Construction and Building Materials 47 (2013) 689–700

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Construction and Building Materials

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Ground penetrating radar investigation of the bell tower of the churchof the Holy Sepulchre

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.05.036

⇑ Corresponding author. Tel.: +30 210 7723276.E-mail address: amoropul@central.ntua.gr (A. Moropoulou).

Kyriakos Labropoulos, Antonia Moropoulou ⇑National Technical University of Athens, School of Chemical Engineering, 9 Iroon Polytechniou Street, Zografou Campus, Athens GR 15780, Greece

h i g h l i g h t s

� GPR identified areas in Bell Tower masonry that entail risk of structural failure.� GPR supported the correlation between weathering and construction technology.� GPR can assess the effectiveness of conservation interventions.� Need for employing GPR to provide information from the interior of structures.

a r t i c l e i n f o

Article history:Received 18 February 2013Received in revised form 19 April 2013Accepted 4 May 2013Available online 11 June 2013

Keywords:Ground penetrating radarNon-destructive testingChurch of the Holy SepulchreBell TowerMasonry structure

a b s t r a c t

The 12th century Bell Tower of the Church of the Holy Sepulchre has sustained significant damage in1545 from an earthquake. Subsequent interventions did not address the structural damage, until 2001,when the Israel Antiquities Authority and The Technical Office of the Greek Orthodox Patriarchate of Jeru-salem implemented a project for the conservation of the Bell Tower. More recently, Ground PenetratedRadar was utilized, as part of a preliminary diagnostic study, and demonstrated the technique’s abilityto evaluate the state of the structural cracks of the Bell Tower and to assess the effectiveness of past con-servation interventions.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction The present Bell Tower of the Church of the Holy Sepulchre

The Church of the Holy Sepulchre (Church of the Resurrection)in the city of Jerusalem is one of the most important historicalsites of Christianity and according to the tradition is the sceneof Jesus Christ’s death and resurrection. The Church dates backto 325 AD, when Emperor Constantine I ordered the constructionof a basilica incorporating the tomb of Christ and the hill ofGolgotha [1,2]. This building was damaged by the fire in 614when the Persians, under King Hosroes II, invaded and destroyedJerusalem. In 617, Patriarch Modestos organized the erection ofthe Holy Places. In 1009, the Caliph of Egypt El-Hakem orderedthe complete destruction of the church. During the reign of Con-stantine Monomachus, Patriarch Nikiforos succeeds in persuadingthe Emperor to offer money for the reconstruction of the HolySepulchre (1027–1048). After the capture of Jerusalem by theCrusaders (1099) the Holy Sepulchre was repaired and furtherreconstructed.

dates back to 1179, and its lower part is incorporated in the Churchof the 40 Martyrs. An 11th century octagonal Bell Tower preceededthe present building but was completely destroyed. In 1545, anearthquake caused damage to the Bell Tower resulting in the col-lapse of the dome and of its base. The Bell Tower remained in thisstate until 1720, when the upper two floors were demolished; partof the floor inside the Bell Room gets raised to enable drainage ofrain water, the window sills at the west side are raised, a new crossvault in the interior of the lower floors is added, and an exteriorarch on the south side is constructed. In the 1808 fire that causedthe dome of the Rotunda of the Church of the Holy Sepulchre tocollapse and sustained significant damage to the building, no firedamage was observed at the northern facade of the Bell Tower.The damaged church was rebuilt by 1810 by the architect KalfasKomnenos. In 1895, a wooden roof was added on the remainingfloors of the Bell Tower. In 1927, an earthquake registering 6.3on the Richter scale and caused significant damage to the church,which was no longer considered safe and parts of it, mainly the Ro-tunda, were supported by metal scaffolding. During the BritishMandate, the Bell Tower was repointed and three metal fastenerswere installed around the structure to strengthen it.

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The Bell Tower is constructed at the west side of the Entrance ofthe Church of the Holy Sepulchre. It is a three-floor structure with aheight of 27.40 m [3]. The first floor (ground level) is incorporatedinto the Chapel of the 40 Martyrs. Its north side is adjacent to theChurch of the Resurrection, whereas its free facade (east) bears apolygonal arch that is closed-faced with masonry and a window.The second floor is free-sided on its east, south and west facades,whereas it is adjacent at its north side with the Church of the Res-urrection at half height (there exists an internal floor – vault of-fice). In each of the east, south and north facades a pointed archis present, closed-faced in the east and north sides, and partiallyopen-faced in the south side. The west facade is not arched, asthe masonry contains, at mid-axis, an internal staircase climbingfrom the ground floor to the top floor. The third floor is the Bellroom and bears two arched openings in every side. The roof ofthe Bell room is wooden.

Although the Bell Tower has been subjected to earthquakesthroughout its history, most structural cracks and arch deforma-tion are attributed [3] to the earthquake of 1545, when the domeof the Bell Tower collapsed. The earthquake caused deformationor splitting of walls (inside the Bell room – east wall), deformationof arches (second floor – all facades) and structural cracks (hori-zontal, vertical and shear cracks), especially at the weakest partsof the building, such as around the windows and around the innerstaircase at the lower west facade.

