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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
315
DOI: 10.4408/IJEGE.2013-06.B-29
LANDSLIDE RISK REDUCTION BY COUPLING MONITORING
AND NUMERICAL MODELING
F
rancesca
BOZZANO
(*)
, I
van
CIPRIANI
(*)
, c
arlo
ESPOSITO
(*)
,
s
alvatore
MARTINO
(*)
, P
aolo
MAZZANTI
(*)(**)
, a
lberto
PRESTININZI
(*)
,
a
lFredo
ROCCA
(*)
& G
abrIele
SCARASCIA MUGNOZZA
(*)1
(*)
Sapienza Università di Roma - Dipartimento di Scienze della Terra e Centro di Ricerca CERI
Piazzale A. Moro, 5 - 00185 Rome (Italy)
(**)
NHAZCA S.r.l., spin-off Sapienza Università di Roma, Via Cori snc - 00177 Rome (Italy)
lution referred to slope instability episodes triggered
by different external factors (e.g. rainfalls, tunneling,
etc) made it possible to better understand the dynam-
ics of the slope-infrastructure system and to refine
the numerical model of the slope by using the finite
difference code FLAC 7.0. Such a numerical model
was implemented by applying a continuum equivalent
approach to the involved jointed rock mass, which
was considered as a visco-plastic material in order to
account for the time dependent behavior. At present
promising results have been obtained, especially in
terms of assessment of stress-strain variations due to
external forces (both environmental and man-induced)
and, thus, of forecasting the activation/reactivation of
slope instabilities.
K
ey
words
: engineering-geology model, monitoring, nume-
rical modeling
INTRODUCTION
The importance of designing structures and infra-
structures on the basis of a robust engineering-geolo-
gy model of a slope is a concept well established in
the scientific and technical community, especially if
facing on large infrastructures (see the “milestones”
such as the observational approach proposed by T
er
-
zaGhI
& P
eck
, 1967 and the papers by F
ooke
, 1997;
h
oek
, 1999 and h
utchInson
, 2001; G
Ibson
& c
howd
-
hury
, 2009). The engineering-geology model is in fact
a fundamental informative layer to understand and
ABSTRACT
1
The importance of the reference engineering-geol-
ogy model of a slope is a concept well established in the
scientific and technical community facing on large in-
frastructures. The engineering-geology model is in fact
a fundamental informative layer to understand and pre-
dict the structure-slope interactions and to design stabi-
lization countermeasures. Such an issue has a relevant
role in the case of unstable slopes: at this regard the
Vajont case history represents a worldwide reference.
Engineering-geology models can be validated
and/or updated by monitoring data. Furthermore, the
harmonization of engineering-geology models and
monitoring data can be achieved by the implementa-
tion of stress-strain numerical models, that represent
a validating tool for the engineering-geology models,
by collecting the monitoring data and by refining, via
calibration analyses, the rheological behaviors, i.e.
the stress-strain constitutive laws. In this frame, our
experience is referred to an unstable slope involved
in a tunnel excavation. A very detailed engineering-
geology model was built by means of several in situ
and laboratory investigations. The availability of
monitoring data with high temporal and spatial reso-
1 This research has been funded by MIUR – PRIN 2009 –
“Analysis, monitoring and control of geological instabili-
ties interacting with human activity”, UR1 “Relationships
between human activities and geologic instabilities by in-
tegrating monitoring data and geological models related
to already studied case histories”. Principal Investigator:
Francesca Bozzano
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F. BOZZANO, I. CIPRIANI, C. ESPOSITO, S. MARTINO, P. MAZZANTI, A. PRESTININZI, A. ROCCA & G. SCARASCIA MUGNOZZA
316
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
puts, etc.). The results – in terms of presence of shear
surfaces or zones and of definition of state of activity
– can then confirm the preliminary conceptual engi-
neering-geology model or modify it according to the
collected data. Once the engineering-geology model is
well constrained, it can be transferred into a numerical
model, which is the most powerful tool for backward
and forward analyses: in a calibration phase, the corre-
spondence between the numerical simulations and the
results of monitoring (then the actual displacements/
deformations) becomes the best criterion for assess-
ing the goodness of the reference engineering-geology
model, which can be eventually modified in a sort of
iterative process until the achievement of a final, sat-
isfactory model.
