Stabilising the leaning Tower of Pisa

Pisa, Italy

Year

2001

Duration

11 years

Cost

£25m

Location

Italy

Project achievements

Solved the problem

Stop the leaning tower of Pisa from leaning or worse, falling over. Previous

Used engineering skill

Implemented a plan to strengthen the base and pull it upright.

Conservation

The tower is hundreds of years old and could easily have collapsed.

Stop Italy's ancient leaning Tower of Pisa from falling over

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 56m 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 3m 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.

"The people of Pisa are delighted that the tower has been restored but not that it has been straightened.”
MARCO FILIPPESCHI, Mayor Of Pisa, speaking in 2013

Stabilising the leaning Tower of Pisa

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 56m tower took nearly 200 years to build. Work started in 1173 and five years later it started tilting.

Did you know …

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.

Difference the project has made

Work completed in 2001 saw the tower straightened by 38cm.

A 2013 study found that 'the bell tower is stable but tending to straighten' and that it had moved a further 2.5cm 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 drill a number of inclined holes just beneath the north side of the foundation and removing small amounts from them in a controlled manner. (The tower leans to the south.)

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 38cm after the work was done and has continued to straighten since. It reopened to the public in 2001.