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Landslides (2014) 11:131140DOI 10.1007/s10346-013-0424-2Received: 5 April 2013Accepted: 26 June 2013Published online: 25 July 2013 Springer-Verlag Berlin Heidelberg 2013
G. GigliI W. FrodellaI F. GarfagnoliI S. Morelli I F. Mugnai I F. MennaI N. Casagli
3-D geomechanical rock mass characterizationfor the evaluation of rockslide susceptibility scenarios
Abstract An integrated methodology based on traditional fieldand remote surveys such as terrestrial laser scanning and terres-
trial infrared thermography is proposed, with the aim of defining
susceptibility scenarios connected to rock slopes affected by insta-
bility processes. The proposed methodology was applied to a rock
slope threatening a coastal panoramic roadway located in western
Elba Island (Livorno district, central Italy). The final aim of the
methodology was to obtain an accurate three-dimensional rock
mass characterization in order to detect the potentially more
hazardous rock mass portions, calculate their volume, and collect
all the required geomechanical and geometrical parameters to
perform a detailed stability analysis. The proposed approach
proved to be an effective tool in the field of engineering geology
and emergency management, when it is often urgently necessaryto minimize survey time when operating in dangerous environ-
ments and gather all the required information as fast as possible.
Keywords Rock mass . Laser scanning . Discontinuity. DiAna .
Thermography. Stability analysis
Introduction
Terrestrial laser scanning (TLS) technique is increasingly used for the
analysis of slopes characterized by instability processes, as it safely
allows in a short time a high detailed and accurate3-D representation
of the investigated rock mass plano-altimetric morphological and
geostructural setting (Abellan et al.2006,2010; Fanti et al.2011;2012;
Gigli et al.2009;2012a,2012b; Jaboyedoff et al.2009; Lombardi et al.
2006; Oppikofer et al.2008; Rahman et al.2006; Slob et al.2002; Slob
and Hack2004;2007; Tapete et al.2012; Turner et al.2006). In order
to perform a spatial analysis for the quantitative description of
discontinuities within rock mass faces with rugged shape, many
authors have been working during the last years on the semiauto-
matic extraction of 3-D rock mass properties from remotely acquired
high-resolution data, mainly digital photogrammetry and lidar (Fer-
rero et al.2009; Gigli and Casagli2011; Jaboyedoff et al.2007; Lato et
al.2009; Sturzenegger and Stead2009; Slob et al.2005).
In engineering geology, terrestrial infrared thermography (TIR)
has been successfully used in some experimental studies for the
detection of features that could lead to hazardous conditions on
rock slopes like subsurface holes (Wu et al.2005), water seepage
zones (Adorno et al. 2009), fractures and unstable protruding
systems (Teza et al. 2012), and open fractures in deep-seated
rockslides (Baroet al.2012). This suggests that TIR, by virtue of
its relatively low cost, fast measurement, and data processing times,
can be profitably used as an ancillary technique (coupled with other
remote sensing techniques, e.g., laser scanning), providing useful
information for the corresponding rock mass geoengineering
characterization (Teza et al.2012).
The proposed approach has been applied to a rock slope sector
affected by instability phenomena overlooking a coastal panoramic
roadway (provincial road 25) located in western Elba Island (Livorno
district, central Italy). This area, due to its geostructural setting and
degree of fracturing, in the past years underwent the detachment of rockblocks and debris, which in some occasion severely damaged the catch-
ment nets and barriers, invading the underlying roadway.
In order to investigate these instability occurrences and collect
the required ISRM (1978) geomechanical parameters, traditional
geological and geomechanical field surveys were integrated with a
TLS survey and its deriving data interpretation mainly based on a
Matlab tool called DiAna (Gigli and Casagli 2011). In addition, TIR
surveys were carried out for the validation of the unstable block
volume calculation and for a rapid assessment of the hydraulic
conditions along the more critical discontinuities of the investi-
gated rock mass, in order to obtain a qualitatively estimate of
ISRM (1978) seepage parameter, contributing together with the
TLS semiautomatic analysis to a more detailed remote 3-D geo-metrical and geomechanical characterization.
The final aim was to establish a methodology for the evaluation of
the rockslide susceptibility scenarios in emergency conditions,
through the following work plan (Fig. 1): (1) traditional geological
and geomechanical field surveys, (2) TLS and TIR surveys, (3) semi-
automatic geomechanical data interpretation, (4) creation of detailed
3-D surface, (5) main unstable rock mass portions detection, shape
extraction, and volume calculation, (6) stability analysis.
