Explain why does the Leaning Tower of Pisa lean?

Foundation origin problems

Initially, the builders planned foundations that were only 3 meters deep, far too light for a marble tower nearly 56 meters tall. The foundations rest on a ground made up of unstable layers of sand, clay, and mud. Not ideal at all! From the start of construction in the 12th century, when the third floor was reached, the tower had already begun to lean due to this fundamental mistake. The poor architects simply had not realized that these small foundations were totally inadequate to properly support the weight of the structure.

Influence of loose soil

The Leaning Tower of Pisa was built on soil primarily composed of clay and sand, with a lot of water underneath. This type of soil, called loose soil, is not really ideal for laying heavy foundations (like that of a massive marble tower...) because it is quite soft, compressible, and tends to shift or settle over time. As a result, when the builders started placing all those tons of stone on top, the ground slowly began to sink under the weight. The problem is that it did not sink uniformly everywhere. Due to differences in the density of the loose soil beneath the tower, it quickly began to lean dangerously to the south.

Consequences of architectural errors

Design errors exacerbated the problem from the outset. First, the walls were far too thin to properly support the significant weight of the tower, especially on the upper floors. Furthermore, there was virtually no foundation capable of evenly distributing the load. In practical terms, these mistakes made the tower particularly sensitive to any ground subsidence. The center of gravity quickly became problematic, as soon as the first floors were completed, the structure began to lean to the south. Instead of immediately correcting the issue, the architects simply tried to compensate by building the upper floors slightly taller on one side than the other, which ultimately worsened the tilt.

Impact of historical interventions

In the early decades after its construction, several correction attempts were made to straighten the tower, but these often only worsened the situation. Over the centuries, engineers have tried to compensate for the tilt by stacking floors with a slight offset, thus creating a subtle curvature in the structure. Other interventions, such as pumping water beneath the tower or reinforcing the foundations with concrete, have sometimes further disrupted the stability of the soil. In trying to limit the damage, each era has left its mark and contributed, in its own way, to the famous lean of Pisa.

Reactions to environmental variations

The tower leans differently depending on the periods and environmental conditions. When it is very hot, the clayey soil, which is particularly loose and sensitive to changes, dries out slightly and hardly moves, altering the inclination. Conversely, after heavy rains, the soil becomes wetter, more flexible, and the monument shifts in another direction, even though these movements remain weak and very slow. It also shifts sometimes during minor earthquakes, which are common in Italy, but surprisingly the structure reacts quite well to these tremors thanks to the flexibility of its foundation. These continuous changes very slightly accentuate the tilt over time, requiring regular monitoring.