The Leaning Tower of Pisa
Group 10
Martina Flanagan
Julian Mannion
Anthony McGowan
Neil O’Donohoe
Introduction
While the Leaning Tower of Pisa is an aesthetically and architecturally impressive
building to begin with, there is no doubt that the iconic status of this landmark is
due in no small part to the 5.5° tilt which has earned the tower its name.
In the past it was widely believed that the inclination of the Tower was part of the
project ever since it’s beginning, but now we know that it is not so. The Tower
was designed to be "vertical" (and even if it did not lean it would still be one of
the most remarkable bell towers in Europe), and started to incline during its
construction, due to a poorly laid foundation and loose substrate, which has
allowed the foundation to shift direction.
However, this tilt could have proven to be the cause of the tower’s near downfall.
Indeed it was calculated that, based on theodolite readings taken since 1911, that
tower would have collapsed by 2050 had remedial work not taken place.
The height of the tower is 55.86m from the ground on the lowest side and
56.70m on the highest side. That’s a difference of 0.84m. The width of the walls
at the base is 4.09m and at the top is 2.48m. Its weight is estimated to be
14,500 tonnes. The tower has 294 steps & it leans at an angle of 5.5 degrees.
This means that the top of the tower is 4.5 meters from where it would stand if
the tower was perfectly vertical.
History
The Leaning Tower of Pisa (Torre pendente di Pisa or La Torre di Pisa) is the free
standing bell tower of the Cathedral of the Italian city of Pisa. Located in the
Piazza dei Miracoli, it is part of a complex of four white marble medieval
buildings. Like the other structures in the piazza, the bell tower was built as an
expression of civic pride and was intended to reflect the stature of the wealthy
city-state of Pisa.
The bell tower of Pisa, Italy is one of the most significant monuments of medieval
Europe and a prized example of Romanesque architecture. In 1172, a wealthy
widow named Berta di Bernado, left sixty coins in her will to buy stones to begin
the construction of the tower. The construction began in the August of 1173 and
due to the fact that the people of Pisa were engaged in many wars at this time
the tower wasn’t completed until almost 200 years later. The architect who
created this masterpiece is still uncertain to this day, but they say it was Bonanno
Pisano.
The tower is also an important icon in the development of modern science; it was
from its belfry that Galileo Galilee proved that the velocity of bodies falling under
the action of gravity is independent of their weights. He is believed to have
dropped two canon balls of different mass to the ground below and measured
their velocities.
Problems
The tower began to sink after construction progressed to the third floor in roughly
1178. This was mainly due to mere three-meter foundation, set in weak, unstable
subsoil. This means that the design was wrong from the very start. The
construction was then halted for nearly a century due to battles taking place
nearby. This rest period allowed the underlying soil to settle. Had this not have
happened, the tower would almost certainly have toppled. In 1272, the
construction resumed. In an effort to compensate for the tilt, the engineers built
higher floors with one side taller than the other. This made the tower begin to
lean in the other direction. Because of this, the tower actually curved.
Construction was again halted in1284 when another battle occurred. The seventh
floor was completed in 1319. The bell chamber was not finally added until 1372.
There are seven bells, one for each note of the musical scale. The largest one was
installed in 1665.
The cause of the lean has been well documented and is due mainly to two factors.
The first problem resulted in the building leaning is that the tower began to sink
after construction progressed to the third floor in 1178. This was due to a mere
three-meter foundation, set in the weak unstable river deposits. This means the
design was flawed from the beginning. Construction of the tower was halted due
to a war the Pisans were engaged in after the completion of the third floor. This
allowed time for the underlying soil to settle. Otherwise, the tower would almost
certainly have toppled during the construction of the tower. Had a much wider
and deeper pad foundation been designed and used at the time, the famous lean
may never have occurred.