Geographical and geological setting
The investigated area is located on the western hillside of Mount
Capanne, overlooking the westernmost portion of the Elba Island
coastline (Livorno district), just north of Punta del Timone and the
village of Chiessi (Fig.2). The area, about 100,000 m2 in extension,
is located along a 250 m stretch of the provincial roadway 25 and is
constituted by steep rock slopes with little vegetation cover rang-
ing from an elevation of about 60 to 160 m a.s.l.
From a geological point of view, the study area is characterized by
complex structural setting (Fig. 2): the Late Miocene Mt. Capanne
monzogranite (MSF) (Farina et al. 2010) crops out with its
thermometamorphic aureole (Dini et al.2002; Ferrara and Tonarini
1993; Juteau et al.1984). The host rock corresponds to an ophiolite
succession with its JurassicCretaceous sedimentary cover (Barberi
and Innocenti1965; Bortolotti et al.2001; Spohn1981; Trevisan1950),
occurring here as metabasalts (MBA) and phyllites interlayed with
discontinuous layers of marbles (CAR). Both the pluton and the host
rocks are intensively crosscut by the Portoferraio porphyry (PMP)
with monzograniticsyenogranitic composition and by the Orano
porphyry (ORA) granodioritic dykes as well as by many
leucogranitic and microgranitic dykelets (LMG) (Bortolotti et al.
2001; Dini et al. 2002; Garfagnoli et al. 2010; Menna et al. 2008;
Rocchi et al.2003; Westerman et al.2003) (Fig.2).
Data collection
Geomechanical field survey
The rock mass characterization and the quantitative description of
discontinuities were obtained by traditional geomechanical
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surveys (scanline method), according to the methods suggested by
ISRM (1978;1985). Three scanline surveys were performed on the
rock slope along the roadway. In order to extend the rock mass
mechanical properties to the other sectors of the investigated rock
mass, random discontinuity orientation measurements were taken
(Fig. 2), confirming the spatial distribution of the identified dis-
continuity sets. Figure3ashows the stereographic projection of the
collected data, where discontinuity pole concentration and main
set modal planes are shown. Five main discontinuity sets were
identified on the investigated rock masses, whose modal orienta-
tion planes are reported in Table1.
JN3 (Fig. 3a) set in particular dips at medium to high angles
towards the western quarters, and includes (1) high persistent
decimetric-spaced discontinuity planes developed in the porphy-
ritic bodies (exfoliation joints (EJ)), which represent slipping
planes that isolate large rock mass portions, and (2) millimetric-
spaced penetrative metamorphic foliation, which is very evident in
the phyllites. In order to determine the intact rock tensile and
compressive strength, a number of point load tests were
performed, following ISRM (1985) suggested methods. Considering
mi=20 for porphyry (PMP and ORA) (Hoek 2007), the resulting
values were: c=91.6 MPa and t=4.6 MPa. The shear strength of
the discontinuities was calculated using Barton's failure criterion
(Barton and Choubey1977).
Laser scanning and thermographic surveys
The laser scanning survey allowed to cover the investigated area
extension in one working day. A long-range 3-D laser scanner
(RIEGL LMSZ410-i) was employed, which is able to determine
the position of up to 12,000 points per second, with a maximum
angular resolution of 0.008, and an accuracy of10 mm from a
maximum distance of 800 m. In order to completely cover the
intervention areas, surveys from different scan positions along the
roadway were performed. Twenty-three laser reflectors were
placed on the slopes and along the roadway, and their coordinates
defined by performing a differential RTK-GPS survey, linking the
different acquired point clouds to a global reference system.
The thermographic survey was performed using a FLIR SC620
tripod-mounted thermal camera. This instrument is characterized
by a microbolometer sensor with a 640480 pixel matrix, which is
able to measure electromagnetic radiation in the thermal infrared
band between 7.5 and 13m, with a thermal accuracy of 2 C and
a 0.65 mrad angular resolution.
Data analysis
Semiautomatic geomechanical survey
The employed Matlab tool (DiAna) (described in detail in Gigli and
Casagli2011) is based on the definition of least squares fitting planes
on clusters of points extracted by moving a sample cube on the point
cloud. The cluster is considered valid if the associated standard
deviation is below a defined threshold. The adopted method, by
selecting the cube dimension and a standard deviation threshold,
has demonstrated its ability to investigate even rock masses charac-
terized by very irregular block shapes. Therefore, discontinuity
planes can be reconstructed, and rock mass geometrical properties
are calculated. In the investigated area, DiAna was used to semiau-
tomatically individuate the main discontinuity plane orientations.
