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29
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2015-02.O-03
N
icola
MORACI
(*)
, M
arileNe
PiSaNo
(*)
, M
aria
c
loriNda
MANDAGLIO
(*)
, d
oMeNico
GIOFFRE’
(*)
,
M
aNuel
PASTOR
(**)
, G
iovaNNi
leoNardi
(*)
& S
iMoNetta
COLA
(***)
(*)
Mediterranea University of Reggio Calabria - Dep. DICEAM - via Graziella Loc. Feo di Vito - 89060 Reggio Calabria, Italy. nicola.moraci@unirc.it
(**)
ETS de Ingenieros de Caminos - Universidad Politécnica de Madrid - Ciudad Universitaria, s/n - 28040 Madrid, Spain
(***)
University of Padua - Dep. ICEA - via Ognissanti, 39 - 35129 Padova, Italy
ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS
FLOWS: RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
ExTENDED ABSTRACT
In Italia diverse regioni presentano un elevato rischio idrogeologico connesso a fenomeni franosi di colata rapida. La Calabria e la
Sicilia sono spesso interessate da tali eventi franosi, che producono ingenti danni alla popolazione, alle strutture e alle infrastrutture presenti
nel territorio.
Una possibile strategia, volta alla mitigazione del rischio connesso a tali fenomeni, può essere perseguita con interventi di tipo
strutturale quali opere di protezione passiva. Tali opere riducono il rischio connesso al fenomeno franoso arrestando o deviando il percorso
della colata detritica.
Al fine di riprodurre le colate di detrito, uno degli obiettivi di ricerca del progetto PON01_01869, sviluppato dal gruppo geotecnico
del Dipartimento DICEAM dell’Università “Mediterranea” di Reggio Calabria, è stato quello di realizzare un modello fisico di grandi
dimensioni in grado di studiare il fenomeno della propagazione dei debris flow.
Il modello fisico è formato da quattro parti: i) una struttura principale costituita da uno scivolo in acciaio, con inclinazione variabile
rispetto all’orizzontale, le cui pareti laterali in plexiglass consentono la videoripresa del flusso detritico; ii) un serbatoio a sezione
rettangolare adibito al contenimento e al rilascio, con un meccanismo di tipo “dam-break”, di miscele acqua-terreno; iii) una struttura per il
sollevamento del serbatoio alle varie altezze di prova; iv) un sistema di misura, trasmissione, registrazione ed elaborazione dei dati di prova
mediante l’impiego di sensori a ultrasuoni, trasduttori di pressione e videocamere ad alta definizione.
Per la progettazione del modello fisico sono state effettuate analisi numeriche, oggetto della presente memoria, allo scopo di valutare
le dimensioni del modello necessarie a riprodurre le velocità di debris flow reali. Le analisi sono state realizzate utilizzando il codice di
calcolo SPH (P
aStor
et alii, 2009) che lavora su un modello non lineare e accoppiato, permettendo la soluzione delle equazioni della
dinamica del continuo in forma lagrangiana:
• il “modello matematico” basato sulle equazioni di conservazione della massa e della quantità di moto è semplificato effettuando
un’integrazione in profondità (lungo l’asse verticale), considerato che i movimenti franosi studiati hanno profondità medie piccole
rispetto alla loro lunghezza o larghezza;
• il “modello numerico” utilizza una tecnica di discretizzazione del campo di moto (metodo SPH, “smoothed particle hydrodynamics”)
attraverso un sufficiente numero di punti mobili, ciascuno rappresentativo di una particella fluida: il campo di moto è ottenuto
interpolando in ogni punto del continuo i valori relativi ai singoli punti mobili attraverso l’uso di opportune funzioni di interpolazione.
Il modello, combinato alle adeguate relazioni costitutive, restituisce le velocità secondo il piano perpendicolare alla direzione di
integrazione e la profondità del materiale in frana.
Per identificare la reologia delle miscele acqua-terreno sono state preliminarmente effettuate delle prove di laboratorio su una canaletta
di piccole dimensioni. Per ricostituire le miscele rilasciate è stato utilizzato un volume solido costante (sabbia con ghiaia medio-fine) al
quale sono stati aggiunti volumi d’acqua tali da ottenere diverse concentrazioni solide in volume. La scelta delle concentrazioni solide in
volume è stata opportunamente operata prendendo in considerazione i valori tipici dei debris flow.
Attraverso una back-analysis numerica dei risultati delle prove di laboratorio, è stato ricavato il legame sforzi-deformazioni che meglio
riproduce il comportamento reologico delle miscele indagate (legge reologica puramente attritiva). Inoltre, la calibrazione del parametro
attritivo che governa tale legge reologica (coefficiente di attrito cinematico) ha consentito di definire una correlazione tra le concentrazioni
solide in volume delle miscele e gli angoli di attrito ad esse corrispondenti.
