Why Pisa’s Tower Leans
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The Leaning Tower of Pisa is over 800 years old. Construction started on it in 1173 and, almost two centuries later (and after two work stoppages), completed in the 1370s. For history’s sake, it is a good thing that there were two work stoppages — otherwise, the structure probably would have collapsed long ago. The Tower is built on, basically, a marsh — a thin layer of alluvial silt atop soft marine clay. The work stoppages gave time for the existing incomplete infrastructure to settle in the soft soil, compacting the ground beneath it and, effectively, letting Mother Earth adapt before construction resumed. A minor miracle, in and of itself.
It therefore makes sense that the Tower is unstable, likely to shift its position any which way as time passes. But the mystery runs deeper. The Leaning Tower of Pisa only leans to the south. Assuming the unstable ground beneath is uniformly so, that did not make a lot of sense.
In 1989, the Tower was leaning roughly 15 feet off its center, when disaster struck — in Milan. Another tower, the Civic Tower of Pavia, crumbled, killing four passersby. Fearing a similar fate for Pisa, the Italian government closed the tower and made a $40 million grant to researchers, hoping to stave off the Tower’s tilt into destruction. The task was daunting, as researchers were split — some believed the problem was with the underlying land; others believed the problem was with the structure itself. Redoubling these problems were the art historians, who insisted on preserving the artistic value of the Tower; in short, propping the Tower up was a non-starter.
Finding a solution took four different tries and almost a decade. In the end, the team used something called “soil extraction” — as described by the Telegraph (UK), this meant “drilling out slivers of soil from beneath the northern side of the tower – away from the lean – and allowing gravity to coax the structure back upright.” It was a nuanced task in and of itself. Professor John Burland, a soil engineer at the Imperial College London, lead the efforts with the help of 120 sensors placed in, around, and outside the Tower, and an army of fax machines. For two years, twice a day, the sensor readings were faxed to his attention; he’d run some calculations and inform the drillers how to continue. The Tower re-opened in late 2001.
In capturing all these data points — two a day for two years — Burland had in front of him the answer to the previously unanswerable question of the Tower’s southward tilt. In 2003, he figured it out. The water table on the north side of the Tower rose up to a foot higher at times during Pisa’s rainy season. In effect, the water tower slowly, slightly, and temporarily boosted the land on the north side, pushing the Tower to lean south. Using a custom drainage system to normalize the water table, Burland has stemmed the Tower’s further tilt entirely — since the solution was put in place seven years ago, the Tower’s tilt has remained the same.
Bonus fact: Groundwater flows can prevent problems, too. In 1958, the town of Concord, Massachusetts opened a landfill adjacent to Walden Pond, the eponymous site of Henry David Thoreau’s On Walden Pond. Because the groundwater table ran from the pond to the landfill (and not vice versa), the pond remained relatively unpolluted by the nearby refuse.
From the Archives: Patent Leather Shoes: How Michael Jackson managed to pull off an incredible, gravity-defying lean of his own.
Related: A 3-d, Leaning Tower of Pisa puzzle.
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