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225
Design of the rockfall protection at the Špičunak location, Gorski kotar,
Croatia
Maroje Sušac(1), Mirjana Vugrinski(1), Dalibor Udovič(1), Davor Marušić(2), Željko Arbanas(3)
1) Geolog savjetovanje Ltd, Samobor, Pod borom 3, Croatia, +385 91 4294 008 (maroje.susac@geolog.hr)
2) Terraforming Ltd., Rijeka, Croatia
3) University of Rijeka, Faculty of Civil Engineering, Rijeka, Croatia
Abstract The Špičunak location at the state road D3, near
the Lokve settlement in the Gorski kotar region, Croatia,
is well – known by numerous traffic interruptions caused
by slide and rockfall occurrences. The rockfalls at the
Špičunak location are mostly predisposed due to
geological setting and heavy jointed rock mass in the road
cut of approximately 180.0 m length and 23.0 m height.
Structural and kinematic analysis of possible future
rockfall were carried out following the modern approaches
and recent techniques in rockfall hazard analysis. These
approaches include application of remote-sensing
techniques enabled to ensure digital terrain models
(DTM) from three-dimensional high-resolution point
cloud (3D HRPC) of the rock cut surface; engineering
geological mapping using combination of remote-sensing
techniques and field mapping. The three-dimensional
high-resolution point cloud (3D HRPC) were established
based on terrestrial laser scanning (TLS) and
photogrammetry survey employing unmanned aerial
vehicle (UAV) using Structure form Motion (SfM)
technique. Based on established 3D models, the cut was
analysed to identify the main characteristics of the rock
mass structure as well as to detect and map the
discontinuities and discontinuity sets, orientation and dip
of discontinuities, spacing of discontinuities, persistence
of discontinuities and roughness of discontinues.
Traditional geotechnical survey was conducted to
determine the characteristics of the main discontinuity
sets at the cliff, as well as to carry out a rock mass
classification using Rock Mass Rating (RMR) system and
Geological Strength Index (GSI). Detailed analyses of field
survey and remote sensing data pointed to three different
zones, based on their properties and rock block standings
according to the general orientation and dip of the cut
face. To identify possibility of failures associated with the
present joint sets and their orientations, the kinematic
analyses of plane, wedge and toppling failure mechanisms
were carried out based on joint sets discontinuity features
data collected by both traditional geological and
geotechnical field survey and remote sensing survey and
data analysis. Based on the kinematic analyses results,
adequate protection measures to prevent further brock
block detachments and rockfalls were selected and
designed. In this paper we will describe field investigation,
establishing of the rock cut model based on remote
sensing and traditional geotechnical investigations,
stability analysis, as well as design element necessary for
ensuring of stability of the rock mass in the cut and safety
of the traffic along the road.
Keywords rockfall, rockfall protection, remote sensing,
3D modelling, kinematic analysis
Introduction
Rockfall, as type of landslide, is one of the most frequent
and dangerous instability type that can cause fatalities and
high economic and social damage. Rockfall process
includes detachment from an almost vertical rock slope,
fall, rolling and bouncing of rock block along a slope
(Dorren 2003; Volkwein et al. 2011), singly or in clusters,
and can be defragmented during impacts (Hungr et al.
2014). A rockfall occurrence can vary from small rocky
fragments to massive blocks of different volumes and
shapes. The high speed, mobility and energy of falling
blocks disable getting a necessary time for fast response
through evacuation or protection (Ritchie 1963; Siddique
et al. 2019; Volkwein et al. 2011).
The Špičunak rock slope is located at the route of the
State road DC3 (Rijeka – Zagreb), about 5 km south from
the Lokve settlement, Fig. 1. The rock slope extends in the
northeast-southwest direction and it was formed by cut
excavation during the road construction in 1950’s. The
length of the investigated part of the cut is approx. 180.0
m, while its height at the highest part is up to 23.0 m, Fig.
2. The existing surrounding terrain has a general elevation
of approx. 865.0 m a.s.l.
