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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
201
DOI: 10.4408/IJEGE.2013-06.B-17
FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF
RUNOUT AND DEPOSITION OF LA MAROGNA ROCKSLIDE
(VICENZA, ITALY)
P
ia
R
osella
TECCA
(*)
, R
inaldo
GENEVOIS
(**)
,
a
ndRea
M
aRia
dEGANUTTI
(*)
& M
aRco
DAL PRÀ
(***)
(*)
CNR - IRPI - Padua, Italy
(**)
Università degli Studi di Padova - Dipartimento di Geoscienze - Padua, Italy
(***)
Professional geologist
K
ey
words
: rock slide/avalanche, FLAC, UDEC, dynamic
analysis
INTRODUCTION
Catastrophic rockslides, damming valleys with a
subsequent dam failure, have always occurred world-
wide. The relevance of this process as a natural hazard
stresses the need for a more complete knowledge of
both the triggering mechanism and the prediction of
life span of rockslide dams.
Rock slope stability depends on the strength of the
rocks, the geometrical and strength characteristics of
the discontinuities (roughness, wall strength and per-
sistence) and on the weathering action on the intact
rock and discontinuities.
Since a rock mass is not a continuum, its behavior
is dominated by discontinuities such as faults, joints
and bedding planes. In general, because the presence
or absence of discontinuities has a great influence on
the stability of rock slopes, their behavior plays a criti-
cal role in a stability evaluation.
Understanding the factors leading to the devel-
opment of rock slopes instability and failure has far
reaching implications for the safe development of both
inhabited alpine valleys and engineering projects.
In this paper, the finite difference (FLAC) and
the distinct element (UDEC) codes have been used
to model the evolution of a rock slope tought to have
failed in response to a seismic shaking. The numeri-
cal simulation is aimed, then, to explain whether the
ABSTRACT
The study of ancient major rock slope instabilities
may help in the detection of the conditions leading to
their development, so that consequences and possible
prevention and mitigation actions can be envisaged.
In this paper, numerical studies have been carried
out to recognize the behavior of a rock slope and the
kinematics of a rock slide/avalanche in the north-east-
ern Italian Alps. The “La Marogna” rock avalanche, in
the Vicenza Province (Venetian Pre-Alps, North-East-
ern Italy), with a volume of about 17
x
10
6
m
3
, still par-
tially dams the narrow valley of the Astico River. Geo-
morphological investigations highlight that the whole
rock avalanche mass is formed by two distinct overlap-
ping bodies and that apparent poor stability conditions
characterize the slope above the present main scarp.
In order to get indications about triggering factors
and present stability conditions, a representative engi-
neering geological model has been built and analyses of
the triggering conditions have been performed using the
bi-dimensional continuum (FLAC) and discontinuum
(UDEC) codes UDEC on the re-constructed original
slope profile. Different situations have been simulated
for gaining a better understanding of the effect of static
and dynamic loads on the modeled rock slope.
The numerical results indicate that the effect of a
contemporary dynamic loading and joint friction de-
crease results in the instability of a rock mass limited
at its bottom by both bedding and a pre-existing dis-
continuity.
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P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ
202
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
a
daMi
(2013, this volume) in order to identify large-
scale instability structures that might have induced the
mass movement.
GEOLOGY AND GEOMORPHOLOGY
“La Marogna” rock avalanche (Fig. 1), detached
from the steep-sided right slope of the Astico River Val-
ley, with a local NW-SE trend. The slope is composed of
the Mesozoic formation of “Dolomia Principale” char-
acterized by well-bedded carbonates. This slope belongs
to the northern limb of an anticline fold so that the bed-
ding dip, NNW trending, increases progressively from
20°-25° at the slope top to about 50° at the bottom (Dal
Prà, 1992). The fold limb is affected in the higher part of
the slope by a normal fault, dipping 70° ENE, by vertical
persistent tectonic lineaments and by some trenches not
directly correlated to the instability phenomenon (Z
aMP
-
ieRi
& a
daMi
, 2013; this volume). The upper part of the
slope is characterized by a sub-vertical scarp, about 600
m long and 160-180 m high.
