Why does the Leaning Tower of Pisa lean?

Find out how the tower was made and how it went wrong…

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Leaning Tower of Pisa

The local architects and city officials designed the complex at Piazza dei Miracoli (the Square of Miracles) as a dedication to art, and as such it is thought the principles of science and engineering were not fully understood.

The tower was built in three stages over a period stretching nearly two centuries. The first part of the tower was built during a time of town prosperity and as such heavy white marble was used for the base and tower, with limestone used for the interior and exterior design features.

Disaster occurred just five years after work began, as the workers finalised the interior of the third floor. The tower was sinking because the weight of the marble building was too much for the extremely insufficient three-metre foundations which had been set in weak and unstable soil that contained a malleable mixture of clay, sand and rubble. The construction was halted for nearly a century to allow the soil to settle. In 1271 work recommenced as engineers began to build the tower’s middle section. To compensated for the continuing problem of its lean, the workers built one side of the wall taller than the other. Subsequently the tower began to lean in the opposite directions and caused it to curve. War caused a break in construction and the seventh floor was not completed until 1319 and the eight level, featuring the belfry, was finally added in 1372.

In 1964 a desperate Italian Government requested aid to stop the tower from toppling. One of the first methods to be tested was to add 800 tons of lead counterweights to the raised end of the base, but this only added to its subsidence. With the problem worsening it was decided to close the tower in 1990 and remove the bells to relive some of the weight.

Cables were cinched around the third level and grounded several hundred metres away to anchor the weight. Work began on removing some 38 cubic tons of soil from under the raised end of the base, which straightened the tower by 18 inches – regaining an angle last recorded in 1838. Ten years of corrective stabilisation followed and the tower reopened to the public in 2001. In 2008 another 70 tons of Earth was excavated and for the first time the structure has officially stopped moving.

Image credit: Johann H. Addicks

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  • Engineer’s Digest

    Great piece of information. An enlightening read underscoring the significance of good engineering knowledge.