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Role of Monitoring in Historical Building Restoration:The Case of Leaning Tower of PisaNunziante Squegliaa & Giuseppe Bentivogliob

a Department of Civil Engineering, University of Pisa, Pisa, Italyb Technical Office, Opera della Primaziale Pisana, Pisa, ItalyPublished online: 20 Aug 2014.

To cite this article: Nunziante Squeglia & Giuseppe Bentivoglio (2015) Role of Monitoring in Historical Building Restoration:The Case of Leaning Tower of Pisa, International Journal of Architectural Heritage: Conservation, Analysis, and Restoration,9:1, 38-47, DOI: 10.1080/15583058.2013.865813

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International Journal of Architectural Heritage, 9: 38–47, 2015Copyright © Taylor & Francis Group, LLCISSN: 1558-3058 print / 1558-3066 onlineDOI: 10.1080/15583058.2013.865813

Role of Monitoring in Historical Building Restoration:The Case of Leaning Tower of Pisa

Nunziante Squeglia1 and Giuseppe Bentivoglio2

1Department of Civil Engineering, University of Pisa, Pisa, Italy2Technical Office, Opera della Primaziale Pisana, Pisa, Italy

A short summary is presented of the studies and the actionsof Committees appointed in past 50 years for the Tower of Pisa.The discussion first addresses the attempts carried out during thewhole history of the Tower to measure its movements and theefforts made to understand the origin and causes of its inclina-tion. A history of foundation rotation has been also deduced bymeans of a precise architectural survey, which has led to a diagno-sis for the inclination and its increase in time. As a consequence,several hypotheses for its stabilization have been proposed. All themeasures for leaning tower stabilization need the application ofobservational method for their implementation. The observationalmethod is strictly based on a comprehensive monitoring system,both described in the second part of the paper. The aims are tostress the importance of a well-conceived monitoring system andto propose the extension of concept of monitoring to construc-tion history details. The data and actions described are from thework carried out by committees appointed by Italian Governmentduring the second half of 20th century, in particular by the com-mittee chaired by Professor Jamiolkowski, appointed in 1991. Theauthors have collaborated with this committee since 1993, and theyare still in charge of the monitoring and maintenance of the Towerof Pisa.

Keywords leaning tower, observational method, soil extraction,monitoring, leaning instability

1. INTRODUCTIONThe monuments of the Piazza dei Miracoli in Pisa were built

in the Middle Ages, during the period of maximum power of theRepublic of Pisa. The leaning tower (Figure 1) is one of best-known monuments of the world. It consists in a hollow cylindersurrounded by six balconies merging from the base cylinderand capped by a bell chamber. At the end of 20th century theinclination of tower axis was approximately 5.5 degrees.

At the beginning of 19th century a heated debate over theinclination of tower took place. The debate arose between who

Received October 2, 2012; accepted November 11, 2013.Address correspondence to Nunziante Squeglia, Department of

Civil Engineering, University of Pisa, Largo Lucio Lazzarino,1–56122, Pisa, Italy. E-mail: squeglia@ing.unipi.it

stated that the inclination was an intentional characteristic oftower versus those who stated that it was an accident of con-struction. No one seemed worried about the equilibrium ofmonument, nor measurements of inclination were carried outin a systematic way up to beginning of 20th century.

As will be explained, the reasons of tower inclination lie inthe nature of subsoil. The ground profile underlying the toweris shown in Figure 2. It consists of three distinct groups of soillayer, named horizons (Cestelli Guidi et al. 1971). Horizon Ais approximately 10 m thick and primarily consists of estu-arine deposits, laid under tidal conditions; as a consequence,rather variable sandy and clayey silts are found. At the bottomof horizon A is a 2-m thick medium dense fine sand layer.

Horizon B consists primarily of marine clay that extends toa depth of approximately 40 m. It is subdivided into four dis-tinct layers. The upper layer is a soft, sensitive clay, locallyknown as the pancone. It is underlain by an intermediate layerof stiffer clay, which in turn overlies a sand layer (termed theintermediate sand). The bottom layer of horizon B is a nor-mally consolidated clay known as the lower clay. Horizon Bis very uniform laterally in the vicinity of the tower. HorizonC is a dense sand (the lower sand) that extends to considerabledepth.

