Document Actions

ijege-13_bs-stead-eberhardt.pdf

background image
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
85
DOI: 10.4408/IJEGE.2013-06.B-07
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
D
oug
STEAD
(*)
& E
rik
EBERHARDT
(**)
(*)
Simon Fraser University - Department of Earth Sciences - Burnaby, British Columbia, Canada (dstead@sfu.ca)
(**)
University of British Columbia - Geological Engineering - Vancouver, British Columbia, Canada (erik@eos.ubc.ca)
networks; numerical modelling; distinct element voronoi;
lattice spring
INTRODUCTION
During the last decade considerable advances
have been made in our understanding of the mechan-
ics of large landslides, yet considerable challenges
still remain to characterize and model the complex
mechanisms often involved. New measurement tech-
niques, notably borehole televiewer tools, photogram-
metry and LiDAR remote sensing, and real-time, high
resolution InSAR monitoring, are contributing un-
precedented amounts of data. Interpreting and apply-
ing this data, however, still remains largely subjective
as geological complexity and uncertainty continue to
pose major obstacles.
This paper will address these challenges by pos-
ing a series of questions focussing on what we per-
ceive to be some of the critical issues in landslide
and rock slope investigations. We will also summa-
rize both the state-of-the-art and the advances made
in recent years in data collection and geomechanical
modelling drawing from our own research and also
published case studies.
WHAT IS THE ROLE OF DAMAGE ME-
CHANISMS IN LARGE LANDSLIDES?
DAMAGE IN ROCK SLOPES
Damage mechanics in landslides studies can be
regarded as the characterization of rock mass and
ABSTRACT
Our understanding of the mechanics of large
landslides has improved considerably over the last
decade with the development of new, innovative data
collection methods, in conjunction with efforts to ac-
quire unique data sets through detailed monitoring of
several large rock slopes and the integration of these
with increasingly sophisticated computer modelling
techniques. In this paper the authors examine these re-
cent developments in the context of three major issues
that are considered fundamental to improved landslide
characterization. Firstly we consider the role of dam-
age mechanisms in large landslides, including what is
meant by damage processes, the types of damage and
the controls on damage distribution within the rock
slope. Secondly we discuss the role of kinematics in
the mechanics of large landslides considering the com-
plex interactions that exist between the influence of
scale, release surfaces and confinement, failure mecha-
nisms, persistence and rock bridges, and groundwater
pressures. Finally we discuss important advances in
the modelling of large landslides and how this contrib-
utes to an improved mechanistic understanding of their
spatial and temporal evolution. In particular, consistent
with our discussion of damage in large landslides, we
emphasise the developments of methods of modelling
brittle fracture on rock slope damage.
K
ey
words
: landslide mechanisms; brittle fracture; damage;
rock bridges; kinematics; remote sensing; discrete fracture
background image
D. STEAD & E. EBERHARDT
86
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
ΔL/L (Fig. 1c) superimposed by a gravitational in-
duced damage ΔRB/L. Failure of a slope in this case
is thus treated as sliding along a non-persistent shear
surface area. In traditional slope stability problems the
failure surface is considered fully persistent (Fig. 1a)
with a uniform factor of safety. In practice the poten-
tial failure surface is often not fully formed and may
require the failure of “patches” or small areas of rock
bridges (Fig. 1b). These rock bridges may reflect the
presence of proto-joints (as discussed by H
EncHEr
et
alii, 2012), sedimentary structures on bedding surfac-
es, or undulations due to tectonic activity. Damage can
similarly be characterized by considering the ratio of
the cumulative failed rock bridge area to the total area
of the potential failure surface.
CRACK PROPAGATION AND DAMAGE
Numerous recent studies in rock engineering have
recognized the importance of crack initiation, crack
damage and crack coalescence during brittle rock fail-
ure. These studies have incorporated different stress or
damage thresholds to derive new failure criteria that are
more applicable to brittle rock under low confinement.
Such tri-linear or S-shaped criteria (D
iEDEricHs
, 1999;
k
aisEr
& k
im
, 2008) have primarily been applied in
underground rock engineering. Recent studies how-
ever have demonstrated their applicability to landslide
(L
EitH
, 2012) and large open pit slopes (W
EssELoo
&
D
igHt
, 2009; E
bErHarDt
, 2009). Figure 2 shows that
the conditions for brittle dominated failure mechanisms
under high gravitational and low confining stresses
may be particularly relevant in the toe (and crest) area
of high rock slopes where stress-induced cracking may
lead to high damage concentration zones. In the inter-
discontinuity damage, and the consideration of the
evolution of the damage variables as a function of
disturbances to the effective stress state. The damage
variable, D, has been defined in numerous ways but
perhaps one the earliest definitions considers the ratio
of the amount of area of cracks/voids, ΔS
D
relative to
a selected area, ΔS, within the slope, i.e. D = ΔS
D
/ΔS.
Using this approach damage varies from zero for an
undamaged material to 1 for a fully failed element.
The development of damage results in a degradation
of the rock mass modulus and strength parameters.
Damage has been defined in numerous ways by con-
sidering the effects of damage such as reduced elas-
tic modulus, hardness or velocity. If we consider the
classical problem of a landslide where non-persistent
joints are present with intervening rock bridges, RB
(see Fig. 1), then the original percentage of rock
bridges is given by:
(1)
As the rock bridges gradually fail under the in-
fluence of downslope gravitational stress the damage
increases until D=1 and ΔRB goes to zero, such that:
(2)
This approach can be interpreted as combining a
pre-existing tectonic damage state due to jointing of
Fig. 1 - Characterization of rock slope damage related to
non-persistent joints and presence of intervening
rock bridges, with examples of: a) sliding surface
coinciding with fully persistent plane of weakness;
b) sliding surface comprised of non-persistent co-
planar joints and cohesive rock bridge elements;
c) classical definition and calculation of damage;
and d) non-coplanar joints and rock bridges
Fig. 2 - S-shaped brittle failure criterion and its applica-
tion to common stress conditions found in rock
slopes. Modified after D
ieDerichs
(1999)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
87
• Geologic processes associated with rock genesis
(intrusion, metamorphism, alteration)
• Geomorphic processes – glacial erosion, glacial
rebound, fluvial down cutting
• Earthquakes
• Precipitation and snowmelt events
• Long term creep
In classical damage mechanics the importance of cy-
clic loading and creep are both considered. The authors
suggest that these processes acting over 100’s to 1000’s
of years play a critical role in landslide development
which has received comparatively little attention particu-
larly when modelling landslide failure mechanisms. Cy-
clic processes result in fatigue of the rock slope and the
gradual accumulation of long term damage until a “criti-
cal slope damage threshold” is attained. Such processes
may be tectonic such as repeated earthquakes of widely
varying magnitude gradually removing rock bridges and
roughness on failure surfaces. They may also be geomor-
phic such as variations in topography (gravitational load-
ing), water pressures, thermal stresses, freeze-thaw and
uplift/erosion over time. g
riffitHs
et alii (2012) empha-
size the dynamic nature of landform evolution processes,
an essential pre-requisite for understanding the mechan-
ics of landslides and developing realistic geomechanical
models. These landslide damage types reflect the varying
stress paths that the slope is subjected to; it is this locally
changing stress-path that is the underlying driving mech-
anism of slope failure and which must be considered in
landslide models.
EXTERNAL AND INTERNAL EVIDENCE OF
SLOPE DAMAGE
Figure 3 shows selected examples of rock slope
damage based on surface observations, what we
term here as “external slope damage”. Y
an
(2008)
and t
uckEY
(2012) have demonstrated the utility of
LiDAR and photogrammetry surveys to map and as-
sess external rock slope damage using fractography
techniques. This includes the characterization of
damage density and differentiation between (1) intact
rock bridges, which describe intervals of intact rock
separating non-persistent discontinuity tips; and (2)
rock mass bridges, which describe larger intervals of
jointed rock between major structures such as faults.
LiDAR data, both terrestrial and airborne, provides
frequent evidence of external slope damage including
tensions cracks and geomorphic features. m
oorE
et
nal zones within landslides damage may be dominated
by shear localization under higher confinement. These
processes agree with the observed early indicators of
instability, with tension cracking at the slope crest and
heave/bulging at the toe, in accordance with the concept
of progressive failure advocated by early workers. The
process of brittle failure through crack propagation in
slopes is hence a fundamental damage dominated fail-
ure mechanism involving degradation of rock bridges,
destruction of asperities and roughness along potential
failure surfaces, and the development of through-going
step path failure. Together, damage processes in land-
slides range from the initial microscale (intra/intergran-
ular microcracking) through to meso- and macroscale
fragmentation and comminution of the rock slope mass
during global slope failure.
SPATIAL AND TEMPORAL DAMAGE CON-
TROLS
The evolution of damage in a slope varies both
spatially and temporally. In consequence certain areas
of a slope may be pre-disposed to increased damage
either in relation to driving forces, water pressures or
due to the existence of pre-existing tectonic damage.
Numerous workers have shown the inter-relationship-
between structures and rock mass quality where the
latter essentially reflects pre-existing damage. How-
ever, the spatial distribution of damage in a rock slope
or landslide has received very little attention to date.
In numerous field-based landslide investigations, the
authors have repeatedly recognized characteristic
damage distributions associated with variations in:
• Slope topography
• Failure surface morphology
• Failure surface geometry
• Failure mechanism
• Lithological variations
• Geological structure
Such damage is evident in field observations,
geophysical surveys, microseismic data and in the re-
sults from geomechanical models. Evidence of spatial
distribution of rock mass damage in the field includes
the location of tension cracks, back scarps, lineaments
and troughs, compression induced bulging/heave, and
outcrop fracturing/fragmentation.
Temporally, numerous processes may increase
damage within a rock slope over time including:
• Tectonics - folds, faults, uplift, deformation phases
background image
D. STEAD & E. EBERHARDT
88
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
alii (2006) reported reductions in seismic velocity to
less than 1500 m/s (compared to intact gneiss veloci-
ties of 3500-6500 m/s) representing an estimated 17%
volume of voids, or damage, in the rock mass. Numer-
ous microseismic surveys in open pit rock slopes have
indicated rock slope damage associated with excava-
tion progress and geological structures such as faults.
INFLUENCE OF SLOPE TOPOGRAPHY ON
ROCK SLOPE DAMAGE
L
EitH
(2012) through field observations and geo-
mechanical modelling showed the influence of exhu-
mation induced brittle fracture (tensile jointing) along
alpine valleys in Switzerland and the influence of this
damage on both glacial U-shaped valley development
and rock slide occurrence. Leith showed that the spatial
distribution of the damage in two dimensions is related
to the present day topography with clear bounding
elevations of rock mass damage on the valley walls.