In other words, therefore, the engineering-geol-
ogy model is a container within which the objective
data, i.e.: actual displacements measured by monitor-
ing, and the results of stress-strain modeling must find
their reciprocal consistency. Once the engineering-
geology model and, then, the numerical simulation of
stress-strain relations is appropriately tested in accord-
ance with the criteria above described, the numerical
modeling becomes a tool of fundamental importance
for the prediction of deformations expected given the
changes of stress state induced – for example – in
predict the structure-slope interactions and to design
stabilization countermeasures. Such an issue acquires
greater importance in the case of unstable slopes: in
this regard, the dramatic effects caused by the occur-
rence of the Vajont landslide on 1963 due to the dam
construction and its rapid drawdown still represents a
strong lesson about the importance of taking into ac-
count interactions between slopes and infrastructure
both in the design phase and in the executive one.
Nowadays the reconstruction of reference en-
gineering-geology models can take advantage of
methods and techniques of investigation (both on site
and in laboratory) able to depict a detailed cognitive
framework for several purposes within the geo-haz-
ard related fields (e.g.: b
IanchI
-F
asanI
et alii, 2008;
G
anerod
et alii, 2008). Notwithstanding, based on the
geological complexity and/or the width of the area to
be investigated as well as on the number of investi-
gations compatible with the dedicated budget, a more
or less significant level of uncertainty still persists. In
the case of unstable slopes, a fundamental help comes
from monitoring activities, which at present benefit
from a wide variety of techniques, both traditional
and innovative, able to measure at different scales,
depths and resolutions the actual displacements and/
or the external acting forces (i.e., rainfall, dynamic in-
Fig. 1 - Geological map of the landslide area. Key to legend: 1 - March 2007 landslide; 2 - Pre-existing landslide; 3 - Debris;
4 - Marine terrace deposits (Pleistocene); 5 - Yellowish sands (Pleistocene); 6 - Gneiss rock involved in the landslide
(RQD < 30); 7 - Gneiss rock involved in the landslide (40 < RQD < 30); 8 - Gneiss rock (RQD > 40); 9 - Landslide
scarp; 10 - Landslide scarp activated during monitoring; 11 - Landslide terrace; 12 - Borehole (2005 geognostic cam-
paign); 13 - Borehole (2007 geognostic campaign); 14 - Borehole (2011 geognostic campaign); 15 - Borehole (test site);
16 - Stratigraphic boundary; 17 - Transgressive stratigraphic boundary; 18 - Deepening channel; 19 -Subsidence; 20
- Fluvial erosion scarp; 21 - Gully; 22 - Cross section trace (see Fig. 2)
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LANDSLIDE RISK REDUCTION BY COUPLING MONITORING AND NUMERICAL MODELING
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
317
GEOLOGICAL SETTING OF THE SLOPE
The case study described in this paper concerns
a section of a major road imperatively planned on
an unstable slope (Fig. 1). In this context during the
start-up works for the realization of a tunnel entrance,
a shallow translational landslide (with a volume of
about 10
4
m
3
) occurred, thus completely destroying
the already constructed structures. Following this
event, the Research Centre for Geological Risks
(CERI) of the University of Rome “Sapienza” car-
ried out detailed engineering-geological investiga-
tions and surveys (geomorphological, geological
and geomechanical field surveys, boreholes, seismic
surveys and laboratory tests of samples) on the slope
in order to define a reference model to explain the
occurrence of the landslide.