The rock avalanche run along the anticline limb and
the sliding surface is well exposed at the base of the main
scarp, where the beds dip 30°-35° as a mean (Fig. 2).
The anticlinalic structure is well exposed on the
300 m high East facing cliff (Fig. 2), where it is also
possible to note the presence of a NNW dipping thrust
(“b” in figure 2) with a “stair case” geometry: steep
reverse fault tracts in stiff layers connect flat tracts
sub-parallel to the bedding.
It is worthwhile to note that the lower flat tract,
located at the transition between the lower thinner and
the upper thicker layers of the “Dolomia Principale”,
rock slope collapse, with the typical features of a rock
avalanche (“La Marogna” rock avalanche), was trig-
gered by the earthquake occurred on 1117.01.03 (I0
IX MCS, M 7.0) (M
addalena
, 1906; G
uidoboni
et
alii, 2005) or the sliding took place just during the de-
glaciation most-unstable situation.
“La Marogna” rock avalanche, located in the As-
tico Valley (Venetian Pre Alps, North-Eastern Italy),
has been analyzed both in static and dynamic condi-
tions to investigate the trigger mechanism and provid-
ing the possibility to evaluate the stability conditions
of the present slope. The geological and structural set-
ting of the area has been investigated by Z
aMPieRi
&
Fig. 1 - Geological schematic map of the “La Marogna”
rock avalanche
Fig. 2 - East facing cliff. a : anticlinalic structure ; b :
main thrust ; c : minor thrust’s element; d: dip of
the outcropping sliding surface
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FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND
DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
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203
tinuum code FLAC (i
tasca
, 2012) mainly investigat-
ing strength degradation leading to failure and calibrat-
ing strength parameters.
Continuum models, however, do not give reli-
able information on internal displacement field and
discontinuity-controlled failures. Discontinuum
modeling approach explicitly simulates the geo-
logical structure treating the problem domain as an
assemblage of independent units, corresponding to
blocks formed by the intersection of joints (l
oRiG
et alii, 1991). The basic difference with continuum-
based methods is that contacts between blocks are
continuously changing with the deformation process.
A more realistic response can be, then, modeled and
the specific failure mechanism, controlled by pre-
defined discontinuities, may be captured .
Discontinuum modeling, such as the distinct ele-
ment 2D-code UDEC (i
tasca
, 2011) probably consti-
tutes the most commonly applied numerical approach
to rock slope analysis.
ESTIMATION OF ROCK MASS AND
JOINTS PROPERTIES
A number of in situ tests and some laboratory
tests have been performed to set representative mean
values of the properties of rocks and discontinui-
ties. Uniaxial compressive tests were conducted in
laboratory, while joint parameters JRC0 and JCS0
were estimated by field Barton roughness profiles
and Schmidt hammer tests in 8 and 140 kPa respec-
tively. Values of the rock mass strength and defor-
mation parameters were obtained using the RocLab
program (H
oek
, 2007). Mechanical properties values
of rocks and intact and weathered rock masses are
shown in Tab. 1.
Observed discontinuities are predominantly unal-
tered. Sub-vertical joints are mostly rough while bedding
planes are prevailingly undulating and less rough. The
residual friction angle (φ
r
) has been estimated respec-
tively to 30° and 28°. Approximate stiffness values of
discontinuities have been back-calculated from data on
deformability of both intact rock and rock mass (i
tasca
,
2011). The scale corrections for in situ block sizes have
been derived using the corrections proposed by b
aRton
& b
andis
(1990), obtaining corrected JRC and JCS val-
ues, respectively 6 and 65 MPa. Shear (G) and bulk (K)
moduli were calculated on the basis of rock mass defor-
mation modulus and a Poisson coefficient of 0.23.
represents the up-hill prosecution of the bedding vis-
ible just down-hill (Fig. 2) (Z
aMPieRi
& a
daMi
, 2013,
in this volume). A secondary shorter tectonic structure,
indicated with “c” in Fig. 2, is present. Three main sub-
vertical joint systems cross the rock mass, trending ap-
proximately N310°-320°, N230° and N260°-270°.