Successivamente, utilizzando la reologia ricavata sperimentalmente, sono state effettuate le analisi numeriche per ricavare le dimensioni
del modello fisico necessarie a riprodurre le velocità dei debris flow reali. In particolare, le analisi numeriche sono state condotte facendo
variare i volumi (in termini di altezza di rilascio) e gli angoli di attrito delle miscele (considerando i casi estremi di solo fluido e materiale
secco e le concentrazioni solide in volume tipiche dei debris flow); la lunghezza e l’inclinazione della canaletta rispetto all’orizzontale.
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
30
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
ABSTRACT
Debris flows are landslides that may involve large volumes
of material and, due to their rapid propagation, they may be
potentially dangerous for human lives and lifelines.
In this paper, numerical simulations carried out for designing
an instrumented large-size physical model are shown. In particular,
a parametric analysis has been performed in order to reproduce,
with the flume tests, the debris flow velocities observed during
real events.
Since the used computational code requires setting the
specific rheological laws, several preliminary experimental tests
have also been carried out, with the aim to find the rheological
behaviour of the debris flow material.
K
eywords
: debris flow, flume test, rheological law, numerical
analyses, design of physical model
INTRODUCTION
Debris flows are important landslides with a flow-like
behaviour. During the flow the volume of landslide increases and
the characteristics of the flow material may change, modifying
the flow mobility. The high velocity that the flow mass can reach
during propagation, due to the characteristics of both the moving
material (i.e. debris) and the type of material in basal surface,
allows that long distances can be rapidly covered. Moreover,
the consequences of debris flow impact are pronounced when
it occurs near infrastructures or other main lifelines, because it
can produce the interruption of traffic or other activity or even
the loss of human lives.
Debris flow materials are complex mixtures of sand, gravel,
cobbles and boulders, often with varying proportions of silt and
clay.
In addition to this, the different components of the debris
flow materials can separate, with larger blocks moving upwards
and onto the front. Moreover, spatial gradational sorting of
debris flows, due to the development of inverse grading or
coarse surge fronts, is common and may be important for the
flow behaviour (e.g., P
ierSoN
, 1986).
Many classifications of flows exist in literature (H
eiM
, 1932;
v
arNeS
, 1954, 1978; c
rudeN
& v
arNeS
, 1996; P
ierSoN
, 2005;
v
allaNce
, 2005; K
eefer
& J
oHNSoN
, 1983; H
utcHiNSoN
, 1968,
1988; H
uNGr
et alii, 2001, etc.). According to H
uNGr
et alii
(2013), the term “debris flow” is used to describe very rapid
to extremely rapid surging flows of saturated debris in a steep
channel with strong entrapment of material and water from the
flow path. It occurs periodically on established paths, usually
gullies and first- or second order drainage channels. Thus,
debris flow hazard is specific to a given path and deposition
area (“debris fan”). This, with the periodicity of occurrence
at the same location, influences the methodology of hazard
studies and contrasts with related phenomena, such as debris
avalanches, whose occurrence is not bound to an established
path. Once debris begins to move in a steep channel, the bed
is subjected to rapid undrained loading, often so sudden that it
could be characterized as impact loading (S
aSSa
, 1985). Under
such conditions, even coarse material can liquefy, or at least
suffer a significant increase in pore-pressure. The bed material
will become dragged in a growing surge. As the surge moves
downstream, erosion undermines the steep banks and further
soil material, as well as organic debris, is added to the flow. The
surges travel down the channel on slopes steeper than 10-20°. In
many cases, it is found that the final mass is much larger to the
initial, because of the entrainment along the path of propagation.
Therefore, the magnitude of debris flows depends primarily on
the characteristics of the channel and it can be estimated by
empirical means (H
uNGr
et alii, 2005).
It is important to notice that entrainment can be much larger
in steep channels, as the bed can become unstable (B
aGNold
,
1966). The bed material can be massively mobilised and
dragged into the flow (H
uNGr
et alii, 2005). Debris surges
spread out when the channel exits onto the surface of the
debris (colluvial) fan, at typical slopes of 5° to 10°. The frontal
boulder accumulation rapidly deposits in the form of levees
or abandoned boulder fronts, while the finer and more dilute
material continues further downslope.
In order to reduce the debris flow risk consequences of both
hazard and vulnerability, structural and non-structural measures
can be used. In both cases, it is important to predict the possible
scenarios in order to propose effective protection works and
safety measures.
Two general approaches are available for mitigating debris
flow risk. The first approach (active approach) consists of
decreasing the destabilizing forces that can trigger landslide;
whereas the second approach (passive approach) is to carry out
containment measures of the movement of the debris.
The most commonly used remedial measures to reduce the
destabilizing forces are for example the modification of the slope
geometry by excavation or toe fill and the drainage of surface
and ground water. In particular, drainage is the most widely
used method for slope stabilization. These remedial measures
are excellent site-specific management tools for landslides if
correctly designed and constructed, for example with regard to
proper design of the filtering transitions (M
oraci
et alii, 2012a,
b, c; M
oraci
et alii, 2014a; M
oraci
et alii, 2014b; c
azzuffi
et
alii, 2014; M
oraci
et alii, 2015).
