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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
255
DOI: 10.4408/IJEGE.2013-06.B-23
UNDERSTANDING LONG-TERM SLOPE DEFORMATION
FOR STABILITY ASSESSMENT OF ROCK SLOPES:
THE CASE OF THE OPPSTADHORNET ROCKSLIDE, NORWAY
R
eginald
l. HeRMannS
(*)
, T
HieRRy
OPPIKOFER
(*)
, H
algeiR
DAHLE
(**)
, T
Rond
eiKen
(***)
,
S
uSan
IVY-oCHS
(****)
& l
aRS
H. BLIKRA
(*****)
(*)
Geological Survey of Norway (NGU) - Trondheim, Norway
(**)
Norwegian Road Authorities, Norway
(***)
University of Oslo - Oslo, Norway
(****)
ETH Zurich- Zurich, Switzerland
(*****)
Åknes Tafjord beredskapsenteret IKS, Norway
(e.g. V
OIGHT
, 1988, A
GOSTINI
et alii, 1991; C
ROSTA
&
A
GLIARDI
, 2003 and references therein). However, his-
torical data on 32 failures from Norway, the European
Alps, and other places indicate that rock slope defor-
mation takes place for at least several years to decades
and accelerates prior to failure (H
ERMANNS
et alii, 2012a
and references therein). This is especially true under
non-seismic conditions. Under seismic conditions the
acceleration phase is in general only a few seconds long
(H
ERMANNS
& L
ONGVA
, 2012). Long-term slip behavior
of rockslides in Norway is not understood due to the
lack of long-term monitoring data (monitoring of rock
slopes has been carried out in Norway on 58 sites for up
to max. 10 years only (H
ERMANNS
et alii, 2013).
The goal of this project was to determine long-
term slip rates of the Oppstadhornet rockslide in or-
der to be able to recognize acceleration of movement
prior to failure in future. The Oppstadhornet rockslide
on the island of Otrøya in western Norway involves
an estimated 10 million cubic meters of schistose
gneiss that has slid towards the fjord as indicated by a
20-m-high sliding surface exposed on the head scarp
of the slide (D
ERRON
et alii, 2005). This is one of 140
unstable rock slopes in Møre og Romsdal county that
are known after several years of systematic mapping
by the Geological Survey of Norway and the County
geologist of Møre og Romsdal (Fig. 1, O
PPIKOFER
et alii, 2013). Failures of such unstable rock slopes
have caused considerable damage and high numbers
of death in the Møre and Romsdal County in historic
ABSTRACT
The Oppstadhornet rockslide is a 10 Mm
3
slide
that occurs on the island of Otrøya in westernmost
Norway. Terrestrial cosmogenic nuclide dating indi-
cates that the Oppstadhornet rockslide became active
ca. 16.6-14.2 kyrs ago when the retreat of the Scandi-
navian ice sheet exposed the island from the continen-
tal ice cover. Sliding along the main sliding surface
was active during the late Pleistocene and Holocene.
Our data suggest that the paleo-slip rate in the Late
Pleistocene was slightly faster than in the Holocene
however large uncertainty margins ask for care with
this interpretation. Present day displacement rates
of ca. 2 mm/year measured with differential Global
Navigation Satellite Systems are similar to the paleo-
slip rates, however they vary over the entire rockslide
body and at several locations after 10 years we could
not yet measure any significant displacement. The
long-term activity of this rockslide suggests that - in
contrast to dynamic stability models - moderate earth-
quake shaking with a recurrence time of 475 years
will not cause the Oppstadhornet rock slope to fail.
K
ey
words
: long-term behavior of rockslides, terrestrial cosmo-
genic nuclide dating, differential Global Navigation Satellite
System, paleo-slip rate, displacement rate, seismic triggering
INTRODUCTION
On the globe only a small number of natural
slopes have collapsed under monitored conditions
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R.L. HERMANNS , T. OPPIKOFER, H. DAHLE, T. EIKEN, S. IVY-OCHS & L.H. BLIKRA
256
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
We followed the approach of B
igoT
-C
oRMieR
et alii (2005) also described in H
eRMannS
et alii
(2012c) and sampled the main sliding surface of the
rockslide for terrestrial cosmogenic nuclide dating
(TCN). We took five samples along the sliding sur-
faces so that we can calculate slip rates based upon
their exposure history.
