The Leaning
Tower of Pisa
Group 10
Martina Flanagan
Julian Mannion
Anthony McGowan
Neil O’Donohoe
Introduction
• The iconic status of this landmark is due in no small part to the tilt
which has earned the tower its name.
• The Tower was designed to be "vertical“ but started to incline during
its construction.
• The tilt could have been the cause of the tower’s near downfall. It
was calculated that, the tower would have collapsed by 2050 had
remedial work not taken place.
• It leans at an angle of 5.5 degrees and the top of the tower is 4.5
meters from where it would stand if the tower was perfectly vertical.
History
• Built as an expression of civic pride and was intended to reflect the
stature of the wealthy city-state of Pisa.
• 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 uncertain to this day,
but they say it was Bonanno Pisano.
• The tower is an important icon in the development of modern
science; from the tower Galileo Galilee proved that the velocity of
bodies falling under the action of gravity is independent of their
weights.
Timeline of Leaning
•
The tower began to sink after construction progressed to the third floor in
roughly 1178.
•
Construction was then halted for nearly a century due to battles taking place.
•
This rest period allowed the underlying soil to settle. Had this not have
happened, the tower would have toppled.
•
In an effort to compensate for the tilt, 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.
•
The tower was closed to the public in 1990 for fear it would collapse and
remedial work began.
Reasons for Leaning
The causes of the leaning are mainly due to two factors:
•
•
Foundations
In 1178 leaning began, due to a mere three-meter foundation, set in the
weak unstable river deposits. Construction of the tower halted due to a war,
allowing time for the underlying soil to settle. Had a much wider and deeper
pad foundation been designed and used at the time, the lean may never
have occurred.
Unsuitable Ground
The tower is built on unsuitable ground for such a heavy and tall building.
The base of the building is only about 2 metres above the water table
Built on an old riverbed
Underlying ground is made up of layers of sand, clay and silt. The layers are
not evenly sorted and the weight of the building has compressed them.
The designers of the time didn’t realise the tower would result in such a
large imposed load. Because the layers of deposits are soft and uneven, as
the ground compressed, it has sunk more in some places than others.
Remedial Actions
•
The first attempt to correct the tower’s inclination occurred during construction. When
construction recommenced for the second time, heavier materials were used on the
north side and slightly taller bricks being used on the south side.
•
•
In 1990 the tower was closed to the public for fear it would collapse.
In 1993, a lead counter-weight weighing approximately 600 tonnes was attached to
the northern side in an attempt to correct the tower’s rotation.
It was concluded that correcting the tower’s lean would lie in removal of soil from
beneath the tower, extensive testing of the soil 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. This
required 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
•
•
Remedial Actions
•
•
•
•
•
•
•
•
In 1990 the tower was closed to the public for fear it would collapse.
In 1993, a lead counter-weight weighing approximately 600 tonnes was
attached to the northern side in an attempt to correct the tower’s rotation.
It was concluded that correcting the tower’s lean would lie in removal of soil
from beneath the tower. Extensive testing of the soil 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.
This required the ground to be frozen to prevent the ground floor walkway
becoming flooded.
Expansion of the ground water which took place during freezing caused the
tower to lurch southward
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.
Remedial Actions
•
In 1998, plastic sheathed cables were wrapped around the first floor to
prevent any further movement or cracking.
•
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, to repair visual damage, mostly corrosion and
blackening.
•
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.
Lessons Learnt
• What we can learn from the leaning tower of Pisa is the
importance of geotechnical research. If the engineers
had researched the surrounding soils properly, they
would have found that 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.