Problems due to sliding and rockfalls at the Špičunak
location existed more than 50 years and caused numerous
traffic interruptions. The road construction was repaired
several times while the slide remediation was completed
in 2021. Rockfall protection measures were applied to the
rock cut several times, last time in 2005, according to then
available techniques (Arbanas et al. 2012) and only to one
part of the cut. Due to further rock mass weathering as
well as freezing and thawing process, numerous rockfalls
were triggered during winters followed by fallen blocks at
M. Sušac, M. Vugrinski, D. Udovič, D. Marušić, Ž. Arbanas – Design of the rockfall protection at the Špičunak location, Gorski kotar, Croatia
226
Figure 1 The map of the Špičunak location.
Figure 2 A view at the rock cut at the Špičunak location.
the road, indicating the need of new rockfall protection
design and implementation which started in 2021.
Geological settings
The Špičunak rock slope is situated in the Gorski kotar
region, south of the Lokve settlement, and the wider area
of the investigated slope belongs to the structure of the
Lokve Lake Dome, within the Gorski Kotar Structural Unit
(Savić and Dozet 1989). The Lokve Lake Dome is built of
Upper Cretaceous and Lower to Middle Permian clayey
and sandy-conglomerate deposits, while the flanks are
built of Upper Triassic dolomite and clastic deposits.
The slope at the location is composed of well-layered
Upper Triassic dolomites of general north-south to
northwest-southeast orientation, with a continuous dip of
layers of 15-30 (40) ° to the west-southwest. Dominant
joint systems are steep to subvertical, oriented N-S, NW-
SE and W-E. One 30 m wide subvertical (108/87) shear
fault zone (108/87) without pronounced vertical
displacement was determined in the middle part of the
slope. In the south-western part of the slope, a local steep
fault (52/73) with approx. 0.5-2.0 m wide fault zone is
registered.
Field investigation
In determination of the detailed geological structure of the
rock cut at the Špičunak location, several campaigns of
field investigations were carried out during the last
decades. The last one carried out for the remedial
measures and rockfall protection design was conducted
using traditional methods of engineering geological survey
and mapping combined with the remote sensing
techniques based on Unmanned Aerial Vehicle (UAV)
photogrammetry data (Froideval et al. 2019; Giordan et al.
2020; Antoine et al. 2020) and terrestrial laser scanning
(Jaboyedoff et al. 2012)in combination with data collected
by traditional survey (Francioni et al. 2019).
Traditional geological and geotechnical field survey
consisted of field mapping of cliff face; direct measuring of
discontinuity orientations and dip direction, persistence,
spacing, aperture, and roughness; as well as determination
of discontinuity wall weathering grades and discontinuity
infilling limited to the accessible zone at the foot of the
rock cut. Field investigations were combined with the
study at the orthophoto (in scale 1:500) and 3D HRPC of
about 35.6 million of points (Fig. 3) for better
understanding of geological and geomorphological
features at the study area. Traditional geotechnical survey
was conducted to determine the characteristics of the
main discontinuity sets at the rock cut, as well as to carry
out a rock mass classification using Rock Mass Rating
system (RMR) (Bieniawski 1989) and Geological Strength
Index (GSI) (Marinos and Hoek 2000). In this study a
combination of traditional geological and geotechnical
field survey (Bolla and Paronuzzi, 2020) and remote
sensing techniques are employed to geotechnical model of
the Špičunak rock cut including the discontinuities and
discontinuity sets, orientation and dip of discontinuities,
as well as other discontinuity features necessary for
analyses of rockfall occurrences and their consequences as
well as representative block volumes for each of
discontinuity sets (Palmstrom 2001).
Detailed analyses of field survey and remote sensing
data pointed to three different zones, based on their
properties and rock block standings according to the
general orientation and dip of the rock cut face. These
zones (Zone I to III) are presented at the 3D HRPC (Fig. 3)
while the borders that separate zones were determined
analysing changes in orientation and dips of the main joint
sets as well as changes in rock block volumes. The Cloud
Compare software (CloudCompare 2015) was employed to
identify discontinuity sets. The rock cut face was divided
in three zones (Fig. 3), and for each segmented zone an
extraction of joint planes using the Cloud Compare Facet
and Compass plugins (Dewez et al. 2016; Nagendran et al.
