# ijege-13_bs-tecca-et-alii.pdf

*Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università*

*Editrice*

*DOI: 10.4408/IJEGE.2013-06.B-17*

**FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF**

**RUNOUT AND DEPOSITION OF LA MAROGNA ROCKSLIDE**

**(VICENZA, ITALY)**

**K**

**ey**

**words***rock slide/avalanche, FLAC, UDEC, dynamic*

*analysis*

**INTRODUCTION**

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.

the discontinuities (roughness, wall strength and per-

sistence) and on the weathering action on the intact

rock and discontinuities.

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.

reaching implications for the safe development of both

inhabited alpine valleys and engineering projects.

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**

their development, so that consequences and possible

prevention and mitigation actions can be envisaged.

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

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.

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.

crease results in the instability of a rock mass limited

at its bottom by both bedding and a pre-existing dis-

continuity.

*P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ*

*International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013*

mass movement.

**GEOLOGY AND GEOMORPHOLOGY**

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

m long and 160-180 m high.

scarp, where the beds dip 30°-35° as a mean (Fig. 2).

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.

the upper thicker layers of the “Dolomia Principale”,

avalanche (“La Marogna” rock avalanche), was trig-

gered by the earthquake occurred on 1117.01.03 (I0

IX MCS, M 7.0) (M

*et*

*alii*, 2005) or the sliding took place just during the de-

glaciation most-unstable situation.

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

*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*

**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*

ing strength parameters.

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

*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 .

to rock slope analysis.

**ESTIMATION OF ROCK MASS AND**

**JOINTS PROPERTIES**

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

shown in Tab. 1.

planes are prevailingly undulating and less rough. The

residual friction angle (φ

discontinuities have been back-calculated from data on

deformability of both intact rock and rock mass (i

been derived using the corrections proposed by b

moduli were calculated on the basis of rock mass defor-

mation modulus and a Poisson coefficient of 0.23.

ible just down-hill (Fig. 2) (Z

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°.

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

sistent with the 1117.01.03 earthquake (b

*et alii*,

5.5x10

collapsed rock volume is about 13

**NUMERICAL MODELING**

compute the interaction between different materials,

site geometry and wave propagation in case of seismic

inputs (b

*et alii*, 2004).

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-

*P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ*

*International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013*

are listed in Tables 2, 3 and 4.

**STABILITY ANALYSES**

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*

**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*

with a seismic event, continuum analysis have been

carried out in dynamic conditions.

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.

to propagate upwards and, for simplicity, isotropic

conditions have been assumed. The input variables

for dynamic analysis have been calculated from the

expressions (b

*max*

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.

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**

continuum numerical methods have been preferred to

preliminarily simulate the mass structure behavior sub-

jected to quasi-static or dynamic loading.

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).

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).

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.

*Fig. 4 - Static continuum analysis with a water table: hor-*

*izontal displacements distribution and plasticity*

*indicators*

*P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ*

*International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013*

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).

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**

analyses, in both static and dynamic conditions, were

carried out using the two-dimensional distinct element

code UDEC (Z

*et alii*, 1997).

*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)*

**FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND**

**DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)**

*Editrice*

ing model for bedding and tectonic discontinuities.

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.

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.

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%.

seismic parameters for the dynamic analysis are dis-

played in Tab. 5.

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).

bedding planes and cross fractures, are represented

in the model.

In dynamic conditions, in fact, the mesh size is con-

trolled by the shortest wavelength of input fluctuation

(i

*Δl*

of seismic wave λ. The highest frequency of the input

wave

*f*

*max*

*V*

*s*

that the maximum input frequency of considered dy-

namic loads is 5 Hz.

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.

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

vertical joints and Coulomb Slip model for the tectonic

discontinuities; vi) in dynamic analysis, Coulomb Slip

*Fig. 8 - Static stability analysis*

*P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ*

of normal load and frictional resistance activation on

the sliding surface and at the blocks contacts.

**DISCUSSION AND CONCLUSIONS**

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.

isting anticlinalic fold and, apparently, partly with a long

thrust cutting the upper portion of the same anticline.

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.

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 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.

(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)*

**FIELD STUDY AND BIDIMENSIONAL NUMERICAL SIMULATION OF RUNOUT AND**

**DEPOSITION OF LA MAROGNA ROCKSLIDE (VICENZA, ITALY)**

*Editrice*

circumstance is confirmed by the different values and

trend of the velocities registered at the surface ground

monitored points (Fig. 11).

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.

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.

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.

scarp and, so, stressing the necessity for accurate and

detailed structural geology surveys in order to repro-

duce the real landslide formation mechanism.

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.

and input frequency is included between 0.13 and

0.26, that is generally considered critical for seismic

amplification (d

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.

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*

*P.R. TECCA, R. GENEVOIS, A.M. DEGANUTTI & M. DAL PRÀ*

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