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The tower on the right looks more inclined than the tower on the left, but the leaning towers in both pictures are exactly the same. This is known as the "Leaning Tower illusion"(Credit: Adriana Olmos/ F. Kingdom et al., 2007) In Chinese reports, the Leaning Tower of Pisa is often described as "200 years old, 600 years old." This sentence perfectly sums up the history of the construction and maintenance of the Leaning Tower of Pisa.

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The story also begins when the Leaning Tower of Pisa was first built. Of course, it was not called "Leaning Tower of Pisa" at that time. As a free-standing bell tower of the equally white and beautiful Cathedral of Pisa, it should have been quietly erected in the Miracle Square in the north of Pisa. In 1173, the clock tower was officially opened, but as the clock tower was gradually built, people found that the tower seemed to be crooked.

But the people of Pisa (in central Italy) didn't seem to think much of it. Perhaps they were "used to it"-it is said that several of the clock towers in Pisa were more or less skewed at the beginning of construction, but most of them remained upright naturally after construction.

This is related to the construction technology of the time, when there was no concept of foundation exploration, but the geological conditions of Pisa were difficult models for construction.

Pisa was built on a coastal plain. A river called Arno runs through the city and borders the Auser River to the north. Every time the river floods for thousands of years, it will leave loose floodplain sediments here, mostly fine sand or clay. These fine sands and clays lay in layers on the plains, and pizza was built on top of these sediments. Such a layer would have been too weak to provide sufficient strength. To add to the problem, the groundwater table in Pisa is very high, especially at the location where Pisa's bell tower was built, and the groundwater table is only more than one meter below the ground level, making the soil more unstable.

In this case, it can be said that as long as there is no collapsed tower, it is a good tower.

Five years after construction began, when the Leaning Tower of Pisa had reached its third floor (the lowest floor was the "zero floor," so the third floor was actually the fourth floor), the tower was clearly leaning north. At this point, the Leaning Tower of Pisa ushered in its first turn of fate: the Leaning Tower of Pisa temporarily stopped construction.

A hundred years later, when the clock tower was opened again, the Leaning Tower of Pisa tilted only about 0.2 degrees north, and the highest point of the tower was 18 centimeters off center. Fortunately, although it seems to have been delayed for a hundred years, this may have just kept the Leaning Tower of Pisa alive. Over the course of a hundred years, the Leaning Tower of Pisa's own gravity compacted the underlying soil, and modern analysis shows that if the construction had been done before the underlying soil had stabilized, the tower would have collapsed.

Obviously, the architect is not willing to let the clock tower tilt forever, decided to "build while repairing." As with building blocks, if a block is skewed, we often shift the next block in the other direction, trying to balance out the previous bias. Architects at the time made similar attempts to compensate by building the north side of each subsequent story higher than the south side, adjusting the thickness of the bearing columns and the thickness of the walls.

Eventually, Pisa's bell tower did not tilt north anymore, but south instead, at a greater angle.

This leads to a cold knowledge-the Leaning Tower of Pisa is not actually straight, but rather "banana shaped" and curved.（M.B. Jamiolkowski. "The leaning tower of Pisa: End of an Odyssey") When the Leaning Tower of Pisa was capped, the construction team also specially built the top floor of the bell tower parallel to the ground, and the "Wish" bell tower could slowly return to its upright position. However, when the Leaning Tower of Pisa was finally completed intermittently in 1370, the southward tilt was already evident. At the time of completion, the top of the nearly 60-meter-high white tower was 2.1 meters off center and continued to tilt.

Sword of Damocles The Leaning Tower of Pisa was finally built after 200 years. This white beauty and magical appearance attracted a lot of visitors. But the government remains concerned that the leaning tower of pisa will tilt and eventually fall. Thus, the Leaning Tower of Pisa ushered in its 600-year maintenance journey. Unfortunately, most efforts make the tilt even worse, the difference being intentional or careless.

For example, in 1838, architect Alessandro della Gherardesca decided to dig a walkway on the south side of the tower to expose the sunken column bases and steps. However, this trail caused the groundwater table to rise to the south, increasing the inclination by 0.25 degrees, and the tower top to be offset by 20 centimeters.

In 1934 Mussolini felt that a leaning tower was a disgrace to Italy, so he took a more simple and brutal approach: 361 holes were drilled around the tower base and 80 tons of cement were injected. This was originally intended to strengthen the structure of the tower base, but after the cement was poured, the tower tilted more severely.

In addition to the effects of such "maintenance" work, the tower itself has been naturally increasing its inclination, which is estimated to increase by an average of 5 arcseconds (1 degree = 3600 arcseconds) per year. Physicists had calculated, based on the tower's weight and height, that the tower would collapse if it tilted more than 5.44 degrees.

For a long time, the bell at the top of the Leaning Tower of Pisa was forbidden to ring for fear of affecting the stability of the clock tower as a whole (Credit: Lonewolf1976). In 1990, the leaning tower of Pisa reached an inclination of 5.5 degrees. From the calculation results, the tower is already a veritable "dangerous building." Meanwhile, another portentous incident sent the government into a panic: the collapse in 1989 of the 12th-century Civic Tower of Pavia, whose masonry resembles the Leaning Tower of Pisa. After all these considerations, the government finally decided to close the Leaning Tower of Pisa to the public, and the task of saving the tower fell to a special committee.