The whole rock avalanche deposit is formed by
two distinct overlapping bodies: the former develops
from the base of the main scarp to the valley bottom. It
is covered by a fan-shaped body rising up the opposite
slope for about 40 m, due to a later process with the
features of a rock/debris flow. The deposits dammed the
Astico River, and the resulting lake should have drained
in a very short time, since no lacustrine sediments have
been found upstream. The deposit is formed by sands,
gravels and pebbles with blocks from 1 to several tens
m
3
and no remarkable granulometric differences are
shown by the two bodies. Two C
14
datings of timbers,
collected at the base of the landslide deposit, are con-
sistent with the 1117.01.03 earthquake (b
aRbieRi
et alii,
2007) to which the rock avalanche is attributed.
The total volume of the landslide deposit re-
sults to be in the order of 16.9
x
10
6
m
3.
The volume
of the first phenomenon has been calculated in about
5.5x10
6
m
3
, while the later one in about 11.4x10
6
m
3
.
Assuming a bulking coefficient of 0.30, the initial
collapsed rock volume is about 13
x
10
6
m
3
, split be-
tween the two slides respectively in 4.2
x
10
6
m
3
and
8.8
x
10
6
m
3
.
NUMERICAL MODELING
Numerical models, both continuum and discon-
tinuum, represent an alternative effective method to
compute the interaction between different materials,
site geometry and wave propagation in case of seismic
inputs (b
ouissou
, 2012; b
oZZano
et alii, 2004).
In the equivalent continuum models for jointed
rock masses (s
inGH
, 1973a,b; G
oodMan
, 1976; G
eR
-
RaRd
, 1982), usually referred to as compliant joint
models or ubiquitous joint models, the original dis-
continuous material is replaced by a hypothetical con-
tinuous material using a homogenization technique.
When coupled with a discontinuum modeling, two-
dimensional continuum modeling may be used to pre-
liminarily examine stress distribution and evolution,
possible phases of stress-induced progressive failure,
and plastic yielding within the rock mass. As such, a
preliminary set of 2-D models were run using the con-
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204
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
The mechanical properties of rocks, ice and joints,
used in the static and dynamic discontinuum modeling,
are listed in Tables 2, 3 and 4.
STABILITY ANALYSES
Both continuum and discontinuum stability
analyses have been carried out as follows: 1) the
original slope has been re-drawn on the basis of the
surrounding morphology and the calculated land-
slide volume; 2) the stresses have been initialized
considering the existence of a 800 m high glacier
progressively reducing its height; 3) in continuum
modeling, ubiquitous-joint and strain-hardening/
softening ubiquitous-joint models have been consid-
ered; 4) in discontinuum modeling, Mohr-Coulomb,
Coulomb slip and continuously yielding models
have been considered; 5) seismic loading has been
Tab. 3 - Rock material properties and models for discontinuum analyses
Tab. 4 - Joints properties and models
Fig. 3 - Static continuum analysis: displacement vectors
in dry conditions and constitutive models. Ubiqui-
tous joint model: grey; Strain softening ubiquitous
joint model: blue
Tab. 2 - Continuum stability analysis parameters and models
Tab. 1 - Initial strength parameters of rock and rock masses. ρ: density; σ
ci
: uniaxial compressive strength; E: elastic modulus; c:
cohesion; φ: friction angle; t: tensile strength
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FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND
DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
205
and the plausibility of a slope collapse in conjunction
with a seismic event, continuum analysis have been
carried out in dynamic conditions.
