Instability mechanisms affecting cultural heritage sites in the Maltese Archipelago

21 Pages·2012·14.25 MB·English

Nat. HazardsEarthSyst. Sci.,12,1883–1903,2012
Natural Hazards
www.nat-hazards-earth-syst-sci.net/12/1883/2012/
and Earth
doi:10.5194/nhess-12-1883-2012
©Author(s)2012. CCAttribution3.0License. System Sciences
Instability mechanisms affecting cultural heritage sites in the
Maltese Archipelago
G.Gigli1,W.Frodella1,F.Mugnai1,D.Tapete1,F.Cigna1,*,R.Fanti1,E.Intrieri1,andL.Lombardi1
1DepartmentofEarthSciences,UniversityofFirenze,Firenze,Italy
*Presentaddress: BritishGeologicalSurvey,NickerHill,Keyworth,Nottingham,UK
Correspondenceto: G.Gigli(giovanni.gigli@unifi.it)
Received: 11October2011–Revised: 9January2012–Accepted: 22January2012–Published: 14June2012
Abstract. The superimposition of geological formations plateausabovethesurroundingcountryside,borderedbyun-
withmarkedcontrastingeotechnicalpropertiespresentsone stable steep cliffs. Such a type of configuration evidently
ofthemostcriticalenvironmentsforslopeinstabilitydueto generates conditions of high hazard associated with a rele-
the different response of the materials to the applied distur- vant vulnerability and related risk for built heritage of sig-
bances. Moreover, the above-mentioned geological setting nificant cultural value; threats to people’s safety can derive
isoftenassociatedwithhighriskconditions,sincemanyiso- aswell(Cestellietal.,1984;Cotecchia,1997;Canutietal.,
latedrockslabslocatedatahigheraltitudethanthesurround- 1999;Luzietal.,2004;Egglezosetal.,2008).
ingcountrysidehavebeensitesofhistoricaltownsorbuild- Weshouldcarefullydistinguishindetailtheoverallmech-
ings. anismwhichgovernsthebehaviorofarockslaboverasoft
The purpose of the present paper is to investigate the substratumfromthefailuresthattakeplaceatitsboundaries.
mechanisms determining instability in rock slabs overlying Only the latter can be classified according to the traditional
a soft substratum, with reference to two cultural heritage landslideclassificationschemes. Theformercanbedefined
sitesinMalta. Accurateinvestigationshavebeencarriedout as a complex mass movement, according to Varnes (1978)
to evaluate the geological, geotechnical and geomechanical and Hutchinson (1988), since it involves a combination of
propertiestogetherwiththemaingeomorphologicalfeatures severaldifferentbasicfailuremechanisms
of the soft clayey substratum and the overlying limestone Therefore, the problem requires a multi-disciplinary ap-
rockmass. proachinvolvingtheapplicationofprinciplesfromdifferent
The main instability processes have thus been identified fields, such as rock and soil mechanics, geology, geomor-
and investigated through kinematic analyses and numerical phology,engineeringgeology,andconservationsciences.
modeling,combinedwitha1992–2001PersistentScatterers According to Casagli (1992), the instability mechanisms
monitoring of ground displacements. The study constitutes affecting this geological environment can be classified into
thebasisforthesubsequentrestorationworks. threedifferentbutstrictlyinterconnectedtypes:
– overallmechanismofinstability;
1 Introduction – soilslopeinstability;
Theoverlappingofhardrockmasses,potentiallysubjectedto – rockslopeinstability.
elasticdeformationsandbrittlecollapse,onaplasticsubstra-
tum is a common geological feature, especially in cultural As regards the overall mechanism, the contrast in stiffness
heritage sites (Cancelli and Pellegrini, 1987; Canuti et al., oftheoutcroppingmaterialscausestheopeningofjointsand
1990,2004; Bertoccietal.,1991; Casaglietal.,1993; Katz newtensilefailures(Casagli,1992);bothisolatelargeblocks
andCrouvi,2007). that can be involved in marginal mass movements (Fig. 1).
Thissuperimpositionfrequentlyleadstomechanicalinsta- Thefracturingandopeningofjointsproducestressredistri-
bilityduetothediverseresponseofthematerialstotheap- bution at the boundary between the rock plate and the sub-
pliedperturbations,suchasman-madeexcavations,weather- stratum. Thus,verticalrelativedisplacementsofrockblocks
ingorerosion.Thelatteractsselectively,withmajorimpacts may also take place due to local elastic-viscous settlements
on the more erodible substratum, thereby forming buttes or andyielding.
