Why Does the Leaning Tower of Pisa Lean Over?

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The leaning tower of Pisa is part of the Cathedral Square in Pisa, Tuscany, Italy, which is a UNESCO world heritage site. Originally it was built as a bell tower (campanile) for the cathedral. The tower has leaned almost since construction started, due to faulty foundation and weak subsoil. It is now secured against toppling over completely.

The leaning tower of Pisa is actually not vertical.
The leaning tower of Pisa is actually not vertical.

History of the Tower

Construction of the tower (in Italian: Torre pendente di Pisa) started in 1173, when the city of Pisa was prosperous. This makes it the third oldest structure of the Cathedral Square (Piazza del Duomo) in Pisa. When construction had reached the third floor, it was halted for a century due to the city being engaged in wars with other Italian cities. In 1272, construction started again, with workers trying to compensate for the tilt by building the floors taller on one side. Construction was halted again in 1294. The seventh floor was completed in 1319, although the final bell chamber was not added until 1372. The current bells in the tower were installed in 1655. From 1990 to 2001, the tower was structurally strengthened and the surface subsequently restored.

The leaning tower of Pisa stands in the cathedral square.
The leaning tower of Pisa stands in the cathedral square.

History of Pisa

The city of Pisa was referred to by the Romans as being an ancient city, funded already in the fifth century B.C., most likely by the Etruscans. Its location at the confluence of two rivers in a lagoon made it easy to defend during the fall of the Roman empire, and it soon became an important trading city.

From the ninth century A.D., persistent piracy forced the city to expand its fleet and ships from Pisa were part of naval expeditions as far as north Africa. The trading power of the city was at its peak in the 11th century, when it was counted among the four main Maritime Republics of Italy. The defeat of its navy by another trading republics, Genoa, in 1284, represented a major naval blow. The city never recovered from the loss of its navy and continued as a minor city until Italian unification.

Apart from the historic buildings, the city also has a large university, founded in the 12th century.

The city of Pisa is situated at the confluence of two rivers.
The city of Pisa is situated at the confluence of two rivers.

Facts about the tower

The leaning tower of Pisa is 55.86 m from the ground at the lowest side, 56.70 m at the highest. The staircase has two steps fewer on the northern side, the south side has 296 steps. The angle with which the tower leans was originally 5.5 degrees, but after restoration, the angle is about 4 degrees, which means the top leans four meters out from where it would be if it was perpendicular to the ground.

The tower is involved in the story about Galileo Galilei dropping two cannon balls with different masses from the tower to check which fell fastest. The truth of the Galileo story is still under debate. However, the tower was almost blown up during World War II, when the Germans used it as an observation post.

The walls are 4 m wide at the base, but only 2.5 m at the top. The tower is built of white marble.

The tower began to sink after construction had progressed to the third floor in 1178. This was due to a mere three-meter foundation set in weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century, because the Pisans were almost continually engaged in battles with Genoa, Lucca and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled. In 1198, clocks were temporarily installed on the third floor of the unfinished construction.

The tower leans four degrees from the vertical.
The tower leans four degrees from the vertical.

Why the tower leans

The tower was built to be vertical, but very soon after construction it started leaning to the southeast, due to inadequate foundations and loose subsoil. It has shifted during construction, and now leans to the southwest.

After the third floor was completed, the construction was stopped for a century. This allowed the tower to settle, and the foundation to stabilize. When construction was resumed, the architect tried to compensate for the tilt, and built it with the floors taller on one side. This made the tower lean in the other direction.

During the 20th century, the city government worried that the tower would collapse, and sought national and international help to stabilize it.

The tower started to lean shortly after construction started.
The tower started to lean shortly after construction started.

Restoration attempts

During the 1960s, the worries that the tower would topple and fall were increasing, as the leaning of the tower had started to increase. This led the Italian government to request international aid in making sure this would not happen. However, as the city is known for the leaning tower, this element was considered important to retain. An international group of architects and engineers formulated a method that included adding 800 tons of lead counterweights at the base. Some of the earth below the leaning side was removed, straightening the tower slightly, and the foundation was generally strengthened. The tower was officially pronounced to have stopped moving in 2008.

Since 2008, the leaning tower is officially stable.
Since 2008, the leaning tower is officially stable.

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