METHOD
SAMPLE COLLECTION AND SAMPLE TREAT-
MENT
On the sliding surface five samples 1 to 5 cm
thick were taken with a chisel from quartzo-feldspat-
ic gneisses on rappel. The position of each sample
site was measured with a handheld GPS, an altimeter
with 1 m precision and the length relative to the top
of the sliding plane determined using a measuring
tape. At each sample location the dip angle of the
sliding plane as well as the average dip angle above
the sample location was measured. The horizontal
shielding was measured at each sample location tak-
ing the average of 30 degree steps. Quartz-samples
were sent to ETH Zurich for quartz purification and
production of nuclide targets.
10
Be nuclides were
measured at ETH Zurich.
AGE CALCULATION
Age calculations were carried out following the
principles outlined in G
OSSE
& P
HILLIPS
(2001), how-
ever for calculating shielding of the cosmic ray flux
due to the cliff exposure principles outlined in D
UNNE
et alii (1999) were applied. No correction for snow
cover was calculated as the sliding plane dips every-
where steeper than 60° and no significant snow accu-
mulation can be assumed.
Ages were calculated using the CRONUS Calcu-
lator (B
ALCO
et alii, 2008) and are reported here as
mean ages and 1 sigma uncertainties of the spread
of outcomes given by the calculator (Tab. 2). We are
well aware that locally determined production rates in
southern and northern Norway exist (F
ENTON
et alii,
2011; G
OEHRING
et alii, 2011). Using the production
rates presented by F
ENTON
et alii (2011) & G
OEHRING
et alii
(2011) the ages reported here would be approx.
10% or 5% older, respectively.
We did not calculated the inherited age (pre-slid-
ing-exposure) of the samples, which results from the
penetration of the cosmogenic radiation into the rock
times. Two catastrophic rock slope failures in Møre
og Romsdal County are among the 10 events with
highest death toll in Norway: the Tafjord rockslide
in 1934 that caused a displacement wave resulting
in the death of 40 persons and the Tjelle rockslide in
1756 that also set off a displacement wave drowning
32 persons in the inner part of the Langfjord (H
eR
-
MannS
et alii, 2012b).
Fig. 2 - Back scarp with sample locations and sample
numbers (person for scale is 1.8 m tall)
Fig. 1 - Unstable rock slopes characterized so far in the
Møre og Romsdal County and location of the Opp-
stadhornet rockslide (star)
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UNDERSTANDING LONG-TERM SLOPE DEFORMATION FOR STABILITY ASSESSMENT OF ROCK SLOPES:
THE CASE OF THE OPPSTADHORNET ROCKSLIDE, NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
257
DIFFERENTIAL GLOBAL NAVIGATION SA-
TELLITE SYSTEM MEASUREMENTS
Differential Global Navigation Satellite System
(GNSS) surveys have been undertaken with one to
three years interval since the establishment of the first
survey points in 2003. A total of three fixed points
were installed in stable areas and 19 rover points in
potentially unstable regions (Fig. 3).
The accuracy of the coordinates are estimated for
each GNSS-point and are generally ca. 1 mm in plan-
imetry and ca. 2 mm in elevation. These values are
found to be too optimistic and a factor of 3 is thus used
to obtain realistic accuracies. Thus, the error on the
total horizontal, vertical and 3D displacement, σ
tot.H
,
mass. This is restricted due to the exponential attenu-
ation of the radiation by rock matter to the uppermost
meters. Our uppermost sample was taken 4.5 m be-
low the top of the sliding plane. Therefore with the
assumption taken here that the surfaces were eroded
by glacial erosion during the last glacial maximum
and covered by ice until deglaciation no inheritance is
expected. The assumption is most likely correct as the
sample site lies at 750 m altitude that in this part of
western Norway was covered by ice sheets for several
thousand years (V
ORREN
& M
ANGERUD
, 2008). The
inherited age is anyhow insignificant in comparison
with the real age and statistical error margins of this
method (Tab. 1).