From the geological viewpoint (Trevisan 1971), the lowersands are marine sediments deposited during the Flandriantransgression. The horizon B is formed by Quaternary depositsof marine origin, dominantly clayey, formed at the time ofrapid eustatic rise. During the past approximately 10,000 years,the rate of eustatic rise decreased and the sediments becameincreasingly estuarine in character. The more recent sedimentsof horizon A mainly comprise sandy and clayey silt; typicallyof estuarine deposits, there are significant variations over shorthorizontal distances.

The piezometric level in horizon A, as shown in Figure 3is between 1 m and 2 m below the ground surface. Pumpingfrom the lower sand has resulted in lowering of piezometriclevel in that stratum, as a consequence a downward seep-age from horizon A to horizon C has been induced resultingin a pore pressure distribution with depth which is slightlybelow hydrostatic. The many borings beneath and around

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ROLE OF MONITORING: TOWER OF PISA 39

FIG. 1. Illustration of a Tower section across the maximum inclination axis(approximately north–south orientation).

FIG. 2. Schematic illustration of the soil profile.

the tower show that the surface of the pancone clay isdished beneath the tower, from which it can be deducedthat the average settlement of the monument is not lessthan 3 m.

FIG. 3. Graph of the ground water elevation in horizons A, B, and C.

Tower construction started in 1173 (Figure 4) and progressedto approximately one-third of the way up the fourth order by1178, when the work was interrupted. At that time, the weightof the tower was approximately 90 MN, with a unit load ofapproximately 315 kPa on the foundation; under undrained con-ditions, the bearing capacity of the foundation was of the sameorder (with an average undrained shear strength of 55 kPa, bear-ing capacity is 330 kPa). As a consequence, if construction hadbeen continued, the tower foundation would fail.

Construction restarted in 1272, after a pause of nearly100 years. Circa 1278, construction had reached the seventhcornice when work again stopped. Once again and for the samereason, had the work continued, the tower would have fallenover. Circa 1360, work on the bell chamber was commencedand was completed circa 1370, two centuries after the start ofthe work. It is known that the tower must have been tilting tothe south when work on the bell chamber began, as it is notice-ably more vertical than the remainder of the tower. Indeed onthe north side are four steps from the seventh cornice up tothe floor of the bell chamber, while the south side has six steps(Figure 5).

Another important detail of the history of the tower is thatin 1838 a walkway was excavated around the foundation, calledthe catino. Its purpose was to expose the column plinths andfoundation steps for all to see as was originally intended. Theoperation resulted in an inflow of water being the bottom of thecatino well below the ground water table. As a consequence,since 1838 the catino has been kept dry by continuous pump-ing. The only two measurements carried out before 20th centuryhave been performed before and after this excavation. Fromthese measurement (Cresy and Taylor 1829; De Fleury 1859),an increase of inclination of 0.5 degrees due to excavation of thecatino has been deduced.

2. FINDINGS OF MONITORING AND DEFINITION OFSOLUTIONS

The history of the tower inclination during its constructionis frozen in the resulting shape of the axis of the Tower due

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40 SQUEGLIA AND BENTIVOGLIO

FIG. 4. Graph of the history of construction.

FIG. 5. Schematic illustration of a detail of the bell chamber.

to the adjustments made to the masonry layers during con-struction. Based on this shape and a hypothesis on the mannerin which the masons corrected for the progressive lean of thetower, the history of inclination of the foundation of the towerversus its weight reported in Figure 6 till 1990 may be deduced.In Figure 7 the same data are reported versus time elapsed sincethe beginning of construction.

During the first stage of construction to just above the thirdcornice (1173 to 1178), the tower inclined slightly northward.The construction stopped for almost a century, and when itrecommenced in circa 1272 the tower began to move south-ward. Work again ceased when the construction reached theseventh cornice in circa 1278, at which stage the deduced incli-nation was approximately 0.6◦ towards the south. During thenext 90 years the construction was again interrupted and theinclination increased to approximately 1.6◦. After the comple-tion of the bell chamber in approximately 1370, the inclinationincreased significantly but the information about the inclinationare scarce. Some rough information on its trend may be obtainedby pictures or documents, such as a fresco painted in 1385 byAntonio Veneziano, just after the completion of bell chamber,or measurement reported by Giorgio Vasari in 1550. In 1817,when Cresy and Taylor (1829) made the first recorded mea-surement with a plumb line, the inclination of the tower wasapproximately 4.9 degrees. In 1859, Rohault de Fleury carriedout another measurement, finding a value of the inclination sig-nificantly higher than that of Cresy and Taylor (1829). In fact,between the two measurements the catino had been excavatedto uncover the base of the monument which had sunk into thesoil due to a settlement greater than 3 m. Digging the catinoseriously threatened the stability of the tower, and caused anincrease of inclination of approximately 0.5 degrees; further-more, the rate of inclination increased and the motion probablychanged from retarded to accelerated.

