Experts are now busy implementing an innovative technique for preventing the leaning tower of Pisa from going overboard
FOR almost 800 years, the Bell tower of
Pisa's cathedral has been tilted a wee-bit
towards south and for almost as long,
engineers, far and near, have tried to
stop the tower from what it does best -
leaning. In 1989, however, when a similar tower fell down, killing five people,
Italian authorities decided that the leading tourist attraction can soon become a
death-trap. They shut off public access
to the tower and appointed an international commission to restructure it.
The first such commission, way
back in 1292, tried to hold the tower in
place by building arches and extra staircases on the sinking side. The structure
ended up being highly disfigured -
almost resembling a giant marble
'banana. The tower, since then, has
leaned more than five metres from the
vertical. The tower is built on an uneven
base of mud, clay and sandy soil and
leans south because that's where the
soft, muddy layer is at its deepest.
The first thing the current commission did was to bind the tower in a
corset - a series of steel hoops to
s d the enormous strain entirely through the first and the second stories.
Then a concrete ring was built around
the base of the tower. Then 750 tonnes
of lead ingots were fastened around the
southern side to make it firmer and
arrest the leaning. The method was
initially successful, bringing the tower
back in line by a few centimeters, but
the method was not very pleasing
aesthetically, to say the least.
Moreover, the remedy was, at best,
temporary. Another idea was then
implemented, based on a new technology - discovered in Japan in the
aftermath of the Mexico city earthquake
in 1994 - to keep unstable buildings
from falling over. The theory was Put to
test on a smaller model, where unstable
ground conditions were simulated by
applying external pressure. Karl 0
Heineger, the person- in-charge for the
trials, says, "The purpose of all this
restoration is not to straighten it (the
tower) up, but to prevent it from further
leaning." Using a technique called
'under excavation', he proposed laying
some clay planes at specific points
under the northern side of the foundation. The clay would subsequently compress, making the north side sink a little
and even up the tilt.
The drilling was stage one - then
something was needed to hold the
tower in place. The team has already
started work to secure the tower to a
hook nailed 50 m below the surface.
Ten anchors of 100-tonne capacity
will be attached to a ring of stressed
concrete running around the tower's
foundation. Once that is done, the ugly
lead ingots will be removed and after a
survey, the tower will again be opened
to the public.
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