The second problem is that the tower is built on unsuitable ground for such a
heavy and tall building. The base of the building is only about 2metres above the
water table and built on an old riverbed. The underlying ground is made up of
layers of sand, clay and silt. It consists of three distinct layers. The top layer is
about 10 m thick and consists of variable, soft, silty deposits laid down in shallow
water many thousands of years ago. The second layer, consisting of very soft,
sensitive marine clays laid down up to 30,000 years ago, extends to a depth of 40
m and is laterally very uniform. The third layer is of dense sand and extends to a
considerable depth. The layers are not evenly sorted and the weight of the
building has compressed them. The designers of the time didn’t think the tower
would result in such a large imposed load. Because the layers of deposits are soft
and uneven, as the ground has compressed, it has sunk more in some places
than others. At the moment the lean is towards the south.
Remedial Actions
There have been many efforts over the years to stop the tower from falling. Some
of them have been almost disastrous and very nearly resulted in collapse.
The first attempt to correct the tower’s inclination towards the south actually
occurred during construction. In 1272, when construction recommenced for the
second time, heavier materials were used on the north side as a remedy as well
as slightly taller bricks being used on the south side to compensate for the lean.
When work began on the bell chamber in 1360, workers put six steps from the
seventh cornice to the bell tower on the south side and only four on the north
side.
In 1934 an Italian engineer drilled 361 holes into the base and filled them with
mortar. For a while it worked.
The tower was closed to the public in 1990 for fear it would collapse and in 1993,
a lead counter-weight weighing approximately 600 tonnes has attached to the
northern side in an attempt to correct the tower’s rotation. John Burland, a
professor in soil mechanics in Imperial College London, concluded that correcting
the tower’s lean would lie in removal of soil from beneath the tower.
Unfortunately, extensive testing of the soil beneath the tower would be required
before any removals could take place. A temporary suggestion of 10 anchors
buried 60m into the ground was suggested as a replacement for the weights
while this took place. However, this would require the ground to be frozen to
prevent the ground floor walkway becoming flooded. Everything did not go
according to plan and expansion of the ground water which took place during
freezing caused the tower to lurch southward (the gaps left behind after the
water had melted further weakened the ground). That is the nearest the tower
has come to disaster. In one night the lean increased as much as it normally
increases in two years. They quickly added another 250 tons of lead and decided
to rethink the whole thing.
Finally in 1998, plastic sheathed cables were wrapped around the first floor to
prevent any further movement or cracking and in February 1999, 40 210mm
diameter augers drilling at a 30°angle around the northern base of the tower
began to tentatively remove soil (the weaker sand above the marine clay). The
idea was that when the drill was retracted, the cavity would shut gently and
provide a cushion as the tower re-settled and the clay became more compressed
and firmer.
After a phase of structural strengthening, the tower is currently undergoing
gradual surface restoration, in order to repair visual damage, mostly corrosion
and blackening. These are particularly strong due to the towers age and to its
particular conditions with respect to wind and rain
The lean has now been corrected by 50cm bringing it back to its 1838 level and
the tower has been declared stable for the next 300 years. It is an interesting fact
that before more recent remedial work began, a computer model could not
replicate the 5.5°tilt as the model collapsed at 5.44°.
Lessons Learned
What we can learn from the leaning tower of Pisa is the importance of
geotechnical research. The architects and engineers at the time of construction
should have noticed that the other buildings on the Pisa plain were suffering from
large settlement; therefore, building a high structure on the unstable ground
conditions would only lead to major problems. This can partly be attributed to the
fact that when the tower was originally built the technology or understanding of
geotechnics was not yet developed.
A second lesson that can be taken from the tower is the effect that remedial
actions have on the overall stability of the tower. Apart from the present solution,
every other attempt to stabilize the tower had a negative out come, and made
the tower lean more.
References:





http://www.architectureweek.com/2001/0711/news_1-2.html
http://www.archidose.org/Jul00/071000.html
http://en.wikipedia.org/wiki/Leaning_tower_of_pisa
http://www.endex.com/gf/buildings/ltpisa/ltphumor/ltphumor.htm
http://www.pbs.org/wgbh/nova/pisa/today2.html