Between 2001 and 2003, the Conservation Department of IsraelAntiquities Authority and the Technical Office of the Greek Ortho-dox Patriarchate of Jerusalem collaborated to restore the BellTower [3]. The restoration program included removal of the ce-ment based repointing of the British Mandate Period, architecturaldocumentation (plans), assessment of the state of preservationprior to conservation work (mapping of stone damage, mappingof bedding planes of the ashlars, mapping of former stone replace-ment and additions, mapping of structural cracks and deformationof arches), repointing with new mortars, strengthening of thestructure with grouting and cleaning of the stone surfaces. As partof a recent National Technical University of Athens (NTUA) preli-minary diagnostic study of the Church of the Holy Sepulchre inJerusalem, non-destructive techniques (infrared thermography, fi-bre-optic microscopy, ground penetrating radar and ultrasonictesting) were employed in order to determine and map the incom-patible materials used for its conservation and structural mainte-nance [4,5]. This work focuses on the use of ground penetrationradar (GPR) to evaluate the effectiveness of conservation interven-tions and to assess the risk of failure of structural cracks present onthe Bell Tower of the Church of the Holy Sepulchre.

2. Experimental procedure

Ground Penetrating Radar (GPR) is an established non-destructive electromag-netic technique that can locate objects or interfaces within a structure [6,7]. GPRuses discrete pulses of radar energy with a central frequency 10 MHz–2.5 GHz to re-solve the locations and dimensions of electrically distinctive layers and objects inmaterials. Specifically, short electromagnetic pulses are transmitted into a mediumand when the pulse reaches an electric interface in the medium, part of the energywill be reflected back, whereas the remaining is propagated further inside the med-ium, until completely dissipated. The reflected energy is collected and displayed asa waveform showing amplitudes and time elapsed between wave transmission andreflection. Surveys are usually conducted along profile lines to produce 2-D sectionsof the subsurface. Low frequency systems penetrate the deepest, but only image lar-ger objects. Higher frequency systems (>250 MHz) image smaller objects but do notpenetrate as deep. The propagation and reflection of the radar pulses are controlledby the electrical properties of the materials, the most important being the relativedielectric permittivity (er) [6,7]. This electrical property is, however, difficult to esti-mate, as it is dependent on the pulse frequency, the material studied, and its watercontent. Knowledge of the value of er can allow calculate the correct depth of thetarget, however, for cases where the substrate layers are unknown, it is sufficientto present 2-D sections based on time elapsed between wave transmission andreflection.

GPR is increasingly used in the field of built cultural heritage protection for theassessment of the preservation state of monuments and historic structures [8], inorder to locate the position of large voids and inclusions of different materials[9], to qualify the state of preservation of the structural system [10–12], to controlthe effectiveness of repair interventions [9], to reveal the morphology/geometry ofwall sections in multiple-leaf stone and brick masonry structures [9,10,13] and toassess the preservation state of mosaics and reveal substrate features such as plas-tered mosaics or sub-surface murals [14–18]. However, lack of knowledge regard-ing the structural characteristics of the surveyed historic structure (constructiontechniques, detailed architecture of all phases, etc.) and the materials involved (his-toric materials, restoration materials and interventions, decay of materials, etc.) of-ten make identification of the observed features in a radargram a difficult task.

The ground penetrating radar system used in this survey was a MALÅ Geosci-ence ProEx system with 1.6 GHz and 2.3 GHz antennae. The MALÅ GeoscienceGroundvision 2 software was used for data acquisition. The MALÅ GeoscienceRadExplorer v.1.41 software was used for data processing.

3. Results and discussion

Permission was granted to inspect areas at the west façade ofthe Bell Tower without the use of scaffold, and areas at the northfaçade with the use of the existing scaffold (Fig. 1).

3.1. Bell Tower – west façade

Fig. 2 indicates the location of the two areas of the GPR surveyat the west façade. Fig. 3 shows structural cracks at this façade, andnoticeable is the one that runs vertically on the masonry [3]. Thecracks were documented during the 2001–2003 conservation workon the Bell Tower and restoration mortar was subsequently used torefill it. Moropoulou et als [4,5] revealed, through the applicationof infrared thermography, the hydrothermal incompatibility be-tween the restoration mortar and the stone of the masonry, wherethe restoration mortar that was used to fill the vertical crack pre-sented higher surface temperature compared to the original jointmortars of the masonry, underlying the different nature of the res-toration materials compared to the historic ones. The GPR surveyat area A focused on the lower end of the vertical crack, aroundthe window at the second level, to evaluate its state and the riskof further expansion.

Fig. 4 illustrates the GPR results for scan BTW01 at area A. Theupper part of Fig. 4 overlays the top-view schematic cutaway, in-scale, of the masonry at the height of the profile, showing the inter-nal wall of the circular staircase. In the middle, a photograph of thescanned area indicates the start and end of the specific GPR scan.The window with the metal bar in the photo is an opening of theinternal staircase that runs from the ground level to the third levelof the Bell Tower. The masonry is one-ashlar layer thick (45–60 cm) along the vertical axis of the staircase window (distance0.7 –2.0 m) and of double leaf construction with rubble core fillingelsewhere. The white dashed outlines in Fig. 4 indicate the cross-section shapes of the external ashlars of the masonry (A1–A4),whereas the white dashed arch indicates the location of the wallof the internal staircase of the masonry. The metal bar of the win-dow does not seem to extend significantly into the stone ashlar,however, the indication of a target at 1.3 m/5.1 ns should be corre-lated with the neighboring presence of the metal bar and its stateof corrosion, both of which could lead to internal cracks to that par-ticular stone block.