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
1884 G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago
presenttheanalysisofthestabilityconditionsoftwoimpor-
tantculturalheritagesitesintheMalteseArchipelago:Mdina
in Malta and Citadel in Gozo (Fig. 2). Due to their natural
and historical importance, both sites are included amongst
themaintouristicdestinationsoftheArchipelago.
2 Descriptionofinvestigatedsites
2.1 Maltesegeology
Fig.1. Simplifiedsketchoftheinstabilitymechanismsaffectinga
rockslaboverlyingasoftsubstratum.
MaltaandGozoIslandsbelongtotheMalteseArchipelago,
central Mediterranean Sea (Fig. 2a), standing on a shallow
submarineelevation, theMaltaplateau, partofthePelagian
Associatedwiththesemechanisms,aslowdownwardand
Platform (Bowen Jones et al., 1961; Pedley et al., 1978;
outwardextrusionoftheclayeymaterialunderlyingtherock
Schembri,1993,1997;Magri,2006)(Fig.2b).
slab may occur, providing potential material involved in
The landscape is geomorphologically characterized by
landslidesandearthflows.Thisclassofmovement,knownas
gently rolling hills with terraced fields (the highest point of
squeezing-out and bulging, is described in detail by Zaruba
the archipelago lies at about 250m a.s.l.), while steep sea
andMencl(1982).
cliffsdelimitthesouth-westerncoastline(Houseetal.,1961;
Iftherockplateiswideandthickandthemechanicalprop-
Vossmerba¨umer,1972;Alexander,1988).
erties of the involved materials are very different, various
The geological setting is constituted by marine sedimen-
largescaleandslowmovingcomplexphenomenamayoccur,
taryrocks,mainlylimestonesOligo-Miocenicinage,subse-
knownintheliteratureaslateralspread(Varnes,1978),cam-
quentlyliftedabovesealevelduringthePlioceneperiodby
bering and valley bulging (Hollingworth et al., 1944; Hor-
asubverticalfaultsystemrelatedtotheopeningofthePan-
swill and Horton, 1976; Parks, 1991), and block type slope
telleria Rift (Reuter, 1984; Finetti, 1985; Alexander, 1988)
movements(Pasek,1974).
(Fig.2).
The overall mechanism of instability also contributes to
Amongthefiveunitslyingalmosthorizontallyacrossthe
the enhancement of the phenomena, which take place both
Maltese Islands (Hyde, 1955; Pedley, 1974; Pedley et al.,
ontheductilesubstratumandontherockplate.
1976, 1978; Bosenceetal., 1981; DebonoandXerri, 1993;
The most common failure mechanisms affecting the un-
Pedley, 1993; Pedley et al., 2002) (Fig. 2c), the following
derlying soft substratum are rotational or compound slides
ones are of particular relevance for the sites of Mdina and
(Hutchinson,1988)andearthflows(Varnes,1978;Hungret
Gozo:
al., 2001). These can also be influenced by the accumula-
tionofmaterialwhichhasfallenfromtherockcliff,causing – Blue Clay Formation (BC): a very soft pelagic blue or
surcharge(ZarubaandMencl,1982;Casagli,1992). Onthe greenishgreymarlandlimeyclay,rangingfrommiddle
otherhand,landslidesatthebaseoftherockslabundermine to upper Miocene in age, forming gentle hillslopes
underlyingsupportandtriggerfurtherblockmovements. mostlycoveredbysoilorscatteredrubble. Itsthickness
Thesubverticalandjointedrockwallscanbeaffectedby variesapproximatelyfromabout20mto75m.
agreatvarietyofmechanisms,suchasrockfalls,rockslides,
toppling(eitherforwardorbacktilting),anddifferentialset-
– Greensand Formation (GS): massive, friable brown
tlements. Rock slope failure at the border of the slab in-
to dark green glauconite and gypsum grain-rich bio-
creases the stress relief, and causes the spread of the insta-
clastic limestone, Tortonian in age. This formation is
bilityphenomena.
rarely more than 1m thick, and can occasionally ex-
Thegreatvarietyofinterconnectedinstabilitymechanisms
pandtoover10matthebaseofhilltopsincentralGozo.
affecting a rock plate – soft substratum system is therefore
abletoproducegreatdamagetostructuresorbuildingsbuilt
either on the rock plate or at the base of the rock walls
– Upper Coralline Limestone Formation (UCL): over
(Fig.1).