From the beginning of 20th century, probably as a conse-quence of Venice bell tower collapse, the inclination of thetower has been regularly monitored by different means. In a firsttime only a geodetic survey was applied. In 1935 a 35-m longpendulum was installed within the tower in order to increasethe precision and especially the frequency of measurement. The

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ROLE OF MONITORING: TOWER OF PISA 41

–1 0 1 2 3inclination of foundation [°]

wei

ght o

f T

ower

[M

N]

50

~1272

100

150

1178

~127

8

~136

0

(Roh

ault

de F

leur

y) 1

859

1990

1817

(C

resy

& T

aylo

r)

4 5 6

FIG. 6. Graph of the history of tilt of foundation.

FIG. 7. Graph of the deduced history of inclination of the Tower.

inclination measured on the tower shaft increases more than therotation of the foundation (Figure 8), implying a steady defor-mation of the tower body. The long-term steady trend is markedby two major perturbations: one in 1935 and another one in theearly1970s. The first one has been caused by cement groutinginto the foundation body and the soil surrounding the catino,

FIG. 8. Graph of the tower inclination as measured by different procedures.

FIG. 9. Graph of the tilt of foundation between 1938 and 1992.

carried out to prevent the inflow of water. The second perturba-tion (Croce et al. 1981) has been related to the pumping of waterfrom deep aquifers, inducing subsidence all over the Pisa plain.The closure of a number of wells in the vicinity of the towerstopped the increase of the rate of tilt.

In any case, even correcting for perturbation (Figure 9), itappears that the rate of tilt was steadily increasing and hadnearly doubled from 1938 to 1993. In the early 1990s, the incli-nation was approximately 5.5 degrees. A careful study of thetower movements led to the conclusion that it was affectedby a phenomenon of instability of the equilibrium, known asleaning instability (Burland 1990, unpublished data [A studyof the motion of the Pisa Tower, internal report]). This phe-nomenon depends on the stiffness of soil-foundation systemrather than its strength (Desideri et al. 1994; 1997; Lancellotta1993).

To demonstrate leaning instability, the simple conceptualmodel of inverted pendulum may be used. It is a rigid verticalpole (Figure 10) with a concentrated mass at the top and hingedat the base to a constraint that reacts to a rotation with a sta-bilizing moment Ms proportional to the rotation. On the otherhand, the rotation induces an offset of the mass and hence an

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42 SQUEGLIA AND BENTIVOGLIO

FIG. 10. Simplified model for leaning instability.

overturning moment M. In the vertical position the system is inequilibrium. Let us imagine that a rotation occurs. If the stabiliz-ing moment is larger than the overturning one, the equilibriumis stable; the system returns to the vertical configuration. If thecontrary occurs, the equilibrium is unstable; the system col-lapses. If the two moments are equal, the equilibrium is neutral;the system stays in the displaced configuration. The stability ofthe equilibrium may be characterized by the ratio FS = Ms/Mbetween the stabilizing and the overturning moment.

Modeling the tower as an inverted pendulum, the restraint atthe foundation may be evaluated by means of elastic solutions.For example the foundation can be modeled by a circular plateof diameter D resting on a linearly elastic half-space, defined byelastic constants E and v. Calling W and M = We, the verticalload and the overturning moment and ρ, α the settlement and

FIG. 11. Illustration for the definition of α and ρ.

the rotation of the foundation (Figure 11), it may be shown inEquation 1 and Equation 2 that:

{ρ

α

}=

∣∣∣∣1kρ

0

0 1kα

∣∣∣∣{

WM

}(1)

with:

kρ = ED

i − ν2; kα = ED3

6(1 − ν2

) (2)

In this simple linear model there is no coupling between set-tlement and rotation, and the stability of the equilibrium is anintrinsic property of the ground–foundation system. It may becharacterized by the ratio FS between the stabilizing and theoverturning moment, as shown in Equation 3:

