soft ground

Wayback Wednesday: Leaning Tower Of Pisa 1173

Posted on August 9, 2023

Wikipedia Wikimedia Pisa Image — Author: Arne Müseler
Photographer
Source
**Wikimedia Commons**
04-02-2022

Eight hundred, fifty years ago, today…

The Leaning Tower of Pisa (torre pendente di Pisa…in Italian) is the campanile or freestanding bell tower of Pisa Cathedral. It is known for its nearly four-degree lean, the result of an unstable foundation. The tower is one of three structures in the Pisa’s Cathedral Square, which includes the cathedral and Pisa Baptistry. The tower has 296 or 294 steps. [The] seventh floor has two fewer steps on the north-facing staircase.

The tower began to lean during construction in the 12th century, due to soft ground, which could not properly support the structure’s weight. It worsened through the completion of construction in the 14th century. By 1990, the tilt had reached 5.5 degrees. The structure was stabilized by remedial work between 1993 and 2001, which reduced the tilt to 3.97 degrees.

Construction of the tower occurred in three stages over 199 years. On January 5, 1172, Donna Berta di Bernardo, a widow and resident of the house of dell’Opera di Santa Maria, bequeathed sixty soldi to the Opera Campanilis petrarum Sancte Marie. The sum was then used toward the purchase of a few stones which still form the base of the bell tower. On August 9, 1173, the foundations of the tower were laid. Work on the ground floor of the white marble campanile began on August 14 of the same year, during a period of military success and prosperity.

The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-meter foundation, set in weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for the better part of a century, as the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca and Florence. This allowed time for the underlying soil to settle […], [otherwise], the tower would almost certainly have toppled.

There has been controversy surrounding the identity of the architect […]. For many years, the design was attributed to Guglielmo and Bonanno Pisano […]. A 2001 study seems to indicate Diotisalvi was the original architect, due to the time of construction and affinity with other Diotisalvi works, notably the bell tower of San Nicola and the Baptistery, both in Pisa.

Between 1589 and 1592, 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.

During World War II, the Allies suspected that the Germans were using the tower as an observation post. Leon Weckstein, a U.S. Army sergeant sent to confirm the presence of German troops in the tower, was impressed by the beauty of the cathedral, and its campanile and […] refrained from ordering an artillery strike, sparing it from destruction.

The tower has survived at least four strong earthquakes since 1280. A 2018 engineering investigation concluded that the tower withstood the tremors because of dynamic soil-structure interaction. [The] height and stiffness of the tower, combined with the softness of the foundation soil, influences the tower’s vibrational characteristics in such a way that it 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.

Wikipedia Summary