160m thick, it consists of pale grey to orange fossil-
The geological setting of the Maltese Archipelago often
iferous coarse grained limestone, ranging from upper
showstheoverlappingofrelativelystiffandbrittlelimestone
Tortonian to early Messinian in age, and it is made up
plates on thick clayey units. This leads to typical geomor-
of four members, the basal one of which is referred to
phological processes, such as slides or flow phenomena in
asGhajnMelelMember(Mgm).
theunderlyingductileunitsandbrittlerupturesinvolvingthe
overlyingrockmasses. Thelatterareoftenaffectedbyhuge
sub-verticaljointsthatisolatelargeblocks. Inthispaperwe
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G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago 1885
1
2 Figure 2
Fig. 2. Geog3ra phic setting (A), simplified geological map (B) and stratigraphic column (C) of the Maltese Archipelago (modified after
Pedleyetal.,420 02) .
UCL hillcaps and plateaus overtopping BC gentle slopes erosionofthelandthatsurroundsthem(Scerri,2003). Allof
are the dominant feature in the landscape of north-western them are characterized by sub-horizontal or gently inclined
Maltaandnorth-easternGozo(Fig.3a,b).Therefore,thege- UCLcapprotectingtheunderlyingsoftBCslopes.Although
ologicalandgeomorphologicalfeaturesofthelatterareasare the geological materials and the stratigraphic sequences are
deeply influenced by the geological characteristics and mu- similar, the investigated sites show quite diverse instability
tualinteractionoftheabove-mentionedformations(Magriet problems, due mainly to the different thickness of the out-
al., 2008; Devoto et al., 2011), while GS, due to its limited croppingformationsandtospecificgeologicalfeatures.
thickness,canonlyaffecttheformofthelandsurfacelocally. UCL,beingatthetopoftheMalteseIslandsstratigraphic
sequence, is particularly subjected to the action of eroding
2.2 Geologicalsettingoftheinvestigatedsites factors: percolatingrainwaterdissolvesthelimestone,form-
28
ing fissures and caves, stopping when it reaches the imper-
MdinaandCitade l(Fig.3c,d)arebuiltontwohillsbelong- meable barrier of the underlying BC. When the contact of
ing to the many that characterize the Maltese and Gozitan thesetwounitsmeetsthelandsurface,springlinesmayde-
landscapes. These reliefs owe their existence to the deeper velop, contributing to undermining the limestone hillcaps’
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1886 G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago
A B
C D
Fig.3. PanoramicviewsofUCLslabsoverlyingBCexposedinGozo(A)andMalta(B),andoftheinvestigatedsitesofMdina(C)and
Citadel(D).
stability. Furthermorerainfallwashesawaytheeasilyerodi- rockplate,thestructuresareoftenbuiltoverthecontactbe-
bleBCcausingtheovertoppinglimestonecapstobreakaway tween the UCL/GS and the underlying BC, thus increasing
andcollapseintoblocksofvarioussizes(Fig.3d)(Pedleyet theinstabilityproblemsduetodifferentialsettlementsofthe
al.,2002). structures.
Forthisreason,mostofthedamagetobuildingsandbas-
2.3 Mdina tion walls is located in the perimetral sectors of the city
(Baratin et al., 2001; Bonnici et al., 2008) (Fig. 4), and is
Mdina, the former capital of Malta, is a fortified town lo-
concentratedinthreespecificareas,namelyVilhenaPalace,
cated in the central – western sector of the Island on a rise
CathedralandCurtainMagazines.
constitutedbytheUCL(Fig.3c). Thankstoitsstrategicpo-
sition,overthepastcenturiesthecityhasbeeninfluencedby 2.4 Citadel
many cultures. In 1693 it was extensively damaged by the
Val di Noto (Sicily) earthquake (Galea, 2007), and most of Citadelisasmallfortifiedtown(Fig.3d)locatedinthecen-
the buildings and bastions were constructed afterwards (De tralpartofGozo,onahilltoplocatedabovethetownofVic-
Lucca, 1993). The city is built on a thin UCL plate (max- toria(alsoknownasRabat). Givenitscentralandpanoramic
imum thickness: 6m) with a rhomboidal shape and a total location within the island of Gozo, this vantage point has
surfaceareaofabout55000m2. hostedurbansettlementssincetheBronzeAge.
