Underexcavation of the Underground Soil of Pisa Tower

8/12/2019 Underexcavation of the Underground Soil of Pisa Tower

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STABILIZATION OF THE UNDERGROUND SOIL BELOW OF PISA TOWER

INTRODUCCTION

The importance to know the problems about the foundation on the towers, especially the Tower ofPisa as well as the towers that were built in Italy led us to study about their problems like what abouttheir leaning , their settlements the structural problems associate with them.The tower of Pisa is one of the most remarkable architectural structures from mediaval Europe. It islocated in the Italian town of Pisa, that is the most visited city in Italy.The tower was built in three stages across almost 200 years. The work of construction began on

August 14 , 1173 during a period of military success and prosperity. The ground floor was built first

and also it is a blind arcade articulated by engaged columns with classical Corinthian capitals.The tower began to sink after construction had progressed to the second floor in 1178.this was due to a mere three metre

The tower began to sink after construction had progressed to the second floor in 1178. This was dueto a mere three-meter foundation, set in weak, unstablesubsoil,a design that was flawed from thebeginning. Construction was subsequently halted for almost a century, because theRepublic of Pisawas almost continually engaged in battles withGenoa,Lucca,andFlorence.This allowed time for theunderlying soil to settle.

In 1272 construction was assumed byGiovanni di Simone,architect of theCamposanto.In an effort

to compensate for the tilt, the engineers built upper floors with one side taller than the other. Becauseof this, the tower became curved. Construction was halted again in 1284, when the Pisans weredefeated by the Genoas in theBattle of Meloria.

There were a lot of attempts to try to reduce the tilt of the tower or at least to try not increase theleaning since 1935 to 1970's. The first attempt was grouting into the foundation body and the soilsurrounding the cantino mainly, to prevent the inflow of water. The second attempt was the pumpingof water from deep aquifers, inducing subsidence all over the Pisa plain. By means of this procedurethe wells in the vicinity of the tower stopped the rate of tilt.

In 1989 there was an spectacular tower collapse occurred in Italy: that was the Civic Tower of Pavia,

with five deaths. A cause of that the attention of security in the Tower of Pisa increase hugely . TheGovernment studied the problem and they decided to close the Tower of Pisa to the visitors followingthe recommendations of the safety laws. In 1990 the Italian Government, concerned about theprogressive increase in the rate of inclination and the risk of sudden structural collapse due to thefragility of the masonry, appointed a multidisciplinary International Committee for the safeguard andthe stabilization of the Leaning Tower of Pisa chaired by a geotechnical engineer and formed by arthistorians, restorers, structural engineers and geotechnical engineers. An exhaustive description of
http://en.wikipedia.org/wiki/Subsoilhttp://en.wikipedia.org/wiki/Republic_of_Pisahttp://en.wikipedia.org/wiki/Republic_of_Genoahttp://en.wikipedia.org/wiki/Republic_of_Luccahttp://en.wikipedia.org/wiki/Florencehttp://en.wikipedia.org/w/index.php?title=Giovanni_di_Simone&action=edit&redlink=1http://en.wikipedia.org/wiki/Piazza_dei_Miracoli#Camposantohttp://en.wikipedia.org/wiki/Battle_of_Meloria_%281284%29http://en.wikipedia.org/wiki/Battle_of_Meloria_%281284%29http://en.wikipedia.org/wiki/Piazza_dei_Miracoli#Camposantohttp://en.wikipedia.org/w/index.php?title=Giovanni_di_Simone&action=edit&redlink=1http://en.wikipedia.org/wiki/Florencehttp://en.wikipedia.org/wiki/Republic_of_Luccahttp://en.wikipedia.org/wiki/Republic_of_Genoahttp://en.wikipedia.org/wiki/Republic_of_Pisahttp://en.wikipedia.org/wiki/Subsoil


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the Committee activities, including those focused on the structural stability and the consequent needsfor masonry strengthening.

