The Leaning Tower of Pisa is a free-standing bell tower in the complex of the Pisa cathedral. It is known for its infamous four-degree inclination, resulting from an unstable foundation. The 57m tall structure on the higher side, is merely based on a 3-meter-deep foundation. The sandy and soggy soil of the region could not bear the weight of this 14,500 tons structure, resulting in a tilt midway through the construction process. The construction was carried out in around three stages which went on for 199 years. The foundation of the structure was laid on the 9th of August 1173. The tower began to sink as the construction progressed to the second floor. After this, the work was on hold for over a century before resuming in 1233. When in 1272 the work resumed under Di Simone, the engineers built the upper floors with one side taller than the other to compensate for the tilt. This resulted in a slight curve in the structure. The seventh floor was completed in 1319, and the bell chamber was added in 1372 by Tommaso di Andrea Pisano.
It is interesting to note that the Pisa tower is just at the critical height that is right at the level before it could have collapsed..
A detailed explanation about the tilt by the lead engineer of the restoration carried out between 1990-2001- https://www.youtube.com/watch?v=kUGJlxIAU0Q(A detailed explanation of the tilt by the lead engineer of the restoration carried out between 1990-2001)
Over the years following its completion, numerous efforts were made to restore the tower’s orientation or at least prevent it from collapsing. All the trials failed, and a few even worsened the tilt bought a sense of worry in fear among people. The tower was closed to the public from 1990-2001 during which work was carried out to reduce the tilt and stabilize the leaning tower before things could get worse. It reached an alarming threshold tilt of 5° in 1993. The team was led by John Burland along with 13 technical experts, who were appointed by the commission to fix the tower.
The restoration work went on in multiple phases. Initially, counterweights were placed on the base towards its northern side to tackle the tilt to the south. The structure was harnessed by strong steel cables (like the ones you see on a suspended bridge) to prevent its collapse during the restoration work. Around 60 cubic meters of clay were excavated by digging wells and then 15 m long concrete pillars were inserted to strengthen the foundation. Once the piles were placed, the cables were strengthened and pulled to begin the straightening process. Accordingly, between December 1999 and January 2000, 41 extraction holes were installed north at a 0.5m spacing between each other with a dedicated auger and casing in each hole.
All of these efforts successfully recovered over 50 cm of lean reducing the to a 4° inclination, exactly as it was 200 years ago. The engineers declared the Tower of Pisa safe for the coming 300 years. This work finished in 2001 when it was again open to the public after it was not an immediate danger. Although no restoration work was done post-2001, engineers continue to study and monitor the tower. They obserevd that the tower was continuing to tilt after the restoration! So much so that the tower recorded an additional 2.5° (from 2001-2013) tilt in 2013. But this was no surprise as it was expected said- Giuseppe Bentivoglio, engineer and Technical Director of the “Opera della Primaziale Pisana“, the body supervising the monuments in the Square of Miracles. Though he thinks that future generations will have to carry out some restorations in the future – the tower is now safe for 200 years. He continues by 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“.