J
ackson
(2002) reports a similar possible influence of
glacial ice elevation and related damage on the lower
alii (2011) have shown through distinct element mod-
elling how such landslide induced damage can cause
amplification of earthquake waves resulting in seismi-
cally induced internal damage. An excellent example
of this is the Madison Canyon rock slide (H
aDLEY
,
1964), which was triggered by the 1959 Hebgen Lake
magnitude 7.5 earthquake in Montana, USA (Fig. 4).
Pre-failure evidence of gravitationally induced frac-
turing was present, which could have served to am-
plify the earthquake effects.
Examples of “internal slope damage” include
both borehole and three-dimensional (3-D) geophysi-
cal observations. Borehole logging using acoustic
televiewers on Turtle Mountain, Alberta, Canada
(s
pratt
& L
amb
, 2005) showed evidence of damage
and disturbance associated with previous rock slope
instability. Geophysical surveys and microseismic
monitoring have provided evidence of internal rock
slope damage in the form of anomalous reductions in
seismic velocities and other geophysical properties. At
the site of the Randa rockslide, Switzerland, g
rEEn
et
Fig. 3 - Examples of external rock slope damage: a) antiscarp associated with deep-seated gravitational displacement; b)
tension cracks associated with head scarp of Frank Slide, Alberta, Canada; c) Palliser rockslide, Alberta, Canada
and associated bucking and toppling damage; d) large, 50+ m long intact rock fracture damage associated with
Newhalem rock slide, Washington, USA
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
89
slopes of Turtle Mountain, Alberta, which has received
little attention in previous modelling studies. Deeply
incised valleys may be. observed at the location of ma-
jor rockslides including Vajont, Italy (g
Hirotti
, 2006)
and Eiger, Switzerland (o
ppikofEr
et alii, 2008) (Fig.
5). The influence of such major geomorphic processes
has yet to be fully integrated into geomorphic stress-
path modelling of landslide mechanisms. E
bErHarDt
et alii, (2004) showed clearly through FEM, DEM
and brittle fracture modelling (FDEM) the influence
of convex glacially over-steepened topography on the
spatial distribution of rock slope damage at the Randa
rockslide. The mechanical influence of glacier retreat
on landslide development has been the subject of some
controversy over recent years particularly with regard
to the concept of glacier-debutressing (m
c
c
oLL
et alii,
2010). Notwithstanding there is considerable evidence
for the onset of slope instability following deglaciation.
The authors suggest that the initiation of failure may
result through a combination of changing kinematics
(kinematic release) and slope damage in the valley
walls due to glaciation related processes (high pore
water pressures, oversteepening, brittle fracture, etc.).
Figure 6a shows the Mitchell Creek landslide in
northern British Columbia, Canada, discovered during
mineral exploration and described by c
LaYton
et alii,
(2013). This landslide is actively deforming and has
been the subject of geotechnical investigations using
cored drillholes, acoustic televiewers, field mapping
and photogrammetry. It is currently being monitored
using combined borehole, surface survey and remote
sensing instruments. The slope damage mechanisms
are complex involving components of topping and
translation with rock mass degradation due to com-
bined tectonic, glacial and gravitationally induced
damage. A particularly interesting facet of this land-
slide is the ability to date the onset of the instability fol-
lowing recent glacial retreat using aerial photography,
Fig. 6b. Numerous studies on deep-seated gravitational
displacements have shown the characteristic external
damage evidence such as antiscarps and grabens.
Many of these landslides are believed to have initi-
ated following deglaciation. In engineering projects
involving proximity to deep-seated gravitational dis-
placements (e.g., hydroelectric and tunneling projects),
internal evidence of slope damage has been noted.
An important feature recognized in large open
pit slopes is the existence of a zone of relaxation
Fig. 4 - The 1959 Madison Canyon rockslide, Montana,
USA, associated with earthquake triggering and
seismic amplification damage
Fig. 5 - Geomorphic damage related to deeply incised
valleys associated with: a) Vajont rockslide (af-
ter G
hirotti
, 2006); and b) Eiger rockslide (after
o
pikoffer
et alii, 2006)
background image
D. STEAD & E. EBERHARDT
90
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
and damage sub-parallel to the pit slope profile. This
zone varies in width and is related to both slope ge-
ometry and blasting disturbance. It is important to
realize that the geomorphic evolution process of a
natural slope, which can be equated to the bench ex-
cavation sequence of an open pit slope, may simi-
larly produce important damage generating stress
relief-stress concentrations behind the slope face. A
practical consequence of the existence of a zone of
relaxation behind the rock slope is that the hydroge-
ology may change as some fluid flow pathways open
and others close, or the rock mass may be subjected
to increased weathering and/or ravelling leading to
surficial instability. It is in this area that we are often
obliged to make estimations of the rock mass quality
(e.g. GSI, RMR, etc.); clearly these estimates are li-
able to be lower bound values with rock mass quality
improving with distance into the rock slope. It can
generally be expected that the external surficial dam-
age should be greater than the internal damage.
DAMAGE RELATED TO FAILURE SURFACE
GEOMETRY AND MECHANISMS
The failure surface geometry has an important in-
fluence on rock slope damage. Figure 7 shows concep-
tual examples of the damage processes associated with
varied failure surface geometries, as they relate to dif-
ferent failure mechanisms. Where rock slope failures
have moved parallel to pre-existing, highly persistent,
planes of weakness (e.g., bedding, faults, etc.), the
amount of induced damage during failure is reduced.
W
oLtEr
et alii (2013) and m
assironi
et alii
(this volume) have investigated in detail the failure
surface morphology at Vajont (Fig. 8). Clearly this
failure surface does not represent the planar surface
morphology shown in Fig. 7a but bears more resem-
blance to Figs. 7b-c. The undulations recorded on the
existing eroded failure scar at Vajont are however
3-D in nature and are formed by two generations of
approximately N-S to NNW and E-W to WNW-ESE
striking folds. The folds result in complex dome and
basin to crescent and mushroom interference pat-
terns. The nature of this folding in the failure surface
varies spatially allowing the definition of failure sur-
face morphological domains (W
oLtEr
et alii, 2013).
The authors suggest that this variation in failure sur-
face morphology may have been reflected variations
in the induced damage preceding and during the fail-
ure at Vajont. Damage would have been associated
with the translation of failure blocks during dilation
of the rock mass as it overrode these fold undula-
tions. The importance of the claybeds at Vajont has
Fig. 6 - The Mitchell Creek Landslide, British Columbia, Canada, showing: a) slide boundaries, instability damage zones and
location of valley glacier; and b) history of glacier retreat and development of landslide damage indicators derived
from air photographs
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
91
Fig. 8 - Failure surface morphology domains at Vajont, Italy, and relation to tectonic folding
Fig. 7 - Failure surface morphology/shape and associated damage mechanisms
background image
D. STEAD & E. EBERHARDT
92
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
phy both along dip and strike. Long term progressive
damage may thus also be influenced by anisotropic
roughness in three-dimensions which when consid-
ered along with varying kinematics may relate to
the movement of the landslide as a series of blocks.
Similar variations in the slide surface morphology
and thickness have been recorded at other landslides
exhibiting discrete block movements (k
aLEncHuk
,
2010) The authors suggest that anisotropic failure sur-
face morphology (or roughness) may be an important
control on 3-D damage mechanisms (and progressive
displacements) in landslides which requires further
consideration in future geomechanical models.
STRUCTURAL CONTROLS ON ROCK SLOPE
DAMAGE
Structural controls on rock slope failure have
been emphasised by numerous authors, both with
respect to rock mass quality and rock slope kinemat-
ics. Folds and faults have an important influence on
pre-existing rock slope damage, often predisposing
areas of a rock slope to react to gravitational stress
through varied failure and damage mechanisms. At
Vajont the presence of the Col Tramontin and Col
delle Erghene Faults in the eastern half of the slide
have resulted in a lower rock mass quality and pre-
dominantly shear type mechanisms of damage. In
the western half of the Vajont slide the rock mass is
less disturbed by faulting, and failure corresponds to
a more active-passive nature due to the higher rock
mass quality (W
oLtEr
et alii, 2012) (Fig. 10). b
ri
-
DEau
et alii (2005, 2009) and D
onati
et alii (2012)
describe the influence of faulting and shearing on
rock slope stability at the Hope Slide. Here the
failure debris is highly fragmented reflecting both
the pre-existing damage and the gravitationally in-
been discussed by numerous workers. We suggest
that the true importance of the beds is a complex
inter-relationship between the fold structure undu-
lation amplitudes and wavelengths, and the claybed
thickness. Using a similar concept of displacement
relative to claybed thickness and roughness used in
the Q rock mass quality system (b
arton
et alii, 1978)
we suggest that movement could have involved a
stick-slip damage mechanism with movement along
the claybeds locking up at the folded bed undulations
(Fig. 9). High pore water pressures at the foot of the
slope and progressive gravitational induce dam-
age could allow the overriding of the fold undula-
tions, producing alternating cycles of clay (slip) and
roughness (stick) dominated displacement-damage
mechanisms. These cycles may have been of short
time duration, less than the displacement monitoring
frequency, resulting in their combined expression in
the overall recorded slope displacement.