DEVELOPMENT

As the one of the most important building and attraction of the architecture . The was always themain concerned about its tilt and its settlement. Because of this from its construction they knew thatthe problems of the tower the first attempt of correct its sink was to build upper floor thinking that bymeans of the weight of the floors above the tower was going to get to their right position. Mostprecise measurement was implement in 1911 when the inclination of the Tower has been increasingin greater values across the year and thy doubled since 1930's . There was very much debate aboutthe reason of the progressive inclination of the Tower. The main reason was mainly attributed tocreep in the underlying soft marine clay and it was a reasonable reason to why the south side of thetower was settling more than the north side.A careful geotechnical and geodetic study was carried out since 1911 showed the motion of thefoundations which was radically different that previously ideas held.The theodolite measurements showed that the first cornice had not moved horizontally apart fromtwo occasions in 1934 and the early 1970s when man had intervened.Also, precision level measurements which commenced in 1928 showed that the centre of thefoundation plinth had not displaced vertically relative to the surrounding ground. Therefore, the rigidbody motion of the Tower could only be as shown in Fig. 5, with an instantaneous centre of rotationat the level of the first cornice vertically above the centre of the foundation. "The direction of motionof points FN and FS are shown by vectors and it is clear that the foundation has been movingnorthwards with FN rising and FS sinking (Burland and Viggiani, 19941)." The discovery thatthe motion of the Tower was as shown in Fig.1 turned out to be crucial in four respects:1. The form of motion is consistent with the phenomenon of leaning instability rather than animminent bearing capacity failure (Hambly, 1985). In simple terms, 2leaning instability of a tallstructure occurs at acritical height when the overturning moment generated by a smallincrease in inclination is equal to or larger than the resisting moment generated by thefoundations.No matter how carefully the structure is built, once it reaches the critical height thesmallest perturbation will induce leaning instability. As pointed out by Hambly: leaning instabilityis not due to lack of strength of the ground but is due to insufficient stiffness.2. The observation that the north side had been steadily rising led directly to the suggestionthat the application of a lead counterweight to the foundation masonry on the north side couldbe beneficial as a temporary stabilizing measure by reducing the overturning moment(Burland et al, 19933).3. The pattern of ground movements depicted in Fig. 1 led to the important conclusion that the seat ofthe continuing long-term rotation of the Tower lies in Horizon A and not within the underlying marineclay as had been widely assumed in the past. It can therefore be concluded that the latter stratum

must have undergone a considerable period of ageing since the end of construction. The ageing

1Geotechics and Heritage Burland, J.B., Jamiolkowski, M.B. and Viggiani, C. (2013). page 215

2Burland, J.B., Jamiolkowski, M.B. and Viggiani, C. (2013). The stabilisation of the Leaning Tower of Pisa. Soils and

Foundations pp. 63-803Burland, J.B., Jamiolkowski, M.B., Lancellotta, R. Leonards, G. and Viggiani, C. (1993). The Leaning

Tower of Pisa, what is going on? ISSMFE News, Vol. 20, No. 3.


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resulted in an increased resistance to yield a conclusion that proved to be of great importance inthe successful computer modeling of the application of the temporary counterweight (Burland andPotts, 1994).4. In the light of the measured motion of the Tower foundation, and consistent with the seat of themovement lying within Horizon A, it was concluded that, in addition to creep, the most likely cause of