The analysis was carried out on a limited sector of the rock mass not
covered by nets, rock bolts, and fences. Figure3breports the poles of
the semiautomatically extracted discontinuities. A total of 1,359
planes were recognized; their density contour lines are very similar
to those obtained by means of traditional surveys (Fig. 3a). Due to
the high number of poles, with the aim of identifying the main
discontinuity sets, each set was assigned a weight (W) based on the
product between its surface area and the number of points consti-
tuting it; the poles were consequently drawn with different symbols
based on the log10W. By observing Fig. 3b, the points with higher
weight are clustered according to seven different discontinuity sets
(labeled from D1 to D7) reported in Fig.4.
The most important parameter for the stability analysis of
planar rockslides is the frictional resistance acting on the sliding
planes, which in turn depends on the uniaxial compressive
strength of the discontinuity walls and on the surface roughness
Terrestrial Laser Scanning
survey
High resolution 3D surface
Semi-automaticgeomechanical survey
Geological and geomechanical
field surveys
Detection and volume calculationof unstable masses
Terrestrial infrared
thermographic survey
Stability analysis
Fig. 1 Logic scheme of the applied methodology
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(Barton1973;1976; Patton1966). Traditional methods for estimat-
ing the roughness of a discontinuity plane (Barton and Choubey
1977; ISRM1978) require a direct accessibility to the discontinuity
plane and are quite time consuming. Moreover, it has been ob-
served that discontinuity roughness is characterized by a very
marked scale effect (Barton and Bandis1982). To overcome these
problems, a 3-D approach using the proposed DiAna algorithm
was pursued, allowing us to rapidly perform quantitative measures
of the roughness of the main discontinuities at various scales. A
searching cube with different dimensions (0.1, 0.2, 0.4, 1, and 2 m
and maximum surface persistence) is moved along the point cloud
representing the selected discontinuity. If the number of points
within the cube exceeds a prescribed threshold (to make sure that
the selection is centered on the surface), the best fitting plane dip
and dip direction are obtained, and the associated points are
extracted from the surface. By plotting the orientation values on
a stereo plot, the discontinuity roughness angles at various scales
can be measured. It is worth stressing that the reliability of this
procedure depends mainly on the accuracy of the point cloud data;
if it is too low, this could lead to an overestimation of surface
roughness (Rahman et al.2006), especially for small-scale analyses
(0.1 and 0.2 m), or low resolution point clouds. Figure 5b reports
Fig. 2 Location and geological maps of the study area (dotted lineshows the provincial roadway; yellow dots indicate random structural data collection points)
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the stereographic projection of the roughness characteristics for
different reference dimensions, calculated from the basal slipping
plane (EJ) which belongs to JN3 (traditional survey) and D5 (semi-
automatic discontinuity set extraction) sets represented in Fig.3a, b.
We can observe the decrease of pole scattering with the increase of
reference cube dimension, while the very high scattering associated
to small reference cube dimensions are probably due to the accuracy
limits of the laser scanning measurements. A roughness angle of 20was calculated for the largest reference dimensions (>1 m).
Worst credible scenario
The 3-D surface model obtained by merging the different point
clouds is shown in Fig. 6a, and the related topographic contour
lines (0.5 m equidistance) in Fig. 6b. Detailed 3-D maps of the
slope dip (Fig.6c) and aspect (Fig.6d) were also created. Both 3-D
models and maps contributed to a complete characterization of
the morphological variability of the investigated area; a rough
morphology, characterized by creek erosion gullies isolating jut-
ting rock mass portions, was revealed.