An alternative landslide risk-mitigating strategy of
engineering solutions is to control the movement of landslide
debris so as to reduce the spatial impact of landslides on
elements sensitive at risk. Mitigation measures consist in the
passive structural barriers usually made with earth reinforced
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ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS FLOWS:
RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
31
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
embankment or dams located to intercept or divert the flow
along the channel.
The knowledge of the physical, kinematic, geometric and
rheological properties of debris flow (e.g., concentration of solid
material, the evolutionary characteristics of viscous water / soil
mass movement, speed profile, thickness) are required to design
protection embankments. The state of knowledge for the design
passive structures for the protection from rapid debris flows,
especially for the design of earth reinforced embankments, is
not yet supported by a comprehensive scientific literature.
Numerical models contribute significantly to describe the
consequences of large mass movements. Several numerical
models have been developed for simulating landslide propagation
and runout (e.g. S
avaGe
& H
utter
1989; Gray et alii, 1999;
c
HeN
& l
ee
, 2000; d
eNliNGer
& i
verSoN
, 2004; M
c
d
ouGall
& H
uNGr
, 2004; Q
uecedo
et alii, 2004; P
aStor
et alii, 2009;
P
irulli
& P
aStor
, 2012; B
orrelli
et alii, 2012). Whatever code
is used, the choice of the correct rheology and of the rheological
parameter values is fundamental. Due to the large dimensions of
real phenomena, back analyses of debris flows already occurred
are the only way to obtain data for runout prediction analyses
(e.g. H
uNGr
& e
vaNS
, 1996). Nevertheless, a lack of knowledge
in geometrical and geomechanical information may lead, in the
back analyses, to wrong interpretations of the mechanics of the
event and inaccurate calibration of numerical models.
In this context, data from measurements made on site
or experimental channel tests are important both in terms of
the theoretical aspects of the problem (determination of the
rheological behaviour, calibration of numerical models) and
in terms of practical aspects (passive barrier or prevention of
phenomenon, definition of alarm systems, etc.). The main
variables that can be measured or calculated are: physical and
mechanical properties, height, velocity, image or video and
mobility of debris flow (S
uwa
, 1989; l
aHuSeN
, 1996; a
rattaNo
et alii, 1997; G
eNevoiS
et alii, 2000a, b; B
erti
et alii, 2000;
a
liPerta
et alii, 2012). Many studies aim at the analysis of
trigger phenomena of landslides (M
uSSo
& o
livareS
, 2003;
o
livareS
& P
icarelli
, 2003; c
aSciNi
& S
orBiNo
2003), at the
analysis of the fluidization of landslide (M
uSSo
et alii, 2004)
and at the analysis of propagation (runout, runup) of debris
flows (i
verSoN
, 1997; P
reStiNiNzi
& r
oMeo
, 2000; M
aNdaGlio
et alii, 2015).
The paper focuses on numerical simulations carried out for
the design of a large-size physical model in order to study the
debris flow propagation.
Specifically, the research aims to reproduce, by means of the
large-size physical model, the debris flow velocities observed
during real events.
In order to calculate the debris flow velocities by means of
the numerical code, it is necessary to know the rheological law
of mixtures which will be used in the research. The rheological
law has been found carrying out several experimental tests.
Fixed the rheological law, by means of the parametric
numerical analysis it has been possible to design the geometric
characteristics of the physical model, necessary to reproduce the
typical velocities of debris flows.
MODEL USED TO SIMULATE THE PROPAGATION
(DEPTH-INTEGRATED COUPLED SPH MODEL)
The distinctive features of this flow-like landslide are
strictly related to the mechanical and rheological properties
of the involved materials, which are responsible for their
long travel distances (up to tens of kilometres) and the high
velocities (in the order of meters/second) they may attain.
The prediction of both run out distances and velocity through
mathematical modelling of the propagation stage can notably
reduce losses inferred by these phenomena, as it provides a
mean for defining the hazardous areas, estimating the intensity
of the hazard (which serves as input in risk studies), and for
working out the information for the identification and design of
appropriate protective measures. In the past decades, modelling
of the propagation stage has been largely carried out in the
framework of the continuum mechanics, and a number of new
and sophisticated numerical models have been developed.
Among the numerical codes developed in the last twenty
years to predict the propagation of flow-like landslide in the
framework of the continuum and discrete element mechanics,
the depth-integrated SPH method proposed by P
aStor
et alii
(2009) is particularly suitable for this kind of analysis.
The mathematical model of SPH method proposed by
P
aStor
et alii (2009) is based on v-pw Zienkiewicz–Biot model,
consisting of:
(i) The balance of mass, combined with the balance of linear
momentum of the pore fluid, which in the case of saturated
soils reads
(1)
where k
w
is the permeability coefficient, v
s
is the velocity of
soil skeleton, D
(s)
refers to a material derivative following
the soil particles and the equivalent volumetric stiffness
Q
is
given in terms of soil porosity n and volumetric stiffnesses of
pore water K
w
and soil grains K
s
as:
(2)
(ii) The balance of linear momentum for the mixture soil
skeleton–pore fluid, given by:
(3)
where r is the density of the mixture, b the body forces and s
the Cauchy stress tensor.
