You must have heard about the Leaning Tower of Pisa. In 1990, the tower was made off limits when engineers pointed out that the weight of people climbing the spiral steps could increase the inclination of the tower, and topple it eventually. After some 11 years of restoration work, a ‘straightened’ Leaning Tower was opened again to the public in November 2001.
Why does the tower lean?
The tower is actually the belfry (tower from which a bell is hung) of the Pisa Cathedral, which stands alongside. The construction of the cathedral began way back in 1064 and completed by the 12th centiry, while work on the tower began later in 1173 and was finally completed as late as the 14th century!
For a long time, it was believed that the tower’s original designer had wanted it to tilt. But, it was later found that the inclination was an accident. It happened because the soil underneath the tower is unstable.
After the first three floors of the tower were built, work had to be stopped because the land underneath the tower began to sink.
It was resumed some time later when the builders thought they could do something about the tilt. They tried to conceal the inclination by using longer pieces of stone on the sinking south side and shorter ones on the north. Five more floors were added, but that only added to the weight pushing down on the soil by several thousand tons and the building sank deeper into the soft ground.
The restoration project
Engineers estimate that the tower has been shifting about one-twentieth of an inch every year. And, over time, the 180 feet tall tower (when measured from the top) has shifted more than 13 feet from the vertical. In the past, many attempts have been made to bring the tower to an upright position with unsuccessful results.
The current restoration project, which started about 11 years ago, involved correcting the tilt by removing soil from under the tower’s northern side (i.e. the higher side).
This exercise has reduced the slant by 16 inches and while the figure may seem minuscule, engineers are now satisfied that the building is safe. However, they point out that all their work may come to naught if there is an earthquake or a particularly vicious storm.