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Repair and strengthening of the foundations of

ancient buildings in Acireale city (Sicily)

F. CastelH/*) M. Maugeri, S. Minaldi,(*) Facolta di Ingegneria, Universita di Catania, Viale Andrea Doha 6,95125 - Catania, Italy. EMail: [email protected]^ Facolta di Ingegneria, Universita di Catania, Viale Andrea Doha 6,95125 - Catania, Italy. EMail: [email protected]^ Genio Civile di Catania, Via Lago di Nicito, 95124 - Catania, Italy

Abstract

This paper describes the strengthening of the foundations of some historicalbuildings in Acireale, a town in the outskirts of Catania in eastern Sicily after aflood in 1995 which caused differential settlements in the foundations anddamage to the masonry structures. After a brief seismological history of the areaand the description of the geotechnical investigation carried out to establish theproperties of the foundation soil, the underpinning of the existing foundation bymicropiles and its enlargement by r.c. beams is reported.The paper ends with the calculation of the bending moment in a micropile due toa horizontal force at its head. The nonlinear behaviour of the soil is taken intoaccount in the calculation.

1 Introduction

The protection of historical cities and the retrofitting of ancient buildings is aproblem of great interest in Italy (A.G.I/), that involves cultural andarchitectural aspects related to the preservation of historic site. So geotechnicalcriteria for stabilising historical buildings is one of the major problems to beconcerned.While the causes of damages are in general of geotechnical nature, the remedialworks fall in the areas of geotechnical and/or structural engineering.

Transactions on the Built Environment vol 38 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

554 Earthquake Resistant Engineering Structures

Acireale is a city of Sicily famous for its baroque churches and historicalbuildings. Due to the flood occurred on the 13 March 1995 in eastern Sicily, a lotof masonry buildings have been affected by foundations settlements that causedconsiderable damages to the overstructure with the risk of their collapse duringan earthquakes.A geotechnical investigation have been performed to measure the foundationdifferential settlements and to design the remedial works of the buildings.Provisional works have been made to avoid the risk of the collapse of themasonry buildings, then the remedial works for the definitive stabilisation werecarried out. As Acireale is located in a seismic area, in the design of repair andstrengthening works seismic aspects have been taken into account.The paper presents a view of ancient construction typologies of the city andreports the retrofitting of foundations, mainly consisting in the strengthening ofthe masonry foundations and in its underpiling.

2 Monitoring of the Damaged Masonry Buildings

The heavy flood occurred on the 13 March 1995 in eastern Sicily, that hascaused 5 victims and great damages to the public infrastructures and civilbuildings in the province of Catania, has required urgent remedial works toeliminate the instability conditions of a lot of masonry buildings (Fig.l) locatedin the historical centre of Acireale (Sicily).

Figure 1: Plan view of the damaged buildings.


Earthquake Resistant Engineering Structures 555

The flood was caused by peculiar meteorological conditions which neveroccurred in the last 150 years.Due to heavy rainfall that to inadequate drainage patterns, some cavities tookplace in the ground foundation causing considerable differential settlements androtation of the front of the structures.The methodology adopted for analysing the conditions of masonry buildings hasrequired preliminary recognisance that included: geometric and photographicsurvey, precision levelling of the ground floor level (Fig.2), detailed survey ofexisting cracks (Fig.3) and other damages induced on the structure and, finally,an analysis of the historical construction typology (Di Mauro ).During remedial works monitoring of deformational behaviour of the structuresby electronic inclinometers and automatic hydraulic levellometers installed onthe masonry has been performed.To guarantee the safety of the structure during the remedial works, this structuralmonitoring is considered a very reliable investigation method for the evaluationof the static conditions of the structures.Settlement surveys were carried out to assess the design of foundationstrengthening and they were extended during the remedial works period to checkthe effectiveness of stabilisation measures. In fact, appropriate geotechnicalstabilisation measures require an understanding of the ground movements andtheir interaction with the buildings be means of observational method. Suchknowledge could be obtained by means of carefully planned long-termmonitoring using simple precise surveying methods.