Such movement might also be expected to be
expressed in irregular microseismic activity. Clearly
the failure surface morphology at Vajont varies sig-
nificantly hence it might be expected that the displace-
ment modes and damage mechanisms likewise vary
in relation to structural morphological controls. At
the Aknes rock slope, similar variations of the schis-
tosity may also be present in the sub-surface topogra-
Fig. 10 - Finite-element model simulating active-passive
zones in the western section of the Vajont rock-
slide, Italy. After W
olter
et alii (this volume)
Fig. 9 - Hypothesized interrelationship between failure
surface morphology and clay bed thickness, and
folded scarp surface at the Vajont rockslide, Italy
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
93
LITHOLOGICAL CONTROLS ON ROCK
SLOPE DAMAGE
The lithology in a rock slope may have an im-
portant influence on damage mechanisms and the de-
velopment of failure. It is suggested that some rock
types may act to concentrate brittle rock mass dam-
age, whereas other rock types may respond in a more
ductile manner dominated by yield and shear mobili-
zation. In rock slopes composed of principally brittle
rock types, behavior may be characterised by sliding
along structures and associated intact rock bridge dam-
age. In other rock slopes involving highly fractured
rock masses or more ductile rock types, damage and
failure may be in the form of shear induced localiza-
tion or “plastic yield” as modelled in most numerical
codes. b
Enko
(1998) clearly showed the importance
of lithology in toppling rock slope failures in Coal
Measures strata (mudstones, sandstones, conglomer-
ates, coal) (Fig. 13). The influence of lithology on the
failure mechanism at Vajont (conglomerate beds) was
duced damage. Similarly, in a detailed study of Tur-
tle Mountain, Alberta, site of the 1903 Frank Slide,
b
riDEau
et alii (2012) and p
EDrazinni
et alii (2008)
correlate the rock mass quality (GSI) and the insta-
bility mechanisms with the influence of fold induced
damage and joint sets. Figure 11a shows modelling
interpretations of fold induced damage and its influ-
ence on the Frank Slide. b
Enko
& s
tEaD
(1998) in
early distinct element modelling clearly showed the
importance of damage in the hinge of the anticlinal
fold and thrusting at the toe on instability. Figure 11b
illustrates the results of research by p
EDrazinni
et alii
(2008) which emphasized the importance of struc-
ture on rock mass quality on the present day stabil-
ity of Turtle Mountain. b
aDgEr
(2002) also empha-
sized the importance of fold-related structures (and
pre-existing tectonic damage) on rock slope failure
mechanisms, Fig 11c. Figure 12 shows the influence
of folding, faulting and foliation on induced damage
for different engineered and natural rock slopes.
Fig. 11 - Structurally induced damage: a) distinct-element model simulating damage in the fold hinge of the Frank Slide, Alberta,
Canada (after B
enko
& s
teaD
, 1998); b) damage and current instability at Turtle Mountain, site of the Frank Slide (after
P
eDrazinni
et alii, 2008); and c) influence of folding on damage and kinematic (after B
aDGer
, 2002)
background image
D. STEAD & E. EBERHARDT
94
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
noted by g
Hirotti
et alii (1992) and is also investigat-
ed by W
oLtEr
et alii (this volume) The weaker mud-
stones and sandstones were observed to have yielded
through shear damage whereas the stronger sandstone/
conglomerate beds acted as concentrators for tensile
damage, playing an important role in the overall slope
stability. J
ackson
(2002) in a study of the factors con-
trolling rockslides in the Canadian Rocky Mountains
emphasised the role of landform evolution where rock
slope instability is promoted by slope retreat of resist-
ant rock masses overlying weaker rock masses. Ero-
sion and yield of the weaker rock types is suggested
to induce brittle damage in the overlying resistant rock
masses leading to progressive and repeated instability.
Potential damage mechanisms within a slope are hence
a complex interaction of variations in lithology and
geological structure.
TEMPORAL DAMAGE
When considering the progressive failure of a
landslide it is important to consider not only spatial
damage variations but also temporal variations in dam-
age. Slide surfaces coinciding with persistent discon-
tinuities with negligible intact rock bridges may fail
in a rapid brittle manner. More complex undulating or
multiplanar failures may involve variations in damage
both spatially and temporally. s
tEaD
et alii (2007) de-
scribe the stages in the brittle failure of a rock slope
with respect to failure processes as primary, secondary
and tertiary. It is suggested that these stages also reflect
similar changes in damage operating in a rock slope.
s
uLLivan
(2007) and m
ErcEr
(2006) describe changes
in the deformation of a rock slope using time-depend-
Fig. 12 - Damage associated with geologic structures: a) fold-related damage in footwall slope in an open pit coal mine in
eastern British Columbia, Canada; b) damage associated with a fault in coal measures; c) foliated structure of the
Aknes rock slope, Norway (map after k
velDsvik
et alii, 2008; photograph by D. s
teaD
)
Fig. 13 - Influence of lithology on distinct element model-
ling of flexural toppling failure in coal measure
rocks. Modified after B
enko
(1998)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
95
form of slope damage maps arising from gravitation-
ally induced displacements
The authors have developed discrete fracture net-
work engineering techniques in an attempt to charac-
terize damage associated with landslides both in situ
and in particularly in geomechanical models. In this
approach damage can be considered in terms of D
10
(number of cracks/unit length), D
21
(length of cracks/
unit area) and D
31
(length of cracks per unit volume).
This approach is particularly useful in characterising
damage during brittle fracture modelling of rock slopes.
WHAT IS THE ROLE OF KINEMATICS IN
LARGE LANDSLIDES?
Appreciation of the kinematic controls is an impor-
tant step toward understanding the mechanics of land-
slides. At their simplest rock slides are often analysed as
two dimensional plane strain problems assuming elastic
blocks and sliding along continuous planes. In practice
the necessary conditions for plane strain analysis are
rarely met with most landslides incorporating there-
dimensional aspects. In this section of the paper we will
consider the kinematic controls on rock slope failures
and their inter-relationship with geological structure, to-
pography, rock mass quality, geomorphology and time.
SCALE AND KINEMATICS
The scale of a rock slope predetermines the useful-
ness of simple stereographic techniques in kinematic
analysis. The latest version of DIPS v.6.0 (r
ocsciEncE
,
2013) incorporates an interactive method of determin-
ing the kinematic feasibility for planar, wedge, flexural
and direct toppling modes of failure. The scale of the
rock slope however usually limits the application of this
method to smaller rock slopes where the likelihood of
joint persistence being of sufficient size to enable kin-
ematic release is possible; joint set persistence provides
an effective cut-off for kinematic block size. As slopes
ent stages. Sullivan recognised five stages- elastic,
creep, cracking and dislocation, collapse (failure)
and post failure. Figure 14 modified after D
ick
et alii
(2013) shows failure divided into regressive, progres-
sive and post-failure stages. The onset of failure occurs
at the transition between the regressive and progressive
stages at which time the rate of damage increases in the
rock slope. Laboratory acoustic emission studies by the
authors on varied rock types have shown similar stages
in the acoustic emission activity associated with creep
mechanisms. This behavior in a low confining stress
environment can be interpreted as brittle damage dom-
inated creep mechanisms. Landslide monitoring can be
interpreted based on these slope deformation stages (or
slope damage stages). Inverse velocity analyses (f
uko
-
zona
, 1985; R
osE
& H
ungr
, 2008; E
bErHarDt
, 2008)
have been used to predict the time to failure of sev-
eral unstable slopes based on displacement monitoring
data. These authors however caution against their use
in the prediction of failure where brittle failure occurs
by sliding along discrete surfaces. Using the analogy
of slope displacement and slope damage stages in brit-
tle rock masses, it is suggested that acoustic emission
and microsesimic data (combined with brittle fracture
modelling) may be amenable to similar data process-
ing methodologies for slope failure prediction.
Previous methods of displacement monitoring in
slopes were based on point measurements, for exam-
ple involving survey prisms. These methods indicate
the regressive and progressive displacement stages at
discrete points and hence localized external or surfi-
cial damage. With the recent introduction of InSAR
and RADAR slope monitoring techniques the dis-
placement is recorded over wide areas at pixels that
can be displayed in point cloud format. Slope radar
technologies allow improved temporal estimates of in-
creasing/cumulative displacements (reflecting external
damage) but also the ability to spatially correlate dam-
age/displacements with rock mass quality variations,
structures and lithology. Linear zones of displacement
have for example been correlated with release struc-
tures indicating progressive damage on these features
with time. g
iscHig
et alii (2009, 2011) used InSAR to
monitor slope displacements at the site of the Randa
rockslide and were able to delineate release structures
through such movements. Interpretation of RADAR/
InSAR data may provide information on the spatial
distribution of surficial/external slope damage in the
Fig. 14 - Stages of slope deformation and damage in rock
slopes. Modified after D
ick
et alii (2013)
background image
D. STEAD & E. EBERHARDT
96
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
increase in height over 50 m, the applicability of kin-
ematic analysis becomes more limited to rock slopes
containing continuous structures such as bedding and
major faults. Kinematic methods are also generally
limited to simple block shapes and through-going frac-
tures. Complex failure mechanisms such as biplanar,
multiplanar, step-path and buckling are not easily han-
dled by simple daylighting oriented kinematic analysis
techniques. Caution is required not to oversimplify the
kinematics of landslide failure mechanisms to fit within
the limitations of conventional kinematic analysis.
BLOCK SHAPE AND KINEMATICS
The shape of blocks related to landslides are funda-
mentally three dimensional. Even a simple mechanism
such as planar failure, often depicted and analyzed in
two-dimensions, involves a hexahedral block with var-
ying orientation of lateral and rear release surfaces. In
a traditional stereographic analysis, the kinematics of
such a hexahedral block will be treated as both a day-
lighting planar failure along a basal surface and wedge
intersections - perhaps not considering appropriately
the block theory aspects of hexahedral failure. The rou-
tine kinematic and limit equilibrium analysis of rock
slope wedge failures has been simplified over recent
decades to consider all wedge failures as either tetrahe-
dral blocks which may or may not be truncated in the
upper part by a tension crack. Failure is normally con-
sidered to be by translation along the line of intersec-
tion or on a more adverse dipping plane. The rotation
of wedges was considered in early rock wedge analyses
but has largely been ignored in recent research. H
ungr
& a
mann
(2011) show the importance of considering
rotational wedge movement of rock wedges. No rou-
tinely available wedge analysis software considers the
kinematic case of a pentahedral wedge where the base
of the tetrahedral block is truncated by a basal slid-
ing surface such as bedding or a fault. Such a failure
mechanism has been increasingly recognized in large
open pits and is also prevalent in natural slopes and has
been termed a “non-daylighting” wedge. In this case
the plunge of the line of intersection would not daylight
and the role of the basal surface is critical.
To treat complex block shapes, block theory has
been used to clearly demonstrate its importance with re-
spect to landslide kinematics and controlling instability.
b
riDEau
(2010) undertook block theory analysis in asso-
ciation with 3-D numerical modelling to investigate the
role of block shape on instability. Discrete Fracture Net-
work approaches have been used to indicate the stability
of kinematic keyblocks in rock slopes (r
ogErs
et alii,
2006; m
ErriEn
-s
oukatcHoff
et alii, 2011) using codes
such as Fracman and Resoblock, respectively. Recent
work by t
Hompson
(2011) and E
LmoutiiE
& p
oropat
(2011) shows great promise in incorporating block
shape, size and orientations into rock slope analysis with
the ability to show the dynamic failure of rock slopes as-
sociated with progressive sliding and removal of unsta-
ble blocks. It is suggested that a combined approach to
rock slide analyses using discrete fracture networks and
geomechanical models will allow improved understand-
ing of landslides failure mechanisms as data collection
and monitoring techniques continue to evolve.