A wide rockfall barrier system has been built through the years
to protect the road stretch at risk from rockfalls. Nevertheless, thefield survey suggested the possible occurrence of more complex
phenomena, involving larger portions of rock mass, which cannot
be retained by the rockfall barriers. Given the geological setting of
the investigated area, and the most probable failure mechanism
occurring (planar failure along JN3 discontinuity set), an iterative
procedure has been applied with the aim of identifying the max-
imum credible scenario. A Matlab routine was built for this pur-
pose by moving on the 3-D surface a plane with the same
orientation of JN3 set. The largest emerging rock mass portions
were, thus, highlighted. By selecting a volume threshold value of
1,000 m3, three protruding rock masses were detected and labeled
from north to south as M1, M2, and M3 (Fig.6c). The latter masses
for their considerable extension, overhanging position, and shape
(mainly elongated along the direction of maximum slope) were
identified as potential critical rock mass sectors with regards to
instability mechanisms. Field survey evidence confirmed the crit-
icality of M1, M2, and M3 rock masses (Fig.7), which is mainly due
to their structural setting; in fact, they are all delimited from thestable portion of the rock slope by highly persistent slope dipping
basal planes constituted by EJ (belonging to JN3 set in Fig. 3aand
D5 set in Figs. 3b and 4). M3 rock mass (Fig.7e, f), in addition to
the basal slipping plane, is delimited southeastward from the
stable portion of the rock slope by a second subvertical plane,
connected to a leucogranitic dikelet (belonging to JN2 set in Fig. 3a
and D3 in Fig.3b). The resulting rock masses volumes (expressed
in cubic meter) are 3,706 (M1), 4,359 (M2), and 1,293 (M3).
Infrared thermographic analysis
A TIR inspection was carried out in correspondence of the critical
rock mass portions (M1, M2,and M3) for a rapid detectionof thermal
anomalies on the discontinuities delimiting them (Fig.8). Within theobtained superficial temperature maps (thermograms) shown in
Fig.8, the surface temperature is represented by means of a color
scale, in which the higher temperatures are displayed by the lighter
colors, whereas the colder temperatures by the darker ones. In order
to obtain a comparison with TLS data, TIR data were acquired from
the same location of two laser scanning positions as follows: ther-
mograms of M1 and M2 masses (Fig. 8a, b) were collected in corre-
spondence of point 2 (Fig.6a), leading to a 3-cm spatial resolution at
a 50-m distance, and thermogram of rock mass M3 (Fig. 8c) was
acquired from point 3 (Fig.6a), leading to a 6-cm spatial resolution
a b
Fig. 3 Stereographic projection of discontinuity poles and modal planes of the main sets collected in the investigated area through the traditional field surveys (a) and thesemiautomatic analysis (b)
Table 1 Geomechanical properties of the rock mass discontinuities from field surveys
Set_id () () X (m) L (m) e (mm) JRC JCS r/R b () p()
JN1 165 71 0.74 2.9 1.6 10 65 0.45 31
JN2 6 73 1.06 3.4 23.35 10.4 58.6 0.52 31
JN3 264 46 1.72 2.8 19 11.1 75 0.53 31 53.2
JN4 250 85 3.88 1.3 1.5 13 60.9 0.68 31
JN5 70 60 2.82 1.3 1.8 9.1 70.2 0.50 31
: dip direction, : dip, X: true spacing, L: persistence, e: aperture, JRC: Joint Roughness Coefficient, JCS: Joint Compressive Strength, r: Schmidt hammer rebound number on
weathered fracture, R Schmidt rebound number on unweathered surfaces, 8b: basic friction angle, 8p: peak friction angle at low stress calculated from: 8p=2JRC+8b
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framework, warm thermal anomalies connected to air circulation
were detected in correspondence of the open portions of the JN3
discontinuities delimiting the detected M1, M2, and M3 critical
masses. The abovementioned discontinuities detected on the ther-
mograms follow closely the basal planes, as result from the iterative
procedure for the definition of the maximum credible scenario. This
interpretation was strengthened by the comparison of the thermo-
grams with the optical images that confirmed no evidence of water
flow along the detected discontinuities, also showing that the only
cooler sectors on the investigated rock mass portions were repre-
sented by vegetation cover (Fig. 8a). For these reasons, dry condi-
tions were diagnosed for all M1, M2, and M3 basal slipping planes,
and the absence of water pressure was considered in the stability
analysis.
Stability analysis
A stability analysis of the main unstable rock masses was carried
out using the input parameters obtained from the geomechanical
field survey (Table1), the semiautomatic elaboration of TLS data,
and the interpretation of TIR thermograms. For a more accurate
estimate of the safety factor, two different scale approaches were
adopted by considering the roughness of the basal slipping plane
obtained at low stress levels, respectively, (1) fine roughnessfrom
the traditional geomechanicalfield survey data (p=2JRC+b; Barton
and Choubey1977; Maksimovic1996) (reference dimension=0.1 m,
see Table 1) and (2) coarse roughnessextracted semiautomatically
from the TLS data (reference dimension >1 m). Since the investigated
mechanisms occur under low stress conditions (the maximum thick-
ness of the potentially slipping blocks are 5, 9, and 12 m for M1, M2,
and M3, respectively), a frictional resistance of 51 was associated to
the coarse roughness, calculated by adding the basic friction angle
(31) to the roughness angle associated to reference cube dimensions
>1 m (20). Considering the volumes of the rock masses and the
detected instability mechanisms, a horizontal acceleration of 0.05 g
due to the possible occurrence of seismic shocks in the Elba island
area was introduced, as reported in the Italian seismic hazard map
(http://zonesismiche.mi.ingv.it/). Because of the different shape and
geometry of the abovementioned masses, two different slope stability
packages were employed: RocPlane (RocScience2004a) and Swedge
(RocScience2004b). Since the discontinuities delimiting the investi-
gated rock masses were all under dry conditions, no water pressure
effects were considered.
