This presentation covers salient features of the stabilisation of the leaning tower of pisa. Presentation is basically based on the account of soil mechanics and other civil engineering techniques.

1. The Enigma of the Leaning Tower of Pisa
‘?’

2. Basic facts about the structure
• Height of the structure: 60 meters
• Diameter of the foundation: 19.6
meters
• Inclination of the tower: 5.5° to
the south
• Estimated weight of the structure:
14,500 tons

3. Cross section through the tower
.

4. Soil Profile beneath the tower

5. Construction timeline
• Construction work began on 9th August, 1173
• About quarter construction was achieved by
1178.
• There was then a halt in the construction for
unknown reasons, but had the construction
continued any further, the foundation would
have experienced a bearing capacity failure.
• The work recommenced in the year 1272.

6. .
• In year 1278, the tower’s construction was
completed up-to the seventh cornice & then
there was again a halt due to the military
action.
• Next recommencement was in 1360 and the
whole construction was finished in 1370.
• It took nearly 200 years to finish the
construction with two intervals of nearly 100
years each which gave the soil to gain
strength.

7. Unpleasant Surprise
• In 1838 an architect, Allesandro
Della Gherardesca excavated a
walk-way around the foundation
of the tower known as Catino.
• It was intended to expose
foundation steps and column
plinths.
• As the excavation was under the
water table, it caused an inflow
of water which gave the tower
an additional tilt of nearly half
degree to the south.

8. History of Tilting
• The commission for the stabilization was headed by Prof.
John B. Burland.
• Due to the absence of any documentary evidence all the
clues for the history of tilt were to be inferred from the
adjustments made to the masonry.
• He put himself in the place of a mason and asked himself,
“What is the most practical thing to do when you arrive
at a given floor and find that the tower is out-of-the
plumb.”
• A widely accepted hypothesis is that the masons would
always try to keep the masonry layers horizontal.

9. Shape of the axis of the tower deduced from relative
inclination of masonry layers

10. Temporary stabilization of the tower
• There were two distinct problems that
threatened the stability of the tower:
1. Strength of the masonry.
2. Stability of foundation against overturning.

11. Temporary stabilization of the masonry
• On the south side of the tower, due to the change in
the cross section the masonry blocks were stressed.
The exact assessment of stresses was not possible
but its effect would have been catastrophic.
• This problem was tackled by using plastic covered
steel tendons which were bound around the first
cornice upto the second floor which helped in
reducing the crack sizes and prevented the tower to
fail due to buckling.

12. Temporary stabilization of the masonry with light
circumferential pre-tensioning

13. Temporary Stabilization for the possible
overturning failure
• This was achieved by placing the counter weight
on the northern side of the tower is four phases
so that the response of the tower could be
recorded.
• Such solution would not have been considered if
it had not been recognized that the leaning
instability rather than the bearing capacity failure
was controlling the behavior of the tower or the
north side of the foundation was settling.

14. Counterweight installation to prevent the
overturning of tower

15. Observed change of inclination of the tower during
application of the counter-weight

16. Choosing the suitable and Permanent
Stabilization
• For bureaucratic and financial reasons the temporary
stabilization took longer than expected.
• Because of the severe restraints that have been
discussed already measures involving application of
concentrated loads to the masonry or underpinning
operations beneath the south side of the foundation
were ruled out.
• Moreover aesthetic and conservation considerations
required that the stabilization effect should have an
absolute minimal appearance.

17. The three possible approaches for the
permanent stabilization
After careful consideration of number of possible
approaches the commission chose to study three in
detail:
• Construction of the ground pressing concrete slab to
the north of the tower which was to be connected with
the post-tensioned concrete ring to be placed around
the periphery of the foundation.
• Consolidation of the Pancone clayby means of carefully
devised electro-osmosis.
• The technique of soil extraction which was at that time
already in practice in Mexico to reduce differential
settlement of number of buildings.

18. Approach #1: Ground pressing slab
• ✗ Numeric modeling of the ground pressing
slab showed that the tower’s response was
uncertain and if anyhow it gave positive
results the results were not large enough and
the settlements were huge & it required costly
civil engineering to proceed with.
• RULED OUT.

19. Approach #2: Electro-osmosis
• ✗ Full scale model showed that the ground
conditions at Pisa were not fit for this method.
• RULED OUT

20. Approach #3: Soil Extraction
• ✔ This method was best suited and was finally
adopted for the final large scale work.
• In this method small amounts of soil from
predefined locations are removed with the
help of inclined drills.
• This leaves a cavity at the drilled location and
which is gradually filled with the soil in
immediate vicinity of the cavity due to the
overburden pressure.

21. Schematic representation of the under-
excavation process

22. Change in the North-South inclination after the
permanent stabilization

23. References
• The Enigma of the Leaning Tower of Pisa, Prof. John
B. Burland, 1998
• http://casehistories.geoengineer.org
• http://home.eng.iastate.edu/