Applying Close Range Non-Destructive Techniques for the Detection of Conservation Problems in Rock-Carved Cultural Heritage Sites

remote sensing

Article Applying Close Range Non-Destructive Techniques for the Detection of Conservation Problems in Rock-Carved Cultural Heritage Sites

William Frodella 1,2,* , Mikheil Elashvili 3, Daniele Spizzichino 4, Giovanni Gigli 1,2, Akaki Nadaraia 3, Giorgi Kirkitadze 3, Luka Adikashvili 3, Claudio Margottini 2, Nikoloz Antidze 5 and Nicola Casagli 1,2,

1 Earth Science Department, University of Florence, Via G. La Pira 4, 50121 Florence, Italy; giovanni.gigli@unifi.it (G.G.); nicola.casagli@unifi.it (N.C.) 2 UNESCO Chair on Prevention and Sustainable Management of Geo-Hydrological Hazards, University of Florence, Largo Fermi 1, 50142 Florence, Italy; claudio.margottini@unifi.it 3 Faculty of Natural Sciences and Engineering, Ilia State University, Kakutsa Ave 3/5, 0162 Tblisi, Georgia; [email protected] (M.E.); [email protected] (A.N.); [email protected] (G.K.); [email protected] (L.A.) 4 Institute for Environmental Protection and Research (ISPRA), ia V. Brancati 48, 00144 Rome, Italy; [email protected] 5 Georgian the National Agency for Cultural Heritage Preservation of Georgia (NACHPG), 27 Atoneli Street, 0105 Tbilisi, Georgia; [email protected] * Correspondence: william.frodella@unifi.it; Tel.: +39-055-275-5979 Abstract: Rock-carved cultural heritage sites are often developed in slopes formed by weak rocks, Citation: Frodella, W.; Elashvili, M.; which due to their peculiar lithological, geotechnical, and morpho-structural features are character- Spizzichino, D.; Gigli, G.; Nadaraia, ized by excellent carvability, which at the same time makes them prone to weathering, deterioration, A.; Kirkitadze, G.; Adikashvili, L.; and slope instability issues. In this context the use of advanced close-range nondestructive techniques, Margottini, C.; Antidze, N.; Casagli, such as Infrared Thermography (IRT) and Unmanned Aerial vehicle-based Digital Photogrammetry N. Applying Close Range (UAV-DP) can be profitably used for the rapid detection of conservation issues (e.g., open fractures, Non-Destructive Techniques for the unstable ledges-niches, water seepage and moisture) that can lead to slope instability phenomena. Detection of Conservation Problems These techniques, when combined with traditional methods (e.g., field surveys, laboratory analysis), in Rock-Carved Cultural Heritage can provide fundamental data (such as 3D maps of the kinematic mechanisms) to implement a Sites. Remote Sens. 2021, 13, 1040. https://doi.org/10.3390/rs13051040 site-specific and interdisciplinary approach for the sustainable protection and conservation of such fragile cultural heritage sites. In this paper some examples of conservation problems in several

Academic Editor: Giuseppe Casula rupestrian sites characterized by different geological contexts, from the mountainous regions of Georgia to the ancient city of Petra in Jordan, are presented, with the aim of evaluating the potential Received: 29 January 2021 of the proposed integrated approach. The ﬁnal aim is to provide conservators, practitioners, and local Accepted: 1 March 2021 authorities with a useful, versatile, and low-cost methodology, to be proﬁtably used in the protection Published: 9 March 2021 and conservation strategies of rock-carved sites.

Publisher’s Note: MDPI stays neutral Keywords: infrared thermography; UAV digital photogrammetry; cultural heritage; landslides; with regard to jurisdictional claims in conservation issues published maps and institutional afﬁl- iations.

1. Introduction Rock-carved settlements were among the ﬁrst man-made works in the history of hu- Copyright: © 2021 by the authors. manity, therefore representing the roots of human society [1]. These sites are not always in Licensee MDPI, Basel, Switzerland. equilibrium with the environment, since they are continuously impacted and weathered by This article is an open access article several internal and external factors, both natural and human-induced, with rapid and/or distributed under the terms and slow onset [2]. These factors include major sudden natural hazards, such as earthquakes conditions of the Creative Commons or extreme meteorological events but also slow, cumulative processes such the erosion Attribution (CC BY) license (https:// of rocks, compounded by the effect of climate change, without disregarding the role of creativecommons.org/licenses/by/ humans, especially in conﬂict situations [3]. Rock-carved cultural heritage sites were often 4.0/).

Remote Sens. 2021, 13, 1040. https://doi.org/10.3390/rs13051040 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 1040 2 of 30

excavated in steep slopes formed by so-called “weak rocks” (e.g., soft sandstones and tuffs characterized by low compressive strength values), which give the advantage of an excellent carvability but on the other hand are affected by major conservation issues in the field of rock slope stability and rock weathering [4]. The peculiar lithological and geotechnical features of these rocks (highly heterogeneous fabric with abundant clayey matrix, high porosity and low compressive strength), together with the natural morphological and structural setting of the excavated slopes (e.g., open fractures, presence of ledges-niches) and the presence of anthropic carved structures (caves, chambers and corridors), make these sites particularly fragile and prone to weathering, erosion, and slope instability processes [5,6]. In this context a further element of damage is represented by moisture expansion, caused by water runoff and groundwater, which reduces the rock uniaxial compressive strength (UCS) [7–9]. The discontinuity pattern also controls the structural stability of slopes, since their geometry with respect to the slope face can determine different kinematic behavior of the rock mass, while their engineering properties are adversely affected by water seepage and moisture, which in turn causes weathering and discoloration films, therefore reducing the wall strength [10,11]. Chemical weathering in particular, leads to the eventual decomposition of silicate minerals to clayey minerals, leading to the loosening of the rock mass and the widening of the joints creating potential instabilities such as open features [12]. Furthermore, the rockmass discontinuity networks are areas of preferential of rainfall infiltration and seepage, enhancing concentrated erosion [13]. In sandstone material in particular, the weathering processes due to the presence of water (e.g., arenitization, crust formation, peeling, decohesion, detachment, exfoliation, and biodeterioration) usually show one-directional water circulation from the outside to inside, with the prevalence of surface moisturizing and rapid evaporation, causing an increase in porosity, which further facilitates water infiltration [14,15]. The combination of the above-mentioned processes causes deterioration and structural instability of the rock-carved settlements as well as landslide processes, representing serious constraints to the future conservation of the sites and to the safety of the visitors [16,17]. Nowadays these sites represent unique cultural heritage sites, often included in the UNESCO World Heritage [18] or Tentative lists [19] which are visited daily by thousands of visitors. This is a resource for the local economies, but at the same time represents both an element of anthropic pressure in a fragile system between natural and ancient cultural elements, and a risk increasing factor. Therefore, the protection and conservation of rock-carved sites are pressing issues not only for the community of conservators, practitioners, and scientists but for the whole society. For an effective and sustainable conservation strategy it is necessary to implement a complex and specific interdisciplinary approach, that should be planned involving nondestructive techniques and consider in turns, the site characteristics (topography, geomorphological- geological setting) and typology of the related hazard [20–22]. In this perspective the use of close-range remote sensing (RS) techniques, such as terrestrial infrared thermography (IRT) and Unmanned Aerial vehicle-based Digital Photogrammetry digital photogrammetry (UAV-DP), represent effective tools for surveying rupestrian environments for slope morpho-structural characterization and detection of criticalities (e.g., weathering, erosion, slope instabilities), since they allow for fast, cost-effective, and easily updatable acquisitions over wide and inaccessible areas. RS techniques offer clear advantages with respect to traditional methods, by optimizing field work and granting the safety of both the operators and the monument [23,24]. On the other hand, traditional methods are still fundamental in order to gather rock geotechnical parameters and validate the outcomes of the remotely sensed data. Rock-carved monuments of Georgia represent a unique cultural heritage: ancient cities, churches and monastery complexes carved in rock, painted and decorated by hermit monks, form a remarkable junction between architectural monument and unique natural-geological landscapes, having their roots deep in the history of Georgian state [25] (Figure1). Remote Sens. 2021, 13,, 1040x FOR PEER REVIEW 3 3of of 31 30

0 500 m 0 1 km

0 500 m 0 500 m

Figure 1.1. Map with the location of the analyzedanalyzed rock-carvedrock-carved cultural heritage sites ( a); locations at a higher scale: Vanis Kvabebi (b); David Gareja area ((c);); UplistsikheUplistsikhe ((dd);); TheThe MonasteryMonastery ofof PetraPetra ((ee).).

OftenOften carved carved in weak weak rocks rocks such as volcanic tuffs and soft sandstones, these sites are particulparticularlyarly vulnerable to to weathering, erosion erosion,, and slope instability phenomena and are thereforetherefore often often inscribed inscribed in in the the UNESCO UNESCO Tentative Tentative List. In recent years, these sites have beenbeen undergoing aa multidisciplinary multidisciplinary research research activity activity coordinated coordinated by theby Georgianthe Georgian National Na- tiAgencyonal Agency for Cultural for Cultural Heritage Heritage Preservation Preservation of Georgia of Georgia (NACHPG), (NACHPG), with with the the support support of ofa teama team of of Georgian Georgian and and international international experts, experts, inin orderorder toto implementimplement an extensive site site-- specificspecific protection and conservation strategies [11,13,20,23].[11,13,20,23]. Arguably Arguably one one of tthehe most iconiciconic monuments monuments of of the the Petra Petra Archaeological Archaeological Park Park in in Jordan, Jordan, World World Heritage site since 1985, Al Deir (also (also called called “the “the Monastery”) Monastery”) [26], [26], is is affected affected by by seepage seepage phenomena phenomena leading leading toto salt salt damage damage processes, processes, one one of of the the main main factors factors of of weathering weathering and and er erosionosion of the famous redred sandstones of Petra (Figure 11).). ThisThis work work shows shows several several applications applications of of close close-range-range remote remote sensing sensing techniques techniques com- com- binedbined with traditional methods (e.g. (e.g.,, field field surveys, laboratory analysis) for the detection andand analysis of of slopeslope instabilities instabilities and and conservation conservation issues. issues. In detail,In detail, IRT IRT and and UAV-DP UAV were-DP wereintegrated integrated with traditionalwith traditional field surveys field surveys and geotechnical and geotechnical laboratory laboratory analyses, analyses, with the with aim theof performing aim of performing a morphological, a morphological, geostructural-geomechanical geostructural-geomechanical characterization characterization and 2D-3D andkinematic 2D-3D analysis kinematic of several analysis cultural of several heritage cultural sites carved heritage in sites rock carvedmasses (Vanisin rock Kvabebi, masses (VanisDavid GarejaKvabebi, and David Uplistsikhe Gareja inand Georgia, Uplistsikhe the monastery in Georgia, of the Petra m inonastery Jordan). of The Petra final in aimJor- dan).was to The provide final aim the basicwas to knowledge provide the on basic the acting knowledge instability on the and acting degradation/weathering instability and degradation/weatheriprocesses, to detectng the processes, main conservation to detect thecriticalities main conservation to provide suggestions criticalities for to the provide plan- suggestionsning and implementation for the planning of and mitigation implementation strategies. of Resultsmitigation demonstrate strategies. thatResults the demon- applied stratemethodology that the canapplied provide methodology fundamental can dataprovide to implement fundamental a site-specific data to implement and interdisci- a site- specificplinary and approach interdisciplinary for the sustainable approach protection for the sustainable and conservation protection of rock-carvedand conservation cultural of rockheritage-carved sites, cultural also showing heritage suggestions sites, also showing for management suggestions plans for in management sites characterized plans byin sitessimilar characterized engineering by geological similar engineering issues. geological issues. 2. Case Studies Geological-Historical Background and Conservation Problems 2. Case Studies Geological-Historical Background and Conservation Problems 2.1. Vanis Kvabebi 2.1. Vanis Kvabebi The monastery complex of Vanis Kvabebi is in southwestern Georgia in Samtskhe- JavakhetiThe monastery volcanic highland complex ofof theVanis Lesser Kvabebi Caucasus is in southwestern (Figures1a,b andGeorgia2). It in is carvedSamtskhe in - Javakhetithe volcanic volcanic steep slopeshighland of theof the Kura Lesser river Caucasus gorge right (Figure bank,s where1a, b and the Javakheti2). It is carved plateau in thedevelops volcanic [27 steep,28]. slopes of the Kura river gorge right bank, where the Javakheti plateau develops [27,28].