ROTATION VS. TRANSLATION
Steeply plunging wedges may rotate out of the
slope or topple. Rotational-translational planar failures
are also commonly observed particularly where plung-
ing folds are present within a rock slope. Simple 3DEC
analyses (Fig. 15) clearly show the rotation of slope
parallel blocks (and hence kinematically non-daylight-
ing) out of a slope with a down plunge component of
displacement. Such down plunge block movements are
evident at Vajont on a small scale and may also have
been important kinematically in the main slope fail-
ure. Rotation in slope failures has predominantly been
considered out of the slope in the slope dip direction
(+/- 20 degrees) for the case of toppling failure and can
be considered as dip rotational mechanisms. b
riDEau
&
s
tEaD
(2011) use 3DEC to investigate the influence of
the orientation of lateral release, rear release and basal
surfaces on the kinematics of toppling failure mecha-
nisms. Considerable differences in slope displacements
and damage inflicted on the rock slope are associ-
ated with oblique dip rotation mechanisms. b
riDEau
&
s
tEaD
(2012) also performed a similar 3DEC kinematic
modelling study on translational planar failure mecha-
nisms showing a similar importance of kinematics in
determining both failure mode and slope displacement.
Few workers apart from H
ungr
& a
mann
(2011) and
s
tEffEn
(Personal communication, 1981) have con-
sidered kinematic rotation about vertical axes or what
might be considered kinematically as plan rotational
failures. Hungr and Amann used a combined approach
of analytical methods, limit equilibrium and 3DEC
analysis while Steffen and other earlier workers used
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
97
during modelling of the Vajont landslide using DDA
and DEM methods, respectively, influences the mobi-
lized shear strength required at failure and the damage
mechanism prior to failure. Internal distortion (dila-
tion) during slope failure is also related to the blocki-
ness of the rock mass and the shape of the failure sur-
face and may be an important damage mechanism in
large landslides. At the Revelstoke dam abutment in
B.C., Canada, c
orkum
& m
artin
(2004) used 3DEC
to clearly demonstrate the important role of blocki-
ness (or number of blocks) in modelling rock slope
displacement and dilation. At Vajont, m
EncL
(1988)
discussed possible internal distortion in the mid slope
where the chair back meets the seat of the failure sur-
face. W
oLtEr
et alii (this volume), Fig. 16, have shown
clearly the influence of block size on potential inter-
nal slope damage (dilation). Internal dilation within
slopes leads to zones of high fracturing throughout
the slope. At the Aknes rock slope, major fractures
are distributed along the slope reflecting significant
damage and internal dilation. Modelling results using
DEM codes have clearly shown the internal dilation
vector algebra limit equilibrium approaches.
The analysis of slope monitoring data frequently
shows evidence of failure kinematics involving both
plan and dip rotation. However our understanding of
such failure mechanisms cannot be furthered through
simplified 2-D models and requires the use of 3-D
codes such as 3DEC, FLAC3D and Slope Model.
JOINT SPACING, PERSISTENCE AND KINE-
MATICS
The importance of considering the influence of
joint spacing and persistence on rock slope kinematics
has been demonstrated by b
riDEau
(2010) and b
ri
-
DEau
et alii (2012). Change in the spacing of joint sets
has long been known to change the kinematics of rock
slope failure and was demonstrated by H
EncHEr
et alii
(1996) and in more recent modelling studies by b
ri
-
DEau
(2010). The spacing and persistence of joint sets/
major structures also controls the number of blocks
that are considered in the landslides. Research by s
i
-
tar
& m
cLaugHLin
(1997) and Wolter et alii (this vol-
ume) has shown that the number of blocks considered
Fig. 15 - Rotational-translation failures and DFN based block removal kinematic analysis: a) simple 3DEC simulations of rota-
tional-translation failure; b) rock slope with rotational-translational failure; c) joint controlled rotational-translational
block failure at Vajont, Italy; d) block movements identified at Aknes, Norway (after k
velDsvik
et alii, 2008); and e)
DFN-based (Siromodel) block removal based on stability of fully formed blocks (after e
lmouttie
& p
oropat
, 2011)
background image
D. STEAD & E. EBERHARDT
98
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
associated with varied failure mechanisms (b
riDEau
& s
tEaD
, 2012; k
inakin
, 2005; a
Lz
oubi
, 2009). It is
suggested that internal dilation within a landslide is
often an essential component of kinematic release that
has received insufficient attention both in field studies
and in geomechanical modelling.
RELEASE SURFACES AND KINEMATICS
Release surfaces are an essential component of
kinematic release for a landslide but are often ignored
in field characterization studies unless 3-D numerical
models are undertaken. Otherwise, they are assumed
to be present but considered to be of negligible impor-
tance. What constitutes a “release surface” is in reality
complex and often related to the scale of the instabil-
ity. In small-scale rock slope failures, joint sets may
provide adequate rear and lateral release. As the scale
of the instability increases, release is less likely to be
provided by discrete joints but may be provided by:
• Discontinuity step paths - along the dip or strike di-
rection
• Major discrete structures - faults or shear surfaces
• Lithological contacts
• Combined jointing/intact rock fracture
• Slope excavation - i.e. anthropogenic
• Topographic features - e.g. gullies
• Previous adjacent instability.
Fig. 16 - Influence of block size on failure mechanism and
damage, Vajont rockslide, Italy. After W
olter
et
alii (this volume)
Fig. 17 - Lateral confinement mechanisms and major landslides. After B
riDeau
(2010)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
99
as assumed in W
YLLiE
& m
aH
(2004). This orienta-
tion minimizes frictional resistance on the lateral re-
leases surfaces (if horizontal stresses are assumed to
be zero). If the lateral releases are sub-vertical in dip
or not parallel to the dip direction of the slope then the
effect of gravitational stresses would be to mobilise
an additional lateral release shear strength component.
Tectonic stresses, if present, may result in elevated
stresses in the horizontal direction acting normal to
the lateral releases surfaces. It is also possible that lat-
eral release surfaces may be stepped (Fig. 18b) and
water pressures may act on lateral release surfaces in
addition to the conventionally considered rear release
or tension crack water pressures
Figures 18c-e, show plan views of Type II con-
verging sub-vertical lateral release surfaces where the
angle (α) between the release surfaces may play an im-
portant role in failure kinematics. If no tension crack or
rear release surface is present, the geometry is that of
a pentahedral non-daylighting wedge. The intersection
between the lateral release surfaces may have a steep
plunge with sliding and failure being determined by
the dip and dip direction of the basal sliding surface.
In practice the pentahedral wedge could vary from an
over-hanging rotational (or toppling) wedge where the
lateral release intersection plunges into the slope to an
active-passive pentahedral wedge with a low plunge
lateral release surface intersection and a daylighting
basal release surface. The pentahedral wedges may be
asymmetric or symmetric, and this would influence
the failure mechanism (sliding/rotation). The angle
(α) could have an important kinematic confinement
Where closely spaced non-persistence joints ex-
ist, lateral release may be provided by stepping be-
tween sub-parallel joints. Release may thus need to
overcome more resistance to sliding than that for
continuous major structures. Release on faults will
be influenced by the orientation of fault surface fea-
tures such as slickensides and fault steps. Lithological
contacts may form release surfaces where high angle
bedding or boundaries between rock types of varying
mechanical properties exist. Where non-persistent
lateral release surfaces are present then failure may
be associated with progressive damage of intact rock
bridges and degradation of surface roughness. Rota-
tional moments greater than the tensile strength of
the rock may allow lateral release of wedges through
extension of pre-existing joints and rock bridge fail-
ure (H
ungr
& a
mann
, 2011). In many cases, slope
excavation or geomorphic processes may remove
kinematic restraint. Such failures may be intimately
linked with slope erosion through undercutting, gul-
lying and drainage incision. b
riDEau
(2010) examined
several major rockslides in relation to their kinematic
constraints and recognized three intuitive kinematic
boundary conditions:
• Failure confined on both sides - bounding structures
required for lateral release
• Failure confined on one side (structure required) and
unconfined on other side due to topographic fea-
ture (gullying)
• Failures unconfined on both sides – no structural
release required (topographic promontory/nose).
The orientation of lateral and rear release surfaces
with respect to landslide failure mechanism and dam-
age has not been widely investigated or reported in the
literature. Figure 17 shows a conceptual classification
of landslides with respect to lateral and rear release
orientation. Three main classes of landslide lateral re-
lease can be recognised:
• Conventional lateral release surfaces at right angles
to the strike of the slope
• Convergent lateral release surfaces intersecting be-
hind the slope crest
• Divergent lateral release surfaces with a notional in-
tersection in front of the slope.
Figure 18a shows plan views of a Type I planar
failure with vertical lateral release surfaces as defined
by H
oEk
& b
raY
(1981). These may fail in a direc-
tion +/- 20 degrees of the dip direction of the slope
Fig. 18 - Plan view diagrams showing: a, b) Type I lateral re-
lease sub-perpendicular to strike of slope; c-e) Type
II lateral release pentahedral wedges/hexahedral
blocks with convergent lateral release surfaces; and
f, g) Type III divergent lateral release surfaces and
influence on damage associated with failure
background image
D. STEAD & E. EBERHARDT
100
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
effect; with obtuse angles of intersection between
the lateral release surfaces, confinement would be
reduced and horizontal forces if present may act to
displace the wedge out of the slope (Fig. 18d). With
an acute angle (α) between the lateral release surfaces
there would be significant confinement acting on the
pentahedral wedge and horizontal forces could act to
clamp the wedge into the slope (Fig. 18e). The size of
the angle (α) is analogous to the ‘wedge angle’ in con-
ventional tetrahedral wedge analysis (H
oEk
& b
raY
,
1981) and the influence of horizontal forces in rela-
tion to the inter-lateral release surface angle is similar
conceptually to the influence of wedge shape noted for
roof wedges in tunnels (g
ooDman
1989). However,
these effects have not been considered in the analysis
of pentahedral non-daylighting wedges particularly
with respect to the inter-relationship between failure
kinematics and slope damage required for failure. It
should also be noted that non-daylighting pentahedral
wedges may also fail through composite basal surfaces
involving failure of intact rock bridges. If a rear re-
lease surface or tension crack is present sub-parallel
to the slope strike then this reflects the geometry of a
hexahedral block failure sliding along the basal surface
and involving varying components of frictional resist-
ance due to the lateral release surfaces. Applying block
theory, the blocks defined by the converging lateral
release surfaces in Fig. 18 would be considered re-
movable. The blocks bounded by the diverging lateral
release surfaces, Type III (Fig. 18f,g), are not however
amenable to block removal and failure could be associ-
ated with increased damage/intact rock fracture. This
trend of release surfaces could result in a pentahedral
failure mechanism controlled not by the lateral release
surfaces but by the spacing of sub-parallel joint sets
(Fig. 18g). It is suggested that failure might be more
liable to occur by progressive unravelling of smaller
pentahedral keyblocks allowing the constraints due to
block theory imposed on the slope to be overcome.