The steep slope is made up of jointed and weath-
ered metamorphic rocks overlaid by Pliocene and
Pleistocene sandy, marine deposits, while few me-
ters of sandy colluviums cover the slope. Moreover,
geological and geomorphological evidence of an old
deep roto-translational slide with a total volume of
about 1
x
10
6
m
3
, which involved jointed gneiss and
the overlying Pliocene and Pleistocene sands, was
also recognized (Figs. 1 and 2). The main sliding sur-
face (up to 50 meters deep) of the old landslide and
many secondary sliding surfaces were reconstructed
by geomorphological surveys and stratigraphic logs
(Fig. 2). Minor shallow translational movements of
about 1
x
10
4
m
3
of colluviums and bedrock have been
observed in the middle-lower part. Among these, we
can include the landslide event which destroyed part
of the already-built excavation structures.
The above described geological model was
used as a conceptual basis to design the stabiliza-
tion works. According to this model, the reactivation
of both the whole old landslide and part of it are
plausible because of slope re-profiling and excava-
tion activities. Therefore, some stabilization work
was performed and a continuous monitoring system
was used to: i) monitor the evolution of the slope
under undisturbed conditions and during construc-
tion; ii) predict the occurrence of critical conditions,
if any. This system would also optimize planning,
design and construction activities and protect the
workers during the tunnel construction (b
ozzano
et
alii, 2011).
various stages of execution of work. In this context
the monitoring returns to be of paramount importance
to check, in the process, the adequacy of the predic-
tions made in terms of deformation/displacement. At
the end of this process it could be possible to identify
critical thresholds of significant monitored values that
can anticipate paroxysmal phases in the slope evolu-
tion, thus making the coupling of numerical models
and monitoring data a unique forecasting and plan-
ning tool that can contribute to undertake efficient risk
reduction policies, such as early-warning procedures.
The recent management of the landslide in Preonzo
- Switzerland (l
oew
et alii, 2012), represents a suc-
cessful and promising result in this sense.
This paper exemplifies the experience of the
CERI - “Sapienza” research team, in the achievement
of conceptual modeling based on validation of the en-
gineering-geology model by the integration of moni-
toring and numerical modeling. The here presented
case-study is referred to an unstable slope involved
in a tunnel excavation. A very detailed engineering-
geology model was built by means of several in situ
and laboratory investigations. In addition, the slope
has been monitored for five years by means of an in-
tegrated monitoring platform made up of “punctual”
and areal displacement monitoring devices, integrated
with piezometers and rain-gauges. The availability of
data referred to slope instability episodes triggered
by different external factors (e.g. rainfalls, tunneling,
etc.) made it possible to better understand the dynam-
ics of the slope-infrastructure system and to refine the
numerical model of the slope.
Fig. 2 - Engineering-geology cross-section of the land-
slide. Key to legend: MRC = Intact rock mass;
LR = Rock mass involved in the landslide; DLR =
Rock mass involved in the landslide (deep part);
Ls = Landslide surface; other symbols in Fig. 1
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F. BOZZANO, I. CIPRIANI, C. ESPOSITO, S. MARTINO, P. MAZZANTI, A. PRESTININZI, A. ROCCA & G. SCARASCIA MUGNOZZA
318
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
- an integrated remote platform made of a Terrestrial
SAR Interferometer (TInSAR) model IBIS-L (by
IDS S.p.A.), a weather station and an automatic
photo camera (from November 2007) (b
ozzano
et alii, 2008);
- three inclinometers, one full screen piezometer and
one ‘Casagrande’ piezometer (from June 2007);
- topographic monitoring by total station of prisms in-
stalled on the bulkheads (from September 2008);
- load cells for monitoring man-made structures after
their construction (from September 2008);
- convergence monitoring of the tunnel (from Novem-
ber 2009).