Before studying the state of the structural crack, one shouldnote also an alteration of the signal between the surveyed ashlars,which is attributed to the orientation of the stylolites present inthe Meleke type of stone of the masonry. Macroscopically, stylo-lites appear as darker, wavy sutured lines, roughly parallel to thebedding and consist of a minute column-like development roughlyat right angles to the bedding. They are believed to have formeddue to solution and pressure acting together along original bed-ding-plane surfaces within the rock. As a result, the insoluble

Fig. 1. Northwest view of the Holy Sepulchre complex with indication of the exterior GPR survey at the Bell Tower (West – BTW; North – BTN) [3].

Fig. 2. Location of the two GPR survey areas at the west façade of the Bell Tower: (a) general view of area A, (b) general view of area B, (c) relative location of areas A and B (d)cutaway plans and plan views with indication of areas A and B. Plans from [3].

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material in the limestone was concentrated along the beddingplanes. The bed and the bedding plane are unique characteristicsof sedimentary rocks (Fig. 5) and generally a stone will resistweathering far better if it is ‘‘in bed’’ than if it is oriented in any

other direction; in effect the thrust of the stone should be at a rightangle to the bedding [3]. Hence, the stone should be laid in its nat-ural bed - the same way that it was deposited or exactly the oppo-site way up. In the Bell Tower most ashlars are placed in the

Fig. 3. Mapping of structural cracks (highlighted for clarity) on the west façade ofthe Bell Tower [3].

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horizontal position. Another stone laying orientation is face-bed-ded, i.e. the bedding layers of the stone are parallel to the masonryplane. Face-bedding’, however, is especially prone to exfoliationand should be avoided. In the Bell Tower, however, face-beddingis rather high (20–40%) [3]. An example is shown in Fig. 4, wherethe ashlar to the left of the starting point of the scan is laid face-bedded and as evidenced macroscopically (also documented in[3]) is heavily weathered. Edge-bedded or joint-bedded refers tolaying of the ashlar such that the bedding layers of the stone areperpendicular to the masonry plane and parallel to the loadingdirection (vertical) of the masonry. In the Bell Tower edge-beddingis very limited; however, in the scanned area BTW01, ashlars A2and A3 appear to be edge-bedded. The fact that ashlar A2, i.e. theone above the window – with the embedded metal bar, is laidedge-bedded, is of significant concern, since the metal bar may ini-tiate and/or propagate cracking in the ashlar under the tensileloading pattern at its bottom surface (right above the windowopening), in effect the metal bar acting as a wedge. Finally, the firstand fourth ashlars of scan BTW01 (A1 and A4 respectively) are laiddiagonally-bedded, an intermediate type of laying between theaforementioned ones. The different GPR signal between the diago-nally-bedded ashlars (A1, A4) and edge-bedded ashlars (A2, A3) isevident in Fig. 4.

Regarding the state of the aforementioned structural crack ofthe masonry (Fig. 3), the GPR survey indicates that there exists arisk of expansion. Specifically, the lower end of the west façade’svertical structural crack coincides with the joint between the firstand second ashlars (A1 and A2 respectively), i.e.at 0.7 m from thestart of scan BTW01. There is an indication of a target T1 at0.7 m/5.6 ns, i.e. at the inner corner of the first ashlar, exactly atthe internal junction with the second ashlar, and at a depth ofapproximately 30 cm (assuming v = 10.48 cm/ns, see calculation

below in area E). As will be discussed below for area B, such a tar-get most possibly corresponds to the grout used to consolidate themasonry; however, no grout was injected at the near vicinity ofthis junction.

It should be noted though, that, as mentioned above, the ma-sonry is one-ashlar layer thick along the vertical axis of the stair-case window and a double leaf construction with rubble corefilling elsewhere. According to [3], the original mortars used weremainly of three types (Fig. 6): (a) a soft, but elastic grey woodash-containing slaked lime mortar, typical building mortar forcore-building from Byzantine, during Crusader till even Mame-luke period; (b) a harder, white slaked lime mortar (with a lowaggregate amount) in the outer part of the joints, which wasmerely used for aesthetical reasons and (c) a soft and elasticlight-brown mortar (a mixture of soil and slaked lime) betweenthe core and the ashlars. No similar building technology wasfound at any other sites in the region, and neither was its naturefully understood. Such a building technology (i.e. use of threetypes of mortars) could indicate that the core and the outer wallwere built separately (although not clear which one first) andthat the hollow area in between filled up with this lime–soil mix-ture [3]. This lime–soil mixture could serve as a water constrain-ing layer between the core and the ashlars, however, theparticular mixture used at the Bell Tower is soft and porous [3].Also, It should be noticed that during the previous conservationwork [3], many deep holes in the core of the building were found,which did not seem to be connected to the earthquake of 1545that caused the collapse of the dome and the demolition of thetwo upper floors in 1720.