The stability of M1 (Figs.6cand7a, b) and M2 (Figs.6cand7c,
d) rock masses, both subject to planar failure mechanism, was
evaluated through a RocPlane analysis. The geometry of the rock
masses were reconstructed considering the dip of the basal slip-
ping plane retrieved from the laser scanning data (48.8 for M1 and
46 for M2) and the thickness and length of the analyzed portions
of rock mass extracted from the high-resolution 3-D surface.
Attributing the parameters obtained from the traditional
geomechanical survey to the discontinuities (fine roughness anal-
ysis), safety factors of 1.04 and 1.14 were obtained for seismic and
non-seismic conditions for M1 and 1.17 and 1.29 for M2 (Table2).
As for the coarse roughness approach, safety factors of 0.98 and
1.08 were obtained for M1, for seismic and non-seismic conditions
respectively, while the corresponding values for M2 were 1.08 and
1.19 (Table2).
According to the laser scanning data, the dip angle of M3 basal
slipping plane is 50.9. The Swedge package was used to investigate
the stability of this rock mass. The resulting safety factors for the
fine roughness analysis were 1.19 and 1.26, for seismic and non-
seismic conditions, respectively, while the coarse roughness anal-
ysis yielded values of 1.07 (under seismic conditions) and 1.14
(under non-seismic conditions) (Table2).
Discussions and conclusions
The provincial roadway 25 plays a key role in the Elba Island
(Livorno district, central Italy) transit conditions, representing the
only linear infrastructure connecting the villages located on the
island western coastline. Being also part of the Tuscan Archipelago
a
1
23 4
56 7
8
M1 M2M3
c
b d
Figure60
0
90
NORMAL
OVERHANGING
N (0)
S (180)
N (360)
E (90)
W (270)
Fig. 6 High-definition 3-D surface and maps of the investigated rock slope (a) (dots mark the different scan positions labeled from1to8), while thesquaredelimitatesthe sector where the semiautomatic geomechanical survey was carried out, 3-D surface-related topographic contour lines (0.5-m equidistance) b), slope (c) (dottedcircles locate the more protruding rock mass portions), and aspect (d) (in detail the overhanging portions of a protruding rock mass sector are shown)
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National Park, this roadway has also a panoramic relevance for
tourism. A 250-m long stretch of the roadway, located just north of
the village of Chiessi, in the recent years has been threatened by the
overlooking steep rock slope, which due to its structural setting and
degree of fracturing underwent the detachment of rock mass por-
tions and rock debris, which in some occasions severely damaged
catchment nets and barriers invading the roadway itself. Considering
the high level of risk and the need to increase the provincial road
transit security conditions, especially during the summer period, a
remote-based procedure was established in order to investigate the
rock slope instability phenomena.
Traditional field surveys allowed a detailed reconstruction of the
complex geostructural setting and the geomechanical properties of the
investigated area. Five main discontinuity sets were identified and
quantitatively described according to the ISRM (1978) suggested
methods. In particular, high persistent slipping planes (constituted
by EJ belonging to JN3 set) were identified as key discontinuities
playing an important role in the stability of the investigated rock mass.
