Repair and Strengthening of the Foundations of Ancient Buildings in Acireale City (Sicily)

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 historical buildings in Acireale, a town in the outskirts of Catania in eastern Sicily after a flood in 1995 which caused differential settlements in the foundations and damage to the masonry structures. After a brief seismological history of the area and the description of the geotechnical investigation carried out to establish the properties of the foundation soil, the underpinning of the existing foundation by micropiles and its enlargement by r.c. beams is reported. The paper ends with the calculation of the bending moment in a micropile due to a horizontal force at its head. The nonlinear behaviour of the soil is taken into account in the calculation.

1 Introduction

The protection of historical cities and the retrofitting of ancient buildings is a problem of great interest in Italy (A.G.I/), that involves cultural and architectural aspects related to the preservation of historic site. So geotechnical criteria for stabilising historical buildings is one of the major problems to be concerned. While the causes of damages are in general of geotechnical nature, the remedial works fall in the areas of geotechnical and/or structural engineering.

554 Earthquake Resistant Engineering Structures

Acireale is a city of Sicily famous for its baroque churches and historical buildings. Due to the flood occurred on the 13 March 1995 in eastern Sicily, a lot of masonry buildings have been affected by foundations settlements that caused considerable damages to the overstructure with the risk of their collapse during an earthquakes. A geotechnical investigation have been performed to measure the foundation differential settlements and to design the remedial works of the buildings.

Provisional works have been made to avoid the risk of the collapse of the masonry buildings, then the remedial works for the definitive stabilisation were carried out. As Acireale is located in a seismic area, in the design of repair and strengthening works seismic aspects have been taken into account. The paper presents a view of ancient construction typologies of the city and reports the retrofitting of foundations, mainly consisting in the strengthening of the 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 has caused 5 victims and great damages to the public infrastructures and civil buildings in the province of Catania, has required urgent remedial works to eliminate the instability conditions of a lot of masonry buildings (Fig.l) located in the historical centre of Acireale (Sicily).

Figure 1: Plan view of the damaged buildings.

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The flood was caused by peculiar meteorological conditions which never occurred in the last 150 years. Due to heavy rainfall that to inadequate drainage patterns, some cavities took

place in the ground foundation causing considerable differential settlements and rotation of the front of the structures. The methodology adopted for analysing the conditions of masonry buildings has required preliminary recognisance that included: geometric and photographic survey, precision levelling of the ground floor level (Fig.2), detailed survey of

existing 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 structures by electronic inclinometers and automatic hydraulic levellometers installed on

the masonry has been performed. To guarantee the safety of the structure during the remedial works, this structural monitoring is considered a very reliable investigation method for the evaluation of the static conditions of the structures. Settlement surveys were carried out to assess the design of foundation

strengthening and they were extended during the remedial works period to check the effectiveness of stabilisation measures. In fact, appropriate geotechnical stabilisation measures require an understanding of the ground movements and their interaction with the buildings be means of observational method. Such

knowledge could be obtained by means of carefully planned long-term monitoring using simple precise surveying methods.

3 Seismicity of the Area

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

Figure 2: Levelling of the ground floor level.

556 Earthquake Resistant Engineering Structures

During the past centuries eastern Sicily was struck by strong earthquakes, mainly related to the earthquakes occurring in the seismogenetic source of the Hyblean foreland 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 been given by CNR/ A brief description of the most significant earthquakes can be summarized as following reported (CN.R/) The Catania earthquake of February, 4, 1169 is one of the oldest shocks of great magnitude whose detailed studies are available. The earthquake took place in the

Southern part of Sicily whose seismicity is characterized by very strong energy releases, which usually occurs after long quiet periods. The shock caused heavy damage The epicentral area was located near the city of Catania in which the intensity seems to have reached the XI degree in the MCS scale. On the 10th December 1542 another earthquake took place in the area, with epicentre located near the city of Sortino where the intensity was about IX degree. The "Val di Noto" earthquake of January, 11, 1693 is considered one of the biggest earthquakes which occurred in Italy. It is thought that more than 1500 aftershocks occurred for about two years more. The strongest earthquake intensity among the most destructive of all the times (in many centers with intensity 1 r degree of the MSK scale), struck a large territory of the southeastern of the Sicily and caused the partial, and in many cases the total, destruction of 57 cities 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, the VII degree. The magnitude was about M= 5.4. Due to the low intensity, damage, this time, interested only several old buildings and ancient monuments, causing only 5 deads.