Furthermore, as verified by the GPR scan BTW01, the aforemen-tioned structural crack (Fig. 3) is located along the vertical axis ofthe ‘‘interface’’ between the double leaf construction with rubblecore filling masonry and the one-ashlar thick masonry, an interfacethat runs vertically along the axis of the internal staircase and per-pendicularly to the masonry plane. Two such interfaces exist in themasonry, the above mentioned one that corresponds to the crackand the other further right, at the junction between the third[A3] and the fourth [A4] ashlars of scan BTW01. Both are character-ized by the ‘‘inherent structural inhomogeneity’’ created by joininga one-ashlar thick laying with a double leaf masonry, and is thusunderstandable why the aforementioned structural crack was cre-ated at exactly such an interface.

These two facts, i.e. (a) the use of a three-mortar-system build-ing technology at the Bell Tower and (b) the presence of the ‘‘inter-face’’ of double leaf construction with rubble core filling masonry/one-ashlar thick masonry, at exactly the same area as target T1, areclosely correlated and aid in the understanding of the origin of tar-get T1 as well as in assessing the risk of expansion of the structuralcrack. Target T1 most possibly corresponds to a cavity or partial fillwith the third type of mortar (the mixture of soil and slaked lime)between the core and the ashlars, present exactly at the double-leaf/one-ashlar-thick ‘‘interface’’ at the masonry. This constitutesan ‘‘inherent structural inhomogeneity’’ which is further intensi-fied, regarding its significance, by the hydrothermal incompatibil-ity between the masonry stone and the restoration mortar thatwas used to fill the crack [4,5]. This cavity area should, therefore,be monitored regularly with non-destructive techniques such asGPR and infrared thermography and correlated with the adjacentedge-bedded ashlar (ashlar A2 in scan BTW01) as well as the pres-ence of the metal bar of the window, as it could readily lead to thedownwards expansion of the masonry structural crack, or reopenthe masonry structural crack upwards.

Fig. 7 shows the GPR survey matrix and an overview of the GPRresults at area B on the west façade of the Bell Tower. The aim of thesurvey at this area was to evaluate the state of the masonry and as-sess the compatibility of previous restoration interventions. Eleven

Fig. 4. GPR results for BTW01 profile at area A of the west façade of the Bell Tower. Upper, top-view schematic cutaway, in-scale, of the masonry at the height of the profile.Middle, photograph of the scanned area with indications of the position of the vertical crack (thick dashed black line) and of the start and end of the GPR scan (light dashedblack lines). The white dashed outlines indicate the cross-section shapes of the external ashlars of the masonry (A1–A4), whereas the white dashed arch indicates the locationof the wall of the internal staircase of the masonry. Target T1 is indicated with a white arrow.

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horizontal (BTW02–BTW12) and four vertical (BTW13–BTW16)scans were performed, as shown in Fig. 7a. The black dashed outlineindicates the area surveyed. In the labeling of ashlars the first lettercorresponds to the area surveyed (in this case B), the first digit cor-responds to the row of the ashlar, and the second digit to the se-quence from left to right. Fig. 7b summarizes the findings of theGPR survey in this area. There are two targets of interest, indicatedin Fig. 7b with black dashed outlines as T2 and T3.

Fig. 8 shows the typical results from two cross-cutting scans(BTW02 and BTW14), focusing around target T2. The white dashedoutlines indicate the cross-section shapes of the ashlars along theprofile of each scan, and the location of targets T2 and T3 (see be-low) are indicated with white arrows. Target T2, present in scans

BTW02, BTW03, BTW04 and BTW14, is observed at approximately4–6 ns, depending on the shape of back-face of ashlars B52 B53 andB42. With a pulse velocity v = 10.48 cm/ns (see calculation in areaE below), the location of target T2 corresponds to approximately20–30 cm from the surface of the masonry. According to [3], the fa-cades are built with diagonal flattened ashlars of 20–40 cm depth,thus target T2 is located right behind the back-face of the aboveashlars.

Target T2 corresponds to an internal void or interface, since itdoes not appear in other parts of area B. Its existence is attributedto the grout material that was used to consolidate the masonry.Specifically, the consolidation of historic masonry often involvesstabilization of walls by filling voids within their thickness. This

Fig. 5. Use of bedding planes of ashlars.

Fig. 6. The three types of original mortars at the Bell Tower: (1) grey wood ash-containing slaked lime mortar; (2) white slaked lime mortar (with a low aggregateamount) and (3) light-brown mortar (a mixture of soil and slaked lime) [3].

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operation is usually needed in cases of thick walls of double leafconstruction with rubble core filling, which have been subjectedto the percolation of water for many years. Such washing actiontends to cause the mortar (especially the one in fill which is usuallyof poor quality) to disintegrate and either wash out from openjoints, or to accumulate as loose fill at the base of the masonrycausing bulging, cracking and displacement of stones [19]. As indi-cated in Fig. 7b, based on records from the previous conservationwork on the Bell Tower [3], close and above target T2, the masonrywas consolidated with liquid grout, inserted at points G1 (1000–5000 cc), G2 and G3 (both 0–1000 cc). Although no scans were pos-sible above BTW02 – due to limited accessibility – it appears thattarget T2 corresponds to the consolidation material which hasfilled the joints at depth, the internal cavities and any internalcracks on the ashlars.

