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Ground penetrating radar investigations for the restoration of historic buildings: the case study of the Collemaggio Basilica (L’Aquila, Italy)

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Ground penetrating radar investigations for the restoration of historic buildings: the case study of the Collemaggio Basilica (L’Aquila, Italy)
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  Case report Ground penetrating radar investigations for the restoration of historicbuildings: the case study of the Collemaggio Basilica (L’Aquila, Italy) Danilo Ranalli, Marco Scozzafava, Marco Tallini *  Dipartimento di Ingegneria delle Strutture, delle Acque e del Terreno, Università dell’Aquila, Monteluco di Roio, 67040 L’Aquila, Italy Received 11 February 2003; accepted 1 May 2003 Abstract Ground penetrating radar (GPR) surveys were applied in the preliminary stage of a project of structural monitoring and restoration of thefacadeoftheCollemaggioBasilica,amedievalchurchlocatedinL’Aquila(centralItaly).GPRsurveyswereveryusefulinevaluatingthestateof conservation of the facade and in identifying the thickness of its walls, the forms and deterioration of its masonry with its ashlar facing andrubble core, and the forms and locations of its middle cornice supports. GPR was demonstrated to be an ideal non-destructive method toinvestigate ancient structures of high cultural and historical value.© 2004 Elsevier SAS.All rights reserved. Keywords:  Ground penetrating radar; Non-destructive method; Italy; Historical building; Medieval masonry technique 1. Research aims Non-destructive survey methods are being increasinglyused in different applications, especially in the conservationof cultural and historical heritage. The main feature of thesemethods is their capability of investigating a site or a struc-ture non-invasively, i.e., without digging, boring or alteringits srcinal composition or shape.Detailed knowledge of the internal masonry structure of historical monuments is key to their restoration. In general,such structure is composed of different types of stones,bricks, with wooden or iron elements inserted into walls andcavities as ties, etc. [1]. Wall thickness and type of founda-tions are also useful information for the planning of struc-tural conservation efforts. Moreover, the recognition of de-tachments and cracks is crucial to verify the stability of buildings.As is obvious, preference should be given to non-destructive techniques while destructive ones (boring anddigging) should be minimised, especially when the buildingsinvolved are highly deteriorated or ancient.Among non-destructive techniques, GPR is the one whichprovides the most interesting results. The complete architec-tural framework of a building, obtained with GPR or othernon-destructive approaches, makes it possible to plan activi-ties of structural monitoring, conservation, restoration andstabilisation. 2. Experimental 2.1. Introduction Non-destructive GPR surveys were conducted as part of aproject of conservation of the Collemaggio Basilica, an im-portant medieval church located in L’Aquila. The surveyswere expected to make available a large set of data on thefacade of the basilica, with a view to planning the restorationof its masonry and mitigating its vulnerability to seismicevents. The project required the collection of data on wallthickness, internal masonry structure and location of detach-ments or cracks. 2.2. Historical and architectural outlineof the Collemaggio Basilica The Collemaggio Basilica (Fig. 1), located in L’Aquilaand built in local and composite Gothic style [2], is the mostfamous medieval church of Abruzzi (central Italy). Its con-struction began in 1287 under Peter of Morrone, the futurePope Celestino V, who was crowned there in 1294. * Corresponding author.  E-mail address:  tallini@ing.univaq.it (M. Tallini).Journal of Cultural Heritage 5 (2004) 91–99www.elsevier.com/locate/culher© 2004 Elsevier SAS.All rights reserved.doi:10.1016/j.culher.2003.05.001  During the 14th and 15th centuries, the Collemaggio Ba-silica underwent several changes, both for improving itsappearance and for repairing the damage caused by earth-quakes (the main ones occurred in 1315, 1349, 1461,1703 and 1915).The wide facade of the basilica is characterised by amagnificent geometrical arrangement of coursed ashlars of white and pink local limestone and marly limestone (Fig. 1).The facade wall consists of inner and outer faces of stonewith coarse-grained rubble core, as confirmed by historicalphotos of the facade damaged by the 1915 earthquake. Themain portal in Gothic style, which was built in the 15thcentury, is decorated with niches, which previously accom-modated statues.The wonderful Gothic rose window of the basilica con-sists of a double row of small spiral-shaped columns and of light-weighttrefoilarches,whileitscorniceisdecoratedwithflowers and leaves. 2.3. The GPR method  GPR is a non-destructive method that uses electromag-netic waves to investigate the underground or the internalstructures of natural or human-made objects [3], especiallymetal objects and structures, as well as caves and voids. Itswave frequency ranges from 10 to 2000 MHz. The GPRsystem consists of a data acquisition unit and of two (trans-mitter and receiver) antennas. The transmitter sends an elec-tromagnetic wave pulse which is reflected back to the re-ceiver by an anomaly, if there is a difference in the dielectricconstant between the anomaly and the surrounding environ-ment. The maximum depth of wave propagation depends onthe wave frequency and on the dielectric constant of theinvestigated medium. The higher the frequency, the smallerthe depth and the better the spatial resolution of the signal.The lower the wave frequency, the higher the depth and thesmaller the spatial resolution. For example, a 100 MHzantenna can investigate a medium down to a depth of about15–20 m with a resolution of about 10–20 cm; a 600 MHzantenna (used in this study) can reach about 4–5 m with aresolution of about 2–5 cm, while a 1600 MHz antenna (theother one used in this study) can reach about 1 m with aresolution of about 1 cm.The output of the GPR survey is a radar section of theinvestigated medium, where a point-shaped anomaly is out-lined by a hyperbolic trace. The  X   axis corresponds to thedirection of scanning, the  Y   axis is the depth. The datacoming from the reflected echoes, saved in the data acquisi-tion unit, are processed by filtering algorithms. The interpre-tation of the radar section permits the recognition of radar Fig. 1 .  