The extraordinary adventure of the tower of Pisa which hangs but never falls

There have been numerous attempts to stabilize the tower of Pisa during its eight centuries of history, made really urgent during the nineties. At the time, in fact, the tower had tilted approximately five and a half sexagesimal degrees, slightly exceeding the threshold at which a collapse was expected. Let’s see then why the tower of Pisa does not fall, retracing briefly the entire history of this legendary Italian monument.

It was 1173 when in a square in Pisa began the construction of a bell tower that would later become famous. Despite the engineering knowledge of the time, however, not much was known about the soil on which they worked, characterized by clay, mud and wet sand, but the architects felt that a base of three meters would be sufficient for the relatively low structure of the tower.

The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled.

Construction resumed in 1272, when attempts were made to compensate for the inclination of the tower by increasing the height of the subsequent floors on the south side. But the weight of the new plans only made that side sink deeper. Upon completion of the seventh floor and the bell room, the angle of tilt was already just over about a degree and a half.

Between 1589 and 1592,[21] Galileo Galilei, who lived in Pisa at the time, is said to have dropped two cannonballs of different masses from the tower to demonstrate that their speed of descent was independent of their mass, in keeping with the law of free fall.

In 1838 they dug around the base of the tower to analyze the sunken foundations, but the removal of carrier sand worsened the situation. Almost a century later, in 1935, the base was reinforced with concrete, but it was not evenly distributed in the foundations. This caused further failure.

In 1992, tunnels were dug to remove about thirty-eight cubic meters of land from under the north side of the tower, temporarily counterbalanced the structure with six hundred tons of lead bars and, finally, anchored the base with steel cables. So the tower was finally straightened, but with an inclination of about four degrees because you didn’t want to lose the characteristic that made it famous all over the world.

At least four strong earthquakes have hit the region since 1280, but the apparently vulnerable tower survived. The reason was not understood until a research group of 16 engineers investigated. The researchers concluded that the tower was able to withstand the tremors because of dynamic soil-structure interaction (DSSI): the height and stiffness of the tower, together with the softness of the foundation soil, influences the vibrational characteristics of the structure in such a way that the tower does not resonate with earthquake ground motion. The same soft soil that caused the leaning and brought the tower to the verge of collapse helped it survive.