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Originally a bell tower designed and built by Bonanno Pisano, the tower was intended to be vertical when its construction started in 1173. However, just before the completion of its third story, the tower began to tilt. The problem was that the completely white marble tower was built on a stone foundation which was just 10 feet thick. The stone foundation rested on a soft surface made of sand and clay. Thus as the construction began, the monument started settling in an unusual manner on the soft surface.
In 1178, the construction work on the monument was brought to a standstill with completion of only three tiers. The construction process was halted almost for a century and then began in 1272 under the architect Giovanni di Simone. In order to prevent any further tilt, the engineers made one side of the building taller than the other. However, this caused the tower to tilt in the other direction causing the construction process to again came to a halt in 1284. An investigation team was appointed to figure out the probable reasons behind the leaning of the tower. Several technicians were employed to raise one side of the gallery on the upper floors which somehow assisted in correcting the lean of the bell tower.
The construction work was at last completed in 1350. During this time the tower was leaning at an angle of four feet, seven inches from the vertical position. The weight of the bells caused the structure to tilt ever further. When in the 20th century, the tower leaned almost seventeen feet, a rescue operation was again called to save the tower. The engineers started removing soil from the opposite side in order to bring the building to equilibrium position. This restoration work reduced the inclination by a miraculous figure of fifteen feet.
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This remarkable building owes its beauty to the architects Nicolo Pisano and his son Giovanni, who together made the foundations of the building three meters deep keeping a base of dry stones, clay and sand. According to Piero Pierotti, a highly renowned person in the history of architecture, the ingredients used for construction were of a particular weight and functional characteristics, which actually helped in saving the masonry work towards the top of the monument.
Eminent scientists like Galileo Galilei later held various experiments in the tower like the one which involved dropping objects in 1589.There are in total seven bells installed in the monument on a musical scale. The largest bell weighs three and a half tons and the oldest bell is known as the Pasquarreccia.
The walls of the Tower at the base have a width of 16.7 ft (4.09 m) and of 8.14 ft (2.48 m) at the top. The overall weight of the tower is estimated to be of fourteen thousand five hundred metric tons. The tower has a total of two hundred and ninety-four steps.