the progressive seasonal rotation was a fluctuating ground-water level due to seasonal heavyrainstorms that occur between September and December every year. Accordingly a number of stand-pipes were installed in Horizon A around the Tower.Measurements made over a number of years have confirmed this hypothesis. Commencement ofrotation each year coincides with very sharp rises in the ground water level in the Horizon A followingeach heavy rainstorm Fig.2 shows the ground water level fluctuations for a selected period of time inthe piezometers located to the North and to the South in the vicinity of the Tower. The insert figuresto the right show the changes in inclination of the Tower in arc seconds as a result of two heavyrainstorms that occurred in September and October of 1995. Each of these events caused a largerrise in piezometric head on the North side than the South side of the Tower. This resulted in asouthward rotation of about one arc second in each case which was only partly reversible.The leaning instability of the Tower has been investiagated by a number of different approaches,including small scales physicla tests at natural gravity and in the centrifuge, and Finite Elementanalyses based on different constitutive models of subsoil. The analyses led to the conclusion thatthe gradual increase of the inclination would have ended in a collapse. Another very significantconclusion was that a decrease of the inclination, even a relatively minor one, results in a substantianincrease in the safety against leaning instability.Understanding the motion of the foundations of the Leaning Tower of Pisa is perhaps the single mostimportant finding in the development of the strategies for both the temporary and long-termstabilization.to begin the stabilization works it was made temporary interventions being aware that thestabilizationn works took a long time measures was conceive. It was designed and implementedpermanent stabilization measures. The Committee took the decision to implement temporary andreversible interventions to improve the safety against overturning and get or gain time to implementthe best permanent solution. A total of 6.9 MN of lead ingots were installed between the years of1993-1994 in the north edge of the base of the Tower. The weights induced a change of inclination of33'' by February 1994: by the end of July it had increased to 48'' and eventually to 53'' by February1994 . The average settlement of the Tower of the surrounding ground was about 2.5 mm. Thesettlement and rotation produced by the counterweight had been predicted by finite element model.The agreement between prediction and observation was satisfactory, increasing the confidence ofthe model.4FIG 3Another measurement of temporary stabilization was made for various reasons, the activity wasinterrupted for periods up to months and the fear that The Committee could dissolve as the precedinghave done and the concern that the ingots stay on the Tower by months even decades induced to

take a medium term temporary measurement was developed to replace the lead weights with tentensionated steel cables anchored in the lower sands at a depth of over 40 m FIG 3 .With the mainadvantage to be invisible and additional benefits of this scheme was the increased lever arm thatwould give a stabilizing moment larger than ingots. The major problem of the ten anchors solutionwas that the anchors had to be connected to the Tower foundation through a ring beam to be

4Geotechics and Heritage ;Burland, J.B., Jamiolkowski, M.B. and Viggiani, C. (2013). page 221


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constructed below the floor of the cantino, and this involved an excavation around the Tower belowground water level and operation pretty delicacy. After many careful comparisons of differentspossibilities it was decided to use local ground freezing immediately below the cantino floor but thewell above the Tower foundations level. According investigations it was drilled cores and it wasdiscovered that below the cantino floor there was a concrete bed of 1 m of thickness, set there

around in 1837 and partly in 1935. The cracks at the interface between the concrete and the Towerfoundation, led the conclusion that the two bodies were nor connected, as a consequence the volumeof variations of the frozen soil during freezing and thawing were expected not to influence the tower.The freezing was commenced on the North side and the northern sections of the ring beam weresuccessfully installed. They were connected to the foundation by means of of stainless steel rods,cemented in the foundation masonry FIG 4During these operations, the water tightness of the cantino was partly destroyed and two pumps hadto be installed to prevent flooding, since a sand layer was provided below the ring beam sections thesystem worked as a ground water level control.In September 1995 freezing was commenced on the south-west and south-east sides, and the Towerbegan to rotate to southward; the movement was also affected by an attempt of installing some micropiles at the south boundary of the cantino. After some attempts of controlling the rotation by theapplication of further lead weights at north, the operation was abandoned.The final intervention was made take in count a deep insight into the behaviour of the tower, throughthe interpretation of its history, the scrutiny of the measurements taken in the last century and theanalysis of the phenomenon of leaning instability. It was concluded that a decrease of the inclinationof the Tower by a half a degree its around to 1800 arc seconds or about the 10% of the inclination thathe tower had 1990 would be sufficient to stop the progressive increase of inclination and tosubstantially improve the stability conditions. At the same time, such a reduction was consideredsmall enough not to be perceived at a first glance. The decrease had to be obtained by inducinf adifferential settlement of the tower opposite to existing one by acting on the foundation soil and not onthe tower. Among other advantages, such a solution is perfectly respectful of the formal, historic andmaterial integrity of the monument.The Committee studied three possible means to achieve the decrease of the inclination