There are two other observations that should be taken into ac-count. First, to the left of T2, and right below point G2, there existsa reinforcing metal ‘‘tie’’ that connects rows 4 and 5 of the sur-veyed area, which was present even before the consolidation workof 2001–03 [3]. This indicates that this part of the masonry hasshown deterioration signs in the past too, that required the instal-lation of this metal tie. Second the ashlar B53 shows considerablesurface roughness due to weathering and is laid face-bedded, dif-ferent from its neighboring ashlars. As supported by infrared mea-surements, it shows different hydrothermal behavior compared toits immediate neighboring ashlars, showing higher temperature[4]. In fact, similar hydrothermal behavior is shown by anothernearby face-bedded ashlar, ashlar B61 at the 6th row, to the leftof grout injection point G1. The correlation of: (a) the close proxim-ity of these face-bedded ashlars, (b) the need to inject significantamounts of liquid grout at three close-proximity points (G1–G3),(c) the need in the past to strengthen the masonry by means of ametal tie and (d) the GPR identification of the presence of an inter-nal void (filled with grout) at T2 leads to the conclusion that thepresence of this internal void was not arbitrary, but in fact the con-sequence of the dissimilar laying of these two face-bedded ashlars(B53, B61). Actually, the void T2 appears to exist at the ‘‘interface’’between face-bedded ashlars and ‘‘non-face-bedded’’ ashlars, i.e. atthe interface between two different hydrothermal behaviors. Aswill be described below, similar phenomena are observed for T3too. Noteworthy is that ashlar B73 is also face-bedded, and groutneeded to be inserted at points G1 and G4, i.e. at a similar ‘‘inter-face’’. Unfortunately, no GPR scans could be provided for this upperpart to identify any similar internal defects like T2. It should benoted that, in general, enhancement of decay phenomena is

observed at interfaces between materials with incompatiblemicrostructure [20].

Fig. 9 shows the typical results from two cross-cutting scans(BTW10 and BTW134), focusing around target T3. The whitedashed outlines indicate the cross-section shapes of the ashlarsalong the profile of each scan, and the location of target T3 is indi-cated with a white arrow. Target T3, present in scans BTW09–BTW12, is observed at a range of 7–10 ns, i.e. at a depth of 35–50 cm from the surface of the masonry (using a pulse velocityv = 10.48 cm/ns). Target T3 most possibly corresponds to the mor-tar/grout at the interface between the back face of the externalashlars (B21, B11, B12) and the interior rubble core filling. Simi-larly to target T2, there are a few observations that need to be ta-ken into account. First, the target appears to be planar, followingthe back-face of ashlars B21, B11 and starting from 35 cm to theleft and moving further to the interior (50 cm) to its right side. Itextends behind two rows of ashlars (1st and 2nd rows), coveringalmost half the length of the surveyed rows. Second, liquid groutwas inserted during the previous conservation work at two points,

Fig. 7. (a) GPR survey matrix at area B of the west façade of the Bell Tower. The black dashed outline indicates the area surveyed. The white arrows indicate the start of eachscan; BTW02–BTW12 (horizontal) and BTW13–BTW16 (vertical). B11–B73 correspond to the labeling of ashlars within this area, as described in text, (b) overview of GPRresults in the same area. Dashed black outlined areas T2 and T3 are described in text. R1 and R2 indicate replacement ashlars. Overlaying of points where liquid grout wasinserted in previous conservation intervention [3]. Black circle G1 = 1000–5000 cc; Grey circles G2–G6 = 0–1000 cc.

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G5 and G6 which are at the target’s rightmost boundaries. Third,ashlars B21 and B11 are replacement ashlars (denoted R1 and R2respectively), as evidenced by their relatively lower surface weath-ering, non-damaged straight edges, and different coloring com-pared to the neighboring ashlars. The replacement was probablyjustified by the extensive weathering of the original ashlars atthe respective locations, weathering that justified further strength-ening of the masonry at this area by consolidation with liquidgrout at points G5 and G6. Fourth and possibly most importantly,ashlars B32 and B22, above and to the right of R1 respectively, ap-pear as face-bedded. It should be noted that, as evidenced macro-scopically and according to records [3], the scaled surface of ashlarB32 has been filled with a stone crack filling mixture (slaked limeputty, hydraulic lime powder, Gil’adi stone powder for a white –beige result and Primal AC 33 acrylic emulsion). Following similardiscussion as above for target T2, enhancement of decayphenomena are observed at the ‘‘interface’’ between different

Fig. 8. GPR results of scans BTW02 (left) and BTW14 (right) at area B of the west façade oashlars along the profile of each scan, and the location of targets T2 and T3 are indicate

hydrothermal behaviors, i.e. between face-bedded ashlars B32and B22 and their neighbors, that led to the replacement of ashlars.

The fact that these weathering enhancement regions appear tobe focusing on the left side of the surveyed area B (see relativelocation of targets T2 and T3) can be attributed to the locationand orientation of area B relative to the Bell Tower. Specifically,in winter, on sunny days, the prevailing wind is from the south-east (a very cold wind, even when the weather is nice and sunnyand the stone surface is heated by a warm sun, but cooled downby the cold wind), whereas on rainy days, from the west ornorth-west (wind changes via the south). In summer, the prevail-ing wind is from the north-west. In addition, only the southernwall is exposed to the sun during most of the day (8:00 am–15:00 pm). These observations [3] show that the south façade ofthe Bell Tower is exposed to environmental factors differently thanthe west façade, and thus, the south-west corner of the Tower isexpected to exhibit an intermediate behavior than the rest of the

f the Bell Tower. The white dashed outlines indicate the cross-section shapes of thed with white arrows.

