side of the monument is more compressible than on the north side. Since 1911, when measurements of the lean began, the Tower has shifted approximately one-twentieth of an inch each year and accelerating.
The Tower was built in stages over two hundred years and the builders realized from the beginning the structure was shifting. Attempting to correct the lean, the builders put taller pieces of stone on the south side and shorter on the north side. For example, the bell tower has six steps up to the base on the south side and only four steps on the north side. As a result, the tower is slightly shaped like a banana.