2019) (Fast Marching procedure) were employed to
identify their orientations (dip directions and dips). The
spacing between the joints for each joint set necessary to
determine block volumes was determined using the Cloud
Compare Distances tool, while the persistence of joints in
accessible rock cut face zones were measured in situ. The
other discontinuity features (separation, infilling,
discontinuity wall roughness and weathering grade) were
obtained by in situ traditional geological and geotechnical
surveys.
Proceedings of the 5th Regional Symposium on Landslides, Rijeka, 2022
227
Figure 1 The three-dimensional high-resolution point cloud (3D HRPC) of the Špičunak rock cut with different zones and stereonet
plots for each zone.
Geotechnical model
Based on previously described field and remote sensing
investigations and analyses, a geotechnical model for each
of determined geotechnical zones was established in order
to conduct the necessary kinematic and slope stability
analyses. In all of determined three geotechnical zones,
three different geotechnical units (GU) were identified
and presented in Tab. 1.
Talus deposits (GU 1) are present at the foot of the
part of the rock cut, as a result of weathering of the
dolomite rock mass. Significant accumulations of talus,
which cover the toe of the investigated part of the cut, are
up to 2.0 m, and are represented below the fault zones with
pronounced shedding of highly weathered rock mass.
Highly to moderately weathered (HW/MW)
dolomites (GU 2) are present along the slope within the
fault zones, and extend from the bottom to the top of the
cut due to subvertical faults extension. Within highly to
moderately weathered (HW/MW) dolomites, the distance
between discontinuities is usually small; from 6 to 20 cm.
The roughness of the discontinuity walls is
Table 1 Description of geotechnical units (GU).
Geotechnical
unit
Geotechnical description
GU 1
Talus, gravelly clay and poorly graduated clayey
gravel with pebbles and blocks.
GU 2
Highly to medium weathered (HW/MW)
dolomites, medium
to high compressive
strength, layered,
disintegrated to a
blocky/disturbed structure.
GU 3
Slightly weathered (SW) dolomites, medium to
high compressive strength, layered, blocky/
disturbed to very block-like structure.
smooth. The discontinuity separation is open to medium
wide (0.5-10.0 mm), in places wide (up to 10 cm), mostly
filled with clay filling, while the interlayer separations are
narrow (<0.25 mm) to partially open (0.25-0.50 mm).
Persistence of discontinuities is long, >20m. The
discontinuity systems represented mainly form
polyhedral, rarely and equidimensional blocks, mostly
small to medium in size (0.1-0.5 m3). The rock structure of
highly to moderately weathered (HW/MW) dolomites is
disintegrated (D) to blocky/disturbed (B/D) (Marinos and
Hoek 2000). The RQD value as an indicator of the rock
mass quality is estimated as 0-25% (very poor to poor).
Laboratory tests determined a wide range of uniaxial
compressive strength of intact rock mass; the interval of
45.0-53.0 MPa was adopted as representative value.
Slightly weathered (SW) dolomites (GU 3) are present
along the slope at the initial (Zone 1 ) and final part (Zone
3) of the investigated rock cut and are locally interrupted
by fault zones with highly to medium weathered
(HW/MW) dolomites. Within the slightly weathered (SW)
dolomites, the distance between discontinuities is usually
small to medium, 6-60 cm. The roughness of the rock
discontinuity walls is predominantly smooth. The
discontinuity separation is mostly partially open to
medium wide (0.5–10.0 mm), predominantly fulfilled with
clay filling and partially without infilling, while the
interlayer separations are narrow (<0.25 mm). Persistence
of discontinuities is long, >20m. The discontinuity systems
represented mainly form polyhedral to equidimensional
blocks; small to large in size (0.2-1.0 m3). The rock
structure of weakly dilapidated dolomite is
blocky/disturbed (B/D) to very blocky (B)(Marinos and
Hoek 2000). The RQD value as an indicator of the rock
mass quality is estimated as 25-50% (poor). The uniaxial
compressive strength of intact rock mass is identical for
M. Sušac, M. Vugrinski, D. Udovič, D. Marušić, Ž. Arbanas – Design of the rockfall protection at the Špičunak location, Gorski kotar, Croatia
228
GU 2 and GU 3; interval of 45.0-53.0 MPa was adopted as
representative value.