From a geomorphological point of view, the main land- ThehillconsistsofmoreorlesscircularUCLrockplate,
formsassociatedwiththegeologicalsystemdescribedabove whichis20mto25mthickandabout150mwide,overlying
are always hidden by anthropic structures, and can only be firstalayerofGSandthengentleBCslopes.
observed on the nearby hills, where the landscape is much The rock plate slopes are sub-vertical, sometimes
lessmodifiedbyhumanactivities. Duetothethinnessofthe overhanging, while the underlying slopes are mostly
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G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago 1887
Fig.4.StructuraldamagesaffectingMdinabastions,wallsandbuidings.(A):St.Paul’sBastion;(B)and(C):VilhenaPalace;(D):Despuig
Bastion.
Fig.5.InstabilitymechanismsaffectingtheUCLrockplateunderlyingCitadel(A:rockmassandunderpinningmasonry;B:rockfalldebris).
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1888 G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago
AREA C
BC2!CCC!C021!!8!CC!!11C!1!C7!C54 B!6 BB!42!!BB!1!3BB!42 AREA B
C!4C!3C!9C!!9 B!5! B!3
C6 B6 B’
Landslide
Upper Coralline Limestone A1!A!2 A!3
!
Blue Clay ! A4
Globigerina Limestone A12! A3A!5
SHcoarirzpontal strata ± A7!!A6
SL 1 A!2
GGeeoomlogeicchaal ncircoasls s-csaenctliinoens (SL) A11!A1!0!A1A9!!A8 AREA A
! Vertical boreholes SL 2
! 01020 40 60 80 (m)
Inclined boreholes
240 240
230 Cathedral 230
220 220
210 Triq San PietruTriq IL-Villegianon 210
200 Curtain Magazine 200
190 C7 C5 Despuig Bastion 190
180 C1 B5 B4 180
170 B3 170
160 160
150 150
(m) (m)
B 0510 20 30 40 (m) B’
E/NE W/SW
Fig. 6. Geological map and cross-section of Mdina. Area A: Vilhena Palace; Area B: Cathedral and Despuig Bastion; Area C: Curtain
Magazines.
characterizedbyverygentletogentlesteepness(from10% From a structural point of view, although extensive ev-
to20%). idence around the fortification walls shows rebuilding of
The rock mass shows a high degree of fracturing, and is structures due to cliff face retreat, the only signs of defor-
constituted by layers having different resistance to erosion, mation are related to differential displacements of the few
leadingtotheformationofledgesandniches(Fig.5a);these structuresbuiltonthecontactbetweendifferentmaterials.
factors cause the detachment and falling of rock blocks of
varioussizes.Onthenorthernsectorofthecliff,alargerock-
falleventtookplaceinDecember2001,andtheresultingde- 3 Datacollection
brisisstillevident(Fig.5b).
Evidenceofpreviousrockfallsisalsovisibleslightlyeast 3.1 Geologicalsurvey
totheeventwhichoccurredin2001. Inordertoinhibitthese
Accurate geological, geomorphological and geomechanical
rockfallsseveralunderpinningmasonrystructureswerebuilt
surveysoftheselectedsiteswerecarriedoutwiththeaimof
in the past, with the purpose of protecting the niches which
investigatingthecausesofthemaininstabilityprocessesand
are the most eroded portions of the outcropping cap rock,
determiningthematerialpropertiestobesubsequentlyused
andtoimproveitsglobalstability. Thesestructures, widely
duringtheanalysisphase.
presentonthenorthernandeasternsideofthecliffface,were
builtindifferentperiods,andmanyofthemarenowinapoor Inparticular, highresolutiongeologicalmapsandrelated
stateofrepair(Fig.5a). cross-sections were drawn utilizing the detailed geological
survey,carriedoutbymeansofadifferentialGPSsurvey,in
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G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago 1889
±

Upper Coralline Limestone j20 Strata dip direction Geomechanical scanlines (SL)
!
Greensand Interpreted fault Vertical boreholes

!
Blue Clay Geological cross-section Inclined boreholes
j
A
d ej
! !