FS = kαα

Wh sin α= ED3

6(1 − ν2)

1

Wh(3)

In the case of the tower of Pisa a rough evaluation of FS maybe obtained using this model by the knowledge of the settle-ment of the tower, ρ ≈ 3 m. Being kρ = W/ρ, one gets E/(1-ν2)≤ 2.85 MN/m2. Accordingly, with h = 22.6 m (height of thecenter of gravity of the tower) and W = 141.8 MN (weightof the tower), FS ≤ 1.12. Even if the simplistic linearly elas-tic subsoil model is not appropriate for the situation, it allowsthe important conclusion that the tower is very near to a state ofneutral equilibrium. The continuing movement, made possibleby the state of neutral equilibrium, is controlled by ratchetingfollowing cyclic actions as the fluctuations of water table inHorizon A. Of course, creep has also some influence on theprocess.

The relationship between the stabilizing moment Ms = kαα

and the rotation α may be linearized over a short interval, butit is certainly non linear and approaches asymptotically a lim-iting value of Ms. In a case as that of the leaning tower, that

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ROLE OF MONITORING: TOWER OF PISA 43

FIG. 12. Graph of the centrifuge experiments by Cheney et al. (1991).

is on the verge of instability, consideration of non-linearityappears mandatory. Further details on stability of tall structuresare reported elsewhere (Desideri et al. 1994; 1997; Lancellotta1993). As a matter of fact, centrifuge experiments by Cheneyet al. (1991) show coupling between settlement and rotation,non-linearity and strain hardening plasticity (Figure 12).

The leaning instability of the Tower has been investigated bya number of different approaches, including small-scale phys-ical tests at natural gravity and in the centrifuge, and finiteelement analyses based on different constitutive models of thesubsoil. The analyses led to the conclusion that the gradualincrease of the inclination would have ended in a collapse.Another very significant conclusion was that a decrease of theinclination, even a relatively minor one, results in a substantialincrease in the safety against leaning instability.

It may be seen in Figure 12 that a decrease of the inclina-tion, bringing on the unloading branch of the curve, stronglyincreases the stiffness of the soil-foundation system, and hencethe stability. As a consequence of the described behavior, adecrease of inclination could be used to stabilize the tower withrespect of leaning instability.

3. THE STABILIZATION OF THE TOWERThe stabilization of the tower was conceived as a two-stage

strategy. Since the tower was on the verge of toppling, a tem-porary and fully reversible intervention has been designed andimplemented in order to slightly improve the safety againstleaning instability. This operation was intended to gain thetime to properly devise, design and implement the long-termsolution.

The first intervention was the application of counterweights.Between 1991 and 1993, a comprehensive analysis of interven-tion has been done and it was established that an inappropriateapplication of counterweight could increase the inclination.

The long term solution has been selected by Committeebetween three possible means to achieve the desired reduc-tion of inclination: a) the construction of a ground pressingslab to the north of the tower; b) the consolidation of the pan-cone clay north of the Tower by electro-osmosis, and c) thecontrolled removal of small volumes of soil beneath the northside of the foundation (underexcavation). All three approacheshave been the subject of intense investigation, and eventuallyunderexcavation was selected.

Both temporary and definitive solutions have been carriedout applying the observational method which is a systematicprocedure developed by Karl Terzaghi (Peck 1969). Today theobservational method is an accepted procedure in geotechnicaldesign so that the recent Italian Code, applied in 2009, consid-ered the method as a design method. As stated by Peck (1969)in his Rankine Lecture, the following ingredients are necessaryfor the application of the method:

• Exploration sufficient to establish at least the generalnature, pattern and properties of the deposits, but notnecessarily in detail;

• Assessment of the most probable conditions and themost unfavorable conceivable deviations from theseconditions, In this assessment, geology often plays amajor role;

• Establishment of the design based on a workinghypothesis of behavior anticipated under the mostprobable conditions;

• Selection of quantities to be observed as constructionproceeds and calculation of their anticipated values onthe basis of the working hypothesis;

• Calculation of values of the same quantities underthe most unfavorable conditions compatible with theavailable data concerning the subsurface conditions;

• Selection in advance of a course of action or mod-ification of design for every foreseeable significantdeviation of the observational findings from thosepredicted on the basis of the working hypothesis;

• Measurement of quantities to be observed and evalua-tion of actual conditions; and

• Modification of design to suit actual conditions.