This simple conceptual consideration shows the
importance of considering the orientation of release
surfaces in landslide investigations and not relying
solely on conventional kinematic planar and tetra-
hedral (wedge) methods of analysis. Further work is
ongoing to characterise major rockslides according
to the lateral release geometry and to constrain rock
slope failure mechanisms by three-dimensional geo-
mechanical models such as 3DEC and Slope Model
(i
tasca
2013a, b) incorporating varied release ge-
ometries.
Fig. 19 - a) Progression of yielded elements with alternating seasonal high and low groundwater tables over a 800 year period
for the Campo Vallemaggia creeping rockslide, Switzerland. b) Progressive development of an internal shear surface
causing the slide to break into two halves and the corresponding redistribution of pore pressures. c) Modelled dis-
placement vs. time plots showing the seasonal stick-slip portions of the landslide motion, most clearly identifiable at
the resolution of 10 years. After s
mithyman
et alii (2009)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
101
High pore water pressure may induce damage during
landslide development and in contrast this damage/
fracturing may eventually lead to a reduction in pore
water pressure and a process of self-stabilization (al-
beit perhaps temporary). This process in a rock slope
may occur in cycles of induced movement/stabilisa-
tion with changes in fluid pressure as observed at Va-
jont. Each cycle could lead to a cumulative increase in
slope damage and catastrophic failure upon reaching a
critical slope damage threshold.
HOW CAN WE MODEL DAMAGE AND
KINEMATICS EFFECTIVELY IN LARGE
LANDSLIDES?
A detailed description of the state-of-the-art in the
numerical modelling of landslides is beyond the scope
of the current paper. The authors would refer readers
to s
tEaD
& c
oggan
(2012) for a more detailed discus-
sion. In this paper we propose to focus on the potential
use of modelling to characterize the principal subjects
discussed so far, that is, damage and kinematics.
As discussed earlier, rock bridges and discontinu-
ity persistence are a fundamental component of under-
standing the mechanisms of failure of large landslides.
Simulating the influence of rock bridges in rock slope
failure can be undertaken implicitly or explicitly.
Limit equilibrium methods consider rock bridges im-
plicitly by assuming an apparent cohesion and friction
associated with the percentage of rock bridges along
a potential failure surface. Continuum mechanical
models also treat rock bridges implicitly by incorpo-
rating rock bridge degradation through an equivalent
(although hard to define) reduction in rock mass prop-
erties. Conventional discontinuum methods assume
continuous through-going or interconnected joints and
hence also use a scaled apparent cohesion and fric-
tion along the joints to allow for rock bridges. Explicit
treatment of rock bridges has received considerable
attention during the last 10 years. In the simplest vari-
ants of the explicit method, discrete non-persistence
fractures are assumed and methods of simulating
intact rock/rock mass fracture between joints are ap-
plied to incrementally model the development of a
continuous failure surface. The most recent variants
of the explicit method of rock bridge simulation incor-
porate discrete fracture networks within a slope and
then simulate rock bridge fracture between the vari-
ously oriented discontinuities. These methods allow
GROUNDWATER AND KINEMATICS
The importance of groundwater pressures in
landslides has received considerable research in soil
landslides but is less well understood in rock slope
failures. p
icarELLi
et alii (2012) provide an excel-
lent summary of the state-of-the-art with respect to
groundwater studies in landslides. Recent research in
large open pits is of direct relevance to our improved
understanding of landslides, particularly in rock.
b
EaLE
(2009) and D
oWLing
et alii (2011) provide an
extremely useful review of the factors controlling
groundwater flow and pressures in rock slopes in ad-
dition to state-of-the-art discussion of the importance
of depressurisation of open pit slopes and the current
trends in groundwater-geomechanical modelling. The
importance of faults is highlighted in relation to their
acting either as conduits for groundwater flow or im-
permeable barriers. Faults in the latter case may act to
compartmentalize the groundwater within the slope;
this effect plays an important role in faulted terrains
(b
onzanigo
et alii, 2007). b
EaLE
(2009) discusses the
varied approaches to groundwater modelling in rock
slopes and concludes that in most cases an equivalent
porous medium approach has proved to be adequate
for slope design.
In landslide studies, although groundwater pres-
sures are routinely considered in two-dimensional
limit equilibrium methods, their consideration in 3-D
analyses is less common. Data uncertainty with re-
spect to measured groundwater levels and model un-
certainty related to complexities in the structural geol-
ogy makes the realistic incorporation of groundwater
pressures and fracture permeability in landslides a
challenging area for future research. In particular, lit-
tle work has been undertaken to consider the role of
groundwater pressure variations on slope damage (a
notable exception is s
mitHYman
, 2007), or the incor-
poration of groundwater into the kinematics of block
movement in 3-D distinct-element models. The rela-
tionship between damage/fracturing and groundwater
pressures requires a coupled hydro-mechanical ap-
proach. The presence of faults and joints controls both
the pathways for fluid flow and kinematic release,
with elevated pore pressures contributing to local-
ized fracturing and damage, which in turn creates new
fluid flow pathways, redistributed pore pressures, and
corresponding slope movements in response to these
evolving changes in the effective stresses (Fig. 19).
background image
D. STEAD & E. EBERHARDT
102
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
two- and three-dimensional simulation of fracture in
rock slopes using a synthetic rock mass approach. In
the following section, we will present a brief overview
of numerical codes and their application to modelling
damage, rock bridge failure and failure kinematics.
MODELLING DAMAGE AND KINEMATICS
Modelling of brittle fracture or damage associated
with rock slope failure prior to 2000 was largely under-
taken using simple boundary element (displacement
discontinuity) methods, s
cavia
et alii (1996). This
early modelling produced some valuable insights into
the role of rock bridges in rock slope instability. More
recently, there has been a significant increase in the
number of studies that have considered brittle fracture
in landslides and rock slopes. Today a wide variety of
codes can be used to simulate brittle fracture allowing
further insight into the mechanics of large landslides.
It is emphasised that considering our current state of
knowledge, brittle fracture methods should be used as
one component of a toolbox approach including con-
ventional continuum and discontinuum codes.
Three principal methods have been used to model
2-D brittle fracture in landslides in both recent re-
search and practice:
• Distinct element (UDEC) Voronoi “Damage” models
• Particle Flow Codes (PFC)
• Hybrid finite-discrete element methods (FDEM)
Two-dimensional UDEC Voronoi methods have
been used for almost 20 years however their applica-
tion to rock slopes is quite recent. Y
an
(2008) used
a combined toolbox approach of UDEC Voronoi,
Fig. 20 - Simulated damage: a) UDEC Voronoi model of rock slope at Checkerboard Creek, British Columbia, Canada (after
a
lzo
'
uBi
, 2009); b) UDEC Voronoi model of rock slope at Highland Valley Copper Mine, British Columbia, Canada
(after a
lzo
'
uBi
, 2009); c) UDEC Voronoi model of active-passive rock slope showing influence of rock bridge content in
the transition zone (after t
uckey
, 2012); and d) preliminary UDEC Trigon model of active-passive transition zone (after
G
ao
, 2013)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
103
ure during the Randa Rockslide, Switzerland (Fig.
22a) and illustrate the importance of brittle rock mass
damage on instability. Figure 22b shows the use of
ELFEN in simulating the damage associated with foot-
wall failures common in high mountain ranges and sur-
face coal mines. The zones of extensile induced dam-
age in the upper slope and the compressive induced
damage in the active-passive toe are clearly visible and
FDEM, boundary element and continuum FEM meth-
ods to simulate rock bridge failure in rock slopes.
f
ranz
(2009) and a
Lzo
ubi
(2009) in perhaps the
most detailed works to date used UDEC Voronoi to
successfully simulate both natural rock slope failures
and open pit mine slopes. Figure 20a shows UDEC
Voronoi models of a high engineered rock slope in-
corporating rock support and realistic inclusion of
joints sets. This work showed the importance of the
degradation of the tensile strength of the rock mass on
slope instability. UDEC Voronoi methods convention-
ally discretize the rock mass into polygons a process
known as tessellation. Properties can then be given to
the polygonal boundaries to represent either joints or
intact rock. t
uckEY
(2012) and t
uckEY
et alii (2013)
used UDEC Voronoi to simulate the influence of var-
ied percentages of volumetric rock bridge content in
the transitional zone between an active-passive wedge
(similar to the situation in the western section of the
Vajont slide) (Fig. 20b).
A recent development in the UDEC Voronoi
method has been developed by g
ao
(2013) where the
polygons are divided into triangular blocks and the
properties are then given to each face of the triangle.
This method appears to provide a much more deform-
able rock mass which is less influenced by the polygo-
nal shape of the blocks.
Two dimensional Particle Flow Code models have
been used successfully to model surface mine slope
failure mechanisms, natural rockslides and runout.
They have the advantage over 3-D PFC codes in being
able to accommodate larger problems but arguably do
not model kinematics as realistically. Recent develop-
ments have seen the incorporation of a smooth joint
model to simulate joints (L
orig
et alii, 2009) and more
recently a flat joint model (p
otYonDY
, 2012), with the
latter allowing the simulation of the correct compres-
sive to tensile strength ratio of the simulated rock. The
strength of the rock mass in a PFC model is simulated
by the bonds between circular particles. Breakage of
the bonds allows realistic simulation of brittle fracture
and damage. Figure 21 shows examples of PFC2D in
the simulation of rock slope failures.
The Finite-Discrete Element code, ELFEN, has
been used successfully by several researchers in simu-
lating both natural and engineered slopes. E
bErHarDt
et alii (2004) describe the use of the ELFEN code in
successfully simulating the stages of damage and fail-
Fig. 21 - Rock slope modelling using Particle Flow Code,
PFC, and Synthetic Rock Mass, SRM: a) PFC2D
model of rock slope (after l
oriG
et alii, 2009); b)
SRM approach (after m
as
i
vars
et alii, 2011); and
c) FLAC 3D model of open pit slope using SRM
derived data (after s
ainsBury
et alii, 2008)
background image
D. STEAD & E. EBERHARDT
104
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
agree with both continuum and other brittle fracture
codes. s
tEaD
et alii (2006) summarize the applications
of ELFEN to varied natural failure mechanisms and
Y
YazmEnskY
et alii (2010) illustrate the integrated use
of a Discrete Fracture Network (DFN), Fig 22d, and
ELFEN in the successful simulation of a 900 m high
failure at the Palabora mine in South Africa (Fig. 22e).