Fig. 3 - Aerial photograph of the unstable rock slope at Oppstadhornet with main structural features of the slide and location
profile for TCN dating as well as of dGNSS stations classified after significance level. The profile A-B is shown in Fig. 4
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R.L. HERMANNS , T. OPPIKOFER, H. DAHLE, T. EIKEN, S. IVY-OCHS & L.H. BLIKRA
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
stretches slope parallel for 1 km. It can be divided in
a more active SW part characterized by internal de-
formation (B
RAATHEN
et alii, 2004) and a rather inac-
tive NE part that is characterized by a depression that
connects to the main sliding surface of the SW part
but that has no significant displacement in the past
10 years and does not show any internal deformation
(Fig. 3). The rockslide in the SW part stretches from a
near shore level up to the peak of Oppstadhornet. The
estimated volume of the unstable rock slope is in the
range of 10 Mm
3
(B
RAATHEN
et alii, 2004; B
HASIN
&
K
AYNIA
, 2004; D
ERRON
et alii, 2005).
It is characterized by crevasses and clefts reflect-
ing block movements in various quartzo-feldspatic
gneisses, some mica-rich and schistose (R
OBINSON
et
alii, 1997; B
LIKRA
et alii, 2001). The field is bounded
by three main structures; an upward bounding main
sliding surface and two marginal flanks to the south-
west and northeast.
The main sliding surface is clearly displayed
as an escarpment, up to 20 m high, forming a half-
graben. This surface is superimposed on the steeply
SE-dipping foliation as well as a narrow zone of cat-
aclasite and carbonate cemented fault breccia (B
RAA-
THEN
et alii, 2004). The southwestern marginal flank
strikes NW-SE and dips steeply NE. It is seen as a
fracture zone, which reactivates an older fault. This
structure is a transfer fault marked by down-oblique
sliding. In the east, the termination of rockslide is
seen as a NNW-SSE striking and steeply SW dip-
ping fracture zone, which acted (or acts) as a trans-
fer structure.
Internally, the rockslide is segmented into large
blocks by two sets of steep fractures (mainly faults),
which strike NW-SE and NE-SW. The NE-SW set is
sub-parallel to the main sliding surface and the folia-
tion in the rocks (Fig. 4). While no breakup of the two
lower blocks could be observed the uppermost block
is breaking apart along scarp-parallel cracks (Fig. 3).
Mapping indicates that the sliding surface of all three
blocks daylight on the slope above sea level (B
RAA-
THEN
et alii, 2004) (Fig. 4).
Sliding in the central and the NE upper part of
the instability is towards the SE and therefore paral-
lel to the schistosity (Figs. 5, 6c). In the central part
displacement is slower in the SW with velocities of
~2 mm/yr and faster in the NE with velocities of ~4.3
mm/yr (Fig. 5). In the NW part sliding is towards the
σ
tot.V
and σ
tot.3D
, respectively, is given (O
PPIKOFER
et
alii, 2013):
where σ
X
, σ
Y
and σ
Z
are the averages of the accura-
cies estimated by the processing software for the
entire time-series and a given point.
Robust linear regressions over the entire time se-
ries were used to calculate average yearly displace-
ment rates, v, as described in B
ÖHME
et alii (2012).
If v exceeds the errors on the displacement, σ
tot
,
divided by the time interval between the first and last
measurement, Δt (in years), then the displacements
are considered as statistically significant from a meth-
odological point of view (O
PPIKOFER
et alii, 2013):
This equation is used for horizontal, vertical or
3D displacement rates using the matching σ
tot
. Further,
all GNSS points were checked for the coherency of
the displacement trends as described in B
öHMe
et alii
(2012). Only GNSS-points with statistically signifi-
cant displacements and coherent trends are considered
as significant in this report. The displacement trend
(horizontal direction) and plunge (vertical angle)
were computed for every GNSS-point with significant
horizontal and/or vertical displacement based on the
regression results.