The Commission's programme consists of two parts: interim stabilization measures and final stabilization measures.

As a temporary stabilization measure, the construction team added a ring of steel "waist" to the first and second floors of the Leaning Tower of Pisa, and laid a ring of concrete on the bottom floor to stabilize the internal structure of the Leaning Tower of Pisa. After that, they piled 600 tons of lead blocks on the north side of the tower as counterweight, hoping to "press" the tower down with this weight.

The effect was surprising. In July 1994, the tower's inclination was reduced by 52 arcseconds-the first time the tower was "straight" back.

But it's not a permanent solution to surround the beautiful white tower with lead. In order to replace the lead block, the committee decided to arrange 10 anchor rods, which were buried more than 40 meters underground. But to bury long anchor rods, groundwater must be controlled. To that end, the committee came up with a "brilliant" idea: freezing groundwater with liquid nitrogen in preparation for installing anchor rods.

But they missed an important point: water changes volume when it freezes. When the groundwater thaws, the tower begins to tilt southward at a rate of 3 to 4 arcseconds per day, increasing the tilt angle in one day directly close to the amount of the previous year. This scared everyone and the plan stopped immediately. Also known as the "Black September" of the Leaning Tower of Pisa, the lead block was not reduced but increased to 900 tons to stop the leaning trend.

The Leaning Tower of Pisa, with its lead blocks added in 1998, is a bit ugly indeed (Credit: Rolf Gebhardt). Like all storylines, after so many twists and turns, the Leaning Tower of Pisa finally ushered in its "fateful moment." This time, the committee brought in John Burland, professor emeritus at Imperial College. Bolan did not choose to use force to "break" the tower back, but decided to dig some soil from the north of the tower, hoping to rely on the tower's own gravity to make the tower straight back.

Starting in 1998, the engineering team carefully drilled several holes in the ground north of the Leaning Tower of Pisa. In 1999, after deploying more than a hundred sensors to detect changes in the tower's body, the team began work carefully. To minimize disruption to the tower, the team removed only a few dozen kilograms of soil per day.

Berland's engineering schematic (Photo credit: John B. Burland, 2002) This was unexpected and surprisingly easy to use. By the time of completion in 2001, 77 tons of soil had been removed from the north side, and the tower had been righted by nearly 40 centimeters, returning it to its 1838 level.

With the help of this construction, Bolan discovered the reason for the tower's continuous inclination. He found that the water table fluctuated dramatically with the seasons. Every year during the rainy season, the water level on the north side rises, causing the tower to tilt southward. To solve this problem, Boland introduced a new drainage system on the north side of the tower, isolating the groundwater.

Finally, the Leaning Tower of Pisa is no longer a dilapidated building. Today, the leaning tower of Pisa tilts slightly less than 4 degrees, and since 2001, the tower has stood upright by about 4.8 centimeters. The committee's team of geotechnical engineers had just reassessed the condition of the Leaning Tower of Pisa last December and were satisfied with its condition: "Considering that this is an 850-year-old patient, leaning about 5 metres and sinking more than 3 metres, the Leaning Tower of Pisa is in excellent health. "

One can still take classic tourist photos with the Leaning Tower, and probably for a long time to come (Credit: Rp2006). Thus, after many wars, large and small, and at least four major earthquakes, the Leaning Tower of Pisa still leans on the Piazza del Marvel. Sadly, the soft soil that caused the tower to tilt may have protected it from earthquakes. A 2019 study assessed for the first time the excellent seismic performance of the Leaning Tower of Pisa. Logically, a moderate earthquake would be enough to destroy such a "fragile" leaning tower. However, the height and stiffness of the tower combined with the soft soil of the foundation constitute a soil-structure dynamic interaction (DSSI), so that the tower body will not resonate with the ground when an earthquake occurs.

After that, just as it slowly tilted, the leaning tower of pisa is slowly "straightening" back. However, this speed was very slow. Even if this trend could continue, it would still take thousands of years for it to completely return. Bolan himself was very satisfied with the result. After all,"there are two results of this project that we absolutely cannot accept. One of them is that the tower is completely straight back."

Your life: Most efforts are just making things worse until that fateful choice comes along.

It's also your life: everything's wrong, but somehow it goes on.

Reference link:

https://www.pbs.org/wgbh/nova/pisa/interventions.html

https://www.geological-digressions.com/the-leaning-tower-of-pisa/

https://www.geoengineer.org/education/web-class-projects/ce-179-geosystems-engineering-design/assignments/the-tilt-of-the-tower-of-pisa-why-and-how

https://www.youtube.com/watch? v=HFqf6aKdOC0

https://www.nature.com/articles/news000914-2

https://www.ndtv.com/feature/leaning-tower-of-pisa-stands-a-little-upright-now-loses-a-tiny-bit-of-its-landmark-tilt-report-3631053

https://www.towerofpisa.org/leaning-tower-of-pisa-facts/

https://www.scientificamerican.com/article/leaning-tower-of-pisa-corrects-itself-a-little/

This article comes from Weixin Official Accounts: Global Science (ID: huanqiukexue)

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