The Leaning Tower of Pisa is undoubtedly one of Italy's most important constructions and a true tourist magnet. However, on more than one occasion it came close to losing both conditions. In fact, the risk of collapse became so high that public access was prohibited in January 1990. With more than two decades of studies focused on its stabilization, experts finally managed to inject another 200 years of life into the tower... But what exactly did they do? The latest video from Practical Engineering invites us to explore the process.
Anyone looking for the Leaning Tower of Pisa will quickly find that it is very close to the Ligurian Sea. The Arno River, also close to the tower, flows into that sea, and the activity of both changed the composition of the soil. A quick rewind to the late twelfth century: the builders had advanced to the second floor of the tower... and that was when they discovered the lack of stability beneath their feet. Pisa's constant wars with its neighbors put the project on pause, and that gave the soil some time (almost a century) to gain a little more firmness.
Stabilizing the Leaning Tower of Pisa
To compensate for the inclination, the engineers of the time decided to build the upper floors with one side higher than the other. That's right: the Tower of Pisa is not only inclined, but also has a curve. The war paused its construction again in the late thirteenth century (Pisa was defeated at the Battle of Meloria, losing its maritime power), and the bell tower had to wait until the late fourteenth century.
All this information allowed historians to estimate the cycles of sinking and stabilization. The first formal measurement in the early nineteenth century gave an inclination of 4.9 degrees. Over time, the studies and analyses on the tower grew more complex. They all pointed to the same result: 5.5 degrees, and the collapse of the tower. The fall of the Civic Tower of Pavia in 1989 only increased pressure to speed things up. In 1993, a study calculated that the safety margin was only 1.07. In other words, 7 percent.
The first action was to build a concrete ring and install gigantic lead ingots as temporary counterweights. This worked quite well at first (the tower corrected its inclination by one hundredth of a degree), and the authorities explored installing an underground version. The problem is that the tower's catino (the walkway at the base) serves as a resting point, and when they removed part of it, the tower began to lean in the wrong direction.
The ingots returned to prevent disaster, but they could not become permanent (for aesthetic and historical preservation reasons). This challenge gave rise to three ideas: extracting groundwater from the north side of the tower (too many variables), electroosmosis (the soil was too conductive), and controlled excavation. Its demo began in 1995, combining a series of tubes and augers under an alternative structure. Later, the tower received additional protection with cables that could be individually controlled to minimize movement in undesired directions, and in 1999, a test with twelve holes began.
The results were very positive. The tower corrected four hundredths of a degree, the twelve holes became 41, and within a year, experts managed to remove 38 cubic meters of material, about 70 tons. In the end, the Tower of Pisa lost more than half a degree of inclination, returning to values from the 19th century. What does that mean? Between 200 and 300 years to continue visiting one of the most emblematic places in Italy... and of course, keep taking photos.
(Editor's note: The photo is by Johann H. Addicks, CC BY-SA 3.0)