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Figure 2.2. Vanis KvabebiKvabebi geological-geomorphological geological-geomorphological features: features: true true color color UAV-DP UAV-DP photogrammetric photogrammetric digital digital models: models: nadiral nadiral view (a); frontal view of the eastern main cliff of the monastery showing the outcropping lithologies of the Godertzi view (a); frontal view of the eastern main cliff of the monastery showing the outcropping lithologies of the Godertzi formation and the 2017 rock fall deposits (b); welded tuffs (dashed red line) (c); lower chapel with ancient rockfall boulder formation and the 2017 rock fall deposits (b); welded tuffs (dashed red line) (c); lower chapel with ancient rockfall (d); upper chapel (e). boulder (d); upper chapel (e). From a geomorphological point of view the site develops in a V-shaped ravine NW From a geomorphological point of view the site develops in a V-shaped ravine NW oriented, bounded byby subverticalsubvertical cliffs, cliffs, up up to to 200 200 m m in in height height (Figure (Figure2a,b). 2a, From b). From a geologic a geo- logicpoint point of view of view the studythe study area area is characterizedis characterized by by the the Godertzi Godertzi formation formation (from(from Upper Miocene to to Lower Lower Pliocene Pliocene in in age), age) which, which is formed is formed by an by upper an upper section section of andesitic, of andesitic, dacitic, daciticand rhyodacitic, and rhyodacitic lava ﬂows, lava withflows, rare with occurrence rare occurren of basalticce of ba rockssaltic rocks (about (about600–700 600 m–700in mthickness), in thickness), and a and lower a lower section section of pyroclastic of pyroclastic deposits deposits such as such volcanic as volcanic breccias breccias and tuffs and of tuffsdifferent of different granulometry, granulometry, with andesitic-dacitic with andesitic composition-dacitic composition (200–250 m (200 in thickness)–250 m in [ 29thick-,30]. ness)On top [29,30]. of the On Goderdzi top of the formation Goderdzi lies form discordantlyation lies discordantly the Akhalkalaki the Akhalkalaki suite: a thick suite: series a thickof dolerites series of and dolerites basalts and of basalts Quaternary of Quaternary age [31]. age Along [31]. Along the cliff the of cliff the of monastery the monastery the theupper upper portion portion of the of Godertzithe Godertzi formation formation is widely is widely exposed, exposed, formed formed from from bottom bottom to top to topby aby layer a layer of coarse-grainedof coarse-grained tuffs tuffs (over (over 30 30 m m in in thickness), thickness), followed followed by by aa 2–3.52–3.5 mm thick volcanic andesitic andesitic ash ash layer layer and and an an upper upper level level of medium-grained of medium-grained tuffs (abouttuffs (about 20 m thick), 20 m thick),including including welded welded tuffs [ 31tuffs,32 ];[31,32]; the basalt the basalt ﬂows flows the of the the of Akhalkalaki the Akhalkalaki suite suite overlie overlie the theformer former (Figure (Figure2b). 2b). Far Far less less studied studied with with respect respect to to the the site site of of Vardzia, Vardzia, just just 2 2 km km to to the the NW [13], Vanis Kvabebi is a cave monastery dating from 8th century, consisting of a maze of tunnels and caves running on 19 levels (Figure 2b).

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NW [13], Vanis Kvabebi is a cave monastery dating from 8th century, consisting of a maze of tunnels and caves running on 19 levels (Figure2b). The monastery consists also of a defensive wall system built in 1204 and two chapels: a newer stone church (still inhabited nowadays by monks) near the top of the defensive wall (Figure2d), and a smaller, domed church that clings to the rock on the level of the highest tunnels (Figure2e). Two levels of pathways are carved in the rock to connect the various levels. Due to its peculiar litho-stratigraphic, structural, and morphological setting the site is undergoing weathering and degradation processes which trigger instability mechanisms, testiﬁed by ancient rock fall deposits constituted by large boulders lying at the bottom of the ravine (Figure2b). In 2017 a rock fall impacted a touristic pathway enhancing the need for detailed multidisciplinary studies to plan a surveying-monitoring strategy and implement a management plan for the protection-conservation of the site and a safe touristic access. Since 2018 Vanis Kvabebi has been undergoing an intensive campaign of protection and restoration activities, involving remote surveying and ﬁeld activities for preliminary landslide hazard assessment for the planning of mitigation measures.

2.2. David Gareja The David Gareja monastery complex is located in Kakheti semidesert region (Eastern Georgia), about 25 km southeast of Tbilisi (Figures1a–c and3). From a geological point of view the area is formed by a thick sequence of soft sedimentary rocks (Lower Miocene to Pliocene in age) characterized by a monocline “hogback type” structure, with strata dipping SW with low-gentle angles (Figure3a), from very coarse-grained, pebbly thick-bedded sandstones to coarse-grained soft sandstones, siltstones, and clays (Figure3b,c) [ 24]. In detail, the stratigraphic sequence is formed by: (i) lower Miocene sandy clays, dark-brown and brownish-gray clays with interbedded sandstones, and rare conglomerate interlayers; (ii) upper Miocene is represented mostly by shallow and coastal sediments, but there are also deep-sea sediments represented by yellowish-gray calcareous sandstones; (iii) Pliocene sediments are represented by continental and marine facies consisting of coarse-grained yellowish-gray sandstones with numerous small pebbles and thick basal conglomerates with volcanic ash interbeds [33,34]. David Gareja is one of the most important landmarks of Georgia and is characterized by a unique combination of historic architecture and peculiar geological/geomorphological features, representing a remarkable example of harmonious interaction of man-made structures with a dramatic landscape. Part of the UNESCO Tentative List since 2007, it includes the complex of 19 medieval monasteries with approximately 5000 cells for monks [35]. Some mural paintings preserved in the cave monasteries (the oldest of which go back to the 6th–8th century), are considered a masterpiece of Georgian medieval art as they contain the portraits of Georgian kings (Figure3a,b). In medieval times David Gareja served as one of the most important monastic and pilgrimage centers of Georgia. The Mongol invasions of the ﬁrst half of the 13th century brought monastic life to an end. New mural paintings were not to be made until the 17th and 18th centuries, when serious attempts were made to revive monastic life. The life of the monasteries ended again in 1921 during the Soviet period, when the monasteries were closed, and some sites used in later years for artillery practice [25]. Within the whole complex the most important monasteries (still inhabited today by monks) are St. David Laura Monastery (Lavra), John the Baptist (Natlismcemeni), and the Virgin (Dodos Rka). The rest of the 16 monasteries (amongst which Sabereebi represents an important example) are currently abandoned. Remote Sens. 2021, 13, 1040 6 of 30 Remote Sens. 2021, 13, x FOR PEER REVIEW 6 of 31

FigureFigure 3.3. TheThe David Gareja area: area: structural structural mono monoclinecline (a ();a );outcropping outcropping lithologies lithologies (silts (silts and and clays clays with with thick thick sand sand layer layer (b) (bcoarse) coarse-grained-grained sands sands-conglomerates-conglomerates (c); ( cLavra); Lavra monastery monastery (d ();d );Examples Examples of of frescoed frescoed cave cave chapels chapels in in the the monasteries monasteries of of Udabno (e) and Sabereebi (f). Udabno (e) and Sabereebi (f). 2.3. Uplistsikhe 2.3. Uplistsikhe Uplistsikhe (“the lord’s fortress” in Georgian) is an ancient cave town-fortress in east- Uplistsikhe (“the lord’s fortress” in Georgian) is an ancient cave town-fortress in ern Georgia about 50 km NW of Tblisi (Figures 1a–d and 4). From a morphological point eastern Georgia about 50 km NW of Tblisi (Figures1a–d and4). From a morphological of view the Uplistsikhe complex is located on the left bank of the Kura River on the slopes point of view the Uplistsikhe complex is located on the left bank of the Kura River on of the Kvernaqi Range. In detail, the site develops on top of a rocky terrace bordered to the slopes of the Kvernaqi Range. In detail, the site develops on top of a rocky terrace the south-west by a rock cliff up to 40 m high and is completely carved into a thick se- bordered to the south-west by a rock cliff up to 40 m high and is completely carved into quence of Upper Oligocene to lower Miocene quartz coarse sandstones, gently dipping a thick sequence of Upper Oligocene to lower Miocene quartz coarse sandstones, gently towards SE (Figure 4a, c) [36]. Being carved on the top of the rock strata in an open space dipping towards SE (Figure4a,c) [ 36]. Being carved on the top of the rock strata in an open of over 8 ha, the site is quite different from most other rupestrian monuments of Georgia, space of over 8 ha, the site is quite different from most other rupestrian monuments of which instead are carved in near vertical cliffs. The uniqueness of Uplistsikhe is repre- Georgia, which instead are carved in near vertical cliffs. The uniqueness of Uplistsikhe is sented by a multilevel archaeological-architectural site with structures dating from the represented by a multilevel archaeological-architectural site with structures dating from early Iron Age up to the late medieval period and is notable for the unique combination the early Iron Age up to the late medieval period and is notable for the unique combination of various styles of rock-cut cultures from Anatolia and Iran, as well as the coexistence of of various styles of rock-cut cultures from Anatolia and Iran, as well as the coexistence of pagan and Christian architecture [37]. pagan and Christian architecture [37].

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FigureFigure 4.4. TheThe UplistsikheUplistsikhe cave town complex: top of the rocky terrace with with the the brick brick church church ( (aa);); examples examples of of cave cave systems systems andand defensivedefensive wallswalls ((bb);); thethe unstableunstable rock cliffcliff bordering the site (red dashed circles circles highlight highlight April April 2019 2019 detachments, detachments, AreasAreas 1–2,1–2, thethe blackblack circlecircle thethe oneone fromfrom JulyJuly 2019,2019, AreaArea 3)3) ((cc);); examplesexamples ofof sandstonesandstone windwind weatheringweathering (corrasion)(corrasion) (d).

Considered as as one one of of the the oldest oldest urban urban settlements settlements in Georgia in Georgia and andan important an important po- political,litical, cultural, cultural, and and religious religious center, center, it itflourished ﬂourished during during Hellenistic Hellenistic and and Roman Roman times, times, while its decline started in the 4th 4th century. century. It It then then acted acted as as a a stronghold stronghold during during the the Muslim Muslim conquest in in the the 8th 8th–10th–10th centuries, centuries, while while th thee Mongol Mongol raids raids in the in the14th 14th century century marked marked the theultimate ultimate eclipse eclipse of the of town, the town, after afterwhich which the town the townwas abandoned. was abandoned. It consists It consists of a defen- of a defensivesive wall, wall,a ditch, a ditch, streets, streets, tunnels, tunnels, water water pipes pipes,, and drainage and drainage channels channels carved carved in rock. in rock. At Atthethe summit summit of the of the complex complex a Ch a Christianristian church church was was built built in in the the 6th 6th century century (Figure (Figure 4a,b).4a,b). The currentcurrent morphologymorphology of of the the site site is is the the result result of activeof active tectonics tectonics (several (several parts parts of the of most the vulnerablemost vulnerable areas areas were destroyedwere destroyed by an by earthquake an earthquake in 1920), in 1920), weathering weathering and erosion and erosion due to waterdue to inﬁltrationwater infilt andration runoff and runoff (also caused (also caused by the by Kura the river Kura action), river action), and wind and erosion.wind ero- In thission. context In this ancientcontext rockfall ancient deposits rockfall atdeposits the slope-toe at the showslope that-toe slopeshow instabilitythat slope phenomena instability havephenomena contributed have tocontributed the shaping to of the the shaping cliff (recent of the minor cliff rockfalls(recent minor were reportedrockfalls fromwere June re- 2018ported to from October June 2019). 2018 to This October led NACHPG 2019). This to led start NACHPG a conservation to start program a conservation in 2000 pro- [38], whilegram in in 2000 2007 [38], the wholewhile complexin 2007 the was whole enlisted complex in the was tentative enlisted list in for the inclusion tentative intolist for the UNESCOinclusion into World the Heritage UNESCO program World Heritage [39]. program [39].