Fig. 9. GPR results of scans BTW10 (left) and BTW13 (right) at area B of the west façade of the Bell Tower. The white dashed outlines indicate the cross-section shapes of theashlars along the profile of each scan, and the location of target T3 is indicated with a white arrow.

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west façade. It is probably this differentiation of the southwest cor-ner (right part of area B), that shifts the weathering phenomena(corresponding to T2, T3, metal bar, replacement ashlars) mostlytowards the left part of the surveyed area.

Summarizing for the surveyed areas A and B of the west façadeof the Bell Tower, GPR identified zones in these areas that entailrisk of failure, either mechanical (crack propagation in area A), orenhanced weathering (as in area B). Therefore, a more systematicscanning and monitoring of the whole façade is proposed for thefuture.

3.2. Bell Tower – north façade

The survey at the North Façade focused on the top level(between scaffolding N1 and N2), at three areas (Fig. 10) wherestructural cracks, increased stone damage and arches deformations

Fig. 10. Location of the three GPR survey areas at the north façade of the Bell Tower. Lefand E. Plan adapted from [3].

(Fig. 11) were documented in the previous conservation work [3].The aim of the GPR survey at these three areas was to evaluate theeffectiveness of the consolidation interventions and assess the riskof structural failure.

Fig. 12 presents the GPR results for scan BTN01 at area C. Area Cis located above the left arch, and exhibited intense stone scalingand horizontal structural cracks (Fig. 11). Due to extensive surfacedamage on the ashlars of the upper row, only scan BTN01 was fea-sible, on the lower row. From available information [3], the stoneblocks at this area (as well as at area D) are ashlars with a slopingsurface and not shallow slabs. They formed part of the respectivearch that was demolished in 1720. Therefore, due to the inclinationof the exterior surface of the ashlars, the targets at depth 7–9 nscorrespond to the bottom surface level of the ashlars and not theirback surface (see upper right schematic in Fig. 12). Of particularinterest is the horizontal crack, outlined in Fig. 12 with a black

t: General view of areas C, D and E. Right: North façade with indication of areas C, D

Fig. 11. (Left) Mapping of structural cracks (highlighted for clarity) and (right) stone damage and arch deformation on the north façade of the Bell Tower. Plans adapted from[3].

Fig. 12. GPR results of scan BTN01 in area C of the north façade of the Bell Tower.The white dashed outlines indicate the cross-section shapes of the ashlars along theprofile of the scan. The upper right schematic shows the cross-section of the slopedashlars; the GPR pulse propagating in-angle imaging the bottom of the ashlars.Overlaying of points where liquid grout was inserted in previous conservationintervention [3]. Grey circles G7–G9 = 0–1000 cc.

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dashed line, that extends along the bottom joint of ashlars C11 andC12 and partially through C13 and which then ‘‘curves’’ downalong the lower side of the arch ashlars CA2 and CA1. An attemptto close this structural crack (as in the case of other structuralcracks) was made during the previous conservation interventions[3] by repointing along its path and by injection of grout materialat nearby points (in this case point G7). However, as shown inFig. 12, it appears that the crack is not refilled completely, butrather superficially, and that there exist internal voids at the bot-tom interface between ashlars C11, C12 and C13 and the ashlarsof the arch (CA1, CA2, CA3, CA4). Of special concern is the internalvoid T4 (0.8–1.2 m/7 ns) present at the area where the crackchanges curvature and follows the geometry of the arch, since itis located precisely at the top of the arch. The observations pointout that the horizontal structural crack extended further horizon-tally at the interior, more than what was externally observableand that the repointing and injection of grout material (injectionpoint G8) was probably not successful in refilling it. Therefore,due to the local geometry and stress intensification (top of thearch), this area entails a significant risk of structural failure andshould be monitored systematically. This example also demon-strates the need for employing non-destructive methods like GPRthat can provide information from the interior of structures, so thatthe conservation and protection interventions employed are de-signed and implemented effectively.

Area D is located above the right arch of the façade, and exhib-ited light stone scaling but substantial structural cracks (Fig. 11).Noteworthy is the significant arch deformation at the upper rightside of the arch, in the form of vertical displacement of rows, whichis the result of the 1545 earthquake that caused the collapse of theabove floors and the weakening of the remaining structure. Theneighboring cluster of structural cracks (Fig. 11) and the verticalstructural crack running approximately from level N4 to N6(Fig. 11) are directly related to this arch deformation. Fig. 13 showsthe GPR scans BTN02 and BTN03 at area D. Scan BTN02 was per-formed at the upper row of ashlars, whereas scan BTN03 at thelower row. As in other areas of the structure, this area was repoint-ed and grout material was inserted at various locations to consol-idate the masonry. The injection points of liquid grout G10 (1000–5000 cc) and G11, G12 (0–1000 cc) are indicated in Fig. 13.

