The Leaning Tower of Pisa has been closed to the public for over a decade (1990 - 2001). During this time, a massive engineering operation took place in the attempt to stop the monument from falling. By the end of 90s, the Tower was in fact slowly heading toward its catastrophic collapse. 

Here is how they fixed it...

In the year 1350, the lean was reported to be 1.4° and it grew over time until it reached an alarming threshold in 1993, passing the 5° marker. 

leaning tower pisa stabilization work sequence

The Tower is leaning because the soil underneath the foundation is not strong enough to carry its weight (read more in this article where foundation details are discussed).

The solution to this problem was then to reinforce the soil under the foundation and to make room for the Tower to compress some of the soil and straighten back up towards the North (the Tower leans to South).

They did this in the course of 10 years with a 30 million EUR project launched in 1990.

NOTE: this was the not the first attempt to fix the inclination of the Tower, but it was the first successful one.
There were two previous attempts (in 1838 and 1934) which resulted in increasing the inclination of the Tower. Those failed attempts are discussed in this article.

The project was led by John Burland, Emeritus Professor and Senior Research Investigator at the Department of Civil and Environmental Engineering of Imperial College London.
Burland led the commission of 13 experts that was appointed to fix the Tower. He started his career in 1967 with his thesis titled "Deformation of soft clay". Given his experience in consolidation works in this type of soil, he was the perfect man for the job.

The Tower tilted because the soil under the foundation wasn't strong enough to carry its weight.

The project went through the following phases:

  1. placing counterweights on the base of the Tower, on the North side, to move its barycenter lower and back to the North, slowing down its tendency to lean towards the South;

  2. harnessing the monument with gigantic steel cables (the kind of cables you find on a suspended bridge) to prevent it from collapsing during the works and to "pull it back" once the soil was prepared for this operation;

  3. digging wells under the foundations of the Tower (60 cubic meters of clay were removed);

  4. draining water from the wells;

  5. reinforcing the foundations with concrete (15-metre concrete pillars were inserted into the ground);

  6. pulling the steel cables to begin the straightening motion.

These engineering solutions proved to be successful as the stabilization works recovered over 50cm of lean, bringing back the Tower around 4° inclination (like 200 years ago).

The works ended in 2001 and the Tower was proclaimed out of danger of falling. It was then reopened to the Public.

After that, no other stabilization work was performed. However, in the year 2013, engineers that are monitoring the Tower reported that the straightening motion hadn't stopped yet and the monument leaned back additional 2.5cm (from 2001 to 2013).

Stabilization works ended in 2001 and the Tower was proclaimed out of danger of falling.

Giuseppe Bentivoglio, engineer and Technical Director of the "Opera della Primaziale Pisana", the body supervising the monuments in the Square of Miracles, declared: "It was expected".

Bentivoglio explained that this spontaneous straightening is in fact due to the works carried out years ago and we should see the Tower leaning back to North for a few more years before it finally settles and eventually starts again falling towards the South.
Bentivoglio believes that future generations will have to carry out more works in order to keep the Tower safe from falling. The Tower is anyway safe for the next 200 years.
He continues saying: "In theory, it would be possible to straighten it completely, but nobody really wants that! ...the Tower was born leaning and leaning it shall remain".

The Mayor of Pisa, Marco Filippeschi, shares the same point of view and he declared: "The people of Pisa are delighted that the tower has been restored, but not that it has been straightened".

...the Tower was born leaning and leaning it shall remain.

This is understandable.
The Tower drives to Pisa a few million visitors each year (they say 3 million of tickets sold yearly + 6 million souls visiting the Miracles' Square). They all come to admire this engineering wonder, to see with their own eyes that it does lean for real and it does seem to behave against the laws of Nature.
With so many tourists pouring in to see a leaning tower... why would the people of Pisa want it straightened up?

The Tower is safe for the next 200 years and this is all that matters to locals, visitors and to the UNESCO, which in 1987 enlisted the Square of Miracles in the World Heritage List.

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