THE LEANING TOWER OF PISA STABILISATION WORK Laurie Wesley University of Auckand, New Zealand

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1 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) 1

2 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) 2

3 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 3

4 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 4

5 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. 5

6 THE DUOMO SHOWING EVIDENCE OF SETTLEMENT PISA WALL WITH BENT GUARD TOWER 6

7 MONITORING RECORDS SINCE 1911 Accurate records commenced in 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. 7

8 INSTRUMENTATION INSIDE THE TOWER PUBLIC ACCESS, ALLOWED FOR NINE CENTURIES, CAME TO AN ABRUPT END IN

9 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 9

10 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 10

11 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. 11

12 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 12

13 TRIAL ON THE TOWER ITSELF Commenced Feb, completed June, 1999 Involved 12 holes, 150 mm dia, inclined at 26 o About 7 m 3 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 13

14 THE CABLE SYSTEM A SAFEGUARD After the black September incident no possible risk could be taken The guy ropes 14

15 ONE OF THE MASSIVE ANCHOR STRUCTURES FOR THE GUY ROPES THE ROW OF 41 FLIGHT AUGURS 15

16 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 16

17 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 17

18 THE FLIGHT AUGURS SETTLEMENT OF FOUNDATION DURING UNDEREXCAVATION 18

19 ROTATION OF TOWER DURING UNDEREXCAVATION (to June 2000) THE EXTRACTED SOIL 19

20 SOIL EXTRACTION RECORDS VIEW FROM THE BAPTISTRY 20

21 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 21