When tilt becomes disaster
Most buildings that start to lean collapse within a few years. Houses subside, old walls bow outward, factory chimneys tip and fall. The famous leaning towers are famous precisely because they are the exceptions. They have been tilting for centuries and have not, yet, fallen over. Understanding why is partly a structural story and partly a story about geology, time, and human stubbornness.
Building Guessr includes several leaners in its database, and they are often the hardest buildings to guess because their silhouette does not look like anything else. This article walks through what keeps them up and what almost took them down.
The story at Pisa
The Leaning Tower of Pisa began settling almost as soon as construction started in 1173. The bell tower for the cathedral next door was planned to stand 60 meters tall on foundations only three meters deep, on soft ground near an ancient estuary. By the time the third of its eight stories was complete, around 1178, the south side had already begun to sink. Work halted; it would not resume until nearly a century later, by which time the soil had compacted enough that construction could continue.
Medieval builders then did something that looks comical until you realize it saved the tower: they built the upper stories with the floors slightly angled to counter the lean, effectively making a banana-shaped building that leans less at the top than at the bottom. If you stand at the base and look up, the tower curves. This accidental compensation kept the center of gravity closer to over the foundations than it would otherwise have been.
The tower was finally completed around 1372. At that point it leaned about one degree. Over the next six centuries, the lean grew to nearly 5.5 degrees, putting the top about 4.5 meters off vertical, and the rate of movement accelerated. By the 1990s, engineers calculated that the tower was months or years from toppling.
Soil: the silent culprit
Pisa's problem was never the stone. The tower itself is well-built marble from the local quarries. The problem was the ground underneath: a complex mix of soft clay, sand, and shell, deposited by the Arno River over thousands of years and unevenly compacted. Pockets of softer material under the south side compressed faster than denser material under the north side. The foundations, only a few meters deep, could not bridge across that difference.
This is the general pattern for leaners. The Suurhusen Church tower in Germany, which officially passed Pisa as the most-tilted tower in the 2000s at 5.19 degrees (Pisa itself has since been corrected downward, but the measurement competition continues), sits on oak foundation beams driven into boggy soil. When the marshland was drained in the nineteenth century for farming, the oak on the drier south side rotted faster than the waterlogged oak on the north. The tower tipped. Two towers in Bologna, the Garisenda and Asinelli, lean because their medieval foundations shifted on the city's alluvial soil; the Garisenda is currently cordoned off and being stabilized because its lean is now accelerating.
The center of gravity test
A leaning building stays up as long as a vertical line dropped from its center of gravity still falls inside its base. That is the entire stability test in one sentence. Pisa survives because its center of gravity, even at 5.5 degrees of tilt, is still over the foundation footprint, though just barely. If the lean kept growing at the rate it was accelerating in the 1980s, the center of gravity would have crossed the base edge sometime in the early 2000s and the tower would have rotated through that axis and fallen.
Wind matters too. Taller leaners have to survive gusts that push the center of gravity further in the direction of tilt, temporarily. Engineers at Pisa had to calculate not just the static stability but the worst-case dynamic stability during storms, and they were cutting it closer than anyone wanted.
Saving Pisa in the 1990s
Between 1990 and 2001, the Pisa tower was stabilized in a project led by British engineer John Burland. The approach was counterintuitive: instead of stiffening the ground under the south side (which had been tried in the 1930s with failed results), they removed soil from under the north side. By drilling out small amounts of clay on the high side, they let the tower settle slightly northward, reducing the tilt by about 45 centimeters at the top. The work was done so carefully that the change is invisible to the casual visitor, but it bought the tower at least another two or three centuries of stability.
The tower was reopened to the public in 2001 and continues to be monitored. It moves a few millimeters per year, sometimes forward, sometimes backward, and is now considered stable for the foreseeable future.
Suurhusen and other quiet leaners
Not every leaner gets the Pisa treatment. Suurhusen Church in northwest Germany was saved by concrete reinforcement of its foundations in 1975 after nearly fifty years of being closed for safety. Bologna's Garisenda is currently being wrapped in steel cables as a temporary measure while a long-term plan is developed. The Nevyansk Tower in Russia, which leans at about three degrees, is said to have been built deliberately crooked as an eccentric stylistic choice, though the evidence is mixed and subsidence is the more likely explanation.
Chicago's old Leaning Tower of Niles, a half-scale replica of Pisa built as a water tower in 1934, leans because the builder wanted it to. That one is safe.
Modern engineering and soft soils
You might wonder why modern buildings on soft soil do not lean. The answer is that they do, but by tiny amounts, and the engineering catches it. Mexico City's Metropolitan Cathedral has been gradually sinking into the ancient lake bed beneath it since the sixteenth century, and in recent decades the Mexican government has injected concrete under parts of the foundation to slow uneven settlement. The leaning Santa Maria del Carmine in Naples, the bell tower of Siena Cathedral, and parts of the Tower of London all have monitoring instruments embedded in their walls.
For newer supertall towers in Dubai, Shanghai, and Taipei, engineers design the foundations first and the building second. Burj Khalifa's foundation is a single giant concrete slab over 192 piles driven 50 meters into the ground. Shanghai Tower uses a mat foundation on thousands of piles. The lesson of Pisa has been thoroughly learned: do the ground work before you start stacking stones.
Keep looking up
Leaning towers are worth seeing in person at least once. They give you a visceral sense of how close some old buildings have come to failure and how hard it is to save them once trouble starts. For a related read on buildings that did not survive, see ten famous lost buildings. For more on castles and other stone medieval structures that share the same foundation problems, try castles: myth vs reality.
Can you place a leaner on the map from a single photo?
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