Ten arc seconds are added to the tilt after boring both into the masonry
foundations and accidentally into the soil beneath them.
With the tower continuing to tilt naturally at the rate of a little over five
arc seconds per year, the Italian government closes the Leaning Tower of Pisa,
sparking outrage by Pisan officials, who fear the loss of tourist revenue and
the resulting impact on the local economy. The closure was provoked by the
collapse of the civic tower of Pavia in 1989, which raised fears for the safety
of the Pisa tower.
the mid-1990s, lead blocks like these helped pull the tower over half an inch
closer to vertical at the top - a significant amount to engineers.
The Pisa Commission stabilizes the masonry by wrapping plastic-coated steel
tendons around the tower up to the second story. This closes many cracks and
reduces the chance of a buckling collapse.
Workers pour a temporary concrete ring around the base of the tower to serve as
a foundation upon which to lay lead counterweights on the north side. Between
May 1993 and January 1994, crews lay down a series of specially cast lead
ingots. By July 1994, the tower has leaned back toward the north - the desired
direction—a full 52 arc seconds.
After deciding to replace the unsightly lead counterweights with an anchored
cable system, the Pisa Commission begins freezing the ground with liquid
nitrogen in preparation for installing the cables. As soon as the freezing
stops, however, the tower begins to lean south at a rate of four arc seconds
per day. This begins in the middle of a September night, which Commission
members will come to remember as "Black September." The operation is
immediately halted, and the search for a permanent solution continues with new
In March, engineers successfully complete a test of a soil-extraction method to
reduce the tower's lean. The method calls for an inclined drill, which creates
cavities that gently close due to the pressure of the overlying soil. For
various political reasons, however, actual soil extraction does not get
underway for almost three years.
In December engineers install temporary cables, which can be tensioned to
steady the tower if detrimental movements occur during stabilization efforts.
With the help of advanced techniques of intervention, officials will hopefully
leave the Leaning Tower of Pisa leaning safely for centuries to come.
In February, engineers begin a very careful process of soil extraction. Using a
dozen boreholes over a width of only about 18 feet, they remove underlying soil
at an extremely conservative pace of about five gallons every two days. By
mid-June, the tower has leaned back toward the north by 90 arc seconds,
equivalent to about one inch at the top, and it continues to move ever so
slowly in a northward direction. By the end of August the lean had decreased by
130 arc seconds (1.5 inches). The response of the Tower to preliminary soil
extraction has been so positive that on September 14th workers removed three of
the lead weights in anticipation of the eventual, progressive removal all the
ingots during full underexcavation. The next step will be for the Commission to
formally approve a full soil-extraction intervention to permanently stabilize
the tower (see Where It Stands Today).
Timeline compiled by NOVA Online Intern Sarah Ince.
Dr. John Burland