THE OPPSTADHORNET ROCKSLIDE
The Oppstadhornet rockslide was mapped out in
the field and it has an up to 20-m-high back scarp that
Fig. 4 - Geological model of the Oppstadhornet rockslide
(after B
raathen
et alii, 2004)
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UNDERSTANDING LONG-TERM SLOPE DEFORMATION FOR STABILITY ASSESSMENT OF ROCK SLOPES:
THE CASE OF THE OPPSTADHORNET ROCKSLIDE, NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
259
aseismic conditions while the dynamic model sug-
gested failure of the uppermost block under seismic
conditions (B
HASIN
& A
MIR
, 2004).
RESULTS
RESULTS OF
10
BE DATING
Unfortunately from the five samples taken only
three could be finally analyzed. This is due to the min-
eral composition of the sampled rock. Also all sam-
ples had visible a high enough quartz content most
samples had also a very high K-feldspar content and
the K-feldspar minerals were much larger than the
quartz minerals. After dissolution of K-feldspar un-
south and therefore oblique with respect to the schis-
tosity (Figs. 5, 6a). Displacement rates are uniform
between 2.1 and 2.7 mm/yr (Fig. 5). In the lower part
the schistosity steepens to nearly vertical and the slid-
ing surfaces cross cut the schistosity, however no sig-
nificant movement can be observed (Fig. 5).
Based upon this geological model static and dy-
namic slope stability models were calculated (B
HASIN
& A
MIR
, 2004). The seismic acceleration with a return
period of 475 years was taken as input data into the
dynamic model that was taken from the seismic zona-
tion map of Norway (N
ORSAR
& N
GI,
1998). The static
stability model indicated that the slope is stable under
Fig. 5 - Mean displacement vectors and rates after 8 years of dGNSS observations. Examples of GNSS time-series for points OT-5
and OT-11 are shown in Fig. 6
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R.L. HERMANNS , T. OPPIKOFER, H. DAHLE, T. EIKEN, S. IVY-OCHS & L.H. BLIKRA
260
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
der HF treatment only three samples had therefore a
quartz quantity that was high enough for dating. The
calculated ages are consistent with sample position as
ages decrease from top to bottom (Tab. 1).
The upper samples at sliding surfaces has an age
of ~12.5 kyrs, while the age of the lower samples are
10.3 and 6.6 kyrs, respectively. The lower sample po-
sitions lie approx. 4 m above ground, hence sliding
continued. As dGNSS data indicate that this block
continues moving we set the age for the lowermost
part of the sliding surface as 0 (just exposed).
PALEO-SLIP RATES RESULTING FROM
10
BE
AGES
Based upon the
10
Be ages obtained, we calculated
the paleo-slip rate between the position of the various
samples, as well as today's base for surface parallel
sliding in orientation of the sliding surface. In ad-
dition, we calculated an uncertainty interval that is
based upon the uncertainty of the ages. The results
indicate that the Oppstadhornet rockslide had a dis-
placement rate of 3.2±2.2 mm/yr between Opp-1 and
Opp-4, 1.5±0.4 mm/yr between Opp-1 and Opp-5,
and 1.1±0.1 mm/yr between Opp-1 and the base of
the sliding surface. These data thus suggest a decel-
eration of displacement (Tab. 2). This is also suggest-
ed when calculating the slip rate between the lower
sample locations and the base of the sliding surface
(0.6 mm/yr) (Tab. 2).
Tab. 1 - Overview of sample locations along the sliding
surface (see Fig. 2) including
10
Be ages and one
sigma uncertainties
Tab. 2 Paleo-slip rates calculated based on TCN expo-
sure ages and uncertainties
Fig. 6 - Graphs of the GNSS time series for: a) horizontal displacements of point OT-5; b) vertical vs. horizontal displace-
ments of point OT-5; c) horizontal displacements of point OT-11; d) vertical vs. horizontal displacements of point OT-
11. Uncertainties on the measured coordinates are shown as error bars. Regression lines are shown as stippled lines
for points with significant and coherent displacements. OT-5 for example has significant 3D displacements (horizontal
and vertical), while OT-11 has only significant horizontal movement
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UNDERSTANDING LONG-TERM SLOPE DEFORMATION FOR STABILITY ASSESSMENT OF ROCK SLOPES:
THE CASE OF THE OPPSTADHORNET ROCKSLIDE, NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
261
freeze and thaw cycles as well as availability of water
during glacial retreat have likely been more intensive
in the Late Pleistocene that might have resulted in the
higher displacement rates. In addition, an initial "faster"
movement could have been related to the debuttressing
of the slope and the relaxation of the rock mass due
to the decay of the ice sheet; under this hypothesis the
rockslide entered a steady state creep movement after
deglaciation. Afterwards the rockslide rather found a
kind of steady-state creep movement.