3 Seismicity of the Area

As Acireale is located in a seismic area, in the design of repair and strengtheningworks, seismicity of the area have been taken into account.

Figure 2: Levelling of the ground floor level.



During the past centuries eastern Sicily was struck by strong earthquakes, mainlyrelated to the earthquakes occurring in the seismogenetic source of the Hybleanforeland and inland and also to the seismogenetic source of the Messina Straits(Azzaro et al.*). A detailed list of the earthquakes which struck this area has beengiven by CNR/ A brief description of the most significant earthquakes can besummarized as following reported (CN.R/)The Catania earthquake of February, 4, 1169 is one of the oldest shocks of greatmagnitude whose detailed studies are available. The earthquake took place in theSouthern part of Sicily whose seismicity is characterized by very strong energyreleases, which usually occurs after long quiet periods. The shock caused heavydamage The epicentral area was located near the city of Catania in which theintensity seems to have reached the XI degree in the MCS scale.On the 10th December 1542 another earthquake took place in the area, withepicentre located near the city of Sortino where the intensity was about IXdegree. The "Val di Noto" earthquake of January, 11, 1693 is considered one ofthe biggest earthquakes which occurred in Italy. It is thought that more than 1500aftershocks occurred for about two years more. The strongest earthquakeintensity among the most destructive of all the times (in many centers with inten-sity 1 r degree of the MSK scale), struck a large territory of the southeastern ofthe Sicily and caused the partial, and in many cases the total, destruction of 57cities between which the greater than the area: Catania with 19.000 inhabitants,Modica (18.000), Siracusa (15.000), Acireale (13.000), Caltagirone (12.000),Noto (12.000), Vizzini (11.000), Lentini (10.000), Ragusa (10.000).On December, 13, 1990 another earthquake struck Sicily (Rovelli et al.*).Anyway the intensity was not too strong and reached, in the epicentral area, theVII degree. The magnitude was about M= 5.4. Due to the low intensity, damage,this time, interested only several old buildings and ancient monuments, causingonly 5 deads.

4 Geotechnical Soil Foundation Properties

The nature of the ground foundation was carefully analised by investigationsperformed under the supervision of the Genio Civile di Catania, as well as bymeans of topographical reliefs and in situ and laboratory tests. Underground soilis constituted for the first 3 meters by fill and gravel and up to a depth of about12 meters by loose fine sand and coarse lava sand, followed by very stiff basalt.The differential settlements and rotation of the front of the masonry buildingswere mainly due to the ground settlements caused by the cavities in the strata ofgravel down of the foundation plain (Fig.3).Soil properties evaluated by laboratory tests are significantly different related tothe various soil nature. From direct shear test, the angle of shearing resistance interms of effective stress ranges between 24° and 44°, with an average value of30° for the loose lava sand and 40° for the basaltic rock. Drained shear strengthis equal to zero, while the rock failure resistance evaluated by compression testsresult ranging between 51.8 and 123.5 MPa.



- ----- fine sand

Joose lava sand

LJILl UilJLLU! l i n n l111JutnLn basalt '

&twnm

Figure 3: Foundation soil profile.

5 Remedial Works

The most typical intervention reinforcement techniques for masonry foundation,derives from the micro-pile technique, which has been developed for almost anykind of remedial and restoration intervention of historic buildings.Especially the technical progress achieved in the field of micro-piles and of smalldiameter borings allows restoration and consolidation works to be performed inalmost every environmental condition, with almost no risk for the overstructuresstability during foundation strengthening.In the examined case history remedial works were carried out in two differentstages: in a first stage, in order to reduce the risk of collapse of the masonrystructures, some provisional works were realized on one side of the buildings. Inparticular, to avoid the possibility of increment of the rotation of the front of thestructures some provisional steel structures were realized along the side of thebuilding facing the street. The steel structures were founded on micro-piles.The micro-piles was also fixed and bored into the masonry foundation formasonry strenghtening and underpinning (Fig.4).The second stage was devoted to the consolidation of the foundation along thewhole perimeter and also along the cross sections. To reduce the stress on soildue to dead and leave load as well as the seismic action a reinforced concretebeam founded on micro-piles were carried out at the base of the masonry (Fig.5).Underpinning by micro-piles is a convenient approach to prevent furthersettlements and possible failure of the structure.The analysis and design of an underpinning pile scheme is a complex problem,



1 \y . _ , _ iI'X: .' -\ X _1 ^ .