The integrated remote platform was hosted in a
specially designed box and installed on the slope op-
posite to the landslide one, at a distance ranging from
700 to 900 m (b
ozzano
et alii, 2008). This platform
has been continuously active from November 2007
with a sampling rate of about 5 minutes. Up to now,
more than 16,000 photos, 250,000 measurements of
weather data and 250,000 SAR maps have been col-
lected. Inclinometric monitoring of the slope is manu-
ally performed since June 2007 with a sampling rate
ranging from 15 days to few days depending on the
working phases. The ‘Casagrande’ piezometers in-
stalled at the depth of 48 m below the ground level
were monitored with the same frequency of the incli-
nometers but they never recorded presence of water.
Topographic and load cells monitoring of man-made
structures (anchored bulkheads) is active since Sep-
tember 2008 with a sampling rate ranging from 15
SLOPE MONITORING
Starting from March 2007 excavations have
been carried out in order to obtain a more stable
profile of the slope. Furthermore, three anchored
bulkheads made of 0.1 m diameter and 10-22 m
long piles were realised. Bulkheads were coupled
with the deeper part of the slope by 35 m long cable
inclined of 35° from the horizontal plane. Anchors
were cemented in the last 12 m and the three bulk-
heads were realised from August 2008 to January
2009 moving downslope (Fig. 3 and monitoring data
in Fig. 4). The tunnel excavation started in Novem-
ber 2009 and was stopped in February 2010 (Fig.
5). A total amount of 28 m of tunnel were excavated
following three main phases before being interrupt-
ed due to the high value of displacement recorded
on the slope (Fig. 5). The slope was continuously
monitored starting from November 2007. The over-
all monitoring system was planned and realized in
order to record both shallow and deep displacement
of the natural slope and man-made structures. The
monitoring system consists of:
Fig. 4 - Results of monitoring from summer 2007 until au-
tumn 2009. Last part of the time interval is referred
to the realization of the three bulkheads
Fig. 5 - Displacement time series (obtained by interfero-
metric and topographic monitoring) of the three
anchored bulkheads (November 2008-June 2011)
and of the middle inclinometer of figure 3 (Sep-
tember 2009-June 2011). The “y” right axes rep-
resents the advancement of tunnel excavation. The
slope crisis during the tunnel excavation in the
period November 2009 - February 2010 is encom-
passed by the rectangle with a red outline
Fig. 3 - Layout of the monitoring system
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LANDSLIDE RISK REDUCTION BY COUPLING MONITORING AND NUMERICAL MODELING
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
319
jointed rock has been derived resulting 35.58-36.19
GPa and 34.04-35.04 GPa for the bedrock and for the
landslide rock mass, respectively. According to the
ISRM standard procedures (ISRM, 2007), the indexes
Ib (block size index) and Jv (number of joint per cubic
metre) were measured by geomechanical scanlines on
the outcropping rock masses. An average Jv value of
about 19 joints for cubic meter and a Ib of about 11 cm
resulted for the bedrock (MRC in Fig. 2), correspond-
ing to the rock mass class H of Table 1, while a Jv of
about 21 joints for cubic meters and Ib ranging from
8.1 to 9.6 cm resulted for the landslide mass (RL and
DRL in Fig. 2), corresponding to the rock mass classes
L and M of Table 1. Moreover a Q rock mass class
was attributed to the landslide surface (Ls) which is
characterised by an average Jv of 30 joints for cubic
meters and an average Ib of 6.3 cm. By using the two
ISRM geomechanical indexes Jv and Ib, a Rock Index
(RI) was determined, which is based on a statistical
algorithm (M
ontaGna
, 2012). This statistical analysis
was developed by the Authors thanks to more than
144 geomechanical scanlines which were performed
on the rock masses outcropping within an area of 100
km
2
which includes the here considered slope. More
in particular, the RI index is based on the transforma-
tion of the data series of Jv and Ib in series of data with
mean equal to zero and variance equal to one by the
relation (M
ontaGna
, 2012):
x
score
= (x
i
– u)/σ
where x
score
is the new series of data, x
i
is the series
of data (related to Jv or Ib), u and σ are the mean and
the standard deviation of the data x
i
, respectively. A
cluster analysis was then applied in order to define the
rock mass classes by using the RI index.