All these steps have different weights in dependence ofnature and complexity of the work. In any case, the majorissues are the definition of quantities to measure and the designof an adequate monitoring system. In 1992 a comprehen-sive monitoring system has been installed on the tower andit is still operative for monitoring of monument in a reducedconfiguration.

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Details about the monitoring system can be found elsewhere(Macchi 2005; Macchi and Ghelfi 2005). During the implemen-tation of stabilizing measures some additional instruments havebeen installed in order to measure the appropriate quantities.

The temporary stabilization has been attained by applying tothe north side of the tower a vertical force, obtained by a stackof lead ingots resting on a post-tensioned ring beam cast aroundthe base of the tower (Figure 13). In addition to the generalmonitoring system, 9 strain gauges piezometric cells have beeninstalled beneath the north side of the foundation in order tocontrol the excess pore pressure just beneath the tower founda-tion (Figure 14). No excess pore pressures have been registeredduring the loading by lead ingots, since the loading had a ratesufficiently low to be fully drained.

The loading has been carried out in five steps, between May1993 and January 1994, with alternating rest periods. Duringthe loading stages a maximum of two ingots per day havebeen installed and the weight of each ingot was approximately90 kN. The rotation of tower foundation during the installationof the lead ingots is shown in Figure 15. It can be seen that theamount of creep between the stages of load is small. However,subsequent to completion of loading, time dependent north-ward inclination has continued. On February 20, 1994, 1 monthafter completion of loading, the northward inclination was 33”.By the end of July 1994 it had increased to 48”, giving a totalof 52” including the effect of the concrete ring.

On the whole, the intervention has been rather successful.The overturning moment has been reduced by 14%; the towerexperienced a northwards rotation of 52.6 arc seconds and asmall average settlement equal to 3.3 mm. An event of theutmost importance is that the progressive southward inclinationof the tower has come to a standstill.

Since the Committee was well aware that studies and experi-ments carried out about soil extraction might not be completelyrepresentative of the possible response of a tower affected byleaning instability, it was decided to implement preliminary soilextraction beneath the tower itself, with the objective of observ-ing its response to a limited and localized intervention. Thispreliminary intervention consisted in 12 holes (Figure 16) to

FIG. 13. Photograph of the lead ingots and concrete beam.

FIG. 14. Schematic illustration of the position of piezometers during leadingots installation.

FIG. 15. Graph of the foundation tilt during lead ingot installation.

extract soil from Horizon A to the north of the tower founda-tion, penetrating southwards under the foundation not more than1 m. The goal was to decrease the inclination of the Tower byan amount sufficient to check the feasibility of underexcavationas a means to stabilize the tower permanently, and to adjust theextraction and measurement techniques.

The underexcavation experiment was carried out betweenFebruary and June 1999. The total volume of soil removed wasapproximately 7 m3, 70% of which was from north of the Towerand the remaining 30 % from beneath the foundation. Since the

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ROLE OF MONITORING: TOWER OF PISA 45

FIG. 16. Photograph of the boreholes for soil extraction as viewed from tower.

observation of foundation vertical movement was mandatory,the monitoring system was integrated with an optical levellingcircuit for quick surveying. Levelling of foundation was car-ried out twice a day in order to strictly control the settlement offoundation, especially of southern edge. Movements of edgesof foundation have been deduced by measurement by consid-ering the foundation mass as a rigid body. The results obtainedare plotted in Figure 17. During the underexcavation period, theTower rotated northwards at an increasing rate, as the extrac-tion holes were drilled gradually ahead near the north boundaryof the foundation and below it. At the beginning of June 1999,when the operation ceased, the northwards rotation of the towerwas 90 seconds of arc; by mid-September it had increased to130”. At that time three of the 97 lead ingots acting on thenorth side of the tower were removed; the Tower then exhib-ited negligible further movements, until the beginning of thefull underexcavation.

The rotation in the east–west plane was much smaller, reach-ing a final value of approximately 10” westwards, as intended.The north side of the Tower foundation underwent an overallsettlement equal to 11 mm; in the mean time the south sidefirst heaved by 1.8 mm and then gradually settled by the sameamount, showing that the axis of rotation was located betweenthe two points, but near the south side. The behavior of thecatino was monitored by precision levelling on a number ofmeasuring points (Figure 18). The old catino underwent sig-nificant distortions, since the connection with the foundationexhibited the same displacement as the foundation itself, whilethe outer perimeter connected to the cylindrical wall had verysmall displacements; in spite of these distortions, no damageoccurred.