Figure 4: Reinforcement and provisional works on the front of the buildings.

that requires to combine the behaviour of the old and new foundations systems.However, generally engineers tend to neglect the contribution of the oldfoundation and design the underpinning piles to support the full load applied bythe structure to the new foundation. Underpinning was carried out with "tubfix"micropiles, grouted at low pressure, with a hole diameter of 130 mm and steeltube reinforcement 10 mm thick with an external diameter of 88.9 mm (Fig.6).The micropiles, starting from ground level, going to a depth of-15 m in the layerof stiff basalt. A loading test on an micro-pile under the working loads of 0.3MN and to 0.95 MN, gave the settlement of 5.6 mm and 13.71 mm respectively;residual settlement on completely unloading of the pile of about 1.74 mm.Evolution of foundation settlements during the execution of micro-piles wasfollowed by means of levelling and by the electronic instrumentation monitoring.The original design of the retrofitting of the masonry structures was modified bythe observational method and the final design was made during the execution ofthe remedial works.

6 Numerical Analysis

In order to analyse the behaviour of the micro-piles subjected to the vertical and



Figure 5: Concrete beam on micro-piles realized at the base of the masonry.

horizontal load due to the seismic action, a pseudo-static numerical analysis wascarried out.The micro-piles settlements due to static loads were evaluated and thencompared to the field test results. A very important aspect for design of a pilesubjected to seismic actions, is represented by the evaluation of lateral deflectionand bending moment distribution along the pile subjected to horizontal pseudo-static force.The behaviour of laterally loaded piles is generally investigated by elastic theory,assuming that the soil behaves as an ideal elastic material, or as a rigid-plasticmaterial, assuming that the ultimate strength of the soil has been reached. On thecontrary, field tests on full-scale piles shows a non-linear soil behaviour and so anon-linear relationship of soil-pile interaction was taken into account.A computer code for the analysis of the behaviour of a single pile subjected tohorizontal loads based on a pile finite element discretization have been employed(Castelli et al7).The code takes into account the non-linearity of the soil-pile interactionemploying hyperbolic or cubic parabolic p-y curves.

micropiles

Figure 6: Particular of underpinning by micro-piles.



Hyperbolic p-y curve is similar to stress-strain relationship proposed by Duncanand Chang® and it is defined as:

y(z)P(z) = - (i)

1 + %vfz)£„• (z) p* (z)

being E,, [FL*] the initial modulus of horizontal subgrade reaction and p« [FL~*]the ultimate horizontal pile load. Cubic parabolic p-y curves can be defined asfollows (Matlock*):

033=0.5 [ - ] (2)Pw(z)

being y o the horizontal pile deflection which occurs at 50% of the ultimate soilreaction. The main difficulty of applying this method is the correct determinationof function's parameters, i.e. the ultimate soil reaction and the initial modulus ofsubgrade reaction, related to hyperbolic p-y curves, and the ultimate soil reactionand the deflection which occurs at 50% of the ultimate soil reaction, related tocubic parabolic p-y curves (Robertson et al. ; Castelli et al/*).By means of the computer code based on the abovementioned p-y curves, thenumerical analysis of a single micro-piles subjected to an horizontal load of 29kN was carried out.Employing the hyperbolic p-y curves, the initial modulus of horizontal subgradereaction was determined according to the relationship proposed by Welch &Reese : Esi = Esoi + (k; z), being z the depth, k; ranging between 4 and 15 N/cm*and Esoi the initial modulus of horizontal subgrade reaction to the ground surface.In the analysis the modulus Eg was assumed constant with depth and equal to2000 N/cm .The ultimate horizontal soil resistance was determined according to Broms^assuming a value of the angle shearing resistance equal to 30°, while the soil unitweight was assumed equal to 22 kN/m .In Fig. 7 is reported the bending moment distribution along the micro-pile length,obtained considering the micro-pile embedded in the concrete beam and thenunable to rotate at the head. The maximum bending moment of 1 1.02 kNm at themicro-pile head was close to the micro-pile yelding moment.Numerical analysis shows also that the horizontal pile deflection at the micro-pile head was 2.5 mm for the given seismic load.

