Miracle OR Mistake?

THE LEANING TOWER OF PISA 

Built in the 12th century, this freestanding belltower is the pinnacle of mystery in construction and architecture. Its construction took place in 3 phases. The first phase began with the fabrication of the ground floor, articulated with a blind arcade format. 

BLIND ARCADE

The peculiar sinking of the tower began with the construction of the second floor, and only worsened with the completion of the project. The tilt reached 5.5 degrees, as the three-meter (weak) foundation was set on unstable subsoil - which could not support the structure's weight. Consequently, the construction was halted for almost a whole century, as the Republic of Pisa was constantly engaged in battles. Fortunately, this halt enabled the underlying soil to settle and strengthen; otherwise the tower was destined to collapse immediately. Construction was completed in 1319 with the 7th floor having seven bells, one for each note of the musical major scale. 

THE COMPLETED TOWER

INTERESTING FACT IS.... 

At least 4 strong earthquakes have hit the region since 1280, but the tower still stands strong. On investigation, it was found that the tower was able to withstand the tremors because of dynamic soil-structure interactions - the height and stiffness of the tower together with the softness of the soil that it stands on influences the vibrational characteristics of the structure in such a way that the tower does not resonate with earthquake ground motion. It is absolutely baffling yet eerie, how the same soft soil that caused the leaning of the tower in first place, also saved it from collapsing at multiple instances. 

Numerous efforts have been made to restore the tower to a vertical orientation or at least keep it from falling over. Most of these efforts failed; some worsened the tilt. Engineers have tried to  compensate the tilt by building the upper floors with one side taller than the other. They also tried to relieve the overall weight of the tower by adding temporary lead counterweights on the opposite side of the tilt. Yet, the tilting did not cease. 

In 1990, the tower was closed to the public because of its increased instability. The selected method for preventing the collapse of the tower was to slightly reduce its tilt to a safer angle by soil removal from underneath the raised end. The government of Italy requested aid in preventing the tower from toppling. It was, however, considered important to retain the current tilt, due to the role that this element played in promoting the tourism industry of Pisa.

After years of structural strengthening and corrective reconstruction to prevent its tilting, in 2001, the tower was declared to be stable for at least the next 300 years; a feat of engineering.