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
32
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Assuming that for flow-like landslides the average depths
are small if compared with their length or width, it is possible
to simplify the 3D propagation model described above by
integrating its equations along the vertical axis.
In this way, the Biot-Zienkiewicz equations for non-linear
materials and large deformation problems are coupled to various
constitutive models (Bingham, Voellmy, Mohr-Coulomb, etc.),
obtaining a 2D depth-integrated model, which presents an
excellent combination of accuracy and simplicity and provides
information about propagation, such as average velocity or depth
of the flow along the path.
The numerical model used for the mathematical problem’s
resolution is the smoothed particle hydrodynamics method
(SPH). The SPH model is a mesh-free method that provides an
interesting and powerful alternative to more classical numerical
methods such as the finite elements method.
Smoothed particle hydrodynamics is based on discretized
forms of integral approximations of functions and derivatives.
The method has been introduced independently by l
ucy
(1977)
and G
iNGold
& M
oNaGHaN
(1977) and applied to astrophysical
modelling, a domain where SPH presents important advantages
over other methods. The SPH method introduces the concept of
‘particles’, to which information concerning field variables and
their derivatives is linked.
In particular, smoothed particle hydrodynamics method is
based on the possibility of approximating a given function f(x)
and its spatial derivatives by integral approximations defined in
terms of a kernel. In a second step these integral representations
are numerically approximated by a class of numerical integration
based on a set of discrete point or nodes, without having to define
any “element”.
The crucial point for simulation of the landslide is therefore to
correctly define the rheological model used for the equivalent fluid.
PHYSICAL MODEL
This paper is part of a wide research developed in the frame
of a National Operative Research Project (PON 01_01869) on
the study of geosynthetic reinforced earth structures’ behaviour
subjected to debris flow impact, currently in progress at the
“Mediterranea” University of Reggio Calabria. The aim of
the research has been to design a large-size flume in order to
simulate debris flows propagation.
The physical model, designed on the base of numerical
simulations’ results illustrated in this paper, consists of four
main parts (Fig. 1).
(i) The principal structure is a steel flume 8 m long and
inclinable, respect to the horizontal direction, with inclinations
ranging between 20° and 45° evaluated according to the slope
inclinations of real debris flows on granular and weathered
cohesive soils (G
ullà
et alii, 2004, 2006), occurred in Calabria.
The flume dimensions have been suitably selected in order to
simulate, at the bottom of the flume, flow velocities comparable
to those ones reported for debris flows in the scientific literature
(r
icKeNMaNN
, 1999).
(ii) A tank 2.5 m high, with a rectangular section 0.5 m x 1.5
m and with a sloping base that can be removed, is placed at the
top of the flume.
(iii) The physical model is equipped with two additional
independent structures: a structure for lifting the tank to the
various heights of test, 8.86 m high, and a reticular structure for
the lifting of the walkway necessary for tank’s inspection.
(iv) The flume has side walls formed by transparent panels
to allow the framing of the debris flow phenomenon using high
definition video cameras. The phenomenon of propagation will
be monitored by pressure transducers, located on the base of
the flume, and by ultrasonic level sensors supported by joists
or aluminium profiles, orthogonally positioned to the bottom of
Fig. 1 -
Schemes of the physical model
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ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS FLOWS:
RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
33
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
the flume at the same sections where the transducers are located.
The test procedure consists of filling the tank with a granular
soil-water mixture (at different concentrations); the mixture is
instantly released, through the rapid opening of a gate, in order
to reproduce the “dam break” trigger mechanism.
In the tests which will be carried out in the research, it has
been chosen to use water-soil mixtures, whose solid matrix
has the grain size distribution shown in Fig. 2. The soil is a
well-graded sand with medium-fine gravel, classified as SW,
according to USCS classification system, and as A1-b, according
to CNR-UNI 10006 classification system, with grain shape from
sub-rounded to rounded, uniformity coefficient U=7.48 and
average grain size D
50
=1.47 mm.
CALIBRATION OF THE RHEOLOGICAL LAW
FOR SELECTED MIxTURES
To identify the rheology of the selected mixtures, several
laboratory tests have been carried out at the University of Padua
(Italy).
The flume is L=2.10 m long (including the tank), B=0.25
m wide, with a slope of i=30°, and it has a rigid bottom. The
triggering of the mixture propagation occurs by means of a
removal gate (Fig. 3).
The experimental tests have been carried out using a constant
solid volume of dry material (the same material that will be used
in the large-size physical model) with different water volumes.
From a constant solid volume of dry material corresponding to
W
s
=30 kg, the volumes of water have been increased in order to
obtain the following solid concentrations by volume:
Fig. 3 -
Flume test apparatus used to evaluate the rheological law of
water-soil mixtures
Fig. 2 -
Grain size distribution of soil
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
34
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
C
v
= 85%
(with 2 liters);
C
v
= 74%
(with 4 liters);
C
v
= 65%
(with 6 liters);
C
v
= 62%
(with 7 liters);
C
v
= 58%
(with 8 liters).
Figure 4 shows the different mixtures selected in the
research, varying the solid concentrations by volume C
v
. The
solid concentrations by volume equal to 74%, 65% and 62%
are those typical of debris flows, according to P
ierSoN
& c
oSta
(1987) (Fig. 5).