The use of ELFEN to date however has been limited to
2-D brittle fracture modelling of rock slopes. Recent
work by H
amDi
et alii (2013) has shown the successful
simulation of 3-D damage at the laboratory scale; the
extension of the code to 64 bits within the near future
may allow simulation of larger scale three-dimensional
brittle fracture problems.
The development of 3-D brittle fracture-damage
modelling methods for large open pit slopes has seen
considerable advances over the last five years as part
of the Large Open Pit (LOP) Project. A Synthetic
Rock Mass (SRM) approach was developed using
PFC3D models and incorporating a DFN derived
from field data (m
as
i
vars
et alii., 2011). This model
was used to investigate the scale effects and anisot-
ropy of rock mass strength for different lithological
units. Samples from the laboratory scale to 80 m in
height were tested in uniaxial, triaxial and tensile
conditions to simulate the rock mass strength. These
synthetic rock mass strengths were subsequently
used to develop FLAC3D models incorporating di-
rectional weakness in a strain-softening ubiquitous
constitutive model. FLAC3D models with strengths
constrained against the SRM modelling were then
used to successfully model mine scale problems at
the Palabora and Northparkes block caving mines
(s
ainsburY
et alii, 2008).
The most recent development in the 3-D brit-
tle fracture/damage modelling of rock slopes has
been the introduction of the Lattice Spring code,
Slope Model (i
tasca
, 2012). This code allows re-
alistic modelling of brittle fracture in large rock
slopes. The code in essence replaces the spheres in
Fig. 22 - a) FDEM ELFEN simulation of Randa rockslide showing stages in slope damage and failure; b) ELFEN simulation of
a high mountain/footwall slope showing zones of extensile and compression-induced rock slope damage (after h
avaej
et alii, 2013); c) massive, 900 m high, open pit rock slope failure at Palabora, South Africa; d) constructed DFN for
Palabora model simulation; and e) FDEM-DFN ELFEN simulation of Palabora pit slope failure (after v
yazmensky
et
alii, 2010)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
105
couple brittle fracture propagation and groundwater
pressures in a fracture rock mass).
A recent development by g
ao
(2013) has been to
incorporate the Voronoi Trigon logic into 3DEC. This
has allowed the use of 3DEC to simulate brittle frac-
ture at both the laboratory and to a lesser extent the
field scale. Although computing overhead may cur-
rently limit full scale 3DEC Voronoi models in size
this development allows the possibility to consider
large scale block movements, with fracture enabling
kinematic mobility in critical areas of the slope.
Kinematic analysis of rock slopes by definition
is predominantly a 3-D technique which has hitherto
often been limited to simple stereographic methods.
The use of 2-D models can only be used in simple
cases to examine failure surface kinematics. The use
of 3-D distinct-element models is an area for future
research in assessing the combined roles of dam-
age, groundwater pressure, and structural control
on instability. The 3DEC code has been shown by
a PFC3D model with nodes. The bonds between the
PFC spheres are replaced by springs, Fig. 23a. A
discrete fracture/joint network can be incorporated
within the lattice spring model and brittle fracture
damage/cracking is simulated by breakage of the
springs. Figure 23b shows a Slope Model simulation
of a non-daylighting wedge as discussed previously
in the paper. It is possible to simulate progressive
failure through the failure of rock bridges either as
patches along the failure surfaces or as failure be-
tween joints, Fig. 23c. Figure 24 shows a preliminary
Slope Model simulation of the Vajont rockslide. As
discussed by W
oLtEr
et alii (this volume), the crack-
ing within the rock slope is concentrated in the zone
between the active-passive blocks in the western half
of the slide. This preliminary work shows the accu-
mulation of internal damage within the Vajont Slide
with numerical time. Future work will investigate the
influence of discrete fracture networks and incorpo-
rate groundwater pressures. (Slope Model is able to
Fig. 23 - a) Lattice Spring treatment of stiffnesses and strengths between particles; b) Slope Model lattice spring simulation of
a non-daylighting wedge failure; and c) simulation of toppling failure with plot showing number of new cracks gener-
ated as a function of calculation time steps
background image
D. STEAD & E. EBERHARDT
106
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
s
ainsburY
et alii (2007), Y
an
(2008), s
troutH
&
E
bErHarDt
(2009), f
ranz
(2009), b
riDEau
(2010),
and others to be particularly useful in kinematic rock
slope studies. The inter-relationship between 3-D
fracture permeability, groundwater pressures and
kinematics has received little attention and is an in-
teresting avenue for future research. The 3DEC code
assumes through-going joints although a persistence
factor has been used to indirectly investigate the in-
fluence of non-persistence on rock slope instability,
b
riDEau
et alii (2012). The influence of non-persist-
ent joints and groundwater on rock slope kinematics
is an important area for future research which could
be approached with a combine 3DEC – Slope Model
approach. The influence of seismic amplification on
rock slope stability (both damage and kinematics) is
also an area where three dimensional distinct-ele-
ment models may find important use building on the
results of 3-D continuum models.
DISCUSSION: CAN WE BETTER OPTIMI-
ZE FIELD DATA COLLECTION?
Detailed descriptions of the state-of-the-art in
monitoring methods for landslides have been present-
ed in several recent works (E
bErHarDt
& s
tEaD
, 2011;
s
tEaD
et alii, 2012). These touch on several critical
parameters and important areas that require further
data to improve our understanding of the mechanics
of landslides:
• Structural geological mapping
• Discontinuity persistence and rock bridges
• Failure surface morphology
• Rock mass quality, block size and shape
• Groundwater and fracture permeability
• Engineering geomorphology
The importance of building an accurate structural
geological map has long been recognized in landslide
studies, as rock slope failure mechanisms are inti-
mately related to the structural geological history. The
Fig. 24 - Preliminary Slope Model lattice spring simulation of the Vajont rockslide. After h
avaej
et alii (2013)
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
107
The ability of remote sensing techniques such as
LiDAR and photogrammetry to cover large and inac-
cessible areas of the rock slope means that methods of
using this data to complement rock mass characteriza-
tion (GSI, RMR, etc.) should continue to be developed.
To-date remote sensing methods have largely been used
to acquire discontinuity orientation data; it is essential
that the scope of data collected remotely be broadened
to maximize the use of the data. This data should ide-
ally provide input for geomechanical models such as
block size and shape. k
aLEncHuk
et alii (2010) describe
methods for treating block size distributions and clas-
sifying block shape. The collection of data required
for Discrete Fracture Network generation (orientation,
trace length, intensity and termination) should be un-
dertaken where possible if more sophisticated SRM
based numerical models are to be used. The considera-
tion of geostatistics when characterising the rock mass
strength using systems such as GSI has also been shown
to be an important issue in geomechanical modelling,
J
EffEriEs
et alii, (2006). Continued laboratory testing
studies are required to provide improved estimates of
parameters used in rock slope modelling including joint
normal and shear stiffness as well as rock mass dilation.
Future developments in constitutive criteria incorporat-
ing brittle fracture under low confinement may also
require a shift in emphasis on laboratory testing proce-
dures toward characterization of damage and associated
developments in geomechanical models.
Collection of groundwater data for landslides re-
quires piezometer installations. Multiple vibrating wire
piezometers (bentonite isolated or grouted-in) are com-
monly used in open pit slopes. Alternatives include mul-
tiple piezometer installations such as the Westbay sys-
tem. Characterization of groundwater in major natural
rock slopes is often subject to significant data and model
influence of tectonic deformation on rock mass qual-
ity, damage and kinematic controls, together with the
stress-path to which the rock slope has been subject-
ed, has been shown to be important in geomechanical
modelling. The usefulness of considering rock slope
fractography in the stability of exfoliation domes has
been discussed by t
uckEY
(2012) using both field
mapping and ground-based LiDAR.
t
uckEY
(2012) also presents an excellent summary
of historic rock bridge research and highlights both the
variation in assumed rock bridge content in previous
publications, Table 1. Numerous workers have consid-
ered rock bridges in the geomechanical modelling of
rock slopes including g
rønEng
et alii (2009, 2010) at
the Aknes rock slope in Norway, and s
turzEnEggEr
&
s
tEaD
(2012) at the Palliser rockslide in Canada. t
uckEY
(2012) clearly demonstrates the advantages, but also the
challenges, in using remote sensing techniques (LiDAR
and photogrammetry) to provide estimates of both
discontinuity persistence and rock bridge content in a
rock slope at several field study sites. These involve the
integrated use of Discrete Fracture Network Engineer-
ing approaches to characterise rock bridge content and
persistence distributions (s
turzEnEggEr
, 2010; t
uckEY
,
2012; t
uckEY
et alii, 2013). T
uckEY
(2012) also provide
detailed characterization of step-path geometry, while
s
turzEnEggEr
et alii (2011) also discuss the challenges
in the production of DFN’s from remote sensing data.
LiDAR and photogrammetry also present a valu-
able means to collect data, post failure, on the char-
acteristics of failure surfaces in large rockslides.
s
turzEnEggEr
& s
tEaD
(2012), W
oLtEr
et alii (this
volume), D
onati
et alii (2012) have all shown clearly
the utility of remote sensing methods in failure surface
characterization at the Palliser (Canada), Vajont (Ita-
ly) and Hope (Canada) landslides, respectively. Excel-
lent examples of the use of geophysics and monitor-
ing in characterizing sub-surface shear surfaces/zones
have been presented by g
anEroD
et alii (2008), W
iL
-
LEnbErg
(2008) and k
aLEncHuk
(2011) for the Aknes
(Norway), Randa (Switzerland) and Downie (Canada)
rockslides, respectively. The ability to import failure
surface morphology such as fold undulations and step-
path geometry into geomechanical models means that
there is no longer a need to over-simply or assume
failure surface geometries. Clearly, it is also important
to use engineering judgement and not to add unneces-
sary detail to the modelling process.
Tab. 1 - Summary of selected case studies quantifying in-
tact rock bridge content
background image
D. STEAD & E. EBERHARDT
108
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
uncertainty due to the complexity of the fracture system
controlling flow and pore pressure distributions, limita-
tions in access, and cost. Remote sensing can be used
to characterize seepage in natural and open pit slopes.
v
ivas
et alii (2013) shows the successful use of both
LIDAR and thermal imaging in the characterization of
seepage in natural and open pit rock slopes and proposes
a seepage intensity method for characterizing seepage.
