History of Interventions
Several times during
its long and storied history, the Leaning Tower has come painfully close to
Built on the shifting sands of a former estuary, the Leaning Tower of Pisa
probably began tilting not long after medieval laborers laid its first
foundation stones. In the 800 years since, a bevy of architects, engineers, and
self-appointed problem-solvers have bent over backwards trying to slow, halt,
or ideally reverse the lean. All efforts have failed - until now. Here we take
a look back at those myriad and often misguided attempts, including the most
recent and promising of all.
The widow Berta of Bernardo leaves 60 "coins" in her will to the Opera
Campanilis Petrarum Sancte Marie to buy stones to build a tower.
On August 9, workers lay the foundation stones for what will become the Leaning
Tower of Pisa. Constructed of marble, lime, and stones, the tower is built in a
circular ditch about five feet deep, on ground consisting of clay, fine sand,
When the tower is just over three stories tall, construction stops for unknown
reasons. Modern analysis reveals that had work continued before allowing
underlying soils to consolidate, the tower would certainly have toppled.
Work recommences nearly a century after the tower was begun, led by Giovanni Di
Simone. Initially the tower leans to the north 0.2 degrees off of vertical, but
by 1278, when workers reach the seventh cornice and construction stops again
(perhaps for military reasons), the tower tilts to the south about one degree,
or roughly 2.7 feet.
Over the next 90 years, the inclination increases to about 1.6 degrees. Work on
the bell chamber begins about this time. Clearly a southward lean is already
apparent, as workers seeking to correct it add six steps from the seventh
cornice up to the bell chamber's floor on the south side, while only adding
four steps on the north side. The tower is officially completed about 1370.
The tower's seven bells
were silenced long ago, for fear their vibrations could trigger a catastrophic
Workers install the largest of the tower's seven bells, a behemoth weighing
Two British architects use a plumb line to determine that the foundation's
inclination is now about five degrees.
The architect Alessandro Della Gherardesca digs out a walkway called the catino
around the base of the tower, to make the formerly buried foundation steps and
column plinths visible.
Measurements made by the Frenchman Ruhault de Fleury reveal that Gherardesca's
1838 excavation led to a serious increase in inclination. Since the catino lies
below the water table on the south side, the excavation triggered an inrush of
water there, along with a possible half a degree increase in the lean.
Precise measurements of the tower's inclination commence using a theodolite.
These measurements will be repeated annually from now on, providing a valuable
record of tower movements in this century.
monitoring of the tower's tilt began in 1911, when officials initiated annual
measurements with a theodolite.
Officials add four level stations around the tower's plinth level to help
monitor changes in the inclination.
Engineers install a plumb line and a spirit level to measure both the
north-south and east-west tilt of the tower. They also drill 361 holes into the
masonry foundation and inject 80 tons of grout. The purpose of the grout is to
strengthen the masonry, yet the result is a sudden increase in the tower's tilt
of 31 arc seconds. (Five arc seconds is equivalent to 1.5 millimeters, or a
little over half an inch, of movement at the top of the tower.)
Soil and masonry drilling within and beneath the foundations engenders a modest
but not insignificant increase in the tilt of six arc seconds.
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© | Updated November 2000