The trends of flow front velocity over time, measured during
the flume test, have been reproduced through different numerical
analyses using the SPH code (Fig. 6). As it can be seen from the
figure, for example for the volumetric solid concentrations 74%
(Fig. 6a), 65% (Fig. 6b) and 62% (Fig. 6 c), the trend of the flow
velocity over the time is quite reproduced.
It has been observed that the rheological model which best
fits the behaviour of the used mixtures is the frictional model
(H
uNGr
, 1995). This model provides, in the case of flume with
rigid bottom, the basal flow resistance stress according to H
uNGr
equation (1995), as follows:
t
= rgh (cos i + a
c
/g) tan f
(4)
with r = density of the flowing material; h = flow depth; i = slope
angle; a
c
= (v
2
/R) = centrifugal acceleration (resulting from the
vertical curvature radius R of the flow path); tan f=(1-r
u
) tan f
bulk friction angle; r
u
= pore pressure coefficient (ratio of pore
pressure to total normal stress at the base of the block); f’ friction
angle.
Therefore, the numerical back-analysis of laboratory results
has allowed to calibrate the parameter m (=tan f) , which
controls the frictional rheological law (equation 4) (Tab. 1), and
it was thus possible to find a correlation between friction angles
and solid concentrations by volume of the mixture (Fig. 7). The
figure shows that the friction angle of mixture sharply increases
for
C
v
ranging from 58% to 65%, whereas the increase is less
pronounced for C
v
higher than 65% (typical values of mixtures
where the solid matrix is predominant).
NUMERICAL ANALYSIS PERFORMED TO
DESIGN THE PHYSICAL MODEL
For designing the physical model, an extensive parametric
analysis has been carried out using the SPH code and the above
mentioned frictional rheological law for the mixtures.
The purpose of the numerical analysis has been to evaluate
the required length of the large-size flume in order to obtain the
typical flow velocity of real events.
The parameters which have been varied are the length of
flume, L; the inclination of flume, i; the released height of
mixture, H and the friction angles of mixture, f.
With respect to the friction angles of mixture f, it has
Fig. 5 -
Rheological classification of flows (P
ierson
& C
osta
, 1987)
Fig. 4 -
Different mixtures used in the research, varying the solid con-
centrations by volume
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ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS FLOWS:
RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
35
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
been chosen to investigate values corresponding to solid
concentrations by volume typical of debris flows (f=20°, 30°),
the friction angle corresponding to the pure fluid (f=0°) and the
friction angle corresponding to the dry material tested in the
current research (f=36°) (Tab. 2).
In the parametric analyses, the released mixture has been
placed at the top of the flume, inside a tank with a 45° inclined
bottom. The tank’s length is 1.5 m and the parametric analyses
have been carried out for different values of released height of
the mixture (i.e. H=1 m and H=2 m) corresponding to different
mixture volumes, V=0.25 m
3
and V=0.94 m
3
respectively (Fig. 8).
Figure 9 shows the 1-D numerical analysis of mixture’s
propagation in flume at different times in the case of flume length
L=8 m.
Fig. 6 -
Flow front velocities: comparisons between experimental and
numerical results for the different mixtures, C
v
=74 % (a),
C
v
=65 % (b), C
v
=62% (c)
Tab. 1 -
Friction angles of the mixture according to the different solid
concentrations by volume
Fig. 7 -
Friction angle values obtained by numerical analyses for the
different mixtures
Tab. 2 -
Parameters considered in the numerical analysis
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
36
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Figure 10, Figure 11 and Figure 12 show the trends of flow
front velocities at the end of the flume versus the flume inclination
for different values of mixture’s friction angle, in case of released
heights H=1 m and H=2 m, considering three different lengths of
flume equal to L=6, 8 and 10 m.
As expected, the flow velocity increases with increasing
flume inclination. Besides, at the same value of flume inclination,
the flow velocity decreases with increasing mixture friction angle.
It can be noticed that the same value of flow velocity can
be obtained with different combinations of mixture friction angle
and flume inclination.
Figure 13, Figure 14 and Figure 15 show the flow heights
at the end of the flume versus the flume inclination for different
values of mixture’s friction angle, in case of released heights H=1
m
and H=2 m, considering three different lengths of flume equal
to L=6, 8 and 10 m.
It can be noticed that the height of the flow increases with
increasing the inclination and, at the same inclination, it decreases
with increasing the mixture friction angle.
By analizing the numerical results in the range of debris flow
typical slopes (i≥35°) and solid concentrations by volume (friction
angles), in the case of lower released height of mixture (H=1 m),
the characteristic velocities of real debris flows (v>5 m/s) can be
obtained with a flume having length equal to L=8 m. Therefore, it
has been chosen to design a 8 meters long physical model.
CONCLUSIONS
The study has provided relevant results for the design of the
large-size physical model.
Regarding the rheological law used in the numerical analysis,
several laboratory tests have been carried out. The test results
have shown that the model which best fits the behaviour of the
selected mixtures is the frictional law.