On the basis of the macroseismic field (s
eRva
,
1990), the 03/01/1117 earthquake intensity has been
estimated in VIII-IX MCS, that is a magnitude of
M=6-7, and the corresponding PGA (Peak Ground
Acceleration) considered equal at least to 0.32g with
frequencies from 2 to 5 Hz. However, the Italian Risk
Map (INGV, 2007) shows, in the same area and for
an excedence probability of 1% in 50 years, a PGA
of 0.25g. The analyses have been, then, performed
considering two values of the maximum acceleration
amax : 0.25g and 0.32g. Using existing empirical cor-
relations, the duration of the seismic loading has been
evaluated in a minimum of 8 seconds.
The dynamic input has been provided applying a
sinusoidal shear wave to the base of the model, free
to propagate upwards and, for simplicity, isotropic
conditions have been assumed. The input variables
for dynamic analysis have been calculated from the
expressions (b
Hasin
& k
aynia
, 2004; i
tasca
, 2012):
where V
max
is the ground motion velocity (m/s), 2pf
the angular frequency, V
s
the shear wave velocity
(m/s) and τ the shear stress, doubled to compensate
for viscous boundaries. The sinusoidal shear wave has
been applied for different periods (8, 12, 24 s) repre-
senting cycles of motion from 16 to 120. Values of
input seismic parameters are summarized in Tab. 5.
Examination of the results obtained with a dura-
simulated applying a sinusoidal stress wave (contin-
uum model) or a sinusoidal velocity wave (discon-
tinuum model), whose frequencies and intensities
have been selected on the basis of the Italian Seismic
Risk Maps (INGV, 2007).
CONTINUUM MODELING
Considering the observed relationship between
rock mass anticlinalic structure and sliding surface,
continuum numerical methods have been preferred to
preliminarily simulate the mass structure behavior sub-
jected to quasi-static or dynamic loading.
Two-dimensional modeling was carried out using
the finite-element code FLAC 7.0 (i
tasca
, 2012), that
is able to efficiently model progressive and time-de-
pendent failure mechanisms. The model incorporated
837 quadrilateral elements and the grid has been drawn
to fit the existing anticlinalic structure. An elasto-plas-
tic constitutive criterion has been assigned to the slope
materials assuming a Mohr-Coulomb yield criterion;
stresses have been initialized assuming a gravity load-
ing and an elastic homogeneous isotropic rock mass.
Before the removal of the glacier, an ubiquitous-joint
model and a strain-hardening/softening ubiquitous-
joint model have been assigned respectively to the base
rock mass and to the rock collapsing mass, to take ac-
count of both material anisotropy and the continuously
changing dip of bedding (Fig. 3).
Strength parameters values have been initially set
to those for intact rocks (Tab. 1). The progressive deg-
radation of rock mass strength with stress variation and
time has been simulated by gradually decreasing the
values of rock properties up to reaching the calculated
properties values of weathered rock mass (Tab. 2).
In dry conditions, the slope is completely stable:
small displacements are essentially restricted to the
steeper parts of the reconstructed slope (Fig. 3). In
case of the presence of a groundwater with a maxi-
mum height of 10 m over the indicated shear sur-
face, the rock slope is still stable but, at the base and
at the top of the potentially collapsing mass, finite
slips along the ubiquitous joints develop (Fig. 4). In
conclusion, the examination of the 2-D finite-element
results in static conditions shows that a condition of
instability cannot be reached, even considering the
presence of a perched water table in the potentially
collapsing mass.
In order to investigate the triggering mechanism
Fig. 4 - Static continuum analysis with a water table: hor-
izontal displacements distribution and plasticity
indicators
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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
tion of 8 seconds (test n. 6) shows that (Fig. 5): i)
maximum horizontal displacements, greater than 6.0
m in the steeper part of the slope, propagate towards
the higher part of the slope, being maximum at the
toe and approximately null at a distance of 550 m;
ii) slips along ubiquitous joints are distributed in all
the collapsing mass; iii) the upper end of the sliding
mass is limited by zones at failure in tension. The in-
stability condition is shown by the horizontal veloci-
ties progressively increasing with time, monitored at
selected points on the shear surface (Fig. 6).