After the very encouraging results of the preliminary under-excavation experiment, the Committee went on to the full under-excavation. This time 41 holes were drilled; the layout in plan isshown in Figure 19. Some lateral holes were prepared to extractsoil just below the bottom of the catino, to make it follow the

FIG. 17. Graph of the effects of preliminary soil extraction.

FIG. 18. Layout of benchmarks for Catino leveling.

movements of the Tower without cracking. As it turned out theywere not used. Between February 21, 2000 and June 6, 2001,when the underexcavation operations ceased, extractions werecarried out, removing a total volume of approximately 38 m3

of soil. Approximately 60 % of this volume was removed from

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FIG. 19. Layout of boreholes for full underexcavation.

below the catino, which is outside the perimeter of the founda-tion. During the final period of soil extraction, the lead ingotsand the concrete beam on which they rested were progressivelyremoved. The results obtained by the full underexcavation areplotted in Figure 20. It may be seen that the goal of decreasingthe inclination of the tower by half a degree has been achieved.The settlement of the north side of the foundation was over160 mm, while the south side experienced a heave of 11 mm.Due to the large displacements of the foundation of the tower,the catino underwent distortions very similar to those experi-enced during the preliminary underexcavation, but much moreintense. In spite of a relative rotation as high as 3 %, no crackingoccurred.

4. CONCLUDING REMARKSThe stabilization of leaning tower of Pisa has been an

interesting challenge for geotechnical engineers for many andperhaps it became its symbol. During last decade of 20th cen-tury the attempts made for stabilization of the monument havebeen successful, but there is the consciousness that the mul-tidisciplinary approach (Burland et al. 2009) applied to theproblem was the base of the success. Geotechnical engineeringsupported this multidisciplinary approach in two aspects: appli-cation of observational method and modeling existing structure.Whereas the first point has been discussed above and it is quiteclear to define the contribution, for the second point some addi-tional explanation are necessary. The general method applied in

FIG. 20. Graph of the effects of full underexcavation.

geotechnical engineering consists in a first stage of investiga-tion about subsoil and eventually existing structures followedby a second stage of modeling. The stage of modeling is usu-ally created ad hoc for any single case, taking into accountthe peculiarities of involved materials both natural and “artifi-cial”. This logical sequence seems particularly appropriate tosolve problems connected to architectural heritage. In fact, anappropriate solution can be defined only by means of a deepunderstanding of the situation in order to properly model thephysical system. A deep understanding of the general situationcan be reached by means of appropriate investigation, extendedboth to monitoring of building (investigation about behaviorto environment factors) and understanding of construction his-tory (investigation about behavior during construction). Theexperience of Leaning Tower of Pisa put in evidence that anappropriate monitoring, intended as knowledge of behavior oftower since the beginning of its construction, lead to appropriatesolutions.

ACKNOWLEDGEMENTThe Authors would like to acknowledge the International

Committee for the Safeguard of the Leaning Tower, who

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gave permission to use information regarding the interven-tions. Further detailed information about the stabilization of theLeaning Tower can be found in MIBAC (2005) and Burlandet al. (2013).

REFERENCESBurland J. B., M. B. Jamiolkowski, N. Squeglia, and C. Viggiani. 2013. The

Leaning Tower of Pisa. In Geotechnics and Heritage, eds., E. Bilotta, A.Flora, S. Lirer, and C. Viggiani, Philadelphia, PA: CRC Press, 207–227.

Burland J. B., M. B. Jamiolkowski, and C. Viggiani. 2009. Leaning Towerof Pisa: Behaviour after stabilisation operations. International Journal ofGeoengineering Case Histories 1(3):156–169

Cestelli Guidi, C., A. Croce, A. W. Skempton, E. Schultze, G. Calabresi, andC. Viggiani. 1971. Caratteristiche geotecniche del sottosuolo della Torre. InRicerche e studi sulla Torre pendente di Pisa ed i fenomeni connessi allecondizioni d’ambiente, vol. I. Firenze, Italy: IGM, 179–200.

Cheney, J. A., A. Abghari, and B. L. Kutter. 1991, Leaning instabil-ity of tall structures, in Journal of Geotechnical Engineering, ASCECXVII(2):297–318.

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