The TLS survey yielded a detailed 3-D remote structural, geo-
metrical, and geomechanical characterization of the investigated
rock masses. In particular, a semiautomatic geomechanical char-
acterization was carried out by means of a Matlab tool called
DiAna (discontinuity analysis) (Gigli and Casagli2011). In partic-
ular, DiAna made possible the automatic calculation of six of the
ten parameters suggested by ISRM for the quantitative description
of discontinuities (orientation, spacing, persistence, roughness,
number of sets, and block size). A total of 1,359 planes were
recognized and clustered according to seven different discontinu-
ity sets, which density contour lines showed to be very similar to
those obtained by means of traditional surveys. Therefore, the
semiautomatic geomechanical survey improved the rock mass
structural characterization, adding two more discontinuity sets
to the five detected by means of the traditional field survey
(Fig.3a). TLS data elaboration provided high-resolution 3-D surface
and morphological slope steepness and aspect 3-D maps, in order to
detect the more protruding rock mass sectors. This led to the recog-
nition of three critical rock masses (namely M1, M2, and M3) and to
the calculation of their shape and volume. The obtained TLS 3-D
products also provided reference morphological maps useful for
both further detailed field inspections and the design and the
Fig. 7 Optical images of the unstable
rock masses taken from the digital
camera integrated in the laser
scanner device from different
scanning positions (A=M1; C=M2;E=M3); related 3-D digital modelwith the detected basal and lateral
slipping planes (B,D, andF)
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location planning of possible future restoration works. The investi-
gated area showed favorable logistic conditions; in fact, the roadwayat the foot of the rock slope investigated wasfundamental in carrying
out up-close the field inspections, the TLS, and the TIR surveys. Had
this condition not existed, the point cloud resolution would not
probably have been high enough for such detailed analyses.
The TIR survey, together with the TLS semiautomatic analysis, led
to a more detailed remote 3-D rock mass geomechanical characteri-
zation and provided more accurate input parameters for the stability
analysis, with regards to the absence of water pressure. The geometric
and mechanical parameters retrieved from the traditional field sur-
veys, the semiautomatic geomechanical data interpretation, and the
TIR survey were used as input data for a detailed stability analysis.
As for the proposed application, two different approaches were
carried out by considering the fine and coarse roughness of the
basal slipping plane, extracted from the field, and the laser scan-
ning data, respectively. Due to the dry condition diagnosed for theinvestigated key discontinuities, no water pressure was consid-
ered. Finally, because of the different shape, geometry, and failure
mechanism of the abovementioned masses, two different slope
stability packages were employed: RocPlane (RocScience 2004a)
for M1 (which is subject to planar failure mechanism) and Swedge
(RocScience2004b) for M3 (which is subject to wedge failure).
The obtained factors of safety for the investigated rock masses
were quite low; with regards to M1, they range from 0.98 (coarse
roughness approach with horizontal acceleration) to 1.14 (fine
roughness analysis in absence of seismic conditions), while M2
presents slightly higher values (ranging from 1.08 to 1.29), mainly
due to a less steep basal plane. Regarding M3, the factor of safety
values range from 1.07 (coarse roughness approach with horizontal
Table 2 Factor of safety values from the stability analysis
Fine roughness Coarse roughness
Seismic Non-seismic Seismic Non-seismic
M1 1.04 1.14 0.98 1.08
M2 1.17 1.29 1.08 1.19
M3 1.19 1.26 1.07 1.14
a ab
c
A1 B1
C1
Fig. 8 Mosaicked thermal images of the detected unstable rock masses (A=M1,B=M2, andC=M3) acquired around 1 p.m.Dotted linesmark the basal slipping planes;white squares on the thermogram allow a comparison with the correspondent sectors in the optical images (A1,B1, andC1), acquired by the built-in digital camera
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acceleration) to 1.26 (fine roughness analysis in absence of seismic
conditions). The factor of safety calculated for M3 resulted a little
higher than the one obtained from M1, despite the higher dipping
angle of the basal plane. This was due to the lateral discontinuity
plane slightly counteracting the wedge failure, as the sliding takes
place along the intersection line of these two planes. The fine
roughness approach led to slightly higher safety factors when
compared to the coarse roughness one.
The volumes calculated for M1, M2, and M3 masses and thecorresponding obtained low factors of safety enhanced the defini-
tion of the risk scenarios in the study area; in addition to the high
risk for both human life and transit conditions, the eventual
failure of any of the three unstable masses would cause a rockslide
that would severely damage the roadway itself. Considering the
steepness of the study area rock slope, the nearness of the coast-
line, and the volumes of materials involved, any of the aforemen-
tioned rock slides would also impact the seaside causing a small
tsunami, locally determining high risk for the local navigation and
bathing, especially during the summer period.
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G. Gigli ()):
W. Frodella:
F. Garfagnoli:
S. Morelli:
F. Mugnai:
F. Menna:
N. CasagliDepartment of Earth Sciences, University of Firenze,
Florence, Italy
e-mail: [email protected]
Technical Note
Landslides 11 & (2014)140
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