Based on the results of the traditional geological and
geotechnical field surveys, as well as the remote sensing
survey and data analysis, the geotechnical model of the
Špičunak rock cut was established, which presented the
main recognized joint sets, their orientations, and all other
discontinuity features that are important for conducting
stability analyses and identification of the causes of
rockfall occurrences. For each of the identified zones, the
mean values of joint set orientations and dip directions, as
well as other discontinuity features were determined.
According to the identified discontinuity features, the
Geotechnical rock mass classification (Rock Mass Rating,
RMR) (Bieniawski 1989) was determined and the mean
RMR values were calculated for each cut zone. The mean
descriptions and values of weathering grade, block
volumes and RMR for each cut zone are presented in Tab.
2.
Stability analyses
After establishing of the geotechnical model of the
Špičunak rock cut, as well as determination of
geotechnical parameters necessary for slope stability
analyses (RMR and uniaxial compressive strength), slope
stability and kinematic analyses were conducted for each
zone of the cut. Stability analyses of the cut in all three
zones conducted using RocScience Slide2 software
(Rocscience 2021) pointed on acceptable values of factors
of safety (FoS) in all three zones of the cut (FoS > 1.7) that
is confirmed with the fact that the Špičunak rock cut has
been globally stable during the last 70 years, while the
instabilities mostly occurred as local detachments of
individual rock blocks or several blocks from the cut
caused by local planar, wedge or topple failures. This fact
implied on necessary kinematic analyses to identify
present rockfall susceptibility.
Kinematic analysis
Once the geotechnical model of the Špičunak rock cut was
established based on the traditional geological and
geotechnical field surveys and remote sensing analysis, it
was possible to analyze the causes of instability and
detachment of rock blocks from the cut face and the
initiation of rockfall occurrences. To identify possibility of
failure associated with the existing joint sets and their
orientations, kinematic analyses of plane, wedge and
toppling failure mechanisms (Wyllie and Mah 2004) were
performed based on the data of the joint set discontinuity
features collected along the rock cut. Kinematic analyses
were performed for each rock cut zone employing the
RocScience Dips software (Rocscience 2021). For each cliff
zone, the exported discontinuity plane (facets),
orientation (dip and dip directions) data extracted using
the Cloud Compare Facet plugin (CloudCompare 2015)
Table 2 Description of zones of the Špičunak rock cut with
descriptions and values of weathering grade, block volumes and
RMR.
Rock Cut
Zone
Weathering
Grade
Block Volume
RMR
Zone 1
SW/MW
0.2-1.0 m3
41
Zone 2
HW/MW
0.1-0.5 m3
37
Zone 3
SW/MW
0.2-1.0 m3
40
were imported into the RocScience Dips software, and
kinematic analyses were performed for each type of failure
mechanism. The results of kinematic analyses are
presented in Fig. 4.
The analyses were conducted for each cut zone as it
follows:
In Zone 1, analyses were conducted for 296
discontinuities that daylighting in the rock face analyses at
dip and dip orientation of 76/330o. The results pointed on
probability of 45.27% for planar failure, 51.16% for wedge
failure and 53.38% for toppling.
In Zone 2, analyses were conducted for 515
discontinuities that daylighting in the rock face analyses at
dip and dip orientation of 82/315o. The results pointed on
probability of 31.65% for planar failure, 47.66% for wedge
failure and 41.94% for toppling.
In Zone 3, analyses were conducted for 364
discontinuities that daylighting in the rock face analyses at
dip and dip orientation of 88/275o. The results pointed on
probability of 16.15% for planar failure, 33.41% for wedge
failure and 33.33% for toppling.