2.4. The Monast Monasteryery of Petra The m monasteryonastery is located high high in in the the hills hills northwest northwest of of the the center center of of the the ancient ancient city city of Petra ( (FiguresFigures 11a–ea–e andand5 5).). From From a a geological geological pointpoint ofof viewview thethe areaarea isis characterizedcharacterized byby the Upper Umm-IshrinUmm-Ishrin red red-pink-pink massive quartzose quartzose subarkosic subarkosic sands sandstone,tone, middle middle to to late late Cambrian in age [40] [40] ( (FigureFigure 55a).a). TheThe monasterymonastery is a monumentalmonumental building carved out of rock in the typical red sandstone of of Petra, dating dating to to the the mid mid-ﬁrst-first century century CE. CE. The The huge huge façade andand thethe innerinner chambers chambers probably probably originally originally served served a complexa complex religious religious purpose purpose and wereand were possibly possibly used used as a churchas a church in the in Byzantine the Byzantine period period [41]. In[41]. April In April 2011, an2011 important, an im- seepageportant seepage coming fromcoming several from pointsseveral on points the rock on the wall rock to the wall left to side the ofleft the side monument of the monu- were detectedment were [42 det], alsoected extending [42], also towards extending the towards base of the the monument base of the (Figure monument5b,c). ( ThisFigure seepage 5b,c). causedThis seepage a widespread caused a salt widespread damage process salt damage on the process monument on the surface monument determining surface seriousdeter- constraintsmining serious for theconstraints conservation for the of conservation the monument. of the This monument. process when This combined process when with com- wind action,bined with as well wind as action, the harsh as well daily as excursions the harsh ofdaily temperature excursions and of temperature relative humidity, and relative are the mainhumidity, factors are of the weathering main factors and of erosionweathering of the and famous erosion red of sandstonesthe famous ofred Petra sandstones (Figure of5). InPetra fact, (Figure most of5). Petra’s In fact, monuments most of Petra’s have monuments deteriorated have at a deteriorated fast pace over at recenta fast pace years over [43];

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forrecent this years reason, [43]; in for 1998 this the reason, World in Monument1998 the World Fund Monument included Fund Petra included in the list Petra of the in onethe hundredlist of the mostone hundred endangered most monuments endangered in monuments the world [ 44in ].the world [44].

FigureFigure 5.5. TheThe MonasteryMonastery ofof Petra: rock mass on the right side side of of the the monument monument showing showing the the a arearea of of interest interest (red (red rectangle) rectangle) andand beddingbedding ofof thethe sandstonessandstones ((aa);); picturespictures ofof thethe areaarea ofof interestinterest duringduring thethe surveyssurveys ofof AprilApril 20112011 ((bb)) andand JuneJune 20142014 ((c).).

3. Materials and Methods During thethe secondsecond halfhalf ofof 20182018 field field inspection, inspection, IRT IRT and and UAV-DP UAV-DP surveys surveys were were carried car- outried inout the in areasthe areas of Vanisof Vanis Kavbebi, Kavbebi, David David Gareja, Gareja and, and Uplithsike Uplithsike [ 24[24,36,36].]. IRT analysis analysis waswas aimedaimed at detectingdetecting conservation problems and and integrating integrating the the field field surveys surveys in in the the geomechanical characterizationcharacterization ofof thethe rockrock slopes slopes (with (with special special regards regards to to fracture fracture opening open- anding and seepage). seepage). UAV-DP UAV-DP was was used use tod to obtain obtain high high resolution resolution 3D 3D models, models, toto be used as as a a detailed topographic base, perform the the modelling of of the the drainage drainage network network and and the the 3D 3D kinematic analysis. In In Petra in June 2014 field field surveys and and IRT IRT inspections were were carried carried out in the framework of the integrated management plan plan of of Petra Petra archaeological archaeological park park to to investigatinginvestigating the hydrogeological setting of the m monasteryonastery sector of rockmas rockmass.s.

3.1. Field Surveys and Geotechnical Analysis Field surveys were aimed at identifying stratigraphic and structural elements representing predisposing factors for slope instability phenomena. Traditional geostructural data were collected and analyzed with RocScience Dips © software [45] in order to detect the bedding planes and the main sets of discontinuities, to be used as input data for both the 2D and 3D kinematic analysis. Rock samples were collected in the ﬁeld during a scientiﬁc mission conducted in Georgia in November 2018. A preliminary set of laboratory analyses were aimed at assessing the main physical and mechanical parameters, to be used in further detailed stability modelling. The rock samples were analyzed in order to detect main physical and mechanical parameters. The following laboratory tests were conducted according to [46]: (1) tilt test, (2) point load test, (3) real unit weight (hydrostatic weighing). Since many samples were easily broken even by touch, some tests (especially those in water immersion) were not carried out.

3.2. IRT Surveys InfraRed Thermography (IRT) is a nondestructive diagnostic technique capable of remotely mapping the investigated scenario’s surface temperature [47]. It can provide detection of subsurface defects on wide areas, to be used as an alternative or complement to conventional inspection technologies in a wide variety of scientiﬁc applications [48]. IRT is accomplished by means of Infrared (IR) calibrated cameras (thermal cameras), capable of detecting the thermal radiation. The product of an IRT survey is a digital image acquired by the thermal camera array detector (called “thermogram” or “thermographic image”),

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which following the correction of the sensitive parameters (such as object emissivity, path length, air temperature, and humidity) is converted by the camera’s built-in processor to a surface temperature map of the investigated scenarios [49]. During recent decades, thanks to the technological development of portable high resolution and cost-effective thermal imaging cameras, IRT has been widely used in the field of civil engineering and cultural heritage for the detection of structural damages, water seepage, plaster detachments, moisture, and surface weathering on different materials (stone, concrete, masonry) [50–54] but rarely in rupestrian sites [13,24]. In the analysis of slope instability phenomena, IRT can lead to the detection of irregular thermal patterns (thermal anomalies) which can reveal the presence of potential criticalities such as: (i) open fractures; (ii) moisture or seepage zones; (iii) ledge-niche systems; (iv) cavities; (v) lava flows [13,47,55–59]. The surveys were performed in hand-held mode using thermal cameras with a focal plane array (FPA) microbolometer sensor [60,61]. The surveying parameters of the sites are reported in Table1. Thermographic image correction, thermal focusing, mosaicking, and histogram equalization were performed by means of FLIR Tools+ software [62]. ESRI ArcMap package was also used to create classified surface temperature maps using quantile function [63].

Table 1. IRT survey parameters of the analysed sites.

Thermal Camera Field of Survey Slope Sensor-Target Image Air Temperature (C◦)— Case Study Camera Resolution (pix) View (◦) Period Aspect Distance (m) Resolution (cm) Relative Humidity (%) November Vanis Kvabebi FLIR SC620 640 × 480 24 × 18 W/E 100 6.6 14.4–63.4 2018 November Natlismcemeli FLIR SC620 640 × 480 24 × 18 S 110/20 7.1/1.2 13.4–65.9 2018 November Sabereebi FLIR SC620 640 × 480 24 × 18 SW 100/20 6.5 22.6–39.8 2018 November Uplistsikhe FLIR SC620 640 × 480 24 × 18 SW 100 6.6 21.3–33.6 2018 Petra FLIR B425 320 × 240 25 × 19 June 2014 SW 20 2.6 26–21

3.3. DSM Creation and Surface Runoff Modelling Digital photogrammetry (DP) is a well-established technique for efﬁcient and accurate 3D geometric data from stereoscopic overlaps of photo sequences captured by a calibrated digital camera [64]. This technique can provide in a short time high resolution 2D and 3D surfaces, allowing for a detailed and highly accurate representation for the analysis of the geometrical-structural setting [65,66]. In the past few years, thanks to the rapid technological development of digital cameras and low-cost UAV, DP has led to new, promising scenarios in cultural heritage applications [67–69]. UAV-DP and TLS surveys were carried out in Vanis Kvabebi, David Gareja, and Uplistsikhe in 2018, with the aim of obtaining a high resolution orthophoto and a complete 3D digital model of the analyzed rock-cut slopes and buildings part of the cultural heritage sites. Close-range aerial photogrammetric surveys were carried out using DJI Phantom 4Pro and DJI Mavic Pro quadcopters. Both UAVs are equipped with quite similar cameras, including the 3-axis gimbal and image stabilization systems (Table2).

Table 2. Technical information of the adopted devices and the obtained DSMs resolutions.

Sensor Accuracy Pixels Field of View Lens Measurement Digital Cameras (‘’) (M) (◦) (mm) Range (m) DJI Phantom 4Pro 1 20 84 35 1 m–∞ DJI Mavic Pro 2 1 20 77 28 1 m–∞ Accuracy Divergence Resolution Angular step Measurement Laser Scanner (mm) (mrad) (arcsec) (◦) range (m) Z-400 5 mm 0.3 1.8 arcsec 0.0024◦–0.288◦ 1.5–280 Study site DSM-Orthophoto Vanis Kvabebi Natlismcemeli Sabereebi Uplistsikhe resolution (cm) (10-5) (8-4) (9-4.5) (7-3.5) Remote Sens. 2021, 13, 1040 10 of 30

Acquisitions were performed using preloaded ﬂight trajectories and controlled overlaps between the images using Pix4Dcapture software. Structure-from-motion (SfM) image processing has been applied, using Agisoft Photoscan software [70]. To achieve high qual- ity of obtained 3D models and precision in georeferencing, ground control points were used with measured absolute coordinates using DGPS (CORS system, with sub-centimeter range horizontal errors in absolute coordinates) or Robotized Total Station devices. Several products of high accuracy were delivered, such as DSM and orthophoto mosaics in both horizontal and vertical projections (Table2), with sub-decimeter resolution (horizontal errors in a decimeter range). Moreover, high-resolution photo textures allowed for true- colored DSMs and for the visual estimate of the damaging effect of water channels and crack systems of water inﬁltration. To obtain complete 3D models UAV-DP was integrated with TLS surveys [24,36], which were carried out by means of a long-range 3D terrestrial laser scanner (Riegl VZ-400) [71] (Table2). ArcMap (ESRI) Hydrology Tools package was applied using Flow Direction, Accumulation and Stream Order functions for the analysis of the slope scale drainage system [63]. This allowed for the modelling of surface water run off trajectories, calculating the corresponding watersheds and categorizing the stream order on the DSMs [13,58].