In conclusion, the dynamic simulation indicates
that an earthquake with the characteristics of that oc-
curred in 1117 A.D. might have triggered the "La
Marogna" rock avalanche but, with continuum mod-
eling, the shape of the collapsed mass does not com-
pletely reflect the present morphology of the slope: the
slip surface follows the local maximum rock strata dip,
but the simulated sliding mass extends far beyond the
location of the present landslide scarp. In any case, the
examined rock slope seems to be more sensitive to the
seismic frequency than to the seismic acceleration, at
least considering a dynamic time of 8 seconds.
DISCONTINUUM STABILITY ANALYSES
In order to investigate the control of pre-defined
discontinuities on slope failure, discontinuum stability
analyses, in both static and dynamic conditions, were
carried out using the two-dimensional distinct element
code UDEC (Z
HanG
et alii, 1997).
The geometry of the joints has been generated ac-
cording to statistical data collected by field measure-
Fig. 7 - Discretization of the blocks into deformable finite-
difference zones and location of the monitored
points PT
Fig. 6 - x-velocities vs dynamic time at monitored points
PT 6, PT 7 and PT 8
T
ab. 5 - Seismic parameters used in continuum modeling dynamic analyses a
max
: peak ground acceleration; V
max
: ground mo-
tion velocity; τ: shear stress
Fig. 5 - x-displacements contours, monitored points PT
and plasticity indicators for dynamic conditions
(a=0.32g, f=5 Hz, duration 8 s, τ=587 KPa). Lo-
cation of the present landslide scarp (red arrow)
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FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND
DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
207
model for sub-vertical joints and Continuously Yield-
ing model for bedding and tectonic discontinuities.
UDEC modeling has been performed through the
following steps: i) the model has been elastically equili-
brated under the gravity force with the obtained rock
parameters values (Tab. 3); ii) the load due to ice sheets
has been applied to the model and later removed in suc-
cessive stages; iii) static and dynamic analyses have been
performed with rock and discontinuities properties and
models shown in Tables 3 and 4 respectively.
In the static condition the slope resulted to be sta-
ble considering both the persistent tectonic discontinu-
ity (“b” in Fig. 2) and the shorter one (“c” in Fig. 2):
computed horizontal displacement rate induced by ice
sheets removal is null. The results diverge only as re-
gards the distribution of the calculated displacements:
in the former case they develop over the persistent tec-
tonic discontinuity up to the top of the modeled slope,
while in the last case, they are limited upslope in cor-
respondence of the present location of the landslide
crown (Fig. 8), representing in a better way the future
collapsing mass.
The dynamic analysis has been performed apply-
ing to the base of the model a S-wave with a peak
amplitude of 0.08, 0.1, 0.13, 0.16 m/s (acceleration of
0.25 g and 0.32 g) at 3 and 5 Hz for 24 and 40 cycles
(duration of dynamic input of 8 seconds). Frequency,
amplitude and accelerations values have been selected
as those used in the continuum modeling dynamic
analysis. Free-field boundaries are invoked along
the left and right boundaries to absorb energy and no
displacement is allowed in the x-direction along the
lateral sides of the model. The Rayleigh damping ra-
tio, which reproduces the energy losses in the natural
system when subjected to dynamic loading, has been
assumed to equal 2%.
Analyses have been performed starting with the
initial static equilibrium condition of the slope. the
seismic parameters for the dynamic analysis are dis-
played in Tab. 5.
The dynamic analysis of the slope featuring the
longest tectonic discontinuity (“b” in Fig. 2), 60 sec-
onds after the end of the dynamic input (test 2), shows
that the slope is entirely unstable. However, the col-
lapsing mass does not match the field observation:
significant displacements involve also the upper part of
the slope, beyond the present location of the landslide
crown (Fig. 9).
ments. Basically, three sets of joints, that include the
bedding planes and cross fractures, are represented
in the model.