Collemaggio facade during GPR data acquisition: note its magnificent light red and white ashlar facing (local limestone and marly limestone).92  D. Ranalli et al. / Journal of Cultural Heritage 5 (2004) 91–99  anomalies and their spatial location. These anomalies mayreflect actual discontinuities, objects or voids.Generally, the sectors in which GPR is successfully ap-plied are Civil Engineering [4,5] and Engineering Geology [6,7], Geology [8] and Geoarchaeology [9–11]. In the past years, GPR has also been applied to the characterisation of buildings of high cultural and historical value for conserva-tion and restoration projects [12]. 2.4. Experimental data The goal of this study was to obtain the largest possibleamount of data on the masonry of the basilica facade.The600MHzantennawasusedtoidentifywallthickness,while the 1600 MHz one was used to detect the internalfeatures of its masonry and the possible detachment of itsashlar facing from the rubble core. Data were collectedthrough a grid of radar sections of the facade for both anten-nas. Scan spacing was equal to about 40 cm. The mesh andlocationofthegridofradarsectionswerechosenonthebasisof the geometrical arrangement of the white and pink ashlars(Figs.1and2).Radarsectionswerelocatedalongtheverticalorhorizontal(course)alignmentoftheashlarswithameshof three (rarely four) courses. The radar scanning lines wereplacedinthemiddleofeachcourse.So,theregulargeometri-cal arrangement of the ashlars facilitated the scanning of thefacade without using a complex coordinate system.The GPR scanned the entire facade (Fig. 2), namely thelowerbandbelowitsmiddlecornice,thepartarounditsthreeportals and its two lateral rose windows.The survey was alsoextended to two square areas located on the right and leftsides of the central rose window above the middle cornice. 2.4.1. Technical details of GPR data acquisition For investigating the facade, use was made of a bistaticGPR with 600 and 1600 MHz antennas and special softwarefor data collection, processing and interpretation [13]. Thewave velocity was set to 10 cm/ns. The signal acquisitiontime was set to 128 ns for the 600 MHz antenna and to 40 nsfor the 1600 MHz one, corresponding to maximum depths of about 6 m (600 MHz) and 2 m (1600 MHz). The collecteddata were processed with the GP0 software [14]. Two filterswere applied to all the radar sections. The first filter (“soilsample”) removed the effect of distortion due to the air–ground interface between the GPR antenna and ground. Asthis phenomenon causes a down-shifting of the radar sec-tions, the filter was necessary to shift the sections upwards.Thesecondfilter(“passband”)removedbackgroundnoiseinthe vertical and horizontal directions. 3. Results 3.1. GPR surveys with medium-frequency antenna(600 MHz) The thickness of the facade wall was measured with600MHzradarsections.Totalthicknesswaswellrecognisedin almost all the facade. The radar signals proved to be lessclear in zones where the inner face of the wall has architec-tural or decorative elements (e.g., columns and arches alongthe nave and the two side aisles) (Fig. 3).The right side of the facade wall proved to be about 20 cmthinner than the left side, except in its lower band, which wasabout 10 cm thinner (Fig. 4).This finding was justified by the presence of the tower(Figs. 1 and 4) on the right side of the facade, making it less Fig. 2 .  Map of GPR survey grid (1600 and 600 MHz radar sections).93  D. Ranalli et al. / Journal of Cultural Heritage 5 (2004) 91–99  susceptible to earthquakes.The thicker left side of the facadewall demonstrated that the tower was surely not built later. 3.2. GPR surveys with high-frequency antenna(1600 MHz) The 1600 MHz GPR survey identified the structural fea-turesofthefacademasonryandthedetachmentsoftheashlarfacing from its rubble core. Also the middle cornice-supporting structures of the facade were identified. The1600 MHz radar sections also showed that the masonrystructureoftheinvestigatedportionofthefacadeconsistedof inner and outer faces of stone with rubble core.The interpretation of the 1600 MHz radar sections wasmore difficult because the masonry structure is heteroge-neous. In the radar sections, there were many radar echoeswhichmightinterferewithandhideanomalysignals(detach-ments, voids, degraded mortar). Fig. 3 .  Radar sections (600 MHz) showing wall thickness: the strong horizontal anomaly (arrow) marks the masonry–air interface. (a)Wall thickness: 120 cm;(b) wall thickness: 110 cm; (c) wall thickness: 90 cm.Fig. 4. 3D image of the basilica facade; thickness values are in cm. The left side and bottom part of the facade wall have the highest values.94  D. Ranalli et al. / Journal of Cultural Heritage 5 (2004) 91–99  Anyway, the interpretation of the radar sections made itpossible to make the following interesting observations.Generally, the radar sections showed the thickness of theashlars (20–30 cm) and of the internal masonry structure(Fig.5).Thescanswerecarriedoutinthezonesofthefacadewheresomeashlarshadbeenpreviouslyremoved.Itwasthuspossible to calibrate and interpret the radar sections withdirect observations (Fig. 6). Fig. 5 .  One thousand and six hundred mega hertz radar section showing anomalies of the ashlar facing thickness (1), middle cornice-supporting through stones(2), joints between the ashlars covering the middle cornice-supporting through stones (3).Fig. 6. Zones where ashlars were removed for a check and their location on the facade: (a) thickness of one of the ashlars (about 25 cm); (b) fractures of ansrcinal medieval ashlar.95  D. Ranalli et al. / Journal of Cultural Heritage 5 (2004) 91–99
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