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On June 16 2001, the Leaning Tower of Pisa in Italy was
officially reopened during a colorful ceremony that
coincided with the feast of Saint Renieri, Pisa's patron
saint.
After ten years of tower
rehabilitation, its ancient key was ceremonially handed
back to the church guardians, who climbed the tower's
297 steps. This marked the start of two days of
celebrations that included a candle-lit procession along
the banks of the River Arno and an open-air concert by
tenor Andrea Bocelli.
The tower will not open to
tourists until November 2001, and even then the number
of visitors will be limited to between 25 and 30 at a
time.
The $30 million project to
stabilize the 12th-century bell tower is being hailed as
one of the great engineering feats of all time, making
the 200-foot- (60-meter-) high tower safe for the next
300 years.
Two years ago, specialists
came up with the solution to slowly remove soil from the
north side of the tower's foundation so that it would
right itself.
This procedure reduced the
lean by 20 inches (50 centimeters) as measured in a
horizontal plane at the seventh cornice. This returns
the tower to the lean it had in 1838.
John Burland, professor of
soil mechanics at Imperial College in London, devised
the soil removal system. Polish-born, Milan-based
engineer, Michele Jamiolkowski, headed the rescue team
through a number of government changes and constant
criticism.
In the 11th century, church leaders in Pisa decided to
place three buildings in the Campo dei Miracoli, or
Field of Miracles. These buildings were the Duomo, the
magnificent cathedral, the circular Baptistery, and the
bell tower itself.
The foundation stones for
the Tower of Pisa were laid in 1173. Constructed mainly
of marble and lime, the tower was built in a circular
ditch, about five feet (1.5 meters) deep, on ground
consisting of clay, fine sand, and shells.
In 1178, with only three
stories of the tower built, work stopped because of
politics and debt, but the tilt toward the south was
already evident. If work had continued before allowing
underlying soils to settle, the tower would certainly
have collapsed.
Work recommenced in 1272
and, to compensate for the lean to the south, heavier
building materials were added to its north side. By
1278, when workers reached the seventh cornice, work
stopped again with the tower tilting to the south by
about one degree, or roughly 2.5 feet (0.8 meters).
Work began on the bell
chamber at the top of the tower in 1360. To compensate
for the southward lean, workers added six steps from the
seventh cornice up to the bell chamber's floor on the
south side, and only four steps on the north side. In
1370 the tower, eight stories and 200 feet (60 meters)
high, was officially completed.
The cause of the lean is
the composite of clay, fine sand, and shells the tower
is built on, which is more compressible on the south
side. Over the years as the tilt increased, the tower
stopped sinking and began to rotate, the north side
moving up toward the surface.
Finally, in 1838, the
architect Alessandro Della Gherardesca dug a walkway
called the catino around the base of the tower to
expose the buried foundation steps and column plinths.
Because the catino was below the water table on the
south side, the excavation triggered an inrush of water,
with a subsequent half-degree increase in the lean.
Based on theodolite
readings since 1911, the tower would have fallen by 2050
if not sooner.
How has it remained
standing? Before remedial work began, the tower was
leaning so far that a computer model could not replicate
the actual position (5.5 degrees off perpendicular)
because the model collapsed at 5.44 degrees.
Burland, who has worked on
similar projects such as the Metropolitan Cathedral in
Mexico City and the Big Ben clock tower in London, said
that one of the reasons it had not fallen was its sheer
mass; it weighs 16,000 tons (14,500 tonnes).
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