the leaning tower of pisa stabilisation work

THE LEANING TOWER OF PISA STABILISATION WORK
Laurie Wesley – University of Auckand, New Zealand
Inducing a wayward structure to be a little more upright
The University of Auckland
tower –(still upright)
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PISA AT THE
MOUTH OF THE
ARNO RIVER
MY STORY AND VITAL STATISTICS
My constructoin was begun in 1173 by Bonano Pisano, and finished
after 99years by Giovanni Simone. Tommaso di Andrew Pisano crowned me
with a bell chamber between 1360 and 1370, so I wasn’t completely finished till
then.
My height is about 60metres
My exterior diameter is 15.48m and my interior diameter is 7.37m
I have 8 floors and 294 steps
I rest on foundations about 3m deep.
External dia. is 19.6m and internal dia. is 4.5m]
My total weight is 14,500 tonnes
My leaning is 4.5m (5.5 degrees)
In my bell chamber there are 7 bells tuned on the seven musical notes
From my summit Galileo Galilei performed his famous experiments about
gravitation
I am one of the “Seven Wonders of the World”
(Authoratative data from a T-Shirt I bought for my daughter)
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CONSTRUCTION HISTORY
The lean appears to have started during the second stage of
construction. (The “stops” during construction were political)
Note: The south edge of the foundation has settled about 2.85m and
the north edge 2.15m (Differential settlement is about 1.7m)
ATTEMPTS AT
CORRECTIONS
DURING
CONSTRUCTION
- evident in the slight
curvature of the tower
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HISTORY OF TILT WITH TIME
Notes:
The tower is thought to have been stable from about 1550 till 1838
The increased rate of lean after that date is thought to have been
caused by excavation of a trench arond the tower and construction
of what is known as the “catino”
THE
“CATINO”
In 1838 an architect had an annular trench (or ditch) dug around the
base of the tower, to improve the aesthetics. The trench was given a
concrete base and concrete outside wall.
The catino extended well below the ground water level and was
permanently pumped to keep it dry
This lowered the ground water level and is believed to have reactivated movement of the tower
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VIEW OF
CATINO
– SOUTH SIDE
SOIL CONDITIONS
The important layer is Horizon B; the upper layer has an undrained
shear strength in the 60 to 70 kPa range, indicating an ultimate bearing
capacity of about 400 kPa. The actual stress of 500 kPa exceeds this.
It is thought that the “successful” competion of the tower was only
possible because of the unintended “stage” construction.
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THE DUOMO SHOWING EVIDENCE OF SETTLEMENT
PISA WALL WITH “BENT” GUARD TOWER
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MONITORING RECORDS SINCE 1911
Accurate records commenced in 1911. These show
“various” perturbations as indicated on the graph.
MONITORING
SYSTEMS
Since 1911:
Precision levelling of settlement markers, and geodetic measurements of
external points. Pendulum inclinometer 30 m long inside tower.
Bubble level, measuring tilt over span of 4.5 m.
Since 1990:
Additional settlement markers and deep datum, and biaxial electrolitic
inclinometers at ground floor. Automatice hydraulic “levelometers”,
also at ground level. Wires (with sensors) diametrically across the tower.
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INSTRUMENTATION
INSIDE THE TOWER
PUBLIC ACCESS,
ALLOWED FOR NINE
CENTURIES, CAME TO AN
ABRUPT END IN 1990
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CLOSURE OF THE TOWER IN 1990 AND
IMPLEMENTATION OF REMEDIAL WORK
Concern for the safety of the tower was increasing as the lean increased. In 1989, the
bell tower of the Pavia cathedral collapsed suddenly – the second occasion of a
catastrophic collapse of a tower. Both were of similar construction to the Pisa tower.