Scan BTN02 indicates the presence of internal voids (e.g. targetT5 at 0.10–0.30 m/8 ns, target T6 (0.5–1.0 m/9–10 ns) and targetT7 (1.1–1.5 m/7–8 ns) at the interface between the upper row

Fig. 13. GPR results of scans BTN02 (upper) and BTN03 (lower) in area D of the north façade of the Bell Tower. The white dashed outlines indicate the cross-section shapes ofthe ashlars along the profile of each scan, and the location of targets T5–T8 are indicated with white arrows. Overlaying of points where liquid grout was inserted in previousconservation intervention [3]. Black circle G10 = 1000–5000 cc; Grey circles G11, G12 = 0–1000 cc.

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(D21, D22 and D23) and the lower row of ashlars (D11, D12 andD13). It should be noticed that, as shown in Fig. 11, a continuousstructural crack was observed externally during the 2001 conser-vation work [3], along the aforementioned joint, which was 1.5–3 cm wide across its length for the portion below ashlars D21and D22, and 5–8 cm for the portion below ashlar D23]. Althoughthe structural crack was filled on the surface during the repointingof the structure, targets T5 and T6 indicate that the crack has notbeen refilled completely at its interior, and since no grout materialwas injected at the upper left part of area D to consolidate the ma-sonry locally (grout injection point G12 is located further belowand at the leftmost side of the aforementioned crack and thus doesnot seem to affect it directly), there appears to be inadequate cohe-sion between the two rows of ashlars, and thus risk of structuralfailure. Even at the right part of the structural crack (1.0 m toend), where locally grout material was injected through pointsG10 and G11, target T7 indicates that the interface between theupper row ashlar D23 and the lower row ashlar D13 appears tobe lacking cohesion. This is not surprising, as the percolation net-work (rubble core filling and intermediate mortar) behind the ash-lars is complex, and any liquid grout material could possibly fillvoids other than the interface of interest. In addition, accordingto records [3], the area around the right arch presented significantstructural cracks also at the interior of the Bell Room, and groutmaterial was injected to the corresponding interior area too. How-ever, the thickness of the masonry (1.85 m) and the amounts ofgrout material injected [3] do not allow filling of the aforemen-tioned structural cracks from the interior of the Bell Room. Theabove observations lead to the assessment that the structural crack

at the joint between the two rows of ashlars in area D is not refilledsuccessfully, and thus entail risk of failure, since this crack is abovean arch, where the stress field is accordingly significant.

Scan BTN03 was performed at the lower row of ashlars (D11,D12 and D13). There also appears to be inadequate cohesion be-tween this row of ashlars and the row below them (DA1, DA2and DA3), and thus significant risk of structural failure. Specifically,as indicated by target T8 at 0.0–1.0 m/8–9 ns, the structural crackthat was macroscopically evidenced on the surface of the corre-sponding joint during the conservation works was not refilled suc-cessfully, and the repointing filled it only on the surface. Incontrast, there appears good cohesion between ashlar D13 andits underlying ashlar DA4, probably due to the fact that the struc-tural crack propagated, instead, vertically along the path of theD22–D12–DA3/D23–D13–D44 joint, thus not opening the D13/DA4 joint. This verifies the macroscopic observations from pastconservation work (Fig. 11), where there was no structural crackobserved on the this joint.

Fig. 14 presents the GPR results for scan BTN04 at area E. Asshown in Fig. 14, intense stone scaling is exhibited at the E21 andE22 ashlars and intermediate stone scaling at ashlar E25. Liquidgrout was inserted at points G13 and G14. In addition, the surfaceof ashlars E23 and E25 has been treated with stone crack filling mix-ture [3]. Scan BTN04 indicates the presence of a series of targets:Target T9 at 0.30 m/4 ns, target T10 at 0.75 m/8 ns, target T11 at1.2–2.1 m/8–10 ns and target T12 at 2.13 m/8.3 ns. Target T11 cor-responds to the interface between the back face of ashlars E24 andE25 and the rubble core filling and appears to present satisfactorycohesion, taking into account the three-mortar-system building

Fig. 14. GPR results of scan BTN04 in Area E of the north façade of the Bell Tower. The white dashed outlines indicate the cross-section shapes of the ashlars along the profileof the scan, and the location of targets T9–T12 are indicated with white arrows. Overlaying of points where liquid grout was inserted in previous conservation intervention[3]. Grey circle G13 = 0–1000 cc. Right: Details of the texture of ashlars E21 and E22 and the thickness of the corner ashlar E25.

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technology employed. Target T12 corresponds to the interface be-tween the right part of ashlar E25 and the ashlar behind it that be-longs to the west façade of the Bell Tower, since ashlar E25 is acorner ashlar (north-west). Ashlar E25 has a thickness of 43.5 cm,therefore the velocity can be calculated as v = 10.48 cm/ns, a valuewhich corresponds well with bibliographical references forlimestone.

Targets T9 and T10 appear to be voids at the interface betweenthe back face of the respective ashlars and the rubble filling. Asmentioned above, based on the three-mortar system building tech-nology employed, a light-brown mortar should be present at theseareas. However, according to records [3,21], the stone masonry andthe roof above this row of ashlars are later additions, dating to the20th century (the roof was built in 1895), as evidenced also by thedifferent color and size of the stones used (see Fig. 14). Therefore,as the rubble core filling at these higher row levels was exposed toweather for centuries, it may not be representative of the rest ofthe structure. Ashlar row E2 is located at exactly this interface be-tween the historic structure and the additional construction in the20th century, and thus, voids cannot be excluded. Moreover,although these targets are adjacent to the three grout insertionpoints, G13–G15, as mentioned above, the percolation network ofthe weathered rubble core filling may direct the grout materialto other areas, leaving the observed voids unfilled. Taking into ac-count the distribution of structural cracks in the north and west fa-cades of the Bell Tower (Figs. 11 and 3 respectively), targets T9 andT10 are evaluated as non-critical for crack initiation or propaga-tion, and despite the possible presence of small internal voids,the structural integrity of the upper northwest corner of the BellTower appears to be satisfactory. This assessment is supportedby the relatively small number of grout insertion points in theupper north-west corner [3], indicating that the structure neededno significant consolidation at this area.