17
fj
j !SL 5 30 !
b
20 SL 4c! ! 1a5!SL g!
jA B ! 6
! 17
L 3 C 2j0
j S
20
j j
TTTeTeexexxtxtttText 20 D
!
12
20
jL 2
S TeTTxeetxxtt
S
j18L 1 E!18j
j
j20
j28 16j 16
15j
A’
0 10 20 40 60 80 (m)
150 Triq San Guzepp Cathedral 150
140 B Triq Bieb l’Imbina 140
130 130
Ditch
120 120
110 E 110
100 100
(m) (m)
A 0 510 20 30 40 (m) A’
NW SE
Fig.7.Geologicalmapandcross-sectionofCitadel.
order to reconstruct, with centimeter accuracy, the 3-D ge- Furthermore,ageotechnicalcharacterizationhasalsobeen
ological model and its geometric correlation with the man- carriedoutbymeansoflaboratorytestsonundisturbedrock
madestructures. and soil samples retrieved from the deep and inclined bore-
The traditional geological surveys have been integrated holes(Tapeteetal.,2012).
with a number of ground (deep vertical boreholes; Figs. 6
and 7) and structural investigations (horizontal and inclined As regards Mdina the stratigraphic contact between the
boreholes), carriedoutbyBallutBlocksServicesLtd.(Md- UCLandtheunderlyingGSandBCisnotregular,andsome
ina)andSolidbaseLtd.(Citadel). groundwateremergesontheclayeyslopesintheformofmi-
nor springs along the scarps. The location of these minor
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1890 G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago
Fig.8.Discontinuitypoles,contourlinesandmodalplanesofthemaindiscontinuitysetscollectedatMdina(A)andCitadel(B).Themean
buriedrock-slopeorientationoftheMdinaareasinvestigatedisalsoreported(redmajorplanes).
springsisusuallymarkedbythegrowthofmanyhydrophilic ite, shows persistent subvertical fissures and joints, which
plants(suchasreeds). Groundwaterseepingoutatthebase have triggered many rockfalls occurring along the cliff face
of the plateau scarps in the form of gravity springs desta- perimeter. For its different internal sedimentary fabric and
bilizes the clay which then undermines the plateau. This reactiontoerosion,ithasbeensubdividedintothreedistinct
processaffectsthescarpreducingshearstrengthandmakes informalsub-members,whichfromtoptobottomare:
itpronetoshearfailureundertheactionofgravity.
Figure 6 reports the high resolution geological map and – UpperBank: havingamassivestructure,isdirectlylo-
a representative cross-section for the Mdina site. Vilhena catedatthebaseofthebastionwalls. Itischaracterized
Palace (Area A), the Cathedral (Area B) and the Curtain byarelativelystrongresistancetoerosionandtoweath-
Magazines(AreaC),whichdisplaythemostevidentinstabil- eringprocesses.
ityproblems(Fig.4),areshownwithwhitecirclesinFig.6.
– ThinlyLayeredBank: itunderliestheUpperBank,dif-
The limestone bedding planes are parallel and almost hori-
fering from the latter for a thinly layered structure and
zontal. Here the GS formation is characterized by a mini-
formuchlessresistancetoerosion. Infactitgivesform
mal thickness (few centimeters), which makes it difficult to
toerosionalniches,oftencoveredbyunderpinningma-
show on the map, and irrelevant with respect to the global
sonrystructures.
slope stability. The geological cross-section also highlights
thethinnessoftheUCLrockplate,beingalmostcompletely
– LowerBank:itconsistsofthelowerportionoftheMgm
coveredwithman-madestructures.Thiscausesseriousinsta-
Member,overlyingtheclifffacebaselevelofGS.Sim-
bilityproblemsduetodifferentialsettlementsofthosestruc-
ilartotheUpperBankitshowsamassivestructureand
tures built over the rock-clay contact. Figure 6 also reports
arelativelystrongresistancetoerosion. Itissometimes
the upper boundary of an area involved in a slope instabil-
coveredbyvegetationandrubblewhichlimitsitsexpo-
ity process, as observed from field surveys and from aerial
sure.