Structural analysis showed the Pisa tower to be severely stressed at the level where the
cross section suddenly reduces, ie at the start of the first “loggia”
INITIAL REMEDIAL WORK
• It was agreed that
urgent action was
required immediately
• In contrast to many
previous occasions,
an effective body
was set up to do the
work
• Structural
strengthening was
the first step
• The second was to
apply lead weights to
the high side of the
foundation
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THE LEAD WEIGHTS
RESPONSE TO THE
LEAD WEIGHTS
AND THE NEXT
STAGE
In view of the good
response to the lead
weights, it was
decided to install a
series of ground
anchors around the
north side of the
tower to take the
place of the lead
weights – still seen
as a further
temporary measure
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THE SEPTEMBER 1995 INCIDENT (BLACK SEPTEMBER)
“ ......... The lean was stopped and the tower even began to straighten. However, in
Sept 1995, the tower moved 2.5 mm in one night (in the wrong direction),
representing 10% of the total amount that had been corrected”.
(Quote from Lonely Planet guide book on Italy)
The problem arose from the way
the catino had been constructed.
In order to install the ground
anchors a new and larger ring
beam had to be constructed
below the one supporting the
lead weights. To construct this
beam part of the catino had to
be removed. It turned out,
unkown to the engineers, that
the catino was structurally
connected to the tower and had
become part of the foundataion
ABANDONMENT OF GROUND ANCHOR PLAN
AND ADOPTION OF A NEW PLAN
After the 1995 “scare” it was resolved that any remedial work from
then on would not involve structural work on the tower. It would be
confined to doing something with or to the soil
In 1962 an Italian engineeer named Terracina proposed the idea of
extracting soil from the high side of the tower. This had been used
successfully to correct large differential settlement of a cathedral of
Mexico city. The committee turned to this proposal.
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FIELD TRIAL ON SPECIALLY BUILT STRUCTURE
The trial was very successful
and indicated some
constraints, especially a limit
on extent of boring beneath
the structure
FIELD TRIAL
BESIDE THE
BAPTISTRY
The success of the trial
gave the committee
confidence to proceed
to a trial on the tower
itself
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TRIAL ON THE TOWER ITSELF
• Commenced Feb, completed June, 1999
• Involved 12 holes, 150 mm dia, inclined at 26o
• About 7 m3 of soil was extracted
• Tower responded as hoped
• North edge settled 13 mm and south side rose 1.5 mm
THE FINAL PLAN
– WHICH WAS
SUCCESSFULLY
CARRIED OUT
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THE CABLE SYSTEM
– A SAFEGUARD
After the “black
September” incident no
possible risk could be
taken
The guy ropes
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ONE OF THE
MASSIVE
ANCHOR
STRUCTURES
FOR THE GUY
ROPES
THE ROW OF 41 FLIGHT AUGURS
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DETAILS OF THE FINAL OPERATION
• Aim was to restore tower to 1817 position – a correction of
about 2000 seconds of arc, ie just over half a degree
(about 0.5m at top)
• 41 flight augers used with I.D.150 mm
• The holes selected for extraction each day and the volume
extracted (average 150 litres) depended on tower’s response.
• Length extracted in one operation on one hole varied, up to 3m.
• To make up 150 litres, 3 to 4 tubes normally operated on daily
• From any one hole, at any one position, extraction was repeated
up to three times (and was still effective)
THE FLIGHT AUGERS AGAIN
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FURTHER DETAILS OF THE OPERATION
• The operation proceeded smoothly without problems
• The lead weights were slowly removed as the operation
progressed, untill all were removed
• The tower rotated about a point approximately 2.5m to 3m
inside the southern edge
• A total of about 37 cubic metres was removed to achieve the
objective.
• The maximum distnce the flight augurs penetrated under the
tower was about 5m.
• Much higher “efficiency” was achieved than in the preliminary
trial - up from 15% to about 75%
THE RIG AND FLIGHT AUGERS
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THE FLIGHT
AUGURS
SETTLEMENT OF FOUNDATION DURING
UNDEREXCAVATION
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ROTATION OF TOWER DURING UNDEREXCAVATION
(to June 2000)
THE EXTRACTED SOIL
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SOIL EXTRACTION RECORDS
VIEW FROM THE BAPTISTRY
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THE END
The operation was sucessful and
the tower is not expected to
require further work for the next
several centuries.
THANK YOU FOR YOUR
ATTENTION
Acknowledgement & Thanks:
Some figure and diagrams used
in this presentation are taken
from papers by Prof. M.B.
Jamiolkowski
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