Moreover, the numerical back-analysis, performed to
reproduce the experimental results, has allowed to find a
correlation between friction angles and solid concentrations by
volume of the mixtures.
Afterwards, the frictional law has been used in the numerical
analysis to reproduce the typical velocities of debris flows.
The numerical analysis has been carried out varying different
parameters. The obtained results have shown that, considering
Fig. 9 -
1-D numerical analysis of mixture’s propagation in the large-size physical model at different times in the case of length L = 8 m
Fig. 8 -
Scheme of large-size physical model used in numerical analysis
background image
ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS FLOWS:
RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
37
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 10 -
Flow velocities versus physical model inclination, for length L=6 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
Fig. 11 -
Flow velocities versus physical model inclination, for length L=8 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
Fig. 12 -
Flow velocities versus physical model inclination, for length L=10 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
38
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 13 -
Height of flow versus physical model inclination, for length L=6 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
Fig. 14 -
Height of flow versus physical model inclination, for length L=8 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
Fig. 15 -
Height of flow versus physical model inclination, for length L=10 m, in the case of released height: (a) H=1 m and (b) H=2 m respectively
background image
ANALYSES AND DESIGN PROCEDURE OF A NEW PHYSICAL MODEL FOR DEBRIS FLOWS:
RESULTS OF NUMERICAL SIMULATIONS BY MEANS OF LABORATORY TESTS
39
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
the ranges of solid concentrations by volume and the slope
inclinations typical of real debris flows, the velocities reach the
debris flows values in the case of flume length equal to L=8 m.
Thus, the numerical analysis has allowed to design the length of
the large-size physical model.
REFERENCES
a
liPerta
a., i
NfaNtiNo
S., l
a
t
orre
a., M
aNdaGlio
G., M
aNdaGlio
M.c. & P
elleGriNo
a. (2012) - Rock slopes: from the analysis to the definition of the
risk mitigation works. The “Scilla Rupe” case (RC, Italy). Rend. Online Soc. Geol. It., 21: 377-378.
a
rattaNo
M., d
eGaNutti
a.M. & M
arcHi
l. (1997) - Debris flow monitoring activities in an instrumented watershed on the italian Alps. Proceedings of the 1
st
International ASCE Conference on Debris-Flow Hazard Mitigation: Mechanics, Prediction and Assessment, San Francisco, CA, August 7-9, 1997: 506-515.
B
aGNold
R.A. (1966) - An approach to the sediment transport problem from general physics: physiographic and hydraulic studies. Professional Paper 422-I.
Washington, DC: US Geological Survey.
B
erti
M., G
eNevoiS
r., l
a
H
uSeN
r., S
iMoNi
a. & t
ecca
P.r. (2000) - Debris flow monitoring in the Acquabona watershed (Dolomites, Italian Alps). Physics
and Chemistry of the Earth, Part B: Hydrology, Oceans & Atmosphere, 25 (9): 707-715.
B
orrelli
l., G
ioffrè
d., G
ullà
G. & M
oraci
N. (2012) - Suscettibilità alle frane superficiali e veloci in terreni di alterazione: un possibile contributo della
modellazione della propagazione. Rend. Online Soc. Geol. It., 2: 534-536.
c
aSciNi
l. & S
orBiNo
G. (2003) - The contribution of soil suction measurements to the analysis of flowslide triggering. In P
icarelli
L. (e
d
.) - Proc. Int.
Workshop on Occurrence and mechanisms of flow-like landslides in natural slopes and earthfills. Sorrento, May 2003: 77-86.
c
azzuffi
d., M
oraci
N., c
alvaraNo
l.S., c
ardile
G., G
ioffrè
d. & r
ecalcati
P. (2014) - The influence of vertical effective stress and geogrid length on
interface behavior under pullout conditions. Geosynthetics, 32 (2): 40-50. ISSN: 1931-8189.
c
HeN
H. & l
ee
C. F. (2000) - Numerical simulation of debris flows. Can. Geotech. J., 37 (1): 146-160.
c
rudeN
d.M. & v
arNeS
D.J. (1996) - Landslide types and processes. In: t
urNer
a.K. & S
cHuSter
r.l. (
edS
.). Landslides investigation and mitigation.
Transportation research board, US National Research Council. Special Report 247, Washington, DC, Chapter 3, 36-75.
d
eNliNGer
r.P. & i
verSoN
r.M. (2004) - Granular avalanches across irregular three-dimensional terrain: 1. Theory and computation. J. Geophys. Res.
109 (F1): 1-16.
G
eNevoiS
r., t
ecca
P.r., B
erti
M. & S
iMoNi
a. (2000a) - Pore pressure distribution in the initiation area of a granular debris flow. In: B
roMHead
e., d
ixoN
N. & i
BSeN
M.l. (
edS
.). Proc. 8
th
Int. Symp. on Landslides, Cardiff, June 2000. ,Vol.2: 615-620. Thomas Telford, London.
G
eNevoiS
r., t
ecca
P.r., B
erti
M. & S
iMoNi
a. (2000b) - Debris flows in Dolomites: experimental data from a monitoring system. In. w
ieczorecK
G.f. (e
d
.).