images. Thislandslideissupposedtoplayasignificantrole
inthestructuralinstability, underminingthelimestoneplate Thestrataorientationscollectedallaroundthecliffshowthat
rightintheCurtainMagazine(AreaC)foundationarea. the bedding planes always dip into the slope from the cliff
As regards Citadel (Fig. 7), the rock outcrops of the cliff perimetertowardsthecentreoftherockplate, withinclina-
facearealmostentirelyconstitutedbyUCL,hererepresented tion angles ranging from 12◦ to 30◦ (Fig. 7). According to
bytheGhajnMelelMember(Mgm),withtheexceptionofa Pedley(1974)thisstructuralsettingisduetoasolutioncol-
GSlayeratthebaseofthehillcap. Fromtheanalysisofthe lapse structure, late Miocene in age, like many others scat-
retrieved borehole cores, this GS layer is about 10m thick teredallovertheMalteseislands. WherethethickMiocene
(Fig.7);thisvalueiscomparabletothethicknessofthesame infill is softer than the surrounding materials, an erosional
FormationoutcroppingatGelmusHill,lessthan1kmWNW hollowiscausedbyselectiveerosion;insteadwhentheinfill
fromCitadel. isharderthanthesurroundingmaterials,becauseofahigher
The Ghajn Melel Member (Mgm), mainly consisting of degreeoflitification,acircularerosionalplateau(mesa)oc-
a yellow to pale brown coarse grained organic calcaren- curs(Pedleyetal.,2002).
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G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago 1891
Table 1. Geomechanical properties of the rock mass discontinuities. Set id: joint set identification code; α: dip direction; β: dip; X:
truespacing;L:persistence;e: aperture;JRC:JointRoughnessCoefficient;JCS:JointCompressiveStrength;r: Schmidthammerrebound
numberonwetandweatheredfracturesurfaces;R:Schmidthammerreboundnumberondryunweatheredsurfaces;φb:basicfrictionangle.
Set id α(◦) β(◦) X(m) L(m) e(mm) JRC JCS r/R φb(◦)
JN1 272 75 3.5 1.3 9.8 11.1 18.3 0.4 32.3
JN2 104 76 7.0 1.6 15.3 12.8 17.4 0.5 32.3
UCLMdina
JN3 314 81 5.4 1.6 15.6 14.6 21.7 0.7 32.3
JN4 075 76 7.8 1.5 21.4 13 20.1 0.5 32.3
JN1 030 89 1.4 2.5 13.3 12.1 15.6 0.4 35.9
JN2 274 84 1.3 2.9 12.9 11.7 17.5 0.8 35.9
UCLCitadel
JN3 329 73 5.1 2.6 17 13.7 16.9 0.6 35.9
BG 0–360 12–17 1.4 Inf. 11 12 15.7 0.7 35.9
3.2 Geomechanicalsurvey stratification, four main sets can be identified on the rock
massunderlyingMdina.
Therockmasscharacterizationandthequantitativedescrip- Fourdiscontinuitysetshavealsobeenidentifiedwithinthe
tionofdiscontinuitieswereobtainedbymeansoftraditional rockslabunderlyingCitadel(Fig.8b).
geomechanical surveys (scanline method), according to the Both sites show medium pole dispersion, with a preva-
methodssuggestedbytheInternationalSocietyforRockMe- lence of high dipping discontinuity planes (excluding the
chanics(ISRM,1978,1985). beddingplanes).
DuetothelimitedrockmassoutcroppingatMdina, only Thesestructuraldatawereemployedinthekinematicanal-
2scanlinesurveys(25mand30minlength)wereperformed ysesandnumericalmodelling,describedinSect.4.
on the rockwall next to the ditch within the Vilhena Palace Theshearstrengthofdiscontinuitieswascalculatedusing
area (Area A) (Fig. 6). In order to extend the rock mass Barton’s failure criterion (Barton and Choubey, 1977). Fi-
mechanical properties to the other investigated areas, some nally,foracompletecharacterizationofthematerialproper-
randomdiscontinuityorientationmeasurementsweretaken, ties to be used in the numerical model, the Mohr Coulomb
confirmingthespatialdistributionoftheidentifieddisconti- equivalentstrengthparametersoftherockmasswerecalcu-
nuitysets. lated through the Hoek and Brown failure criterion (Hoek
andBrown,1980;Hoeketal.,2002). Thesepropertieswere
As regards Citadel, a total of 6 scanline surveys (varying
usedasinputparametersforthekinematicanalysesandnu-
from 15m to 33m in length) were performed according to
mericalmodelling(Table1).
