The Leaning Tower of Pisa's historical tilt originated from its initial construction on uneven sediment deposits.

• A river in Italy deposited sand and clay unevenly in the area where Pisa now stands, creating unstable ground.
• Construction of the tower began around 1173, and its tilt became apparent early on, leading builders to compensate during construction.
• The tower's inclination reached 4.9 degrees by 1817 and continued to increase until modern times.

By 1990, the tower's tilt had reached a critical point, prompting the Italian Government to seek a stabilization solution.

• The tilt reached a peak in 1990, leading to the closure of the tower and the formation of an expert committee.
• The committee conducted extensive testing, including soil analysis and computer modeling, to assess the tower's stability.
• Immediate stabilization measures included a concrete ring and lead counterweights to halt the increasing lean.

Temporary stabilization measures were successful, but a later anchoring plan resulted in increased tilt.

• The lead counterweights initially reduced the tilt, which encouraged the committee to consider deeper anchoring.
• The anchoring plan was abandoned when the tower began to tilt further due to structural dependencies on the surrounding walkway.
• Additional lead weights were added to counteract the renewed tilt, and focus shifted to a permanent solution.

The committee faced challenges in devising a permanent solution that would preserve the tower's historical appearance.

• Solutions had to consider historical preservation and were complicated by the need to maintain a specific amount of lean.
• Ideas included groundwater removal and electro-osmosis, but these were rejected due to unpredictability and soil conductivity.
• The final idea, underexcavation, involved removing soil beneath the tower to correct the tilt.

Underexcavation was tested and implemented, resulting in a significant reduction of the tower's tilt.

• After testing on a model, underexcavation began with a safeguard system of cable stays to control unwanted movement.
• A preliminary trial showed positive results, and the final plan involved 41 holes and the removal of 38 cubic meters of soil.
• The tower's tilt was reduced by about half a degree, and the lead counterweights and drainage system were adjusted accordingly.

The presenter reflects on the ingenuity of the stabilization project and recommends educational content from independent creators.

• The precision of the underexcavation process is highlighted as a remarkable engineering feat.
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• Nebula, an ad-free streaming platform for independent educational creators, is promoted as a source of in-depth content.