The Leaning Tower of Pisa is not just some cranky Disneyland tourist attraction. It is an
architectural gem and would be one of the most important monuments of medieval Europe even
if it were not leaning. Standing in the Piazza dei Miracoli, it is part of the complex of
four major gleaming white medieval buildings comprising: the Cathedral (Duomo), its bell
tower (the Leaning Tower), its Baptistry and the Cemetery (Camposanto).
As with the other buildings in the Piazza, the bell tower was intended to represent the
civic pride and glory of the wealthy city state of Pisa and as such it is beautiful, unique
and enigmatic. In 1990 the Tower was closed to the public because of fears for its safety
and in the same year a Commission was established by the Italian Prime Minister to
implement stabilization measures. There can be no doubt about the importance of such an
operation to Pisa, to Italy and to World Heritage.
History of construction:[BR][BR]Construction of the Tower began in August 1173. By about 1178 it had progressed to one
quarter the way up the fourth storey and then the work stopped. The reason for stopping is
not known, but had the construction continued much further the soil in Layer B would not
have been strong enough to carry the load and the Tower would have fallen over.
Work recommenced nearly 100 years later in about 1272, by which time the strength of the
clay had increased due to consolidation under the weight of the Tower (although this would
not have been known). By about 1278 construction had reached the seventh level when work
again stopped. There can be no doubt that, had the Tower been completed at this stage, it
would have fallen over. In about 1360, when further consolidation of the underlying clay
had taken place, work on the bell chamber commenced and was completed in about 1370 –
nearly two hundred years after commencement of construction.
Another important detail is that in 1838 the architect Alessandro della Gherardesca
excavated a walk-way (catino) around the base of the Tower so as to reveal the column
plinths and foundation steps, as was originally intended. The result of this was an inrush
of water on the south side, since here the excavation is below the water table, and there
is evidence to suggest that the inclination of the Tower increased by more than a quarter
of a degree at this time.
In 1995 it was found that Gherardesca had placed a 0.7 m thick
ring of concrete in the floor of the catino.
History of tilting:[BR][BR]The vertical axis of the Tower is not straight – it bends to the north. In an attempt to
correct the lean, tapered blocks of masonry were placed at the level of each floor to bend
the axis of the Tower away from the lean. Careful analysis of the relative inclinations of
the masonry layers has revealed the history of the tilting of the Tower.At the end of the
first phase the Tower was actually leaning northwards by about one quarter of a degree.
Then as construction advanced above the fourth storey it began to move towards the south
and accelerate so that by 1278, when the seventh level had been reached, it was inclining
southwards by about 0.6 of a degree. This had increased to about 1.6 degrees by 1360 when
work on the bell chamber commenced.
In 1817 two British architects used a plumb line to measure the inclination, which by then
was 5 degrees. Thus the construction of the bell chamber caused a very significant increase
in inclination. Advanced computer analysis has revealed that the rapid increase in
inclination as the seventh level was reached and the bell chamber was added is directly
analogous to constructing a tower from model bricks on a soft carpet. It is possible to
build to a certain critical height, but no higher, however careful one is – a phenomenon
known as leaning instability. The Tower was just at its critical height and was very close
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