A Foundation on Marshy Land
The Leaning Tower of Pisa began tilting almost immediately after its construction started in August 1173. The source of the centuries-long problem lies directly beneath it. The city of Pisa sits on soft, unstable ground of clay, sand, and marine deposits from the Arno and Serchio rivers. The name "Pisa" is even thought to come from a Greek word for "marshy land". The tower's foundation, a shallow ring only three meters deep, was entirely inadequate for the 14,500-tonne marble structure it was meant to support.
The ground consists of three distinct layers. The top ten meters are a mix of sandy and clayey silts. Below that, down to 40 meters, is a layer of soft marine clay known as the Pancone, the layer responsible for the lean. A final layer of dense sand exists below 40 meters, but the tower's foundation never came close to reaching this stable ground. The soil under the tower's south side is more compressible than under the north. As the weight of the tower increased, the south side began to sink faster, causing the differential settlement that resulted in the now-famous lean. By the time construction reached the third story in 1178, the tilt was already noticeable, and work was halted for nearly a century.
The Underexcavation Solution
By 1990, the tower's lean had reached a perilous 5.5 degrees from the vertical, with the top overhanging the base by 4.5 meters. Fearing an imminent collapse, the Italian government closed the tower to the public and assembled a commission of engineers, led by British expert John Burland, to stabilize it. After years of analysis, the team rejected invasive methods like propping or grouting that could damage the fragile masonry. Instead, they chose a delicate technique called soil extraction, or "underexcavation."
The plan involved removing small, controlled amounts of soil from beneath the foundation on the north side, the high side. The principle was simple: by creating small cavities under the elevated end, the tower's own immense weight would cause it to settle gently back toward the north, reducing the overall inclination. As a preliminary measure in 1993, engineers placed 600 tonnes of lead counterweights on the north side to temporarily reduce the overturning force.
The main intervention began in 1999. Forty-one steel tubes were drilled at an angle into the ground, stopping just under the foundation. A rotating auger inside each tube carefully extracted soil. Over the next two years, the team removed a total of 38 cubic meters (about 70 metric tons) of earth. The process was monitored constantly with a vast array of sensors. The tower responded exactly as predicted. By the time the project concluded in 2001, the lean had been reduced by 45 centimeters, returning it to its 1838 position. The inclination was corrected from 5.5 degrees to 3.97 degrees. Engineers declared the structure safe for at least another 200 years.