Proc. Second Int. Conf. on Debris Flow Hazards Mitigation: Mechanics, Prediction and Assessment, Taipei, Agosto 2000; 283-292.
G
iNGold
r.a. & M
oNaGHaN
J.J. (1977) - Smoothed particles hydrodynamics: Theory and application to non-spherical stars. Monthly Notices of the Royal
Astronomical Society, 181: 375-389.
G
ray
J.M.N.t., w
ielaNd
M. & H
utter
K. (1999) - Gravity-driven free surface flow of granular avalanches over complex basal topography. Proc. R. Soc.
London A 455: 1841-1874.
G
ullà
G., M
aNdaGlio
M.c. & M
oraci
N. (2006) - Effect of weathering on the compressibility and shear strength of a natural clay. Canadian Geotechnical
Journal, 43: 618-625.
G
ullà
G., M
aNdaGlio
M.c., M
oraci
N. & S
orriSo
-v
alvo
G.M. (2004) - Definizione degli elementi generali dei modelli geotecnici per l’analisi delle instabilità
superficiali per scorrimento-colata in Calabria Jonica. Atti XXII Convegno Nazionale di Geotecnica, Palermo, 22-24 Settembre 2004:127-134.
H
eiM
a. (1932) - Landslides and human lives (Bergsturz and Menschenleben). In: S
KerMer
N. (
ed
). Bi-Tech Publishers, Vancouver, BC, 196 pp.
H
uNGr
o. (1995) - A model for the runout analysis of rapid flow slides, debris flows and avalanches. Canadian Geotechnical Journal, 32 (4): 610-623.
H
uNGr
o. & e
vaNS
S.G. (1996) - Rock avalanche run-out prediction using a dynamic model. Proceeding 7
th
International Symposium on Landslides,
Trondheim Norway, 1: 233-238.
H
uNGr
o., e
vaNS
S.G., B
oviS
M. & H
utcHiNSoN
J.N. (2001) - Review of the classification of landslides of the flow type. Environ Eng. Geosci. VII: 221-238.
H
uNGr
o., l
eroueil
S. & P
icarelli
l. (2013) - The Varnes classification of landslide types, an update. Landslides DOI 10.1007/s10346-013-0436-y. ©
Springer-Verlag Berlin Heidelberg 2013.
H
uNGr
o., M
c
d
ouGall
S. & B
oviS
M. (2005) - Entrainment of material by Debris Flows. In: J
aKoB
M, H
uNGr
o (
edS
). Debris flow hazards and related
phenomena. Chapter 7: 135–158. Springer, Heidelberg (in association with Praxis Publishing Ltd)
H
utcHiNSoN
J.N. (1968) - Mass movement. In: f
airBridGe
rw (
ed
). Encyclopedia of geomorphology. Reinhold Publishers, New York, 688-695.
H
utcHiNSoN
J.N. (1988) - General report: morphological and geotechnical parameters of landslides in relation to geology and hydrogeology. In: Proceedings
of the 5
th
International Symposium on Landslides, Lausanne, 1: 3-35
ACkNOWLEDGMENT
All authors have contributed in equal manner to the
development of research and to the extension of memory.
The research described in this paper was financially supported
by the Project PON01_01869 (TEMADITUTELA).
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N. MORACI, M. PISANO, M.C. MANDAGLIO, D. GIOFFRE’, M. PASTOR, G. LEONARDI & S. COLA
40
Italian Journal of Engineering Geology and Environment, 2 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
i
verSoN
r.M. (1997) - The physics of debris flows. Reviews of Geophysics, 35: 245-296.
K
eefer
d.K. & J
oHNSoN
a.M. (1983) - Earthflows: morphology, mobilization and movement. USGS Professional Paper 1264.
l
aHuSeN
r.G (1996) - Detecting debris flows using ground vibrations. U.S. Geological Survey Fact Sheet 236-96.
l
ucy
l.B. (1977) - A numerical approach to the testing of fusion process. Astronomical Journal, 82: 1013-1024.
M
c
d
ouGall
S. & H
uNGr
o. (2004) - A model for the analysis of rapid landslide runout motion across three-dimensional terrain. Can. Geotech. J., 41 (6):
1084-1097.
M
aNdaGlio
M.c., M
oraci
N., G
ioffrè
d. & P
itaSi
a. (2015) - Susceptibility analysis of rapid flowslides in southern Italy. International Symposium on
Geohazards and Geomechanics, ISGG 2015. University of Warwick, United Kingdom. 26 (1), September 2015.
M
oraci
N., M
aNdaGlio
M.c. & i
elo
d. (2012a) - A new theoretical method to evaluate the internal stability of granular soils. Canadian Geotechnical
Journal, 49 (1): 45-58.
M
oraci
N., M
aNdaGlio
M.c. & i
elo
d. (2012b) - Reply to the discussion by Dallo and Wang on “A new theoretical method to evaluate the internal stability
of granular soils”. Canadian Geotechnical Journal, 49 (7): 866-868.