Finally, the authors stress the importance of under-
taking engineering geomorphological mapping as part
of a landslide investigation. Such mapping not only
provides information on past geomorphic process that
may have influenced the instability, but also emphasiz-
es the importance of considering landform evolution. A
landslide may have been influenced by a geomorphic
stress-path whereby processes such as river incision,
tectonic uplift and glaciation, glacier retreat, weather-
ing, etc., serve to bring the current slope to a critical
damage threshold. At Vajont, W
oLtEr
et alii (this vol-
ume) clearly show the importance of considering en-
gineering geomorphology and how geomorphic proc-
esses can be used to constrain geomechanical models.
CONCLUSIONS
In this keynote paper the authors have deliberately
made no attempt to provide a detailed discussion of
all aspects of landslide failure mechanics, but instead
have concentrated on the important role of rock slope
damage as a major driver of rock slope instability. Our
paper introduces the concept of rock slope damage and
then attempts to place it in context with the wide array
of controls in which damage is manifested in a slope.
Our intention is to stimulate a different way of view-
ing landslides which will hopefully lead to an improved
understand of the complexity of landslide mechanics.
The kinematics of a landslide is emphasized here as
being a critical element in any landslide study, which
is all too often simplified to fit with simplistic models.
Our discussion on kinematics is intended to emphasise
that landslides are invariably three dimensional entities
with three–dimensional structure, rock mass quality,
topography and groundwater characteristics. Numeri-
cal modelling has advanced to include sophisticated
two and three - dimensional codes - it is however in the
hands of the user to choose the correct code and to treat
the problem in its correct kinematical manner.
The development of geological, hydrogeologi-
cal, structural, rock mechanics and geotechnical mod-
els (as used in the open pit mining sector) is crucial
to understanding the mechanics of landslides. Recent
developments in data collection are also intrinsically
three-dimensional providing point clouds of 3-D data
which must be optimised if their benefits are to be max-
imised in understanding the mechanics of landslides.
Finally we end by emphasising that our future under-
standing of the mechanics of landslides will require a
more effective way of visualising the hidden interior of
a rock slope. State-of-the-art remote sensing methods
now provide rock slope surface data, and must in turn
be integrated with surface and subsurface monitoring
data, 3-D geomechanical models, and most importantly
future developments in geophysical techniques, if our
understanding of the mechanics of complex landslides
is to continue to improve. A major obstacle to improv-
ing our understanding of the mechanics of major land-
slides is the high level of existing model and parameter
uncertainty. The use of a toolbox approach in forensic
investigations of major landslides is an essential step
in reducing model uncertainty whereas integrated geo-
mechanical modelling-rock slope characterization and
modelling will allow a reduction in data uncertainty.
ACKNOWLEDGEMENTS
Research was funded by NSERC Discovery
Grants to D. Stead and E. Eberhardt. The authors
wish to thank Golder Associates for provision of the
FRACMAN code, Rockfield Software for provision
of the Elfen code, and Loren Lorig, John Read and
Itasca Software for providing the Slope Model code.
Slope Model was developed as part of the Large Open
Pit (LOP) project, and is sponsored by companies
including Anglo-American, AngloGold Ashanti, Bar-
rick, BHP Billiton, Compañia Minera Doña Ines de
Collahuasi, De Beers, Newcrest, Newmont, Ok Tedi
Mining Ltd., RioTinto, Teck Resources Ltd, Vale, and
Xstrata Copper.
REFERENCES
a
Lzo
ubi
a.m. (2009) - The effect of tensile strength on the stability of rock slopes. Ph.D. thesis, University of Alberta.
b
aDgEr
T.C. (2002) - Fracturing within anticlines and its kinematic control on slope stability. Environmental and Engineering
Geoscience, VIII (1): 19-33.
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
109
b
arton
n., L
iEn
r. & L
unDE
J. (1978) - Engineering classification of rock masses for the design of tunnel support. Rock Mechanics
and Rock Engineering, 6 (4): 189-236.
b
EaLE
g. (2009) - Groundwater in rock. In: r
EaD
J. & s
tacEY
p. (E
Ds
.). Guidelines for open pit slope design. CSIRO Publishing,
Collingwood, Victoria.
b
Enko
B. (1997) - Numerical modelling of slope deformations. Ph.D. thesis, University of Saskatchewan.
b
Enko
b. & s
tEaD
D. (1998) - The Frank Slide: a re-examination of the failure mechanism. Canadian Geotechnical
Journal, 35 (2): 299-311.
b
Likra
L.H. (2012) - The Åknes rockslide, Norway. In: c
LaguE
J.J. & s
tEaD
D. (E
Ds
.). Landslides: types, mechanisms, and modelling.
323-335. Cambridge University Press, New York.
b
onzanigo
L., E
bErHarDt
E. & L
oEW
s. (2007) - Long-term investigation of a deep-seated creeping landslide in crystalline rock - Part
1: Geological and hydromechanical factors controlling the Campo Vallemaggia landslide. Canadian Geotechnical Journal, 44
(10): 1157-1180.
b
riDEau
M.-A. (2010) - Three-dimensional kinematic controls on rock slope stability conditions. Ph.D. thesis, Simon Fraser University.
b
riDEau
m.-a., c
Havin
s., a
nDriEux
p. & s
tEaD
D. (2012) - Influence of 3D statistical discontinuity variability on slope stability
conditions. In: E
bErHarDt
et alii (E
Ds
.). Landslides and engineered slopes: protecting society through improved understanding,
1: 587-593, CRC Press, London.
b
riDEau
m.-a., p
EDrazzini
a., s
tEaD
D., f
roEsE
c., J
aboYEDoff
m. & v
an
z
EYL
D. (2010) - Three-dimensional slope stability analysis
of South Peak, Crowsnest Pass, Alberta, Canada. Landslides, 8: 139-158.
b
riDEau
m.-a. & s
tEaD
D. (2010) - Controls on block toppling using a three-dimensional distinct element approach. Rock Mechanics
and Rock Engineering, 43: 241-260.
b
riDEau
m.-a. & s
tEaD
D. (2012) - Evaluating kinematic controls on planar translational slope failure mechanisms using three-
dimensional distinct element modelling. Geotechnical and Geological Engineering, 30 (4): 991-1011.
b
riDEau
m.-a., s
tEaD
D., k
inakin
D. & f
Ekova
k. (2005) - The influence of tectonic structures on the Hope slide, British Columbia,
Canada. Engineering Geology, 80: 242-259.
b
riDEau
m.-a., Y
an
m. & s
tEaD
D. (2009) - The role of tectonic damage and brittle rock fracture in the development of large rock
slope failures. Geomorphology, 103 (1): 30-49.
c
LaYton
m., s
tEaD
D. & k
inakin
D. (2013) - The Mitchell Creek Landslide, B.C., Canada: investigation using remote sensing and
numerical modeling. In Proc. 47
th
U.S. Rock Mechanics/Geomechanics Symposium, San Francisco.
c
orkum
a.g. & m
artin
C.D. (2004) - Analysis of a rock slide stabilized with a toe berm: a case study in British Columbia, Canada.
International Journal of Rock Mechanics & Mining Sciences, 41: 1109-1121.
c
unDaLL
p. & D
amJanac
b. (2009) - A comprehensive 3D model for rock slopes based on micromechanics. Proceedings, Slope Stability
2009, Santiago.
D
ick
g.J., E
bErHarDt
E., s
tEaD
D. & r
osE
n.D. (2013) - Early detection of impending slope failure in open pit mines using spatial and
temporal analysis of real aperture radar measurements. Proceedings, Slope Stability 2013, Brisbane.
D
iEDEricHs
m. (1999) - Instability of hard rock masses: the role of tensile damage and relaxation. Ph.D. thesis, University of Waterloo.
D
onatti
D., s
tEaD
D., g
Hirotti
m. & W
oLtEr
a. (2012) - A structural interpretation of the Hope Slide, BC., using terrestrial
photogrammetry with rock mass characterization. Rend. Online Soc. Geol. Italy., XX. Società Geologica Italiana, Roma.
D
oWLing
J., r
EiDEL
J. & b
EaLE
g. (2011) - A review of key factors affecting mine dewatering and slope depressurization. Proceedings,
Slope Stability 2011, Vancouver.
E
bErHarDt
E. (2008) - Twenty-Ninth Canadian Geotechnical Colloquium: The role of advanced numerical methods and
geotechnical field measurements in understanding complex deep-seated rock slope failure mechanisms. Canadian
Geotechnical Journal, 45 (4): 484-510.
E
bErHarDt
E. & s
tEaD
D. (2011) - Geotechnical instrumentation. In D
arLing
p. (E
D
.). SME mining engineering handbook. 3
rd
ed., 1
(8.5): 551-572. Society for Mining, Metallurgy & Exploration.
E
bErHarDt
E., s
tEaD
D. & c
oggan
J.s. (2004) - Numerical analysis of initiation and progressive failure in natural rock slopes: the
1991 Randa rockslide. International Journal of Rock Mechanics and Mining Sciences, 41: 69-87.
E
LmouttiE
m.k. & p
oropat
g.v. (2011) - Uncertainty propagation in structural modeling. Proceedings, Slope Stability 2011,
Vancouver.
background image
D. STEAD & E. EBERHARDT
110
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
f
raYssinEs
m. & H
antz
D. (2006) - Failure mechanisms and triggering factors in calcareous cliffs of the subalpine ranges (French
Alps). Engineering Geology, 86 (4): 256-270.
f
ranz
J. (2009) - An investigation of combined failure mechanisms in large scale open pit slopes. Ph.D. thesis, University of New
South Wales.
f
ukuzono
t. (1985) - A new method for predicting the failure time of a slope. In 4
th
International Conference and Field Workshop on
Landslides, Tokyo. Japan Landslide Society: 145-150.
g
anEroD
g.v., g
ronEnt
g., r
onning
J.s., D
aLsEggE
E., E
LvEbakk
H., t
onnEsEn
J.f., k
vELDsvik
v., E
ikEn
t., b
Likra
L.H. & b
raatHEn
a. (2008) - Geological model of the Åknes rockslide, western Norway. Engineering Geology, 102 (1-2): 1-18.
g
ao
f. (2013) - Investigation on the failure mechanisms of underground coal mine roadways through discrete element modelling. Ph.D.
thesis, Simon Fraser University.
g
Hirotti
m. (1992) - Aspetti geomeccanici e modellazione numerica della frana del Vajont. PhD thesis, Universitá di Parma, Ferrara.
g
Hirotti
m. (2006) - Edoardo Semenza: the importance of geological and geomorphological factors in the identification of the ancient
Vaiont Landslide. In: E
vans
et alii (eds.). Landslides from massive rock slope failure. 395-406.
