Where So Many Others Have Failed
Then in 1989 another smaller and straighter bell tower in the nearby city of
Pravis collapsed into an 800 year old pile of rubble. The collapse killed four
people and served as a warning bell to Italy's Prime Minister Giulio Andreaotti.
He closed the Tower of Pisa to visitors and, in March of 1990 assembled a crack
team of engineers, geotechnical experts, architects, and historians to hash out,
for once and for all, the Tower of Pisa's stability problem. The 14-member team
worked tirelessly. They battled innumerous public outcries, averted near catastrophe,
negotiated miles of red tape, resolved funding crises, risked their careers,
and took criticism from all quarters. Finally, eleven years and $30 million later,
the tower was returned to a safe angle. Not straight, but safe.
For the engineering enthusiasts, the behavior of the tower over the centuries
was analyzed by using a suite of finite element geotechnical computer programs.
Remarkable agreement between computer prediction and actual records was achieved.
A two-phase approach was adopted, first temporary stabilization and then a longer
term solution. First the integrity of the masonry was tackled by binding lightly
pre-stressed plastic covered steel tendons around the tower at the first cornice
and at intervals up the second story. This was done back in 1992. Then a pre-stressed
concrete ring was cast around the base of the tower at plinth level. This ring
acts as a base for supporting specially cast lead ingots placed one-at-a-time
at suitable time intervals. The first was placed in 1993 and subsequently the
stability of the tower was improved.
The second phase solution again invoked heated debate, but finally the supervising
committee opted for a "soft solution". This involved reducing the inclination
of the tower by up to half a degree by induced subsidence under the north side
of the tower foundation, yet without touching the structure. The technique, known
as "soil extraction", involves removing small volumes of soil from
the sandy silt formation by inclined drilling. The technique was in fact first
used in 1832. This work on the tower commenced in 1999 and so far the results
are encouraging. However, the tower will have to be carefully monitored and work
applied on a continuing but intermittent basis.
Making the tower straight would in fact have been an easier task than maintaining
it at merely a safe angle, but to that idea the people rebelled. What had been
known as the Tower of Pisa had come to be known as the Leaning Tower of Pisa,
as if that had been the intention of its designers all along. The underlying
construction mistake had been incorporated into the very identity of the structure.
Without a lean, there was no Leaning Tower. Thus, it would stay.
On December 14, 2001, 828 years after its inception, The Leaning Tower of Pisa
was declared safe. On that day it was reopened to the public with a promise that
this fix, though not permanent, would buy the tower another 300 years. After
that, it would be up to some future project team to take over its salvation.
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Not
On My Watch