1. to construct a ground pressing slab to the north of the tower2. to induce the consolidation of the Pancone clay north of the Tower by electro-osmosis;3. to remove a small controlled volumes of the soil beneath the north side of the foundation

UNDEREXCAVATION.

the three preceding procedures was subjected of extensive numerical modelling.The electro-osmosis showed that cannot be completely controlled and could cause a dangerousphenomena such as pore pressure increase may occur. For this reason this option was ruled out.Small scale model test of a favourable response, encouraging the Committee to undertake a largescale experiment. To explore the operational procedures and developed the field of equipment.

The main purpose of the large-scale trials was to develop the drilling technology for soil extraction. Adrill was developed which consisted of a hollow-stemmed continuous flight auger housed inside acontra-rotating 168mm diameter casing (Fig8). The arrangement permits the drill to be advanced withminimum disturbance to the surrounding ground.When a chosen location is reached the drill isstopped and withdrawn by about a meter leaving a cylindrical cavity. The trials showed that thecavities formed in the silty soil of Horizon A closed gently and that repeated extractions could bemade from the same location. The trial foundation was successfully rotated by about 0.25o anddirectional control was maintained even though the ground conditions were somewhat non-uniform.


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Very importantly, an effective system of communication, decision making and implementation wasdeveloped. This system consisted of a daily report from the site to the responsible engineer of theresponse of the foundation to the previous days soil extractions. The responsible engineer thenissued a signed document in which the previous days response was summarized and analysed, theobjectives of the coming days soil extraction were set out and instructions given for the locations and

volumes of the next soil extractions.its response. This preliminary underexcavation was to be carried out over a limited width of 6m northof the Tower using twelve bore holes lined with 219mm diameter casings (Fig. 9). On 9th February1999, in an atmosphere of great tension, the first soil extraction took place. The Tower slowly beganto rotate northwards. When the northward rotation had reached about 80 arc seconds by early June1999 the preliminary soil extraction was stopped. Northward rotation continued at a decreasing rateuntil October 1999.The success of the preliminary underexcavation persuaded the Committee that it was safe toundertake soil extraction over the full width of the foundationFIG5. Accordingly, between December1999 and January 2000, 41 extraction holes were installed north of the Tower at 0.5m spacing with adedicated auger and casing in each hole . Full underexcavation commenced on 21st February 2000and the Tower was steered northwards in a remarkably straight path. Towards the end of May 2000progressive removal of the lead ingots was commenced. Although this resulted in an increase ofoverturning moment the soil extraction continued to be effective.To prevent any unexpected adverse movement of the monument, a safeguard structure wasnecessary FIG6.On 16th January 2001 the last lead ingot was removed from the post-tensioned concrete ring andthereafter only limited soil extraction was undertaken. In the middle of February 2001 the concretering itself was removed and at the beginning of March progressive removal of the augers and casingscommenced with the holes being filled by a bentonitic grout. The final extraction and auger removaltook place on 6th June 2001 at which time the Tower had been rotated northwards by about 1800 arcseconds the response of the Tower are given by Burland et al, (2003).5

The goal of reducing the inclination of the tower by half a degree has been reached.The intervention brought the Tower back to the position it had at the begining of the XIX century ,just after the excavation of the cantino. It can be seen as a reparation to the incautious undertaking ofthe architect Gherardesca, with another well- concieved and carefully conducted excavation.It is important to add that the study of the movements of the Tower revealed that the oscillations ofthe ground water table consequent to heavy rainfalls exerted a small negative influence on themonument. As a matter of fact, the ground water table at the south of the foundation is around 0.4 mhigher than that at north, so that the net result of the underpressure on the Tower is a smallstabilizing moment.During intense rainfalls events the two levels tend to equalize, thus producing a small overturning

moment on the monument; it is believe that the cumulative effects by ratchettig of these repeatedimpulses has been one of the factors producing the steady increase of the inclination in the longterm.