3.4. Kinematic Analysis In order to define the main rock slope instability processes, a spatial kinematic analysis can be performed by using discontinuity orientation data, semiautomatically and manually extracted from the obtained slope 3D surface models. Given the slope geometry and the discontinuity orientation, this kind of analysis is capable of mapping the spatial probability of occurrence for the following instability mechanisms: (i) plane failure (PF) [72]. (ii) wedge failure (WF) [72]. (iii) block toppling (BT) [73]. (iv) flexural toppling (FT) [73,74]. Ref. [75] introduced a kinematic hazard index for each instability mechanism. These values are calculated by counting poles and discontinuities in the critical areas of the stereographic projection: (v) Npf = number of poles satisfying plane failure conditions. (vi) Iwf = number of intersections satisfying wedge failure conditions. (vii) Nbt = number of poles satisfying block toppling conditions. (viii) Ibt = number of intersections satisfying block toppling conditions. (ix) Nft = number of poles satisfying flexural toppling conditions. The kinematical hazard index is calculated as follows: (i) Cpf = 100 × (Npf/N) for plane failure. (ii) Cwf = 100 × (Iwf/I) for wedge failure. (iii) Cbt = 100 × (Nbt/N) × (Ibt/I) for block toppling. (iv) Cft = 100 × (Nft/N) for flexural toppling. where N and I are the total number of poles and intersections, respectively. By using specific software, such as KARS [75], Rock Slope Stability [76], or DiAna-k [77], it is possible to load a great number of discontinuities with different friction angles. In- tersection lines are calculated automatically, together with the equivalent friction angle, based on the friction angles of the intersecting planes and the shape of the wedge [75]. The analysis can be performed for specific slope orientations or for each cell of a 3D surface (true 3-D kinematic analysis). This method overcomes many limitations of the traditional approaches, as it is possible to employ true 3D surfaces, and the kinematic conditions leading to the investigated instability mechanisms have been extended to overhanging slopes. Moreover, a global kinematic index (GKI) can be introduced, to quantitatively define the rock instability hazard for each sector of the slope, whatever the effective instability mechanisms. The input data of this method are the high-resolution 3D meshes obtained Remote Sens. 2021, 13, 1040 11 of 30

from terrestrial laser scanner or photogrammetric surveys and the discontinuities extracted with the manual and semiautomatic [78] methods. As the orientation of fractures is related to the tectonic processes that have been acting in the investigated areas, a kinematic analysis can be useful to highlight the rock wall sectors which are more prone to instability processes. This approach was used for the obtained 3D surfaces using CloudCompare © software [79].

4. Results 4.1. Geotechnical Data The analyzed Georgian monastery complexes are carved in soft rocks such as tuffs (Vanis Kvabebi) and sedimentary transitional-marine formations (David Gareja and Uplith- sikhe). From a geotechnical point of view all the collected samples are characterized by very low strength parameters (many samples were easily broken even by hand), particularly for the David Gareja samples, showing that these rocks are very weakly cemented (Table3 ). The geotechnical characterization of volcaniclastic tuff material at the in-situ scale is affected by several issues, such as the large dimensions of the clasts, reduced strength contrast between the matrix and the clasts themselves [80,81]. Regarding the Monastery of Petra thin sections of Upper Umm Ishrin sandstone shows multicolored, ﬁne to medium, sub- angular to sub-rounded grained sandstone [82]. Mineralogical-geotechnical data from [44] report iron oxides and kaolinite are the main rock matrix, while quartz grains make up over 80% of the stone content. Porosity is moderate, with total porosity around 15% and mainly medium (1–10 µm) to coarse pores (10–100 µm). According to [16] the UCS values of the monastery are around 25 MPa, showing that all the analyzed rupestrian sites are also constituted by weak rocks.

Table 3. Average values of the obtained geotechnical parameters of the collected samples.

Friction Angle Uniaxial Compressive Unit Weight Case Study (◦) Strength (MPa) (KN/m3) Vanis Kvabebi 28◦ 2.70 16.76 Natlismcemeli 27◦ 0.49 16.56 Sabereebi 23◦ 0.52 15.09 Uplistsikhe 40 3.55 19.5

4.2. Field-Remote Surveys and Kinematic Analysis 4.2.1. Vanis Kvabebi Besides the sub-horizontal layer bedding, three main subvertical joint sets were identified (Figure6a ): (1) J1: dipping W and N-S oriented, forming the main slope faces (Figure6d ); (2) J2: dipping NW and SW-NE oriented, it is responsible for the formation of a master joint intersecting all the upper section on the eastern rock wall of the complex; (3) J3: dipping SE and trending NW-SE, it represents persistent fractures along which ephemeral creek erosion has caused deep channel cuts in the southern sector of the complex (Figure6e). The field surveys highlighted differential erosion as an important local predisposing factor of instabilities, creating large niches along the bedding, especially in correspondence of the softer and more erodible ash layer and the upper section of the medium-grained tuffs, therefore undermining the overlying overhanging cliff sectors. In this framework the main identified high angle joint sets (Figure6a), combined with the stratigraphic features worsened the cliff instability: J1, dipping W parallel to the cliff face determines planar failures (Figure6b,d) especially affecting the overhanging sectors within the tuff layers, while the combination of J2 (dipping NE) and J3 (dipping SW) cause wedge failures (Figure6c), occurring especially in correspondence of the more competent welded tuff sectors (Figure6f). The 2D-orthoprojection and the performed hydromodelling allowed the investigation of the complex drainage pattern of the ravine, with streams categorized Remote Sens. 2021, 13, x FOR PEER REVIEW 12 of 31

Table 3. Average values of the obtained geotechnical parameters of the collected samples.

Friction Angle Uniaxial Compressive Unit Weight Case Study (°) Strength (MPa) (KN/m3) Vanis Kvabebi 28° 2.70 16.76 Natlismcemeli 27° 0.49 16.56 Sabereebi 23° 0.52 15.09 Uplistsikhe 40 3.55 19.5

Remote Sens. 2021, 13, 1040 4.2. Field-Remote Surveys and Kinematic Analysis 12 of 30 4.2.1. Vanis Kvabebi Besides the sub-horizontal layer bedding, three main subvertical joint sets were iden- tifiedby catchment (Figure 6a): area (1) and J1: dipping order (Figure W and7 a).N-S Very oriented, low orderforming rivulets the main develop slope onfaces the (Figure main 6d);cliff ( (eastern2) J2: dipping sector) NW along and SW the-NE J3 fracturesoriented, (Figureit is responsible7a–c), as for well the asformation the main of streamsa master jointlocated intersecting in the southern all the upper sectors, section which on deeply the eastern cut into rock the w volcanicall of the sequence complex; along (3) J3: large dip- pingerosion SE channelsand trending involving NW- theSE, wholeit represents cliff height persistent (Figures fractures6e and7 a).along Inparticular, which ephemeral stream creekerosion erosion acts along has caused the J3 master deep channel joint creating cuts in potentially the southern unstable sector nichesof the atcomplex the base (Figure of the 6medium-grainede). tuffs (Figure7d).

Figure 6.6. Structural setting and kinematickinematic analysis at Vanis Kvabebi: identified identiﬁed sets ( a); kinematickinematic analysis for planar ( b) and wedge (c) failures; 2017 rock fall deposits (d); subvertical fracture where creek channel erosion has concentrated ( (e); welded tuff portionportion affectedaffected byby wedgewedge failuresfailures ((ff).).

The IRTfield surveys surveys were highlighted carried out differential from a 100 erosion m distance, as an leadingimportant to alocal 1120 predisposing× 1460-pixel factorimage of with instabilities, 6.5 cm pixel creating size. large Several niches thermograms along the werebedding, mosaicked especially creating in correspond- a surface encetemperature of the softer map ofand the more main erodible part of ash the layereastern and cliff the face upper (Figure section8a). of A temperaturethe medium- grainedgradient tuffs, is visible therefore from top undermining to bottom (Figure the overlying8b): the overhanging upper basalt ﬂowcliff sectors.portions In show this frameworkhigher temperatures the main identified (3.4 ◦C), due high to angle a more joint direct sets sunlight(Figure 6a), radiation combined with with respect the strati- to the graphiclower medium-grained features worsened tuff the level cliff (2.4 instability:◦C), and J1, especially dipping theW parallel coarse-grained to the cliff tuffs, face more de- terminesshaded and planar cooler failures (1.9 ◦ C),(Figure also due6b, d) to especially the presence affecting of grass the cover overhanging at bottom sectors of the within ravine. theWarm tuff anomalies layers, while were the highlighted combination in of correspondence J2 (dipping NE) toand the J3 ledge-niche(dipping SW) systems cause wedge in the failuresupper basalts (Figure and 6c), the occurring cave system especially (4.4–6.5 in correspondence◦C), the eroded of lower the more part competent of the basalt welded ﬂows tuff(4.2–5.6 sectors◦C), ( andFigure along 6f). which The 2D the-orthoprojection second level pathway and the develops performed but especially hydromodelling where the al- lowedmaster thejoint investigation belonging to J2 of intersects the complex the whole drainage lower pattern basalt ofportion. the ravine, Regarding with this streams open fracture (over 40 m persistent) the IRT data allowed for the description of the aperture geomechanical parameter as from extremely wide (10–100 cm) to cavernous (>100 cm) according to [12]. In correspondence to the 2017 rock fall niche (AOI 1 in Figure8c), no evidence of moisture/seepage phenomena was detected; therefore, dry conditions were assessed during the survey (Figure8a,c). Remote Sens. 2021, 13, x FOR PEER REVIEW 13 of 31

categorized by catchment area and order (Figure 7a). Very low order rivulets develop on the main cliff (eastern sector) along the J3 fractures (Figure 7a–c), as well as the main streams located in the southern sectors, which deeply cut into the volcanic sequence along Remote Sens. 2021, 13, 1040 large erosion channels involving the whole cliff height (Figures 6e and 7a). In particular,13 of 30 stream erosion acts along the J3 master joint creating potentially unstable niches at the base of the medium-grained tuffs (Figure 7d).

FigureFigure 7 7.. VanisVanis Kvabebi Kvabebi surface surface water water run run off off model: model: (a (a) )14 14 cm cm resolution resolution DSM DSM with with streams streams categorized categorized by by catchment catchment areaarea (ovals (ovals highlight highlight details details in in b b-e;-e; stream stream legend: legend: L L = = left left cliff; cliff; F F = = front front cliff; cliff; R R = = right right cliff; cliff; CH CH = = channels); channels); (b (b) )top top-view-view of the lower chapel area; (c) Large niche created by erosional features of the J2 set Master Joint; (d) erosional channel in of the lower chapel area; (c) Large niche created by erosional features of the J2 set Master Joint; (d) erosional channel in correspondence of J3 fracture; (e) stream gullies on the slope of the ravine opposite the monastery. correspondence of J3 fracture; (e) stream gullies on the slope of the ravine opposite the monastery.

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The IRT surveys were carried out from a 100 m distance, leading to a 1120 × 1460- pixel image with 6.5 cm pixel size. Several thermograms were mosaicked creating a surface temperature map of the main part of the eastern cliff face (Figure 8a). A temperature gradient is visible from top to bottom (Figure 8b): the upper basalt flow portions show higher temperatures (3.4 °C), due to a more direct sunlight radiation with respect to the lower medium-grained tuff level (2.4 °C), and especially the coarse-grained tuffs, more shaded and cooler (1.9 °C), also due to the presence of grass cover at bottom of the ravine. Warm anomalies were highlighted in correspondence to the ledge-niche systems in the upper basalts and the cave system (4.4–6.5 °C), the eroded lower part of the basalt flows (4.2–5.6 °C), and along which the second level pathway develops but especially where the master joint belonging to J2 intersects the whole lower basalt portion. Regarding this open fracture (over 40 m persistent) the IRT data allowed for the description of the aperture geomechanical parameter as from extremely wide (10–100 cm) to cavernous (>100 cm)

Remote Sens. 2021, 13, 1040 according to [12]. In correspondence to the 2017 rock fall niche (AOI 1 in Figure 814c) of, no 30 evidence of moisture/seepage phenomena was detected; therefore, dry conditions were assessed during the survey (Figure 8a, c).