CONDITIONING OF SLIDING
Our
10
Be data from the Oppstadhornet rockslide
suggest taht the main sliding surface indicate further
more that the main sliding surface probably became ac-
tive between 16.6 and 14.2 kyr. This fits well with the
time when the island of Otrøya melted out of the de-
caying ice sheet after the last glacial maximum as sug-
gested by V
ORREN
& M
ANGERUD
(2008) in a map show-
ing the retreat of the Scandinavian Ice sheet. Following
that map Otrøya became ice free between 14.3 and 13.7
kyr, however the top of the mountains at 750 m altitude
might have melted out of the ice earlier. Therefore this
result is very similar to the Skjeringahaugane rockslide
in Lusterfjord, in the inner fjord region of Sogn og Fjor-
dane county. That area became ice free at ca. 10 kyr
and sliding started immediately at that rock slide (H
ER-
MANNS
et alii, 2012c).
Based on the
10
Be data alone it is impossible to
discuss the process that caused the initiation of sliding
whether it was glacial toe erosion, glacial debuttress-
ing or meltdown of the permafrost or a combination of
those. However, sliding of the main sliding surface is
along the schistosity while it is cross cutting the schis-
tosity in its lower part (B
RAATHEN
et alii, 2004) (Fig.
4). This suggests that sliding started from the top, sub-
sequently increasing stress on intact putting exerting
stress on intact rock bridges in the lower part of the
slope. Those might have failed during seismic loading
that might have been stronger than today due to the fast
rebound of Norway after deglaciation.
Present displacement velocities measured with
dGNSS are relatively even over the slope with 2.0-
4.3 mm/yr. In contrast is the slide direction varying
between different compartments. While only a minor
block in the upper slope slides parallel to the schistosity
most of the rock mass in the upper slope slides with
an angle 30° oblique towards the dip direction of the
ESTIMATION OF THE START OF SLIDING
BASED ON
10
BE AGES AND CALCULATED
SLIP RATES
The start of sliding can be estimated by adding the
quotient of the length of sliding above the first sample
to the age of that sample (H
eRMannS
et alii, 2012c). If
we take into account the slip rate between Opp-1 and
Opp-4 we obtain an age of start of sliding of 14.2 kyr.
Using the slip rate between Opp1 and the base of the
sliding surface results in an age of 16.6 kyr.
DISCUSSION
POTENTIAL OF THE METHOD
Today there exist multiple tools picturing the defor-
mation within and measuring the displacement veloc-
ity of rockslides including ground-based and satellite-
based remote sensing techniques and geophysical tools
describing deformation of rock mass at depth (e.g.,
G
ANERØD
et alii, 2008; L
AUKNES
et alii, 2010, O
PPIKOFER
et alii, 2011). However, these tools do not allow us to
understand the rate of deformation in the past and to
understand a rockslide in its evolution over time. One
possibility to understand the development of rockslides
over time is trenching of disturbed soils on the rockslide
in combination with dating of deformed soil horizons
(G
UTIÉRREZ
et alii, 2010). However at rockslides, where
no soil has formed along the sliding surfaces, TCN dat-
ing is the only tool to understand rockslide development.
TIMING OF SLIDING AND SLIDING RATES
Our
10
Be ages of the main sliding surface indicate
that ~11 m of displacement took place within the Pleis-
tocene, while only 6.2 m of displacement took place
in the Holocene, suggesting a decrease of displacement
rate. Unfortunately, is the analytical uncertainty mar-
gin on the age from the sample that dates the end of
the Pleistocene, is very high so that all slip rates cal-
culated with the upper sample are indistinguishable.
