Tower of Pisa: Lessons learned by observation and analysis
The Leaning Tower of Pisa is one of the world’s best known and most treasured monuments; it is founded on highly compressible soils and started leaning since the time of construction. In the 1990s the overhang had reached the value of 4.7 m and was increasing at a rate of 1.5 mm per year; a collapse appeared possible and even imminent, either by overturning or by brittle failure of the masonry. The International Committee appointed by the Italian Government in 1990 conceived and implemented a stabilisation intervention consisting in a small decrease of the inclination by underexcavation. Nowadays the stabilisation of the monument appears to have been attained, as confirmed by subsequent monitoring. A number of monographs and papers report the history of the monument and the progress of its inclination, the studies and investigations carried out since the early 20th century and the interventions of the Committee; these are shortly summarized at the beginning of the present chapter. A number of interventions carried out during the entire life span of the tower are discussed in detail, also taking advantage of advanced numerical analyses; it is shown that the Tower is very sensitive to any small disturbance and that some interventions, carried out with the best intentions, actually had definite detrimental effects. Possible future scenarios are finally briefly discussed.