4 Geotechnical Soil Foundation Properties

The nature of the ground foundation was carefully analised by investigations performed under the supervision of the Genio Civile di Catania, as well as by means of topographical reliefs and in situ and laboratory tests. Underground soil is constituted for the first 3 meters by fill and gravel and up to a depth of about

12 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 buildings were mainly due to the ground settlements caused by the cavities in the strata of gravel down of the foundation plain (Fig.3). Soil properties evaluated by laboratory tests are significantly different related to the various soil nature. From direct shear test, the angle of shearing resistance in terms of effective stress ranges between 24° and 44°, with an average value of 30° for the loose lava sand and 40° for the basaltic rock. Drained shear strength is equal to zero, while the rock failure resistance evaluated by compression tests result ranging between 51.8 and 123.5 MPa.

Earthquake Resistant Engineering Structures 557

------fine sand

Joose lava sand

LJILl UilJLLU! linn l111 JutnLn 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 any kind of remedial and restoration intervention of historic buildings.

Especially the technical progress achieved in the field of micro-piles and of small diameter borings allows restoration and consolidation works to be performed in almost every environmental condition, with almost no risk for the overstructures stability during foundation strengthening. In the examined case history remedial works were carried out in two different

stages: in a first stage, in order to reduce the risk of collapse of the masonry structures, some provisional works were realized on one side of the buildings. In particular, to avoid the possibility of increment of the rotation of the front of the structures some provisional steel structures were realized along the side of the building facing the street. The steel structures were founded on micro-piles.

The micro-piles was also fixed and bored into the masonry foundation for masonry strenghtening and underpinning (Fig.4). The second stage was devoted to the consolidation of the foundation along the whole perimeter and also along the cross sections. To reduce the stress on soil due to dead and leave load as well as the seismic action a reinforced concrete

beam 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 further settlements and possible failure of the structure. The analysis and design of an underpinning pile scheme is a complex problem,

558 Earthquake Resistant Engineering Structures

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

Figure 4: Reinforcement and provisional works on thefron to f 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 old foundation and design the underpinning piles to support the full load applied by the structure to the new foundation. Underpinning was carried out with "tubfix" micropiles, grouted at low pressure, with a hole diameter of 130 mm and steel tube 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 layer of stiff basalt. A loading test on an micro-pile under the working loads of 0.3

MN 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 was followed by means of levelling and by the electronic instrumentation monitoring. The original design of the retrofitting of the masonry structures was modified by the observational method and thefina ldesig n was made during the execution of the remedial works.

6 Numerical Analysis

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

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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 was carried out. The micro-piles settlements due to static loads were evaluated and then compared to the field test results. A very important aspect for design of a pile subjected to seismic actions, is represented by the evaluation of lateral deflection and 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-plastic material, assuming that the ultimate strength of the soil has been reached. On the contrary,fiel d tests on full-scale piles shows a non-linear soil behaviour and so a non-linear relationship of soil-pile interaction was taken into account. A computer code for the analysis of the behaviour of a single pile subjected to horizontal loads based on a pilefinit eelemen t discretization have been employed (Castelli et al7). The code takes into account the non-linearity of the soil-pile interaction employing hyperbolic or cubic parabolic p-y curves.

micropiles

Figure 6: Particular of underpinning by micro-piles.

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Hyperbolic p-y curve is similar to stress-strain relationship proposed by Duncan and 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 as follows (Matlock*):

=0.5 [- 03]3^ (2)

Pw(z) being y^o the horizontal pile deflection which occurs at 50% of the ultimate soil reaction. The main difficulty of applying this method is the correct determination of function's parameters, i.e. the ultimate soil reaction and the initial modulus of subgrade reaction, related to hyperbolic p-y curves, and the ultimate soil reaction and the deflection which occurs at 50% of the ultimate soil reaction, related to cubic parabolic p-y curves (Robertson et al.^; Castelli et al/*). By means of the computer code based on the abovementioned p-y curves, the numerical analysis of a single micro-piles subjected to an horizontal load of 29 kN was carried out. Employing the hyperbolic p-y curves, the initial modulus of horizontal subgrade reaction 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 to 2000 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 unit weight 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 then unable to rotate at the head. The maximum bending moment of 1 1.02 kNm at the micro-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 executed on the masonry buildings located in the historical centre of Acireale (Sicily), affected by considerable settlements of foundation and rotation of the front due

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Bending Moment [kNm]

-15 -5 0 5 10

E,4-

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 reduce the risk of collapse of the structures some provisional works by steel structures placed along the side of the buildings were realized. During the second stage the retrofitting of masonry foundation was carried out by micro-piles underpinning, taking into account the seismic actions. The retrofitting design of the masonry structures was modified by the observational method during the remedial works.

Aknowledgement

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

References

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

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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 University of Texas, Austin, May, 1978.

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