Figure 8.8. Vanis KvabebiKvabebi IRTIRT data:data: mosaicked surface temperature mapmap ofof thethe mainmain sectorsector ofof the eastern cliff ((aa)) (the(the whitewhite rectanglerectangle representsrepresents the the Area Area of of Interest Interest 1 1 = = AOI AOI 1 1 shown shown in in (c ());c)); vertical vertical surface surface temperature temperature proﬁle profile of theof the cliff cliff (b) ( (Li1b) (Li1 in ain); a); detail of the 2017 collapse sector (AOI 1) (c); surface temperature map of the cliff’s southern sector (d). detail of the 2017 collapse sector (AOI 1) (c); surface temperature map of the cliff’s southern sector (d).

Cold thermal anomalies were detected along creek erosion channels (Figure8a), especially in the cliff’s southern sector (Figure8d), due to the presence of moisture connected to ephemeral rivulets. The 3D kinematic analysis performed shown in Figure9 covers the area represented by the 3D model of Figure2b. The color scale represents the 3D spatial probability of occurrence (expressed in %) for the different instability mechanisms, with colors ranging from blue (very low probability) to red (high probability), for the most relevant detected instability mechanisms affecting the rock cliff, which are: wedge failure (Figure9a), followed by plane failure (Figure9b), and subordinately free fall (Figure9c). Remote Sens. 2021, 13, x FOR PEER REVIEW 15 of 31

Cold thermal anomalies were detected along creek erosion channels (Figure 8a), especially in the cliff’s southern sector (Figure 8d), due to the presence of moisture connected to ephemeral rivulets. The 3D kinematic analysis performed shown in Figure 9 covers the area represented by the 3D model of Figure 2b. The color scale represents the 3D spatial probability of occurrence (expressed in %) for the different instability mechanisms, with colors ranging from blue (very low probability) to red (high probability), for the most relevant detected instability mechanisms affecting the rock cliff, which are: Remote Sens. 2021, 13, 1040 15 of 30 wedge failure (Figure 9a), followed by plane failure (Figure 9b), and subordinately free fall (Figure 9c).

Figure 9. Results ofof thethe 3D3D kinematic kinematic analysis. analysis. Wedge Wedge failure failure =48% = 48% (a); ( planara); planar failure failure = 31% = 31% (b); free(b); fallfree = fall 31% = (white31% (white ovals ovalshighlighting highlighting the detected the detected unstable unstable niches) niches) (c); GKI (c); up GKI to 62%up to ( d62%). (d).

TheThe latter latter can can be be locally locally predominant predominant in in overhanging overhanging sectors, sectors, such such as as the the larg largee niche createdcreated by by erosional erosional features features of of the the J2 J2 set set master master joint joint in in Figure Figure 77cc (Niche 1 in FigureFigure 99c),c ), andand the the 2017 2017 collapse collapse niche niche (=AOI (=AOI 1 in 1 inFigure Figure 8a,8 a,c;c; Niche Niche 2 in 2 inFigure Figure 9c,).9c,). In Ingeneral, general, the cliffthe cliffis highly is highly prone prone to kinematic to kinematic instability instability mechanisms mechanisms (GK (GKII max max > 60%, > 60%, Figure Figure 9d),9 d),es- peciallyespecially along along the the lowermost lowermost basalt basalt flow ﬂow portion portion and and the the ash ash layer layer within within the the tuff, tuff, along along thethe niches niches and and the the master master joint. joint.

4.2.2.4.2.2. David David Gareja Gareja HereafterHereafter the the outcomes outcomes of of the the surveys surveys on on the sites of Natlismcemeni and Sabereebi areare presented. presented. Like Like many many of of the the monasteries monasteries of of David David Gareja, Gareja, the the Natlismcemeni Natlismcemeni complex complex (still(still inhabited inhabited by by a a monastic monastic community) community) is is located located on on top top of of a a hogback hogback ridge ridge overlooking overlooking aa low low hilly landscape andand isis mainlymainly carvedcarved in in a a 40 40 m m high high rock rock cliff. cliff. The The latter latter is formedis formed by bya 20 a 20 m-thick m-thick sequence sequence of of thin thin alternating alternating layers layers of of ﬁne-grained fine-grained sands sands and and clays clays capping capping a a20 20 m-thick m-thick level level of of coarse-sand coarse-sandlayers; layers; aa debrisdebris talustalus discontinuouslydiscontinuously mantled by sparse bushy vegetation covers the lowermost part of the sequence (Figure 10a). The monastery’s main spaces (chapels and monks’ cells) are represented by three levels of over 30 caves cut into the thick weak sandy layer, while additional brick construction works are placed either on top of the sequence (tower) or adjacent to the cliff face (a new chapel and storage rooms; Figure 10a). The main discontinuities are organized in two main joint sets: J1 is related to the bedding and dips NE against the slope with low angles; J2 is a mainly subvertical set dipping SW along the slope; J3 is a subvertical fracture sparsely cutting the cliff face (Figure 10b). These fractures have a fundamental control on the stability of the cliff, as testiﬁed by detached large blocks deposited at the foot of the cliff on the northern sector of the cliff face (Figure 10a). The performed 2D kinematic analysis allowed for assessing the Remote Sens. 2021, 13, x FOR PEER REVIEW 16 of 31

bushy vegetation covers the lowermost part of the sequence (Figure 10a). The monastery’s main spaces (chapels and monks’ cells) are represented by three levels of over 30 caves cut into the thick weak sandy layer, while additional brick construction works are placed either on top of the sequence (tower) or adjacent to the cliff face (a new chapel and storage rooms; Figure 10a). The main discontinuities are organized in two main joint sets: J1 is related to the bedding and dips NE against the slope with low angles; J2 is a mainly sub- Remote Sens. 2021, 13, 1040 vertical set dipping SW along the slope; J3 is a subvertical fracture sparsely cutting16 ofthe 30 cliff face (Figure 10b). These fractures have a fundamental control on the stability of the cliff, as testified by detached large blocks deposited at the foot of the cliff on the northern sector of the cliff face (Figure 10a). The performed 2D kinematic analysis allowed for as- smainessing instability the main mechanisms instability mechanisms affecting the affecting rock cliff, the which rock are cliff, represented which are byrepresented plane failure, by planefollowed failure, by wedge followed failure by wedge and subordinately failure and subordinately by toppling (Figure by toppling 10c–g). (Figure 10c–g).

FigureFigure 10.10. Outcomes of the ﬁeldfield surveys in thethe NatlismcemeniNatlismcemeni monastery:monastery: local geological features (a) andand structuralstructural discontinuities ( b). Main instability phenomena phenomena detected detected in in the the field ﬁeld surveys and corresponding 2D kinematic analysis: planar (c,,g g)) and and wedge wedge ( d(d,h, )h failures;) failures; topples topples (e (,ei)., i Open). Open fracture fracture (f ).(f).

Wedge failures failures can can be be locally locally predominant predominant in overhanging in overhanging sectors, sectors, such such as the as cave the nichescave niches (Figure (Figure 10d–h), 10 whiled–h), open while fractures open fractures parallel parallel to the slope to the can slope develop can developtopples (Fig- top- ureples 10e, (Figure i). Very 10e,i persiste). Very persistentnt and spaced and wide spaced-open wide-open fractures fractures are visible are within visible the within monas- the tmonasteryery caves and caves can and potentially can potentially damage damage the complex the complex seriously seriously (Figure (Figure10f). The 10 IRTf). The survey IRT wassurvey carried was carriedout from out different from different acquiring acquiring positions positions (at a 110 (at m a and 110 m20 andm respectively; 20 m respectively; Table 2)Table in order2) in orderto cover to coverthe whole the whole complex complex (Figure (Figure 11a) and 11a) make and close make-ups close-ups on critical on criticalsectors (AOIsectors2, 3 (AOI2, in Figure 3 in 11b, Figure c).While 11b,c).While the sound the soundrock slope rock shows slope shows average average values values between between 10.2– ◦ ◦ 1010.2–10.8.8 °C, warmC, warm thermal thermal anomalies anomalies (13.8 (13.8–14.1–14.1 °C) wereC) were widely widely detectable detectable both both at the at the top top of tofhe the thick thick sandy sandy layer layer related related to toJ1, J1, and and at at the the topmost topmost sector sector o off the the cave cave accesses, accesses, high- high- lighting potentially unstableunstable ledge-nicheledge-niche systemssystems (Figure (Figure 11 11a–c). a–c). Warm Warm thermal thermal anomalies anoma- liesalso also enhance enhance wide wide open open fractures fractures located located parallel parallel to the to slopethe slope (related (related to J2; toFigure J2; Figure 11b,c). 11 No cold thermal anomalies were detected; therefore, dry conditions were assessed at the time of the survey for the site. The 3D kinematic analysis conﬁrms how the more unstable slope sectors are located in correspondence to the ledge-niche systems, where GKI reaches up to 64% (Figure 11d). The monastery of Sabereebi (currently abandoned) is composed of a series of 15 frescoed caves (Figure 12a) developed on three levels carved in sandy layers. As for Natlism- cemeni, Sabereebi is located on top of a hogback ridge carved in material formed by a transition between very weak sandstones and partially cemented sediments, represented by a sequence of conglomerates at the top, a 10 m-thick level of coarse-grained sands overlying a sequence of alternating ﬁne-grained sands and clays; a sparsely vegetated Remote Sens. 2021, 13, 1040 17 of 30

debris talus covers the slope toe (Figure 12a,b). In fact, some sandy blocks were found at Remote Sens. 2021, 13, x FOR PEER REVIEW 17 of 31 the foot of the talus during the 2018 ﬁeld survey, one of them evidently freshly detached from a pillar formed by the intersection of J2-J3 (Figure 12a,c,d). The bedding (J1) dips NE towards the slope with low angles, while two not widely spaced but persistent discon- b,tinuity c). No sets cold cut thermal the slope anomalies parallel were (J2) and detected orthogonal; therefore (J3), todry the conditions cliff face, were dipping assessed E and atSSW the respectively, time of the survey deeply for affecting the site. the The stability 3D kinematic of the cliff analysis (Figure confirms 12e). The how 2D kinematicthe more unstableanalysis conﬁrmsslope sectors high are planar located and in especially correspondence wedge to failure the ledge indexes-niche along systems, these where planes, GKIsubordinately reaches up toppling to 64% ( (FigureFigure 11d).12c–f).

FigureFigure 11.11. OutcomesOutcomes ofof thethe IRTIRT surveys surveys of of the the Natlismcemeni Natlismcemeni monastery: monastery: IRT IRT mosaicked mosaicked image image (2075 (2075× 505× 505 pixels pixels with with 7.1 cm7.1 pixelcm pixel size) size) showing showing the the detected detected AOI AOI 2–3 2 (white–3 (white rectangles) rectangles) and and the the warm warm thermal thermal anomalies anomalies (dashed (dashed white white ovals) ovals) (a); AOI(a); AOI 2 (1160 2 (1160× 480-pixel × 480-pixel image image size) size) (b )(b and) and AOI AOI 3 (21203 (2120× ×590-pixel 590-pixel imageimage size) ( c), both both having having 1.3 1.3 cm cm spatial spatial resolution; resolution; 3D3D kinematickinematic analysisanalysis (GKI(GKI index up to 64%) (d).