The geomechanical parameter values for the nu-
days to few days depending on the working phases.
The convergence monitoring of tunnel had a sampling
frequency ranging from a day to a week, with a step of
7 m for depth of excavation.
Before the beginning of the tunnel excavation the
anchored bulkheads showed an almost constant veloc-
ity of displacement on the order of 0.05 mm/h. Immedi-
ately after the beginning of the excavation the velocity
of bulkheads suddenly increased reaching maximum
values of 0.75 mm/h, with acceleration and decelera-
tion peaks on the order of 0.02 mm/h
2
(Fig. 4 and Fig.
5). During the three excavation phases discussed above
a maximum displacement of about 100 mm was record-
ed on the first order of bulkhead. In the last two phases,
the interferometric monitoring allowed us to clearly
recognize a typical creep behaviour.
NUMERICAL MODELING
ENGINEERING-GEOLOGY MODEL
The engineering-geology model of the landslide
was developed from on-site geomechanical surveys,
stratigraphical logs and CID triaxial tests. The geome-
chanical characterization of the old deep roto-trans-
lational slide was achieved by data coming from the
stratigraphic logs (Fig.1). The CID triaxial tests were
performed on some samples of gneiss at the “Labora-
torio della Provincia Autonoma di Trento” geotechni-
cal laboratory to evaluate both strength and deforma-
tional parameters of the intact rock (M
ontaGna
, 2012).
The Young's modulus values measured for the intact
rock from triaxial laboratory tests vary in the range of
50–60 GPa and the UCS resulted about 94 MPa (b
oz
-
zano
et alii, 2012). So, by the use of an equivalent
continuum approach (r
aMaMurthy
, 1993; s
ItharaM
et alii, 2001), the equivalent Young’s modulus of the
Tab. 1 - Rheological parameters used for the stress-strain numerical modeling, calibrated by the monitoring results. The up-
per box exemplifies the adopted visco-plastic model
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F. BOZZANO, I. CIPRIANI, C. ESPOSITO, S. MARTINO, P. MAZZANTI, A. PRESTININZI, A. ROCCA & G. SCARASCIA MUGNOZZA
320
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
merical simulation are summarized in Table 1 and
they were derived for each rock mass class after a
stress-strain calibration process via numerical mod-
eling, by assuming a creep rheology according to the
visco-plastic Burgers model.
NUMERICAL MODELING
A 2D finite difference model was implemented
along three different sections obtained across the
slope, by the finite difference code FLAC 7.0 (ITAS-
CA, 2011). Geological features as well as structural
elements (such as anchored bulkheads, retaining
walls) were also modelled in order to follow the
stress-strain changes induced within the slope by the
on-going human activities. The rheological behaviour
of the involved rock mass was reproduced according
to an equivalent continuum approach (s
ItharaM
et
alii, 2001), i.e. taking into account by equivalent pa-
rameter values the intact rock mass properties as well
as the jointing conditions (Hoek and Brown failure
criterion combined to Mohr-Columb failure criterion).
Use was made of two different creep models to
simulate the rheology of the MRS and of the DRL,
RL and Ls lithotechnical units (Fig. 2). Respectively:
1) a Burgers visco-plastic model was assumed for the
MRS;
2) a Burgers visco-plastic model coupled with a plastic-
ity threshold was assumed for the RL, DRL and Ls.
Tab. 1 summarises the parameter values attrib-
uted to the adopted rheological models. According to
b
ozzano
et alii (2012), the viscosity values for both
the visco-elastic and the visco-plastic elements of the
Burger model were selected in the range 1019-1023
Pa•s and calibrated to match morphological evidenc-
es and the displacements recorded as a consequence
of the landslide.
For calibrating the viscosity values of the Burger
model referred to the MRC, the viscosity values of
the Kelvin-Voight visco-elastic element were always
assumed to be one order of magnitude higher than
the ones used for the visco-plastic Maxwell element.
