The Leaning Tower of Pisa is the bell tower of the Pisa Cathedral, built in the Field of Miracles. A little history:
Its construction began in 1173 AD with a 3-meter-deep slab foundation, and walls were built on top of it. When the structure reached the third floor, it became evident that the tower was beginning to tilt.
We are talking about a 14,500-ton tower built on a 3-meter-deep foundation resting on sand and mud. Imagine geotechnical studies before construction at that time—barely any. After detecting the tilt, the engineers of that era built the walls of the remaining floors in such a way that the height of the sinking side was greater than the other side, attempting to compensate for the tilt angle.
A clever move! However, this actually caused the tower to lean even more because the increased load led to greater foundation settlement on the already sinking side.
Construction lasted for 199 years, and the various pauses during this period allowed the soil to settle. The compacted clay soil is the reason the tower did not collapse and has withstood four earthquakes without falling. However, if it remained like that on such ground, this architectural gem would eventually collapse. So, it was time to “play” with modern engineering techniques.
Modern engineers calculated the tower’s center of gravity and concluded that it would collapse if it reached an inclination of more than 5.44 degrees.
The tower was closed in 1990 for restoration. Engineers drilled 361 holes, each 40 meters deep, and injected 90 tons of concrete into the ground through these holes. (This is equivalent to 361 columns, each 13 stories tall, “planted” into the ground.) At the foundation, piles were placed beneath the slab, essentially acting as columns that penetrate the unstable soil layer and “anchor” into deeper soil layers to stabilize the slab.
Next, soil was removed from the less sunken side of the tower, causing it to settle further and reducing its tilt back to the original four-degree angle. Finally, steel reinforcements were used to secure the base of the tower to the stable ground.
