3-D Geomechanical Rock Mass Characterization

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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

Landslides 11 & (2014) 131

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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

Technical Note

Landslides 11 & (2014)132


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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)

Landslides 11 & (2014) 133


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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)

Technical Note

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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)

Landslides 11 & (2014) 137


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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

Technical Note



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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]

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3-D Geomechanical Rock Mass Characterization

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