Based on the performed numerical calibration, the
here considered viscosity values result the lowest for
justifying the landslide activation in the present slope
shape with respect to the river valley evolution. On
the other hand, for calibrating the viscosity values for
RL, DRL and Ls a best fit was performed between
the monitored displacements, referred to the different
excavation and re-shaping steps within the landslide
mass and the numerical modeled ones.
Fig. 6 shows the outputs in terms of displacements
resulting from the sequential numerical modeling
performed for calibrating the rock mass rheological
behavior of the landslide mass. A slope re-shape was
realized starting from March 2007 and three orders
Fig. 6 - Sequential numerical modeling performed for
calibrating the rock mass rheological behavior
Fig. 7 - Simulated vs. measured displacements at the in-
clinometer (violet and yellow lines) and at the
anchored bulkheads (brown lines)
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LANDSLIDE RISK REDUCTION BY COUPLING MONITORING AND NUMERICAL MODELING
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
321
In the here proposed case-study, a stress-strain nu-
merical modelling was performed by reproducing the
monitored displacements due to the re-shaping of the
considered slope and the realization of reinforcements
(bulkheads). During the here considered time interval
the slope experienced a total amount of displacement
of more than 100 mm. By this back-simulation the
best rheological model describing the rock mass me-
chanical behavior was determined and its parameters
were calibrated. The stress-strain effects related to the
slope re-shaping and reinforcement were referred to
the engineering geology model and it can now pro-
vide scenarios of possible effects due to destabilizing
actions external to the slope as well as to the manage-
ment of the road tunnel.
Finally, we tuned up a numerical tool able to ma-
nage strain effects on slopes constituted by jointed
rock by applying the continuum equivalent appro-
ach to the time dependent behavior. Up to now, such
an approach has been experienced and validated by
other authors (r
aMaMurthy
, 1993; s
ItharaM
et alii,
2001; s
ItharaM
& M
adhavI
l
atha
, 2002; s
rIdevI
&
s
ItharaM
, 2000) for an elasto-plastic behaviour as
an useful application for tunneling; in this paper we
demonstrated the reliability of such an approach also
for the visco-plastic behavior applied to sliding slopes
emphasized by coupling it with time and space high
resolution monitoring data.
ACKNOWLEDGEMENTS
The Authors thank Dr. Alfredo Montagna for his
precious field work.
of anchored bulkheads were installed to stabilize the
slope after the cut of corresponding parts of the slope.
Horizontal displacements up to some centimeters in-
volved the landslide mass after the occurrence of the
March 2007 landslide (Fig. 6a); further centimeter-
scale displacements added to the previously occurred
ones as a consequence of the re-shaping and of the cut
of the trenches (Fig. 6b and 6c).
The graphs reported in Fig. 7 illustrate the final
modeling results, i.e. the horizontal displacements re-
sulting by the numerical model after the rheological
calibration (i.e. according to the rock mass parameters
summarized in Tab. 1); the numerically computed
displacements are compared with the ones measured
along the inclinometer borehole and at the anchored
bulkheads during the re-shape and consolidation
works. The comparison demonstrates the very good
fit among the modeled and the measured displacement
values, thus providing a stress-strain scenario which
strongly reproduces the actual behavior of the slope,
observed during the consolidation works.
CONCLUSIONS
This study reports the experience of the CERI -
“Sapienza” research team on the interaction of moni-
toring and numerical modelling in the achievement of
a conceptual model based on a engineering-geology
one with reference to a specific case-study. In this fra-
me, the coupling of numerical models and monitoring
data could represent an effective forecasting and plan-
ning tool that can contribute to undertake efficient risk
reduction policies.
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F. BOZZANO, I. CIPRIANI, C. ESPOSITO, S. MARTINO, P. MAZZANTI, A. PRESTININZI, A. ROCCA & G. SCARASCIA MUGNOZZA
322
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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