4. Conclusions

The survey of the Bell Tower of the Church of the HolySepulchre demonstrated the ability of ground penetrating radar

to evaluate the state of structural cracks and to assess the effective-ness of conservation interventions. Other complementary non-destructive techniques such as infra-red thermography [5,8,14], fi-bre-optic microscopy [5,8,14,16], digital image processing [8,20],or digital photogrammetry [11] by their principle of operationmainly provide information about the state of the surface of thestructure (e.g. surfacial cracks, roughness of the surfaces, micro-structure of the building material, decay of the materials, etc.) orabout phenomena that occur within the structure and are observa-ble at its exterior surface (e.g. water transport phenomena/humid-ity, salt crystallization leading to salt efflorescences). Other non-destructive techniques such as ultrasonic testing [8] or sonictomography [13], which utilize propagating mechanical pulses,do provide information about the interior of the structure but theirapplicability is often limited due to their sensitivity on the micro-structure of the building materials (porosity, inhomogeneities,inclusions, etc.) [8,13], the state of preservation of their exteriorstructure (roughness, decayed layers, etc.) [13] which have detri-mental effects on the signal dissipation and thus the depth of pen-etration or the resolution of information. Moreover, limitationsexist when direct (through wall) setups are required (transmitterand receiver being opposite each other) in order to provide atomography of the structure, as this is not always feasible, as inthe case of this work, and thus the indirect approach is used (trans-mitter and receiver being on the same exterior surface of the struc-ture). Ground penetrating radar, which uses electromagneticpulses offers more advantages in providing information from theinterior of structures [8,10,13].

Specifically, GPR identified areas in the masonry of the BellTower that entail risk of structural failure, as in the case of areaA (west façade), where a cavity between the core and the ashlarsis present exactly at the double-leaf/one-ashlar-thick interface atthe masonry, at the end of the vertical structural crack. This inher-ent structural inhomogeneity can be intensified by the hydrother-mal incompatibility between the masonry stone and therestoration mortar that was used to fill the crack, and consequentlyentails the risk of a downwards expansion of the masonry struc-tural crack, or reopening of the masonry structural crack upwards.

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GPR can support the correlation between the weathering phe-nomena acting on the building and specific elements of the con-struction technology employed, such as the dissimilar laying ofthe ashlars. In particular, at the interfaces between face-beddedashlars and non-face-bedded ashlars, as in the case of area B,where differential hydrothermal behaviors are observed, theweathering phenomena are intensified and may lead to the crea-tion (through wash-out of the intermediate mortar) of internalvoids – as verified by GPR, or to a focused weathering of neighbor-ing ashlars – as verified macroscopically, necessitating theirreplacement.

GPR can also assess the effectiveness of conservation interven-tions, such as the injection of consolidation grout at various loca-tions of the structure and the repointing of the masonry. Afterthe removal of the British Mandate period repointing and the his-toric joint mortars, the extent and distribution of the structuralcracks was revealed and strengthening of the Bell Tower structurewas required. This was accomplished by the use of liquid grout thatwas injected at various locations on the Bell Tower, aiming to con-solidate the masonry and fill most of the cracks/voids present thatoriginated either from the 1545 earthquake or from the weatheringof the three-mortar-system employed. However, GPR demon-strated that when this consolidation intervention is performedwithout a detailed knowledge of the interior structure of the ma-sonry and the extent and distribution within the volume of thestructure, the effectiveness of the consolidation intervention canbe limited. This was the case in areas C and D, where althoughgrout was injected at various locations in order to consolidatethe structure and fill the structural cracks, and although followedby repointing of the joints, GPR indicated that the structural crackswere in fact unfilled at the interior of the masonry, and due to thelocal geometry and stress intensification (top of the arches) therewas a significant risk of structural failure.

Overall, this work demonstrates the need for employing non-destructive methods like GPR that can provide information fromthe interior of structures, so that the conservation and protectioninterventions employed are designed, implemented and assessedeffectively. In the case of the Bell Tower of the Church of the HolySepulchre, a systematic survey with GPR and other complementarynon-destructive techniques is required in order to assess the effec-tiveness of past interventions, and allow for the identification ofareas in the structure with significant risk of failure that need tobe monitored and treated appropriately with possible additionalinterventions.

Acknowledgements

The authors would like to thank His All Holiness, BeatitudePatriarch of Jerusalem and All Palestine, Theophilos III and theHead Manager of the Technical Office of the Patriarchate of Jerusa-lem Dr. Theodosios Mitropoulos for the kind invitation, the fullsupport and the valuable information they gave to the Research

Team of the National Technical University of Athens for the imple-mentation of the preliminary diagnostic study of the Church of theHoly Sepulchre.

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