The leaning tower of Pisa
Group 10
Alan Milton
Jordan Verity
Eoin Lambe
Abdulhadi Mohsen
Introduction/history
Even if it did not lean, the bell tower of Pisa, Italy, would be one of the most significant
monuments of medieval Europe and a prized example of Romanesque architecture. As it
stands today, the tower is an architectural wonder. Located in the Piazza dei Miracoli, it
is part of a complex of four white marble medieval buildings, the others being a
cathedral, a baptistery, and a cemetery. 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.
Started in 1174 by the architect Bonanno Pisano, the tower took over 200 hundred years
to complete, probably owning to Pisa’s perpetual state of warfare. The bell tower rises to
a height of 53.3 m above ground level and comprises of eight levels. The circumference
of the tower at its base is 48.8 m. The tower weighs 14,500 Mg and rests on a ring-shaped
masonry foundation that is 19.6 m in diameter and at its deepest is 5.5 m below ground
level. The foundation slopes toward the south at 5.5 degrees to the horizontal, and the
seventh level overhangs the ground by about 4.5 m on the south side. The exterior of the
tower is dominated by the 30 columns on each level. The tower’s inner structure consists
of an outer cylinder of heavy stone blocks, an inner cylinder of porous, weaker stone
blocks and a fill of stone chips and lye mortar. A spiral stairway consisting of 294 steps
leads upward within the walls of the tower to the belfry.
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.
Problems/cause
By about 1178 work had reached the fourth story, but already at this early stage the tower
had started to lean, work then stopped on the tower until 1272. At the end of the first
phase of construction (1178) the tower was leaning northward by about a quarter of a
degree. But as construction advanced above the fourth story it began to move toward the
south and then accelerated so that by 1278, when the seventh level had been reached, it
was inclining southward by about 0.6 degree. This had increased to about 1.6 degrees by
1360, when work on the bell chamber began.
Research has shown that had construction continued much further then the fourth story
the shear strength of the soil beneath the structure would not have been sufficient to carry
the load and the tower would have collapsed. When work began again in 1272 the shear
strength of the clay in the soil had increased through consolidation under the weight of
the tower. Although consolidation theory wasn’t properly understood at that time it is
possible that experience had taught the builders that constructing towers too quickly on
the plain on which Pisa is located resulted in collapse.
The tower’s inclination can be understood by examining the ground upon which it is
built. 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 under lagoonal, fluvial, and
estuarine conditions less than 10,000 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 water table is also just 1 to 2 m below the surface
Remedial actions
Through out history there have been many attempts to stabilize the tower. Although none
of these involved complete straightening of the tower as it is most famous for its lean and
eliminating this would be in essence taking away some of it character.
The first remedial action undertaken was during the first stage of construction. In an
attempt to correct the lean, tapered blocks of masonry were used at each floor level to
bend the axis of the tower away from the lean. This also had the effect of increasing the
stress in the second level to a critical level.
In 1838 the architect Alessandro della Gherardesca excavated a walkway (catino) around
the base of the tower in order to expose the column plinths and foundation steps that had
become buried through the settlement of the south side of tower. The excavation
extended below the water table so that water rushed into the catino. There is evidence to
suggest that the inclination of the tower increased by more than a quarter of a degree at
that time. It was later discovered that Gherardesca had placed a 0.7 m thick ring of
concrete in the floor of the catino, presumably to stabilize the base of the catino and
hence the tower. Gherardesca's excavation had brought the tower very close to collapse.
In 1934 consolidation of the foundation masonry through grout injection resulted in a
sudden movement south by about 10 mm at the top of the tower, and groundwater
abstraction from the lower sands in the 1970s increased the movement by about 12 mm.
These responses showed how very sensitively poised the tower was.