The results of kinematic analyses are expressed as
probability occurrence for any of possible failure
mechanisms (planar sliding, direct toppling and flexural
toppling; wedge failure was excluded because of very low
probability of occurrence). Probability is expressed as the
ratio of number of planes (or combination of planes)
meeting the conditions for a failure related to the number
of all planes (or combination of planes). An expression of
probability of failure occurrence was presented as
kinematic hazard index (KTI) by Casagli and Pini (1993),
as a number of failures meeting kinematic conditions of
failure relative to total number of possible failures. Here is
applied the same approach, but the probability is related
to each failure mechanism separately. As the temporal
component of rockfall occurrences is not included, the
calculated probabilities indicate on rockfall susceptibility
rather than rockfall hazard.
The results of the analyses indicated a nearly high
probability of all three failure mechanisms, that it was
expected according to hard jointed rock mass and
relatively small volumes of rock blocks at the cut face. The
results and rock mass structure in all three zones of the
cut, indicate on an approach that will include protection
of the entire rock cut face and does not consider measures
to support individual blocks at the slope, except in Zone 3,
where bigger rock blocks are present.
Proceedings of the 5th Regional Symposium on Landslides, Rijeka, 2022
229
Figure 4 Kinematic analyses of the Špičunak rock cut by cut zones according to planar sliding, wedge and direct toppling, conducted
using Rocscience Dips software. The results are presented in rows for each zone (e.g., Zone Z1 is in the first row). The results of planar
sliding are in the first column, wedge failure in the second column, and direct toppling in the third column.
Slope protection systems
According to the results of slope stability and kinematic
analyses, as well as rock mass structure in the cut, rockfall
protection measures were designed. Two design
approaches were considered (Arbanas et al 2012): (i) the
prevention of rockfalls by removing potentially unstable
rock mass or by installing rock mass support systems and
(ii) the reduction of rockfall mass energy and suspension
of running rockfall mass using rockfall protection diches,
walls or barriers (Volkwein et al 2011). After analyses of
technical and economic aspects of possible remedial and
protection measures, application of rock mass support
system in combination with rockfall protection walls in
the foot of the cut was selected.
In Zone 1, with the lowest cut heights and relatively distant
from the road, the rock cut protection is designed using
rockfall protection fences that enable detachment of small
blocks and their fall to the slope toe, but without reaching
the road. In Zone 2, where the rockfall hazard and a
possibility that detached running blocks reach the road
and endanger vehicles, the support system consisted of
rockbolts and multi-layered reinforced shotcrete that will
cover overall cut face was designed. Rockbolts designed on
a raster of 3.0 x 3.0 m, lengths of 6.0 and 9.0 m, and bearing
capacity of 240 kN in combination with two-layered
shotcrete lining will have significant impact on complete
prevention of further rockfall occurrences as well as an
increasing of global slope stability, Fig. 5.
M. Sušac, M. Vugrinski, D. Udovič, D. Marušić, Ž. Arbanas – Design of the rockfall protection at the Špičunak location, Gorski kotar, Croatia
230
Figure 5 Results of global rock cut stability analysis with applied
slope protection measures in Zone 2, conducted using RocSlide
software.
In Zone 3, where the kinematic analyses indicate the
lowest probability of rockfall occurrences, but with the
biggest rock blocks in the cut and closest to the road, a
combination of individual rock blocks support by
rockbolts, support system consisted of rockbolts and
multi-layered reinforced shotcrete and protective gabion
wall along the road edge was designed.
Application of these rockfall measures at the
Špičunak rock cut will practically completely eliminate
further rockfall hazard in Zone 2 and 3, while the further
rockfall processes associated with rock mass weathering
process, rinsing of the discontinuity infilling related to
freezing and thawing processes as well as temperature
effects on rock mass, will take place in Zone 1. Anyhow,
engineering judgment and analyses indicated on
acceptable rockfall risk and rockfall protection using
protection fences was adopted as an adequate and
economically justified protection measure.
Acknowledgments
The part of this research is carried out in the frame of the
UNIRi Project uniri-tehnic-18-276-1448 Research of
Rockfall Processes and Rockfall Hazard Assessment
supported by University of Rijeka, Croatia. This support is
gratefully acknowledged.
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