7 Conclusions

In the paper are reported the remedial, repair and strengthening works executedon the masonry buildings located in the historical centre of Acireale (Sicily),affected by considerable settlements of foundation and rotation of the front due


-15


Bending Moment [kNm]-5 0 5 10

E,4-

IH

6 -

7 -

8 -

Figure 7: Bending moment distribution along the micro-piles lenght

to the flood occurred on the 13 March 1995 in the eastern Sicily.The buildings retrofitting has been made in two stages: in the first stage to reducethe risk of collapse of the structures some provisional works by steel structuresplaced along the side of the buildings were realized. During the second stage theretrofitting of masonry foundation was carried out by micro-piles underpinning,taking into account the seismic actions. The retrofitting design of the masonrystructures was modified by the observational method during the remedial works.

Aknowledgement

Authors wish to thank the Structural Engineer Mario Granata and the GraduateStudent Filippo Di Mauro for their contribution.

References

1. Associazione Geotecnica Italiana (A.G.I.), The Contribution of GeotechnicalEngineering to the Preservation of Italian Historic Sites. Proc. X ECSMFE,Florence, 1991.



2. Di Mauro F, Consolidamento delle fondazione di alcuni edifici nel centrostorico di Acireale. Graduated Thesis, University of Catania, (Italy), 1999.

3. Azzaro R, Barbano M.S., Moroni A., Mucciarelli M, Stucchi M., Theseismic history of Catania. Paper submitted to Journal of Seismology, 1999.

4. Consiglio Nazionale delle Ricerche (CNR), Un catalogo parametrico deiterremoti di area italiana al di sopra della soglia del danno. Linea di Ricerca"Sismicita" eG.d.L. "Macrosismica", G.N.D.T.,\991.

5. Consiglio Nazionale delle Ricerche (C.N.R), Atlas of isoseismal maps ofItalian earthquake., Sottoprogetto Rischio Sismico e Ingegneria Sismica, EdPostpischl,\991.

6. Rovelli, A., Boschi, E., Cocco, M., Di Bona, M, Berardi R., Longhi, G, //terremoto del 13 Dicembre 1990 nella Sicilia Orientale: Analisi dei datiaccelerometrici. Contributi allo studio del Terremoto della Sicilia Orientaledel 13 Dicembre 1990. Istituto Nazionale di Geofisica. Roma, 1991.

7. Castelli F, Maugeri M., Motta E, Nonlinear analysis of the deflection of apile subjected to horizontal loads. Rivista Italiana di Geotecnica, XXIX, (4),pp.289-303, (1995) (in Italian).

8. Duncan J, Chang C.Y., Nonlinear analysis of stress and strain in soil.,Journal SMFE Div., ASCE, (XCVI), SM-5, pp. 1629-1653.

9. Matloch H. Correlations for design of laterally loaded piles in soft clay.Proc.il Offshore Tech. Conf., Houston, Texas, (1), pp.577-594.

10. Robertson P.K, Davies M.P., Campanella R.G. (1989). Design of laterallyloaded driven piles using flat dilatometer. Geotechnical Testing Journal,GTJODJ, XII, (1), March,30-38.

11. Castelli F., Maugeri M., Motta E., Modeling of an horizontal loading test ona pile under an earthquake damaged building. Proc. 7° Convegno NazionaleL 'Ingegneria Sismica in Italia", Siena, (I), pp. 185-194 (1995) (in Italian).

12. Welch R.C., Reese L.C. (1972). Laterally loaded behaviour of drilled shafts.Research Report n.3-5-65-89, Center for Highway Research, The Universityof Texas, Austin, May, 1978.

13. Broms B.B,, Lateral resistance of piles in cohesionless soils. Journal SMFEDiv., ASCE, (XCI), SM-3, pp. 123-156.


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