The dimension of blocks has been established
considering that dynamic analyses will be carried out.
In dynamic conditions, in fact, the mesh size is con-
trolled by the shortest wavelength of input fluctuation
(i
tasca
, 2011). Thus the maximum size of the grid Δl
should be less than (1/10÷1/8) the shortest wavelength
of seismic wave λ. The highest frequency of the input
wave f
max
, which cannot make waveform distorted, can
be written as:
where V
s
is the shear wave velocity (m/s). The grid size
of the discrete elements has been selected considering
that the maximum input frequency of considered dy-
namic loads is 5 Hz.
The slope was divided into blocks and the interior
region was discretized into 722 fully deformable trian-
gular elements. The base is assumed to be flexible but
the boundary is fixed in the y-direction. Figure 7 dis-
plays the model of the jointed slope and the ice sheets.
The analyses have been performed considering: i)
fully persistent and interconnected discontinuities; ii)
presence of a persistent tectonic discontinuity (“b” in
Fig. 2) and a shorter and steeper discontinuity (“c” in
Fig. 2); iii) material properties deriving from the con-
tinuum modeling analyses; iv) glaciation and deglacia-
tion processes have been modeled after the application
of the rock mass properties values (Tab. 4, Static analy-
sis), in order to consider the modification of stresses
in the transition from glacial to non-glacial conditions
(I
veRson
, 2012); v) in static analysis, Mohr-Coulomb
constitutive model for rock material, bedding and sub-
vertical joints and Coulomb Slip model for the tectonic
discontinuities; vi) in dynamic analysis, Coulomb Slip
Fig. 8 - Static stability analysis
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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
velocities that could be the consequence of the variation
of normal load and frictional resistance activation on
the sliding surface and at the blocks contacts.
DISCUSSION AND CONCLUSIONS
This paper presents the numerical study of the “La
Marogna” rockslide, probably earthquake triggered,
based on geological observations, field data, and labora-
tory tests. In order to analyze the influence of geological
and seismic factors on slope failure, numerical simula-
tions were performed using both continuum (FLAC)
and discontinuum (UDEC) bi-dimensional approaches.
The detailed geological investigation has shown that
the sliding surface coincides partly with the dip of the ex-
isting anticlinalic fold and, apparently, partly with a long
thrust cutting the upper portion of the same anticline.
The initial estimate of the impact of geological set-
ting (anticlinalic structure and long thrusts) and geome-
chanical characteristics on slope stability was made un-
der static conditions. Both continuum and discontinuum
modeling revealed the stability of the slope with the
strength parameters values obtained by field and labora-
tory tests and realistic water pressures on sliding surface.
However, simulation results do not match satis-
fyingly the morphology of the present slope: the real
uphill extension of the collapsed rock mass may be
simulated only considering the existing secondary
tectonic discontinuity, shorter and more tilted than the
more evident and long one. The discontinuum approach
simulates effectively this discontinuity, showing that
The analyses have been done again considering the
observed shorter discontinuity (“c” in Fig. 2).
The sequence of displacements at different times
after the end of the dynamic loading (Fig. 10) indicates
the slope instability but, in this case, the collapsing mass
matches rather well the present slope geometry. The dis-
placement dynamics, highlighted also by the amplifica-
tion of blocks deformation, shows that the collapsing
mass is split in two main parts as indicated by the sub-
vertical joint progressive aperture, the most upward part
being characterized by a delayed dynamics.