5Burland, J.B., Jamiolkowski, M.B. and Viggiani, C. (2000). Underexcavating the Tower of Pisa: Back to the future.

GEOTECH-YEAR 2000, Developments in Geotechnical Engineering,


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As a final intervention, a drainage system was installed in April 2001 at north side of the Tower ,essentially aimed at stabilizing the groundwater level in the vicinity of the Tower. It produced a furtherreduction of the inclination of around 60 seconds of arc that can be clearly seen in the FIG. 7.As a result of the full under excavation and of the implementation of the ground water control inHorizon A, the Tower undulation mid 2002 had reduced its tilt by 1880 arc seconds - about 10% of

the maximum value reached in 1993. In the succeeding six years the Tower has continued rotatingnorthwards at a reducing rate, so that by September 2008 the accumulated reduction of thefoundation tilt reached 1948 arc seconds, see Fig.16. In the two years from September 2006 oSeptember 2008 the residual rate of rotation northwards has reduced to less than 0.2 arc second peryear.

The settlements of the south edge, centre and north edge of the foundation generated by thestabilization operations, and mainly by full under excavation are shown in the FIG8.In September 2008 the center of the foundation had settled around 90 mm and is continuing to settleat a that rate, over the last two years is less than 1.0 mm per year.to predict a future scenario and predict the behaviour is not simple because is a complex phenomena. It can be present two possible scenarios the optimist that we can hope that the progressive increaseof the inclination has been definitely stopped and the monument keeps withour any motion, apartfrom the cyclic movements connected to the environmental action such as the seasonal groundwatertable fluctuation, The drainage system is kind of effective if it have a proper maintenance.The pesimist scenario sees the towe staying motionless for a period to decades followed by aresumption of the southward rotation with steadily increasing rate and approaching againt the valuehad before.The geotechnical stabilization has been finally attained, the behaviour of the monument across theyears will confirm it.


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FIGURES:

FIG Tower of Pisa Source:Wikipedia

FIG 16CROSS SECTION

6


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FIG 3 SOIL PROFILE

FIG 4 MOTION OF THE TOWER FOUDATION


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FIG 5

LEAD COUNTER WEIGHT IN THE NORTH SIDE OF THE TOWER

FIG 6 THE UNDEREXCAVATION


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FIG 7 SCHEME OF THE SAVEGUARD STRUCTURE WITH STEEL STAYS


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FIG 8 THE DRAINAGE SYSTEM

FIG 8 SETTLEMENT OF THE TOWER FOUNDATION AS A RESULT OF STABILIZATION

WORKS


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REFERENCES

BURLAND, J.B., JAMIOLKOWSKI, M.B. AND VIGGIANI, C. (2003). THE STABILISATIONOF THE LEANING TOWER OF PISA. SOILS AND FOUNDATIONS

INTERNATIONAL JOURNAL OF GEOENGINEERING CASE HISTORIES , VOL. 1, ISSUE3, P. HTTP://CASEHISTORIES.GEOENGINEER.ORG

HAMBLY, E.C. (1985). SOIL BUCKLING AND THE LEANING INSTABILITY OF TALLSTRUCTURES. THE STRUCTURAL ENGINEER,

GEOTECHICS AND HERITAGE ;BURLAND, J.B., JAMIOLKOWSKI, M.B. AND VIGGIANI,

GEOTECHNICAL ENGINEERING FOR THE PRESERVATION OF MONUMENT

ANDHISTORIC SITE

HTTP://CASEHISTORIES.GEOENGINEER.ORG/VOLUME/VOLUME1/ISSUE3/IJGCH_1_3

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Underexcavation of the Underground Soil of Pisa Tower

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