TheIRT surveysmonastery were of Sabereebi carried out (currently from 100 aband m andoned) 20 m is (Table composed2) to map of a all series the monasteryof 15 fres- coedcliffs andcaves make (Figure detailed 12a) analysisdeveloped on on AOIs. three IRT levels analysis carved showed in sandy average layers. temperatures As for Nat- of lismcemeni,26.8 ◦C of the Sabereebi slope face is and located warm on thermal top of a anomalies hogback ridge (from carved 31 to 42 in◦C) material in correspondence formed by ato transition the recent between collapse very sector weak and sandstones the underlying and partiallyslope talus cemented (AOI 4), sediments, where erosion repre- is sentedexposing by barea seque soilnce (Figure of conglomerates 13a,b,d). Sharp at cold the thermaltop, a 10 anomalies m-thick level (15.4–17.7 of coarse◦C)- highlightgrained sandsa large overlying system ofa sequence open cracks of alternating connected fine to J3-grained at the rightsands endand ofclays; the a complex sparsely (AOI vege- 5; tatedFigure debris 13a,c,e talus). As covers for Natlismcemeni, the slope toe no (Figu coldre thermal 12a, b). anomalies In fact, some were sandy detected; blocks therefore, were founddry conditions at the foot were of the assessed talus during at the timethe 2018 of the field survey. survey, one of them evidently freshly detached from a pillar formed by the intersection of J2-J3 (Figure 12a, c). The bedding (J1) dips NE towards the slope with low angles, while two not widely spaced but persistent discontinuity sets cut the slope parallel (J2) and orthogonal (J3) to the cliff face, dipping E and SSW respectively, deeply affecting the stability of the cliff (Figure 12d). The 2D kinematic analysis confirms high planar and especially wedge failure indexes along these planes, subordinately toppling (Figure 12 c–e).

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Figure 12. The Sabereebi monastery geological features: cliff face showing the chambers and the detached block in 2018 (a); Remotestratigraphic Sens. 2021, 13,sequence x FOR PEER (b );REVIEW the detachment sector in 2020, showing the collapsed pillar (c); 2D kinematic analysis: wedge19 of 31 failure mechanism (d); planar failure mechanism (e); direct toppling (f).

Figure 13. Sabereebi IRT data:data: Mosaicked surface temperature map (950 ×× 480480 pixels with 6.5 cm resolution ( a); verticalvertical and horizontal surface temperat temperatureure proﬁlesprofiles Li1 (b) andand Li2 (c). Classified Classiﬁed surface temperature maps in correspondence of AOI 4 (d) and AOI 5 (e). AOI 4 (d) and AOI 5 (e).

The UAV UAV-DP-DP survey and thethe hydromodellinghydromodelling analysis provided a high-resolutionhigh-resolution DEM and derivedderived productsproducts such such as as slope slope and and the the drainage drainage maps maps (Figure (Figure 14a,c). 14a, These c). These latter lshowedatter showed main main cataclinal-type cataclinal- streamstype streams draining draining the hogback the hogback of the of monoclinal the monoclinal structure struc- to turethe NE, to the while NE, small while anaclinal-type small anaclinal streams-type erodestreams the erode debris the talus debris underlying talus underlying the monastery, the monastery, as shown by the bare soil sectors highlighted by IRT. Therefore, linear erosion 1 does not affect the cliff face of Sareebebi, although field surveys highlighted traces of mud drips on the cliff face coming from the top of the hogback, showing that overland flow can affect the monastery during rainfall events (Figure 14b).

Figure 14. UAV-DP high resolution DEM derived products: slope map (a) (the monastery complex is located by the red rectangle); drainage pattern map (b) (the monastery complex is located by the yellow rectangle); UAV close-up on the detachment sector showing light brown colored mud drips on the dark brown sandstones (c).

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Figure 13. Sabereebi IRT data: Mosaicked surface temperature map (950 × 480 pixels with 6.5 cm resolution (a); vertical and horizontal surface temperature profiles Li1 (b) and Li2 (c). Classified surface temperature maps in correspondence of AOI 4 (d) and AOI 5 (e). Remote Sens. 2021, 13, 1040 19 of 30 The UAV-DP survey and the hydromodelling analysis provided a high-resolution DEM and derived products such as slope and the drainage maps (Figure 14a, c). These latter showed main cataclinal-type streams draining the hogback of the monoclinal struc- as shownture to the by NE, the while bare soilsmall sectors anaclinal highlighted-type streams by erode IRT. Therefore,the debris talus linear underlying erosion doesthe not affectmonastery, the cliff as face shown of Sareebebi, by the bare although soil sectors ﬁeld highlighted surveys by highlighted IRT. Therefore, traces linea ofr mud erosion drips on does not affect the cliff face of Sareebebi, although field surveys highlighted traces of mud the cliff face coming from the top of the hogback, showing that overland ﬂow can affect the drips on the cliff face coming from the top of the hogback, showing that overland flow can monasteryaffect the duringmonastery rainfall during events rainfall (Figure events 14 (Figureb). 14b).

FigureFigure 14. UAV-DP 14. UAV high-DP high resolution resolution DEM DEM derived derived products: products: slope map map (a (a) )(the (the monastery monastery complex complex is located is located by the by red the red rectangle); drainage pattern map (b) (the monastery complex is located by the yellow rectangle); UAV close-up on the rectangle);Remote Sens. drainage2021, 13, x FOR pattern PEER mapREVIEW (b ) (the monastery complex is located by the yellow rectangle); UAV close-up20 onof the31 detachment sector showing light brown colored mud drips on the dark brown sandstones (c). detachment sector showing light brown colored mud drips on the dark brown sandstones (c).

4.2.3.4.2.3. Uplistsikhe Uplistsikhe FourFour main main identified identified discontinuity sets sets were were identified identified in the in field: the field: J1 (bedding) J1 (bedding) dips dips SESE with with low low angles, angles, J2J2 jointjoint set dips dips SW SW parallel parallel to tothe the slop slopee face face with with high highangles, angles, J3 is a J3 is a subvertical joint set trending WNW-SSE and J4 dips NW with mid-high angles (Figure subvertical joint set trending WNW-SSE and J4 dips NW with mid-high angles (Figure 15a). 15a). The 2D kinematic analysis highlighted two equally predominant instability mecha- Thenisms: 2D kinematic plane (Figure analysis 15b) highlighted and wedge failures two equally (Figure predominant 15c). In general, instability the rock mechanisms: cliff is planehighly (Figure prone 15 tob) kinematic and wedge instability failures mechanisms (Figure 15c). (GKI In general,max = 50%, the Figure rock cliff15d), is particu- highly prone to kinematiclarly the areas instability affected by mechanisms the June 2018 (GKI and max the November = 50%, Figure 2019 failures15d), particularly (Figures 4c and the areas affected15d). by the June 2018 and the November 2019 failures (Figures4c and 15d).

FigureFigure 15. Structural 15. Structural data data in the in the site site of of Uplistsikhe: Uplistsikhe:main main discontinuity sets sets (a ();a 2D); 2D kynematic kynematic analysis analysis for planar for planar (b) and (b ) and wedgewedge failures failures (c); 3D (c); kynematic 3D kynematic analysis analysis (GKI (GKI index index up up to to 49.9%) 49.9%)( (dd),), showingshowing the the areas areas affected affected by by rockfalls rockfalls (Areas (Areas 1–2 1–2 in in the red ovals), and the AOI highlighted in Figure 16 (black square). the red ovals), and the AOI highlighted in Figure 16 (black square). The IRT survey highlighted warm thermal anomalies along persistent joints belonging to J4, where minor rock falls occurred in 2019 (Figure 16a). General dry conditions were assessed along the rock cliff except for the left sector, where a cold thermal anomaly (as well as the presence of vegetation and weathering coating) revealed the presence of an ephemeral stream, which concentrates along J4, the rock cliff’s most persistent discontinuity set (AOI 6 in Figure 16b, d). This is confirmed also by the hydromodelling analysis (red arrow in Figure 16c), which also shows how the high order ephemeral streams concentrate mainly along J3-J2 sets, while low order ephemeral streams mainly along J4. The exception is represented by the detected cold thermal anomaly located in correspondence to a high order main stream developed in a very persistent and eroded discontinuity belonging to J4. AOI 6 is also characterized by warm thermal anomalies enhancing cavities of various size caused by corrasion (Figure 4d, spots = Sp1–4 in Figure 16b) and is particularly prone to instability, as shown by the high GKI values (Figure 15d).

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Figure 16. Outcomes of the IRT surveys:surveys: mosaickedmosaicked thermogramsthermograms (2800 ×× 850850 pixel) pixel) ( aa)) showing open fractures along J4 (white(white dashed ovals) and AOI 6 highlighted in b, d; detail of the cold thermal anomaly ( b) with spots showing the warm thermal anomalies where the corrasion cavities develop (Bx = box plot showing min., max., avg. temperature); modelled thermal anomalies where the corrasion cavities develop (Bx = box plot showing min., max., avg. temperature); modelled ephemeral drainage network of the site (c) (red arrow highlights the ephemeral stream developing the cold thermal anom- ephemeral drainage network of the site (c) (red arrow highlights the ephemeral stream developing the cold thermal anomaly aly in b); correspondent optical image of b (d). in b); correspondent optical image of b (d). 4.2.4. The Monastery in Petra The IRT survey highlighted warm thermal anomalies along persistent joints belonging to J4,The where IRT minorsurveys rock allowed falls occurredthe mapping in 2019 of evident (Figure widespread 16a). General cold dry thermal conditions anomalies were alassessedong an alignment along the rockwhich cliff extends except continuously for the left sector, from wherethe left a-bottom cold thermal sector anomalyof the of the (as rockwell wall as the topresence the left basal of vegetation part of the and monastery weathering (Figure coating) 17a). revealedSeepage theis particularly presence of evi- an dentephemeral in the stream,left sector which of the concentrates rock wall along with J4,respect the rock to cliff’sthe adjacent most persistent lower lef discontinuityt part of the monument,set (AOI 6 in as Figure shown 16 alsob,d). by This the isvertical conﬁrmed surface also temperature by the hydromodelling profiles (Figure analysis 17 b, c (red, e– g).arrow The inanalysis Figure of16 c),the which average also temperature shows how confirms the high this order trend: ephemeral while on streams the dry concentrate rock wall temperaturesmainly along J3-J2range sets, from while 20.1 low to 20.3 order °C ephemeral (Bx 1 in Figure streams 17 mainlyd, h), in along the damp J4. The area exception varies fromis represented 19 to 19.4 by°C the(Bx detected2 in Figure cold 17 thermald, h), while anomaly there is located a marked in correspondence drop in correspondence to a high toorder the mainseepage stream sectors developed (from 13.4 in a °C very to persistent15.5 °C), especially and eroded in discontinuity the rock mass belonging at the left to of J4. theAOI m 6onastery is also characterized (see Bx 3 in Figure by warm 17 d, thermal h). anomalies enhancing cavities of various size caused by corrasion (Figure4d, spots = Sp1–4 in Figure 16b) and is particularly prone to instability, as shown by the high GKI values (Figure 15d).

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4.2.4. The Monastery in Petra The IRT surveys allowed the mapping of evident widespread cold thermal anomalies along an alignment which extends continuously from the left-bottom sector of the of the rock wall to the left basal part of the monastery (Figure 17a). Seepage is particularly evident in the left sector of the rock wall with respect to the adjacent lower left part of the monument, as shown also by the vertical surface temperature proﬁles (Figure 17b,c,e–g). The analysis of the average temperature conﬁrms this trend: while on the dry rock wall temperatures range from 20.1 to 20.3 ◦C (Bx 1 in Figure 17d,h), in the damp area varies from 19 to 19.4 ◦C (Bx 2 in Figure 17d,h), while there is a marked drop in correspondence Remote Sens. 2021, 13, x FOR PEER REVIEWto the seepage sectors (from 13.4 ◦C to 15.5 ◦C), especially in the rock mass at the left22 of 31 the

monastery (see Bx 3 in Figure 17d,h).

FigureFigure 17.17.Petra Petra MonasteryMonastery IRTIRT data:data: mosaicked thermal image of of the the low low-left-left corner corner of of the the monastery monastery (68 (6800 ×× 240240 pixels; pixels; correspondingcorresponding optical optical image image in in Figure Figure5c) 5c) ( a ();a); close-up close-up single single thermograms thermograms (320 (320× ×240 240 pixels): pixels): AOI AOI 5 5 ( b(b––dd)) and and AOI AOI 6 6 ( e(e––h). h).

