The Leaning Tower of Pisa is the freestanding bell tower of the cathedral of the Italian city of Pisa. Famous for its unintended tilt, the 56 m tower took nearly 200 years to build - work started in 1173. Five years later it started tilting.
The tilt was noticeable as workers built the second floor. The shift was caused by shallow foundations of 3 m and unstable subsoil.
Engineers working on the tower in later years built upper floors with one side shorter than the other to compensate. This means the structure is curved as well as tilting.
Work was slow and sometimes abandoned for decades as Pisa was often at war with nearby Florence, Genoa and other city states. The tower was finally completed in 1372.
Attempts were made throughout the tower's history to correct the tilt. Some of them made the problem worse and by the 1990s there were fears the tower could collapse altogether.
The Italian government appointed a 'committee of 13 experts' to come up with a plan to save the tower in 1990. The team was led by British engineering expert John Burland.
 Difference the project has made
Work completed in 2001 saw the tower straightened by 38 cm.
A 2013 study found that 'the bell tower is stable but tending to straighten' and that it had moved a further 2.5 cm vertically since 2001.
The structure is expected to straighten another couple of millimetres and then start to lean again – but at a much slower rate.
Engineers believe the tower's future is now secure for at least 200 years.
 How the tower was straightened
The tower was closed to the public in 1990 for engineers to come up with a way to stop the tower collapsing.
The team hit on a solution that would reduce the tower's inclination by about half degree - reducing stress on the building's masonry and stabilising its foundations.
The method – known as soil extraction – saw engineers dig a series of tunnels on the north side of the tower and remove small amounts of earth. (The tower leans to the south.) Steel cables helped pull it back into its original position.
Around two lorry-loads of soil needed to be extracted for the foundations to right themselves.
The project team also found that the tower tilted more in winter as the north side water table was higher than the south when it rained. This lifted the north side even more.
Engineers dug drains so the water could flow away into wells.
The tower straightened itself by 38 cm after the work was done and has continued to straighten since. It reopened to the public in 2001.
 Fascinating facts
There are other buildings that lean more than the Tower of Pisa. The Capital Gate building in Abu Dhabi, UAE is the world's most tilted man-made tower. It has an 18-degree slope – 5 times more than Pisa – although it was deliberately constructed to slant.
Pisa is not even the most accidentally lopsided building in the world. Medieval towers in the German towns of Bad Frankenhausen and Suurhusen both lean more than the Italian structure.
There's also the Leaning Tower of Wanaka – part of the Puzzling World tourist attraction near Wanaka, New Zealand. The tower balances on one corner and leans at an angle of 53-degrees to the ground.
This article was originally published here in 2018 by ICE.
--The Institution of Civil Engineers