However, after 60 s from the end of the seismic
shaking, the two blocks move with the same velocity
(monitored points PT 2 and PT 3, Fig. 11). It is also
worthwhile to note the pulsating trend of the horizontal
Fig. 10 - Sequence of block deformation 5-times amplified and displacement vectors in the unstable rock mass 0 sec (a), 5 sec
(b), 10 sec (c), 30 sec (d) and 60 sec (e) after the end of dynamic
loading
Fig. 9 - Block deformation 5-times amplified and dis-
placement vectors 60 sec after the end of dynamic
loading (a: 0.32g; f: 5 Hz)
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FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND
DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)
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209
slowly, separating in this way from the former one. This
circumstance is confirmed by the different values and
trend of the velocities registered at the surface ground
monitored points (Fig. 11).
It is possible, then, to suppose that the earthquake
triggered the front part of the rock mass that slid down
as a rock avalanche and deteriorated the rear part that,
however, remained on site. Only later this rock mass
was destabilized probably by an increase of pore pres-
sure due to heavy rain, so accounting for the fan shape
of its deposit. This hypothesis seems to be validated
also by the observation that the first phenomenon is
smaller than the later one and this results also by the
numerical simulation.
This sequence of events must be checked consider-
ing: i) different initial morphologies of the slopes; ii)
the different nature of the two parts constituting the
sliding surface: the tectonic discontinuity in the upper
part and the bedding planes in the lower one; iii) the
non uniform distribution of the shear resistance on the
sliding surface, due to the different characteristics of
these two parts and to the presence of rock bridges at
the transition between them; iv) the presence of a real-
istic groundwater level in the rock mass.
The study is likely to better estimate possible future
seismic landslides on the upper slope and run outs, that
represents the major risk factor for the valley bottom
villages: type of earthquake source and site conditions
need to be, then, better understood both from field evi-
dence and modeling.
the rockslide detaches in correspondence of the present
scarp and, so, stressing the necessity for accurate and
detailed structural geology surveys in order to repro-
duce the real landslide formation mechanism.
Dynamic analyses, carried out considering historic
data and seismic characteristic of the region, showed
that the shear resistance of the controlling slip surface
may be exceeded by the shaking-induced inertial forces
due to, at least, a medium intensity earthquake, that is
lower than that (M=7.0) of the 1117 A.D. earthquake.
The duration of the seismic loading seems to be not so
relevant as collapse may be modelled with a seismic
shaking duration of only 8 s. In general terms, the slope
collapse occurs for source frequencies in the range of
those that may be attributed to this earthquake and large
continuous post-seismic displacements develop with
increasing velocities. Maximum horizontal velocities at
the surface of the landslide body reach 0.6 m /s after 60
s from the end of the seismic loading (Fig.10) increas-
ing, then, abruptly so that “La Marogna” rockslide may
be categorized as a high-speed landslide.
For the studied slope height (160-180 m) and in-
put frequencies (3 and 5 Hz) the ratio of slope height
and input frequency is included between 0.13 and
0.26, that is generally considered critical for seismic
amplification (d
Hakal
, 2004).
The results of numerical analyses show that, in
case of rock slopes, the discontinuum approach may
be considered more reliable, but it requires more accu-
rate and detailed field surveys that not always can be
easily and completely obtained. Continuum approach
is, on the other hand, useful in order to preliminar-
ily simulate the mass structure behavior subjected to
quasi-static or dynamic loading and analyze the cor-
responding stresses distribution.
However, while the triggering mechanism of the
rockslide has been correctly reproduced, the dynamics
is the main object of coming researches. As a matter
of facts, the field observation indicates the presence of
two different landslide deposits attributable to differ-
ent processes, while the numerical simulation shows a
unique sliding phenomenon. It should be highlighted,
nevertheless, that the sequence of displacements fol-
lowing the seismic loading (Fig. 10) displays a rock
mass that, before the general collapse (Fig. 10 e) is bro-
ken up in two parts with different dynamics: the small-
er front part, where the reconstructed slope is steeper,
moves earlier, while the larger rear one moves more
Fig. 11 - Test n. 2: time histories of horizontal velocities at
monitored points PT 1, PT 2, PT 3. First 8 seconds
refer to the seismic loading
background image
P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ
210
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
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