The fact that the Leaning Tower of Pisa, Italy (it. La Torre
di Pisa) is still standing eight hundred years after commencement
of its construction, is remarkable. The 58 m (191 ft.) structure leans off
to the south at an inclination of approximately 5.5 degrees. This means
that the seventh (top) cornice protrudes an astonishing 4.5 meters
(14.8 ft.) over the first cornice. Indeed, since its construction the
tower has been at an ever increasing risk of either buckling,
or tipping over.
The real history of the tower starts a few thousand years before
its construction; actually even before urbanization of the area that
would later become the city of Pisa. Geographical surveys have shown
that between the second half of the third millennium, and the VIII
century B.C.. (when the earliest settlers moved in), a river ran
through the area where the Cathedral Square and adjoining tower would be
built. Over time, the river bed moved eastwards, leaving a loose soil of
soft river sediments. This unsturdy foundation is the primary
reason of the inclination of the Tower.
In 1172, the widow Berta of Bernardo leaves 60 coins in her will to
the Santa Maria institution to purchase stones for a tower; a campanile (bell tower) associated with the cathedral. One year
later, construction commences. It is not clear who the original
architect of the tower is; classical texts identify Bonanno Pisano as
the architect, although recent studies show that either Diotisalvi or
Biduino are more likely candidates. However, what is known is the
initial construction date; August 9, 1173, as evidenced by the
inscription in the foundation: A.D. MCLXXIV. CAMPANILE HOC FUIT FUNDATUM
MENSE AUGUSTI (note that the historic Pisan calendar system differs one
year from our current calendar system). The foundation of the tower
was built in a circular ditch, about five feet deep, on ground
consisting of clay, fine sand and shells. The tower itself
was constructed from marble, lime and other stones.
Construction of the tower was halted several times. The first halt
was already in 1178, due to a war with Florence. During that time, the
tower was 10.6 m. (35 ft.) tall, and consisted of three stories. At this
height, the incline of the tower was already apparent to the
constructors. A second war with Florence in 1185 added another
interruption to the construction schedule, which would last until 1272.
In 1284, construction was halted again; this time due to a war with
Genoa. These unscheduled stops are the main reason the tower still
exists today. Modern analysis has revealed that the interruptions
allowed the underlying soils to consolidate. Had this not happened
the tower would certainly have toppled.
Over time, the tower began to lean over at an ever increasing angle.
Initially, the incline was only 0.2 degrees, but in 1278 the incline was
already 1 degree, with an increase to 1.6 degrees during the following
90 years.
Around 1370, the tower is officially completed. Seven bells are added
to the tower, and in 1655 the largest bell is installed, weighing around
three and a half ton. Without doubt, this further adds to the
inclination of the tower. Nowadays, the bells are silenced, since they
could easily trigger a collapse of the tower. In 1817, the incline is
measured by two British architects. The tower now leans over at around 5
degrees.
In 1838, the architect Alessandro Della Gherardesca nearly
kills the tower by digging a trench (the catino)
around the base of the tower. The architect's intention was to expose
the foundation and bottom of the columns to the public. In a matter of
days, the tower leans over an additional 0.5 degrees (approximately 1 m,
or 3 ft.). Since the trench was below the water table, it triggered an
inrush of water on the south side, resulting in additional instability.
In 1839, a scientific conference commemorates Galileo Galilei, with
the installation of a memorial stone that reads: "From the top of this
Tower, Galileo used to do his experiments on the fall of heavy
bodies.". Although Galileo lived and worked in Pisa for several
years, this probably never happened.
Serious efforts begin in the early 20th century to measure the
inclination and its rate of rotation, and plans are proposed to halt the
tower's potential collapse. However, those initial attempts have adverse
effects. In 1934, engineers drill holes in the foundation, and inject
grout to strengthen the masonry. The tower immediately leans over an
additional 31 arc seconds. Additional drilling in the soil
and masonry in 1966 and 1985 further worsen the situation. In 1990, the
government officially closes the Leaning Tower of Pisa.
In the nineties, several more scientifically sound efforts are
undertaken to stop a further deterioration. But these efforts have a
mixed success due to technical complexities, and also political
obstacles. First, the base of the tower was wrapped with steel belts, to
reduce the danger of buckling. Next, a concrete foundation was
laid down on the north side with several counterweights to tilt the
tower's axis back up. This work has a positive effect on the stability
of the tower. In 1995, engineers try to replace the ugly
counterweights with anchored cables. In order to anchor
the cables, engineers treat the loose soil with liquid nitrogen. The
results of this procedure were disastrous, and the tower was again heading for earth...
In 1996, Engineers evaluated a soil extraction method below the tower's
foundation. Holes were drilled on the north side of the tower, and soil
was removed. This method proved to be the most successful so far, and
further soil extraction was conducted in 1999. The engineering team
managed to reduce the incline angle back to where it was in 1970, and stabilize the inclination. On
June 17, 2001 the tower will be opened again for the public.
http://torre.duomo.pisa.it (official website)
http://www.pbs.org/wgbh/nova/pisa/
http://www.endex.com/gf/buildings/ltpisa/ltpisa.html | 