5. Discussion 5.1. Conservation Problems Affecting the Analysed Rupestrian Sites Authors like [13] and [16] report the relationships between UCS and porosity in the rocks of some important rupestrian sites part of the UNESCO world heritage or tentative lists, including Vardzia in Georgia (geologically very similar with respect to Vanis Kvabebi) and Petra. They demonstrate how the low UCS values are generally coupled with high porosity, especially in volcanic materials, for example the ones of Vardzia, which are part of the same geological formation of Vanis Kvabebi. On the other hand, the continental/sedimentary geological formations, such as the soft sandstones of Bamiyan (very similar to the ones of David Gareja) and Petra, exhibit a relevant vertical heterogeneity due to the formation’s stratigraphy. From a geotechnical point of view the arrangement between the tuff components, their different fabric, and the normally large amounts of clay minerals in the matrix opens a very wide spectrum of porosities. This can play a key role in the behavior of the rock resistance to weathering and deterioration due to

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5. Discussion 5.1. Conservation Problems Affecting the Analysed Rupestrian Sites Authors like [13,16] report the relationships between UCS and porosity in the rocks of some important rupestrian sites part of the UNESCO world heritage or tentative lists, including Vardzia in Georgia (geologically very similar with respect to Vanis Kvabebi) and Petra. They demonstrate how the low UCS values are generally coupled with high porosity, especially in volcanic materials, for example the ones of Vardzia, which are part of the same geological formation of Vanis Kvabebi. On the other hand, the continental/sedimentary geological formations, such as the soft sandstones of Bamiyan (very similar to the ones of David Gareja) and Petra, exhibit a relevant vertical heterogeneity due to the formation’s stratigraphy. From a geotechnical point of view the arrangement between the tuff components, their different fabric, and the normally large amounts of clay minerals in the matrix opens a very wide spectrum of porosities. This can play a key role in the behavior of the rock resistance to weathering and deterioration due to moisture [6,7]. Moisture expansion under determinate relative humidity (hygric dilatation) is caused by the swelling and shrinking of clay minerals (originating from the chemical decomposition of silicate minerals of the weak rocks) and can be recognized as one of the most important factors contributing both to the weathering and deterioration of both volcanic tuffs and sandstones [5,8,9,83]. In particular [8] demonstrated how the intensity of moisture expansion and related swelling pressure cannot be attributed only to the swelling of clay minerals, suggesting that the micropores and the resulting disjoining pressure during wet/dry cycles also play an important role. Moisture expansion also leads to the reduction of the uniaxial compressive strength of the tuff when saturated [10,11]. Therefore, water runoff, concentration, and seepage can trigger these degradation phenomena within weak and porous material such as the tuffs and sandstones of the analyzed sites. Furthermore, in rupestrian sites areas of preferential water runoff are governed by the slope morpho-structural setting, being caused by the combination of the fracture system (especially the open features), and the slope-scale morphology-roughness (e.g., ledges and niches and unstable protruding-overhanging sectors) [13]. Joints and cracks in particular can also affect the structural stability of tuff slopes, since their geometry with respect to the slope face generates different kinematic behavior of the rock mass, while their engineering properties are adversely affected by water seepage and moisture, causing weathering and discoloration films reducing the wall strength [9,10,17]. This process can lead to the loosening of the rock mass and the widening of the joints creating potential instabilities such as open features [12]. Water is also a major factor in the deterioration process in Petra, due to flood damage, rainwater, runoff water, capillary action, and their subsequent effects such salt encrustation on the sandstone surface [40,82,84]. The rising of the water table in the area in particular has had a major impact on many other deterioration mechanisms such as salt crystallization due to water evaporation and the dissolution or leaching of materials (clay minerals, for example, since most of the Petra monuments were carved from sandstone with a high clay content) [85]. Salt-crystal encrustation can also expand and widen existing fissures, triggering rock-falls and collapses; this process is known as salt-wedging and is a great contributor to the degradation of Petra’s archaeology and natural features [86,87].

5.2. Conservation Criticalities and Proposed Mitigation Measures 5.2.1. Vanis Kvabebi The performed analysis conﬁrmed that the 2017 rock fall event occurred in presence of a strong structural control due to the collapse of a protruding sector of the basal sector of the medium-grained tuffs; this was undermined by an erosional niche in the underlying ash layer, J1 joints, and a master joint belonging to J2, characterized by high persistence involving the whole upper cliff sector (from the medium-grained tuffs to the basalt ﬂows; Figures6–9). The modelled stream pattern is connected to rainfall and is governed by the interaction of water runoff with the morpho-structural setting of the slope, allowing analysis of damage pattern and detection of substantial water driven erosion. The J2 master- Remote Sens. 2021, 13, 1040 23 of 30

joint is a key fracture for the stability of the cliff, since the hydromodelling shows how it acts as a concentration area for water runoff, which in turn causes linear stream erosion that keeps widening the joint. This process has gone to the point of creating a concentration of water runoff which in turn has generated an unstable niche in the lower portion at the contact with tuffs, where basalts are more erodible (Figures 7c and9c ). Vanis Kvabebi in general shows distinct marks of rainwater weathering and erosion processes: nearly all recent collapses are connected to heavy rainstorms, as reported by local monks. Water stream gullies observed in situ completely match with the stream trajectories outlined in the hydromodelling, confirming water runoff/concentration as the most damaging factor Remote Sens. 2021, 13, x FOR PEER REVIEW 24 of 31 in the site. Based on these outcomes a water drainage system was designed using the obtained high-resolution 3D model as a topographic reference base, in order to design appropriate plans and cross-sections. TheThe modelled modelled streams streams were were classified classified in in a a GIS GIS environment environment [[63]63] according according to the numbernumber ofof confluencesconfluences and and the the watershed watershed areas. areas. The The main main streams streams are arerepresented represented by two by twomain main channels channels (Ch1 (Ch1–2–2 in Figure in Figure 7a, of7 a,which of which watershed watershed areas areasare 5.5 are and 5.5 6.5 and Ha, 6.5 Figure Ha, Figure18a). Both 18a). channels Both channels are fed are from fed the from steep the slopes steep slopes(approximate (approximatelyly 200 m vertically) 200 m vertically) on top onof topthe ofVanis the VanisKvabebi Kvabebi volcanic volcanic tuff-breccias tuff-breccias in which in which they cut they deep cut deepgullies gullies (Figure (Figure 8) and8 ) andare arenot notmuch much affected affected by runoff by runoff coming coming from from the theJavakheti Javakheti Plateau Plateau (only (only 1 Ha 1 Haof the of the6.5 6.5Ha Hawatershed watershed comes comes from from the Javakheti the Javakheti Plateau; Plateau; Figure Figure 18a). Due18a). to Duethe depth to the of depth the gul- of thelies, gullies, it is practically it is practically impossible impossible to divert to water divert from water them. from Smaller them. Smallerstreams streams(L1–4 and (L1–4 F2- and3 in F2-3Figure in 7a) Figure develop7a) develop on the monastery’s on the monastery’s main eastern main cliff, eastern their watershed cliff, their surface watershed var- 2 surfaceying from varying 330 to from 2670 330 m2 to(Figure 2670 m 18a).(Figure The act 18iona). Theof these action streams, of these with streams, special with regards special to 2 regardsthe ones to with the oneswatershed with watershed areas 770,areas 550, and 770, 2230 550, andm2 have 2230 a m greathave impact a great on impact the weather- on the weathering,ing, erosional erosional features features,, and stability and stability of the ofmonastery. the monastery. As shown As shown by the by analysis the analysis of the of theDEM, DEM, despite despite the the complex complex natu naturere of the of the relief, relief, most most of the of the observed observed water water streams streams can can be bediverted diverted from from the the main main cliff cliff by by directing directing th themem to to other other parts parts of of the the rock rock cliff cliff ( (FigureFigure 18b).18b). BasedBased on on thethe existingexisting DEMDEM thethe alterationalteration ofof hydrographicalhydrographical model for the proposed water divertingdiverting wallwallwas was calculated.calculated.

FigureFigure 18. 18.Vanis Vanis Kvabebi Kvabebi mitigation mitigation measures: measures: hydrographic hydrographic model model (a) showing(a) showing the watershedthe watershed surfaces, surfaces, the rock the cliffrock areas cliff (redareas rectangles), (red rectangles), the area the where area the where water the drainage water drainage system (shown system in(shownb) is planned in b) is (dashedplanned black (dashed square), black and square), the Javakheti and the PlateauJavakh divideeti Plateau (dashed divide green (dashed line), green (map line) of the, (map planned of the water planned diverting water wall diverting (dashed wall red (dashed line) (b red). line) (b).

TheThe resultsresults showshow thatthat thethe proposedproposed wallwall willwill ensureensure thethe waterwater divertingdiverting of an area of 27702770 mm22 area,area, whilewhile thethe minorminor streamstream catchmentcatchment areasareas willwill bebe reducedreduced toto valuesvalues rangingranging fromfrom 276276 to to 460 460 m m22 (Figure(Figure 1818b).b). TheThe waterwater divertingdiverting wall (about(about 90 m inin length)length) will be constructedconstructed using using local local material material toto minimizeminimize itsits visualvisual impact.impact. The area is easily accessible by a pedestrian path from the opposing side of the cliff, while only the first 15 m section will require scaffoldings, being placed on a rock cliff. Thus, mitigating rainwater impact will considerably reduce active hazards on the given area.

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by a pedestrian path from the opposing side of the cliff, while only the ﬁrst 15 m section will require scaffoldings, being placed on a rock cliff. Thus, mitigating rainwater impact will considerably reduce active hazards on the given area.

5.2.2. David Gareja (Natlismcemeni and Sabereebi) Generally, there is not a predominant instability process acting in the analyzed sites: they can all be attributed to rock collapses directly dependent on the local structural setting differently interacting with the slope face and stress release. In Natlismcemeni the performed activities proved to be effective tools for the location of potentially unstable ledge-niche systems and open fractures and to define the more landslide-prone sectors as priority areas for sealing/reinforcement. In Sabereebi erosional processes caused by ephemeral streams in the debris talus can undermine the stability of monastery pillars (as shown by the recent collapse; Figures 12–14). Future mitigation works to be planned are drainage channels to avoid both overland flow on top of the rock slope and stream erosion of the underlying debris talus. Rock samples collection and laboratory tests are still ongoing, and they will define the main strength and deformation parameters useful for future stability models. Nonstructural mitigation actions, such as the implementation of a monitoring system to measure the displacements, such as robotized topographic stations and Ground-based Synthetic Aperture Radar systems are being implemented (Figure 19a,b). Such action is important for establishing the priorities in the areas of intervention, in the framework of the site’s preservation, as well as for the safety of local monks, for the staff of Remote Sens. 2021, 13, x FOR PEER REVIEW 25 of 31 rock-climbers which will be operating in reinforcement/conservational tasks in the near future and for visiting tourists.

FigureFigure 19.19. Nonstructural measures, measures, such such as as displacement displacement monitor monitoringing systems, systems, planned planned inin David David Gareja:Gareja: RobotizedRobotized TotalTotal Station Station ( a(a);); ground ground based based interferometric interferometric synthetic synthetic aperture aperture radar rada systemr system (b ). The(b). scaffoldingThe scaffolding constructed constructed for the for anchoring the anchoring phase phase in Uplistsikhe in Uplistsikhe (c). (c).

