The historical symbol of a massive construction error, the Leaning Tower of Pisa, has a unique property. In its 850 years of existence, it has suffered four major and countless small earthquakes, but it still stands still. The answer to the question "Why?" was looking for an international team of 16 engineers and architectural historians.
When the second floor of the tower was built in 1178, its creators already realized that they were wrong. A weak foundation and soft soil gave a deviation of the axis of the structure from the vertical by 5 degrees, but then this was considered insignificant. The foundation was strengthened and continued to be built with interruptions for another 200 years. Today, with a height of 56 m, the top of the tower is deviated from the vertical by 5 m and, until the restoration work in 2008, it “fell” by 1 mm per year. But for half a millennium it did not fall.
Researchers have collected information on a wide range of parameters: seismic activity, structural features of the structure, geotechnical data, soil and groundwater composition. And they came to the conclusion that the Leaning Tower of Pisa is indeed a wonder of the world. A unique combination of factors that could hardly have been implemented on purpose.
The soft ground that caused the tower to tilt initially is so different from the rigid frame of a stone tower that they do not interact with each other. And with the tremors of the earth's crust, vibrations are hardly transmitted to the tower through the soft earth, so there is no resonance in it. And since at the beginning of construction, to correct the error, the structure was strengthened in every possible way, then small loads are practically not afraid of it.