themethodssuggestedbyISRM(1978, 1985). Thesescan-
lineswereperformedallaroundthecliffface,tocoverboth
3.3 Laserscanningsurvey
themassiveUpperandLowerBankandtheThinlyLayered
BankoftheGhajnMelelMemberwithintheUCLFormation
Nowadays, advanced techniques are available for obtaining
(Fig.7).
high resolution 3-D representations of land surface, such as
Foreachscanlineasheetwasfilledbymeasuringtheori-
digital photogrammetry (Chandler, 1999; Lane et al., 2000)
entation,persistence,aperture,roughness,rockwallstrength,
and laser scanning (both terrestrial and aerial) (Kraus and
filling,seepageofthediscontinuitiescrossingthesurveyline.
Pfeiffer,1998;FrohlichandMettenleiter,2004).
InordertodeterminetheUCLintactrocktensileandcom-
Themainproductofalongrangelaserscanningsurveyisa
pressive strength for both investigated sites, a number of
highresolutionpointcloud,obtainedbymeasuringwithhigh
pointloadtestswereperformed,followingthemethodssug-
accuracy(millimeterorcentimeter)thedistanceofameshof
gested by ISRM (1985). The resulting σ values for UCL
c points on the object, following a regular pattern with polar
intactrockare: 22.3MPa(Mdina)and6.7MPa(Citadel).
coordinates. The high acquisition rate (up to hundreds of
With the aim of identifying the major discontinuity sets, thousands points per second) makes the detailed 3-D shape
orientation data were plotted on a stereographic projection oftheobjectimmediatelyavailable.
(lowerhemisphere). The advantage of employing remote and high resolution
Figure 8a reports the stereographic projection of the col- surveyingtechniquesisthatitallowsustoperformdetailed
lected data: discontinuity poles concentration, main set analyses and to rapidly obtain information on inaccessible
modal planes and mean buried rock-slope orientation in the rock exposures. This is very important in the field of en-
investigated areas are shown. Excluding the sub-horizontal gineering geology and emergency management, when it is
www.nat-hazards-earth-syst-sci.net/12/1883/2012/ Nat. HazardsEarthSyst. Sci.,12,1883–1903,2012
1892 G.Giglietal.: InstabilitymechanismsaffectingculturalheritagesitesintheMalteseArchipelago
A B
Fig.9. LaserscannerinactionatVilhenaPalace,Mdina(A);Truecoloured3-DpointcloudofVilhenaPalace,Mdina(B).
N
Mtarfa
Mdina
A B
Fig. 10. (A): Example of ERS2 image acquired along descending orbit on the Maltese Archipelago (displayed in terms of amplitude).
Footprintofthewholesceneisshownwithintheinsetasadashedblackpolygon. (B):Subsetof(A)displayedinSARcoordinatesand
includingtheurbanareaofMtarfaandMdina.
often advisable to minimize survey time in dangerous envi- tures and the slopes, making it possible to reconstruct the
ronmentsandatthesametimegatheralltherequiredinfor- plano-altimetric relationship between the local geology and
mationasfastaspossible. theman-madestructures(Figs.6and7). Thankstothehigh
The Laser Scanning technique is being more frequently resolution of the laser scanning survey we can also extract
used for instability analyses in cultural heritage sites even the smallest features, such as the structural crack pat-
(Boehler et al., 2001; Arayici, 2007; Lambers et al., 2007; tern, the crack opening direction (Gigli et al., 2009), and
Yastikli, 2007; Yilmaz et al., 2007; Al-kheder et al., 2009; theorientationofcriticaldiscontinuitieswithintherockmass
Fanti et al., 2011, 2012), as it allows detailed and high ac- (GigliandCasagli,2011).
curacy3-Drepresentationofboththeunderlyinggroundand A laser scanning survey of the investigated Mdina areas
theoverlyingstructuresinashorttime. was carried out, by employing a long range 3-D laser scan-
A detailed 3-D model of the investigated sites is useful ner (RIEGL LMSZ410-i) (Fig. 9a). This device can deter-
for building a complete digital model including the struc- minethepositionofupto12000pointss−1withamaximum
Nat. HazardsEarthSyst. Sci.,12,1883–1903,2012 www.nat-hazards-earth-syst-sci.net/12/1883/2012/