In 1989 a commission was established to make recommendations regarding the safety of
the tower and implement stabilization measures. Temporary stabilization of the
foundations was achieved during the second half of 1993 by applying 600 Mg of lead
weights to the north side of the foundation via a posttensioned, removable concrete ring
cast around the base of the tower. This caused a reduction in inclination by about one
minute of arc. Of greater importance, it reduced the overturning moment by about 10
percent. In September 1995 the load was increased to 900 Mg to control the movements
of the tower during an unsuccessful attempt to replace the unsightly lead weights with
temporary ground anchors. The second-level masonry problem was tackled in 1992 by
attaching a few lightly posttensioned steel tendons around the tower at the second level.
A permanent solution was sought that would result in a small reduction in inclination
(half a degree). Various methods were investigated such as drainage by means of wells,
consolidation beneath the north side by electroosmosis, and loading the ground around
the north side of the tower by means of a pressing slab loaded by ground anchors.
A method involving soil extraction gradually gained favor. This involves installing a
number of soil extraction tubes just beneath the north side (high side) of the foundation.
This method had been used successfully in the past on reducing the amount of settlement
within Mexico City's Metropolitan Cathedral, but never on a tower on the point of
topping. The method was studied first by means of physical models, then by numerical
modeling, and finally by a large-scale trial. Trials were carried out on a 7 m diameter,
eccentrically loaded trial foundation close to the tower in similar ground conditions. A lot
was learnt from the trials such as the existence of a critical line located about half a radius
in from the northern edge of the foundation (away from the leaning side). If soil
extraction from beneath the foundation took place north of this line, the response of the
tower would be positive. If, however, extraction took place south of the line, the tower
would move south and become unstable. A specific drill was developed from the trials. It
consisted of a hollow-stemmed continuous flight auger (also known as an Archimedes'
screw) housed within a counterrotating casing 180 mm in diameter. When the drill was
withdrawn the cavity formed closed gently and repeated extraction could be made from
the same location.
In August 1996 limited soil extraction was carried out beneath the tower. As a safety
precaution cables were attached to the third story of the tower. In the event of untoward
movements of the tower these cables were to be tensioned by lead weights to hold the
tower steady.
At first the rate of soil extraction was very slow (20 L each day). Once it was known that
the drilling was achieving the desired results the rate of extraction was increased and on
the 9th of February 1999 full-scale drilling commenced. The full-scale extraction caused
an average movement of about 6 seconds of arc per day, reflecting an average soil
removal rate of 120 L per day.
Towards the end of May 2000 the tower was stable enough to start to remove the
unsightly lead weights attached in 1993 and by January 2001 the last lead weights were
removed and drilling ceased. The tower was now out of intensive care.
The target reduction of inclination for the tower had been half a degree, an amount not
visible to the casual observer but sufficient to stabilize the foundations and reduce the
stresses in the masonry by a significant amount. By 2001 a total reduction of 1,830
seconds of arc had taken place. This reduction in inclination is equivalent to a northward
movement of the seventh level of about 442 mm. The tower has now been returned to the
angle of inclination it exhibited in the 1830s, around the time that Gherardesca dug the
catino and before the dramatic lurch south.
Lessons learnt
The main lesson that can be learnt from the leaning tower of Pisa is the importance of
geotechnical research. When the tower was originally built the technology or
understanding of geotechics was not yet developed, but the original architects and
engineers should have noticed that other building on the Pisa plain suffered from large
settlements and hence building such a high structure on very unstable ground conditions
could only lead to major problems.
A second lesson that can be taken from the tower is what effect remedial actions have on
the overall stability of the tower. Every attempt apart from the present solution at
stabilizing the tower had in fact a negative out come, and made the tower lean more.
References
1) Why buildings fall down: how structures fall, Matthys Levy and Mario Salvadori,
2002
2) “The leaning tower of Pisa phases of construction”
[www.endex.com/gf/buildings/ltpisa/ltphases.html] 20/10/06
3) Preserving Pisa’s Treasure, Civil Engineering Magazine, March 2002