Practical Engineering - How Engineers Straightened the Leaning Tower of Pisa

Short Summary

Main recommendation: The transcript does not recommend a product or method for viewers to adopt; its central conclusion is that engineers stabilized the Leaning Tower of Pisa by removing soil from beneath the north side using inclined drilling, a process called underexcavation, which reversed part of the tilt and reset the tower's lean to roughly its early-1800s level.

Key Points

• The tower began leaning as soil under it compressed unevenly; by 1990 its average tilt was about 5.5 degrees, and it was closed to the public over collapse fears.
• A 1993 safety estimate of 1.07 meant the soil could generally withstand about 7 percent more load than the tower imposed, leaving almost no margin.
• Temporary fixes included a concrete ring and about 600 tons of lead counterweights on the north side (later increased by 350 more tonnes); the planned anchor solution caused the tower to tilt the wrong way and was abandoned.
• Electro-osmosis, which uses electric current to drain water from clay, failed because the soil was too conductive.
• Underexcavation was tested first in 1995 and then in a 12-hole trial in 1999 before the final 41-hole program starting in 2000.
• The final project removed about 38 cubic meters (roughly 70 tonnes) of soil over about a year, cut the tilt by about half a degree, and kept some lean to preserve the tower's historical character.

Details Worth Noting

• Concrete instructions: The engineers' method was to drill inclined boreholes beneath the footing, advance a hollow stem auger inside two casings, and retract it so the cavity closes and the soil settles. They tightened adjustable cable stays between the tower and anchoring frames as a safeguard, and controlled the sequence of underexcavation to steer movement. This is a historical engineering method, not advice for viewers.
• The tower's curvature results from builders compensating for the lean during construction, with the south side of the belfry having six stairs and the north side four.
• Historians estimated foundation settlement over time from the construction timeline and the known variation of tilt with height.
• The final work also added a drainage system to control fluctuating groundwater and structurally attached the tower to the Catino walkway to enlarge the foundation area.

Bottom Line

Engineers saved the Leaning Tower of Pisa by carefully removing soil beneath it, deliberately keeping enough of its famous tilt to preserve its historical identity.