However, also the comparison of the slip rate calculated
for the upper and the lower samples suggest a reduc-
tion of the paleo-slip rate in the Holocene. This might
have been related to the climatic difference between
the Late Pleistocene and the Holocene. Although we
cannot penetrate with our few data into the temporal
fluctuation of the climatic changes at the end of the
Late Pleistocene it is obvious that climatic shifts such
as the Younger Dryas have been much more dramatic
than climatic variability within the Holocene. Therefore
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R.L. HERMANNS , T. OPPIKOFER, H. DAHLE, T. EIKEN, S. IVY-OCHS & L.H. BLIKRA
262
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
rock slope failures that might have large economic con-
sequences or might cause a significant loss of life more
elaborate geological models are needed to improve the
dynamic stability models. In addition, including long
lasting progressive failures and sliding into improved
stability models would allow imaging slope stabilty
conditions better.
Therefore we postulate based on our longterm slip
rates that the slope is likely more stable under seismic
loading with a recurrence of 475 years than previously
estimated.
CONCLUSION
Although our
10
Be ages are limited they indicate
that the Oppstadhornet rockslide became active when
the island of Otrøya melted out of the Scandinavian
ice sheet and continued to be active through the Late
Pleistocene and the Holocene. The data further sug-
gest that sliding was faster in the Late Pleistocene and
that the rate slowed down in the Holocene. Today's
displacement rates obtained from dGNSS coincides
well within uncertainties with paleo-slip rates on the
main sliding surface. Finally these data suggest that
the slope is more stable than suggested by dynamic
stability models and that the next earthquake with 475
years recurrence interval will similar to earlier such
events not cause the slope to fail.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support
of the excellence centre "International Centre for Geo-
hazards" that initially brought Reginald Hermanns to
work in Norway. The autors also want to acknowledge
the GeoForschungsZentrum Potsdam that financed
Reginald Hermanns position in 2003 when the sam-
ples were taken and for processing at ETH laborato-
ries. In the past years the Norwegian Water and En-
ergy Directorate financed mapping of unstable rock
slope in Møre og Romsdal County.
schistosity. This change of slide direction goes together
with the inclination of sliding suggested by our dGNSS
data. While the upper part is sliding with the same dip
as the orientation of schistosity (and the sliding surface)
the lower part is sliding with a less inclined angle and
cross cutting the schistosity (Figs. 5, 6a). This could
be explained by conditioning for sliding in the upper
slope by slope parallel debuttressing and by glacial toe
erosion in the lower slope. Furthermore, this could in-
dicate that today's displacement direction on the main
sliding surface is different from the Pleistocene due to
the drop of velocity over the slope and that today the
blocks are moving 30° oblique to the dip direction of
the schistosity. This would be an alternative explana-
tion that slide velocity apparently has decreased. If the
slide would have changed from schistosity parallel to
oblique to schistosity the full slip rate would not result
in length of surface exposed on the sliding plane since
reorientation of sliding direction.
IMPLICATION OF SLIDING HISTORY ON
THE STABILITY MODEL
Our
10
Be ages also help to evaluate the dynamic
stability model calculated by B
HASIN
& A
MIR
(2004).
Those authors postulate that the slope should fail under
seismic shaking that could be expected in relation with
an earthquake with a recurrence period of 475 years.
However, seismic accelerations as used in the dynamic
slope stability model have occurred repeatedly in the
past and stronger earthquakes with larger recurrence
times also did not cause the slope to fail in the past thou-
sands of years. This might be due to the continuous slip
over 15 kyr, which just in the past hundred years lead to
conditions that the rock may fail under seismic loading
today. However, we rather suspect that the geological
models for the Oppstadhornet rockslide are simplifica-
tions of the real geological conditions. Parameters such
as hydrological conditions and the number of intact
rock bridges can only be estimated. Hence for those
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UNDERSTANDING LONG-TERM SLOPE DEFORMATION FOR STABILITY ASSESSMENT OF ROCK SLOPES:
THE CASE OF THE OPPSTADHORNET ROCKSLIDE, NORWAY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
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