Why does the leaning Tower of Pisa lean

The Leaning Tower of Pisa is a famous bell tower in the Italian city of Pisa. It is not attached to its cathedral, but rather it is situated behind it, making it an oddity for architecture at the time. It is also the third oldest structure in the Cathedral Square. Interestingly enough, the tower was originally designed to stand vertically, but due to poor foundation construction, the tower began to lean toward the southeast around 1173. Currently, the tower leans at an angle of 3.99 degrees, which is a result of on-going reconstruction work in 1990 and 2001. Before then, the tower had a lean of 5.5 degrees.

The Tower of Pisa was built and designed to be a work of art which would accompany the cathedral in Cathedral Square, Pisa. Construction of the tower took place over the course of 177 years and was performed in about three stages. The first floor was made of white marble campanile and it began on August 9, 1173.
The tower began to sink soon after construction began on the third floor in 1178. This was caused entirely by the tiny three meter foundation which was laid for the tower, coupled with the fact that it was set in weak, unstable soil. Because of this, construction on the tower was stopped for nearly a century, as Pisans were further occupied by battled with Genoa, Lucca, and Florence. This period of halted construction is surely what saved the tower from toppling, as the soil settled over the years, the foundation slowly became more stable, even if it was still too small.

After more than a century of being left unfinished, in 1272, Giovanni di Simone finally decided to try and correct the tilt of the tower by building upper floors in which one side is taller than the other. This resulted in the tower then leaning from the southeast, to the southwest, which is where it leans today. Because of this form of compensated construction, the tower is actually curved. Construction didn’t last long, as in 1284, it was halted again when Genoa defeated Pisa at the Battle of Meloria. Construction on the tower didn’t begin again until 319, and the bell chamber by which many people recognize the tower was not added until 1372. This was the last time there was any major construction work done on the tower, which means it’s stood for nearly 700 years without any outside construction taking place.

Over the years, there have been numerous suggestions made in how to correct the lean of the tower. Finally, in 1990, a major restoration plan began which hoped to help right some of the lean the tower had been gaining in the past 700 years. When the restoration was finally finished in 2001, there was a two degree difference in lean. In addition to the correction of lean, several lead counter weights were installed on the opposite end of the tower in order to prevent it from regaining its former position.