M
oraci
N., i
elo
d. & M
aNdaGlio
M.c. (2012c) - A new theoretical method to evaluate the upper limit of the retention ratio for the design of geotextile filters
in contact with broadly granular soils. Geotextiles and Geomembranes, 35: 50-60.
M
oraci
N., M
aNdaGlio
M.c. & i
elo
d. (2014a) - Analysis of the internal stability of granular soils using different methods. Canadian Geotechnical Journal.
Manuscript ID: cgj-2014-0006. Published on the web 22 April 2014, DOI 10.1139/cgj-2014-0006.
M
oraci
N., c
azzuffi
d, c
alvaraNo
l.S., c
ardile
G., G
ioffrè
d. & r
ecalcati
P. (2014b) - The influence of soil type on interface behavior under pullout
conditions. Geosynthetics, 32 (3): 42-50. ISSN: 1931-8189.
M
oraci
N., M
aNdaGlio
M.c. & i
elo
d. (2015) - Reply to the discussion by Ni et alii, on “Analysis of the internal stability of granular soils using different
methods”. Canadian Geotechnical Journal, 52: 1-7. DOI.org/10.139/cgj-2014-0495.
M
uSSo
a. & o
livareS
l. (2003) - Flowslides in pyroclastic soils: transition from “static liquefaction” to “fluidization”. In: P
icarelli
L. (e
d
.). Proc. Int.
Workshop on Occurrence and mechanisms of flow-like landslides in natural slopes and earthfills. Sorrento, May 2003: 117-128.
M
uSSo
a., f
ederico
f. & t
roiaNo
G. (2004) - A mechanism of pore pressure accumulation in rapidly sliding submerged porous blocks. Computers and
Geotechnics, 31 (3): 209-226.
o
livareS
l. & P
icarelli
l. (2003) - Shallow flowslides triggered by intense rainfalls on natural slopes covered by loose unsaturated pyroclastic soils.
Géotechnique, 53 (2): 283-288.
P
aStor
M., H
addad
B., S
orBiNo
G., c
uoMo
S. & d
reMPetic
v. (2009) - A depth-integrated, coupled SPH model for flow-like landslides and related
phenomena. Int. J. Numer. Anal. Meth. Geomech., 33: 143-172.
P
ierSoN
t.c. (1986) - Flow behavior of channelized debris flows, Mount St. Helens, Washington. In: a
BraHaMS
a.d. (
ed
.). Hillslope processes. Allen &
Unwin, Boston, 269-296.
P
ierSoN
t.c. (2005) - Hyperconcentrated flow-transitional process between water flow and debris flow. In: J
aKoB
M. & H
uNGr
o. (e
dS
). Debris flows and
related phenomena. Vol. 8: 159-196. Springer, Heidelberg.
P
ierSoN
t.c. & c
oSta
J.e. (1987) - A rheological classification of subaerial sediment-water flows. In: c
oSta
J.e. & w
ieczoreK
G.f. (e
dS
.). Debris flow/
avalanches: process, recognition and mitigation reviews in engineering geology. Volume VII: 1-12. Boulder, CO. Geological Society of America.
P
irulli
M. & P
aStor
M. (2012) - Numerical study on the entrainment of bed material into rapid landslides. Geotechnique, 62 (11): 959-972.
P
reStiNiNzi
a. & r
oMeo
r. (2000) - Earthquake-induced ground failures in Italy. Engineering Geology, 58 (3-4): 387-397.
Q
uecedo
M., P
aStor
M., H
erreroS
M.i. & f
erNa
´
Ndez
M
erodo
J.a. (2004) - Numerical modelling of the propagation of fast landslides using the finite
element method. Int. J. Numer. Methods Engng, 59 (6): 755-794.
r
icKeNMaNN
d. (1999) - Empirical relationships for debris flows. Natural Hazards, 19 (1): 47-77.
S
aSSa
K. (1985) - The mechanism of debris flows. In: Proceedings 11
th
International Conference on Soil Mechanics and Foundation Engineering, San
Francisco, 1: 1173-1176.
S
avaGe
S. B. & H
utter
K. (1989) - The motion of a finite mass of granular material down a rough incline. J. Fluid Mech., 199: 177-215.
S
uwa
H. (1989) - Field observation of debris flow. Proc. Japan-China (Taipei) Joint Seminar on Natural Hazard Mitigation, Kyoto: 343-352.
v
allaNce
J.w. (2005) - Volcanic debris flows. In: J
aKoB
M. & H
uNGr
o (e
dS
.). Debris flows and related phenomena. Vol 10. Springer, Heidelberg: 247-271.
v
arNeS
d.J. (1954) - Landslide types and processes. In: e
cKel
e.B. (e
d
.). Landslides and engineering practice, special report 28. Highway research board.
National Academy of Sciences, Washington, DC, 20-47.
v
arNeS
D.J. (1978) - Slope movement types and processes. In S
cHuSter
r.l. & K
rizeK
r.J. (
edS
.). Landslides, analysis and control. Special report 176:
Transportation research board, National Academy of Sciences, Washington, DC., 11-33.
Received July 2015 - Accepted November 2015
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