5.2.3.5.2.2. UplistsikheDavid Gareja (Natlismcemeni and Sabereebi) SeveralGenerally, critical there areas is not (Figures a predominant 15 and instability16) among process which acting the rockfall-affected in the analyzed areassites: (Areasthey can 1–3 all in be Figure attributed 15d) wereto rock selected collapses as the directly highest dependent priority for on the the consolidation local structural works setting differently interacting with the slope face and stress release. In Natlismcemeni the performed activities proved to be effective tools for the location of potentially unstable ledge-niche systems and open fractures and to define the more landslide-prone sectors as priority areas for sealing/reinforcement. In Sabereebi erosional processes caused by ephemeral streams in the debris talus can undermine the stability of monastery pillars (as shown by the recent collapse; Figures 12–14). Future mitigation works to be planned are drainage channels to avoid both overland flow on top of the rock slope and stream erosion of the underlying debris talus. Rock samples collection and laboratory tests are still ongoing, and they will define the main strength and deformation parameters useful for future stability models. Nonstructural mitigation actions, such as the implementation of a monitoring system to measure the displacements, such as robotized topographic stations and Ground-based Synthetic Aperture Radar systems are being implemented (Figure 19a, b). Such action is important for establishing the priorities in the areas of intervention, in the framework of the site’s preservation, as well as for the safety of local monks, for the staff of rock-climbers which will be operating in reinforcement/conservational tasks in the near future and for visiting tourists.

5.2.3. Uplistsikhe Several critical areas (Figures 15 and 16) among which the rockfall-affected areas (Ar- eas 1–3 in Figure 15d) were selected as the highest priority for the consolidation works and for monitoring. These started from Area 1 with the reinforcement of a critical block on the rock cliff, by means of deep anchoring. A special hanging scaffolding was constructed and fifteen horizontal drillings up to 9 m in depth were conducted, where the still anchors were fixed and cemented (Figure 19c). The integrity of the rock surface was

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Remote Sens. 2021, 13, x FOR PEER REVIEW and for monitoring. These started from Area 1 with the reinforcement26 of of a 31 critical block on the rock cliff, by means of deep anchoring. A special hanging scaffolding was constructed and fifteen horizontal drillings up to 9 m in depth were conducted, where the still anchors were fixed and cemented (Figure 19c). The integrity of the rock surface was restored, leaving restored, leavingno visualno visual effects. effects. A water-drainage A water-drainage structure structure will will be also be also planned planned in correspondence in cor- to the respondence toephemeral the ephemeral stream stream highlighted highlighted in Figure in Figure 16c. This 16c. first This stage first ofstage mitigation of mitiga- works will allow tion works willthe allow opening the opening of the historical of the historical entrance entrance into the into Uplistsikhe the Uplistsikhe rock town, rock located town, at the bottom located at the bottomof the rock of the cliff. rock cliff.

5.2.4. The Monastery5.2.4. The of Petra Monastery of Petra As confirmed Asby IRT confirmed surveys by and IRT the surveys field inspections and the field the inspections water seepage the waterin the seepagerock in the rock mass occurs notmass from occurs single not point froms but single is generally points but diffused is generally along diffusedan alignment along connected an alignment connected to the bedding,to below the bedding, which belowthe rock which mass the is rockprobably mass isfully probably saturated. fully This saturated. is in agr Thisee- is in agreement ment with thewith hypothesis the hypothesis of [44], which of [44 ],states which that states the seepage that the detected seepage detectedin 2014 (Figure in 2014s (Figures5b,c 5b, c and Figureand 17) 17 i)s isprobably probably the the result result of of the the rock rock mass mass water water circulation circulation from from a a back back-side- reservoir side reservoir (as(as shownshown byby thethe dampnessdampness ofof thethe rockrock mass),mass), wherewhere a a dam dam was was recently recently constructed in constructed ina a gorge gorge passing passing on on the the left left side side of of the the monument monument and and a a nearby nearby old old Nabatean Naba- cistern [88]. tean cistern [88]The. The increase increase of seepage of seepage from from 2011 2011 to 2014 to 2014 canbe can likely be likely related related to the to exceptional the rainfall exceptional rainfalloccurring occurring from Decemberfrom December 2013 to2013 March to March 2014 (Figure2014 (Figu 20),re that 20), may that havemay produced the have producedstorage the storage inside inside the nearbythe nearby cistern cistern or relevantor relevant seepage seepage in in the the rock rock mass mass [ 89]. Ref. [42] [89]. Ref. [42] identifiesidentifies another another possible possible cause cause for for the the seepage seepage in a in mixed a mixed influence influence of the of the dam and dam and rainfall.rainfall. Nevertheless, Nevertheless, the relationship the relationship between between seepage seepage vs. rainfall, vs. cisterns rainfall,, and cisterns, and the the recently realizedrecently reservoir realized with reservoir the construction with the construction of a dam, ofshould a dam, be should better beinvesti- better investigated. gated. Other cisternsOther cisternsare also arepresent also in present the same in the area same [89] areabut, according [89] but, according to [44], they to [do44 ], they do not not play a majorplay role a majorin water role seepage in water inseepage the monastery. in the monastery. The performed The performed analysis suggests analysis suggests the the need to mitigateneed to the mitigate impact the of impactrising dampness of rising dampness (especially (especially along the alongmonastery the monastery left left side side basement)basement) and to verify and tothe verify potential the potential weathering weathering of the rock of the also rock in terms also in of terms geome- of geomechanical chanical strengthstrength reduction. reduction.

Figure 20. Rainfall data in Petra between September 2010 and 2014 [90]. Figure 20. Rainfall data in Petra between September 2010 and 2014 [90].

5.3. IRT and UAV-DP for Protection/Conservation Strategies of Rupestrian Cultural Heritage Protecting rupestrian cultural heritage from weathering, erosion, and slope instability using advanced RS techniques is a fundamental step towards a sustainable conservation strategy, especially when traditional knowledge is being progressively abandoned and/or forgotten [16]. In the protection and conservation of the investigated sites, the

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applied integration of close-range remote sensing techniques provided a versatile ancillary low-cost tool for identifying the main critical sectors of the analyzed sites by means of a rapid morpho-structural and hydrographic characterization. In particular IRT was used to integrate field surveys for remote geomechanical surveying of aperture and seepage parameters and to confirm the ephemeral drainage network patterns obtained by performing the hydromodelling on the UAV-DP 3D models. These latter were also combined with the field collected structural data to obtain a 3D kinematic analysis for identifying the main instability phenomena and mapping their spatial probability of occurrence on the rock slopes. One of the limits of the graphic kinematic analysis is that of considering an infinite persistence, zero friction angle, not taking into account the variability of the parameters that determine the friction angle; in fact, the roughness, the uniaxial compression strength and the eventual filling of the discontinuities differ according to the fractures. These are identified by combining fracture dip and dip directions with the local slope orientations. The adopted methodology can rapidly and successfully lead to identifying the presence of conservation criticalities such as moisture zones connected to surface water runoff/concentration areas, ephemeral drainage networks, ground water seepage, open fractures, and potentially unstable ledge-niche systems. In the case studies of Georgia large areas of the slope surface were inaccessible; therefore, the remote sensing approach provided a time-effective mapping while granting the safety of operators. The adopted remote sensing techniques are not time consuming (all the surveys were carried out in one working day, as well as the data processing), have relative low cost and grant a nondestructive approach for the safety of both the monument and the operator. Nevertheless, field surveys are still necessary to collect geomechanical and geotechnical parameters and collect samples for laboratory analysis.

6. Conclusions The present paper describes the support activities for rupestrian cultural heritage protection and conservation activities carried out in Georgia and in Petra (Jordan) between 2014 and 2018. Different rupestrian monasteries carved in weak rocks were surveyed by means of traditional surveys and close-range nondestructive remote sensing techniques, such as IRT and UAV-DP, in order to collect geomechanical parameters, perform preliminary landslide hazard assessment and identify the main conservation problems, with the final aim of defining preliminary sustainable mitigation measures. In particular, IRT and UAV-DP were integrated to provide an accurate slope-scale mapping of potential criticalities with respect to weathering and degradation and slope instability, such as water runoff concentration areas, moisture/seepage sectors, and open fractures. 3D kinematic analysis was also performed to detect the areas characterized by higher susceptibility with respect to the different instability phenomena. Field surveys and geotechnical analysis were carried out to validate the outcomes of the RS activity and improve the geotechnical characterization of the sites. Sustainable preservation of rupestrian cultural heritage sites carved in weak rocks such as tuffs and soft sandstones depends on several natural factors, among which the most important are avoiding surface runoff, -ground water infiltration, sealing open fractures, and reinforcing ledge-niche systems. Reducing surface water interaction is also fundamental so as to preserve unique mural paintings and wall inscriptions. Nevertheless, only a multidisciplinary approach can define a new paradigm for protection and conservation strategies of rupestrian cultural heritage sites: understanding the ongoing weathering/erosion phenomena as well as the instability processes is the main target in order to define a general master plan of sustainable mitigation measures and choose the most suitable monitoring system. The performed method can be used in similar geological contexts where the areas at risk are vulnerable and hardly accessible even for professional rock climbers.

Author Contributions: W.F. conceived the article structure, collected, processed, and analyzed the IRT data in Georgia; M.E. supervised the Georgian team and provided insights on the mitigation strategies; D.S., and G.G. performed the ﬁeld surveys, provided structural-geotechnical data and Remote Sens. 2021, 13, 1040 27 of 30

performed the kinematic analysis; A.N. and G.K. performed the UAV-DP surveys and processed the topographic data; L.A. provided the site’s historical setting and conservation criticalities; C.M., N.A. and N.C. coordinated the project work, the paper’s writing with special regards to the reviewing process. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by “State Program of study, Monitoring and Conservation of Rock Cut Monuments of Georgia, National Agency for Cultural Heritage Preservation of Georgia”; “Fundamental Research grant #FR17_290 of Shota Rustaveli National Science Foundation of Georgia, Complex Monitoring and Diagnostic System of Cultural Monuments”; “PhD research grant of Shota Rustaveli National Science Foundation of Georgia, Project Number #PhDF2016_223, Characterization of the Construction Material of Georgia’s Historical Monuments, localization of the Potential Quarries and The Modern Methods of Conservation”. Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and and amended by the 64th WMA General Assembly, Fortaleza, Brazil, October 2013. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data supporting the reported results are openly available in (1) 16. Margottini, C.; Gigli, G.; Ruther, H.; Spizzichino, D. Advances in Geotechnical Investigations and Monitoring in Rupestrian Settlements Inscribed in the UNESCO’s World Heritage List. Procedia Earth and Planetary Science, 2016, 16, 35–51, https://doi.org/10.1016/j.proeps.2016.10.005 (accessed on 1 March 2021); (2) Frodella, W.; Elashvili, M.; Spizzichino, D.; Gigli, G.; Adikashvili, L.; Vacheishvili, N.; Kirkitadze, G.; Nadaraia, A.; Mar-gottini, C.; Casagli, N. Combining InfraRed Thermography and UAV Digital Photogrammetry for the Protection and Con-servation of Rupestrian Cultural Heritage Sites in Georgia: A Methodological Application, Remote Sensing, 2020, 12(5), 892, https: //doi.org/10.3390/rs12050892 (accessed on 1 March 2021). The rainfall data on Petra can be found at: https://clim-engine-development.appspot.com (accessed on 1 March 2021). Acknowledgments: This work was carried out in the framework of a bilateral agreement between the Earth Science Department of the University of Florence (Italy) and the Faculty of Natural Sciences and Engineering of the Ilia State University (Tblisi, Georgia). The authors are very grateful to the National Agency for Cultural Heritage Preservation of Georgia (NACHPG) and to all the staff members, for their support in the present research. This preliminary assessment was made possible thanks to the support of the university student Simone Corradini, from the Faculty of Architecture of Rome University “La Sapienza”, involved during the second June 2014 field survey, who oversaw the acquisition and processing of IRT images. Conflicts of Interest: The authors declare no conflict of interest.

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