g
iscHig
v., a
mann
f., m
oorE
J.r., L
oEW
s., E
isEnbrEiss
H. & s
tEmpfHubEr
W. (2011) - Composite rock slope kinematics at the current
Randa instability, Switzerland, based on remote sensing and numerical modeling. Engineering Geology, 118: 37-53.
g
iscHig
v., L
oEW
s., k
os
a. m
oorE
J.r, r
aEtzo
H. & L
EmY
f. (2009) - Identification of active release planes using ground based
differential InSAR at the Randa rock slope instability, Switzerland. Natural Hazards and Earth Systems Science, 9: 2027-2038.
g
ooDman
r. E. (1989) - Introduction to rock mechanics. 2
nd
edition. John Wiley & Sons.
g
riffitHs
J.s., s
tokEs
m., s
tEaD
D. & g
iLEs
D. (2012) - Landscape evolution and engineering geology: results from IAEG Commission
22. Bulletin Engineering Geology and the Environment, 71: 605-636.
g
rEEn
a.g., m
aurEr
H., s
piLLmann
t., H
EinckE
b. & W
iLLEnbErg
H. (2006) - High-resolution geophysical techniques for improving
hazard assessments of unstable rock slopes. The Leading Edge: 311-316.
g
rønEng
g., n
iLsEn
b. & s
anDvEn
r. (2009) - Shear strength estimation for Åknes sliding area in western Norway. International
Journal of Rock Mechanics and Mining Sciences, 46: 479-488.
g
rønEng
g., L
u
m., n
iLsEn
b. & J
EnsEn
a.k. (2010) - Modelling of time-dependent behaviour of the basal sliding surface of the Åknes
rockslide area in western Norway. Engineering Geology, 114: 414-422.
H
aDLEY
J.b. (1964) - Madison County Rockslide, Montana USA. In: v
oigHt
b. (E
D
.). Rockslides and avalanches, part 1, natural
phenomena. Dev. Geotech. Eng., 14a: 87-128, Elsevier Sci., New York.
H
amDi
p., s
tEaD
D. & E
Lmo
D. (2013) - Numerical simulation of damage during laboratory testing on rock using a 3D-FEM/DEM
approach. In Proc. 47
th
U.S. Rock Mechanics/Geomechanics Symposium, San Francisco.
H
avaEJ
m., s
tEaD
D., L
orig
L. & v
ivas
J. (2012) - Modelling rock bridge failure and brittle fracturing in large open pit rock slopes. In
Proc. 46
th
US Rock Mechanics/Geomechanics Symposium, Chicago.
H
EncHEr
s.r., L
EE
s.g, c
artEr
t.g. & r
icHarDs
L.r (2012) - Sheet joints: Characterization, shear strength and engineering. Rock
Mechanics and Rock Engineering, 44: 1-22.
H
EncHEr
s.r., L
iao
Q.H. & m
onagHan
b.g. (1996) - Modelling slope behavior for open pits. Trans. Inst. Min. and Metall., Sect. A:
Mining Industry, 105: A37-47.
H
EnDron
a.J. & p
atton
f.D. (1985) - The Vaiont Slide, A geotechnical analysis based on new geologic observations of the failure
surface. US Army Corps of Engineers, Washington, DC, Technical Report GL-85-5.
H
oEk
E. & b
raY
J. (1981) - Rock slope engineering. 3
rd
ed. Institution of Mining and Metallurgy, London.
H
ungr
o. & a
mann
f. (2011) - Limit equilibrium of asymmetric laterally constrained rockslides. International Journal of Rock
Mechanics and Mining Sciences, 48: 748-758.
i
tasca
(2012) - 3-Dimensional Distinct Element Code (3DEC, v.4.1). Itasca Consulting Group, Minneapolis.
i
tasca
(2012) - Slope Model. Itasca Consulting Group, Minneapolis.
i
tasca
(2012) – Universal Distinct Element Code (UDEC, v.5.0). Itasca Consulting Group, Minneapolis.
J
EffEriEs
m., L
orig
L. & a
LvarEz
c. (2006) - Influence of rock-strength spatial variability on slope stability. In: H
art
et alii (2008,
E
Ds
.). Continuum and distinct element numerical modeling in geo-engineering. Paper: 01-05.
J
ackson
L.E. (2002) - Landslides and landscape evolution in the Rocky Mountains and adjacent foothills area, Southwestern Alberta,
Canada. Reviews in Engineering Geology, XV: 325-344.
background image
UNDERSTANDING THE MECHANICS OF LARGE LANDSLIDES
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
111
k
acHanov
L.m. (1986) - Introduction to continuum damage mechanics. Kluwer Academic Publishers.
k
aisEr
p.k. & k
im
b-H. (2008) - Rock mechanics challenges in underground construction and mining. In: p
otvin
et alii (E
Ds
.),
SHIRMS 2008. 23-38.
k
aLEncHuk
k.s. (2010) - Multi-dimensional analysis of large, complex slope instability. PhD thesis, Queen's University.
k
aLEncHuk
k.s., D
iEDEricHs
m. & m
ckinnon
s. (2010) - Characterizing block geometry in jointed rock masses. International Journal
of Rock Mechanics & Mining Sciences, 43: 1212-1225.
k
inakin
D. (2004) - Occurrence and genesis of alpine linears due to gravitational deformation in south western British Columbia.
M.Sc. thesis, Simon Fraser University.
k
vELDsvik
v., E
instEin
H.H., n
iLsEn
b. & b
Likra
L.H. (2008) - Numerical analysis of the 650,000 m3 Aknes rock slope based on
measured displacements and geotechnical data. Rock Mechanics and Rock Engineering, 22: 689-728.
L
Ebaron
a.m. (2011) - The effect of rock bridges on blasting fragmentation in sedimentary rock. M.Sc. thesis, University of Utah.
L
EitH
k.J. (2012) - Stress development and geomechanical controls on the geomorphic evolution of alpine valleys. Ph.D. thesis, ETH
Zurich.
L
orig
L., s
tacEY
p. & r
EaD
J. (2009) - Slope design methods: Three-dimensional model for rock slopes based on micromechanics. In:
r
EaD
J. & s
tacEY
p. (E
Ds
.). Guidelines for open pit slope design. CSIRO Publishing, Collingwood, Victoria.
L
orig
L., c
unDaLL
p.a., D
amJanac
b. & E
mam
s. (2010) - A three-dimensional model for rock slopes based on micromechanics. Proc.,
44
th
US Rock Mechanics Symposium, Salt Lake City.
m
as
i
vars
D., p
iErcE
m.E., D
arcEL
c., r
EYEs
m
ontEs
J., p
otYonDY
D.o., Y
oung
r.p. & c
unDaLL
p.a. (2011) - The synthetic rock mass
approach for jointed rock mass modelling. International Journal of Rock Mechanics and Mining Sciences, 48: 219-244.
m
artin
c.D. & k
aisEr
p.k. (1984) - Analysis of a rock slope with internal dilation. Canadian Geotechnical Journal, 21 (4): 605-620.
m
ccoLL
s.t., D
aviEs
t.r.H. & m
csavEnEY
m.J. (2010) - Glacier retreat and rock-slope stability: Debunking debuttressing. In:
W
iLLiams
et alii (E
Ds
.). Geologically active. Proceedings of the 11
th
IAEG congress, Auckland: 467-474.
m
EncL
v. (1966) - Mechanics of landslides with non-circular slip surfaces with special reference to the Vaiont slide. Geotechnique, 16
(4): 329-337.
m
ErcEr
k.g. (2006) - Investigation into the time dependent deformation behaviour and failure mechanisms of unsupported rock slopes
based on the interpretation of observed deformation behaviour. Ph.D. thesis, University of Witwatersrand.
m
ErriEn
-s
oukatcHoff
v., k
orini
t. & t
HoravaL
a. (2011) - Use of an integrated discrete fracture network code for stochastic stability
analyses of fractured rock masses. Rock Mechanics and Rock Engineering, 45 (2): 159-181.
m
oorE
J.r., g
iscHig
v., b
urJanEk
J., L
oWE
s. & f
aH
D. (2011) - Site effects in unstable rock slopes: dynamic behavior of the Randa
instability (Switzerland). Bulletin of the Seismological Society of America, 101 (6): 3110-3116.
o
ppikoffEr
t., J
aboYEDoff
m. & k
EusEn
H.r. (2008) - Collapse at the eastern Eiger flank in the Swiss Alps. Nature Geoscience, 1:
531-535.
p
aronuzzi
p. & s
Erafini
W. (2009) - Stress state analysis of a collapsed overhanging rock slab: a case study. Engineering Geology,
108: 65-75.
P
icarELLi
L., L
ErouEiL
s., o
LivarEs
L., p
agano
L., t
omassi
p. & u
rciuoLi
g. (2012) - Groundwater in slopes. In: c
LaguE
J.J. & s
tEaD
,
D. (Eds.). Landslides: types, mechanisms and modelling. Cambridge University Press.
p
otYonDY
D.o. (2012) - A flat-jointed bonded-particle material for hard rock. Proc. 46
th
US Rock Mechanics/Geomechanics
Symposium, Chicago.
r
EaD
J. & s
tacEY
p. (2009) - Guidelines for open pit slope design. CSIRO Publishing, Collingwood, Victoria.
r
istau
J. (1994) - Field verification of a step-path simulation model for rock slope stability analysis. M.Sc. thesis, University of Idaho.
r
ocsciEncE
(2012) - DIPS (V. 6.0). Rocscience Inc., Toronto.
r
ocsciEncE
(2012) - Phase2 (v. 8.0). Rocscience Inc., Toronto.
r
ogErs
s., m
offit
k. & c
HancE
a. (2006) - Using realistic fracture network models for modelling block stability and groundwater flow
in rock slopes. Proceedings of Canadian Geotechnical Society Conference, Vancouver: 1452-1459.
r
osE
n.D. & H
ungr
o. (2007) - Forecasting potential rock slope failure in open pit mines using the inverse-velocity method.
International Journal of Rock Mechanics & Mining Science, 44: 308-320.
s
ainsburY
b., p
iErcE
m.E. & m
as
i
vars
D. (2008) - Analysis of caving behaviour using a synthetic rock mass-ubiquitous joint rock
mass modelling technique. Proc. 1
st
Southern Hemisphere International Rock Mechanics Symposium, Perth, 1: 243-253.
background image
D. STEAD & E. EBERHARDT
112
International Conference on Vajont - 1963-2013 - Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
s
ainsburY
D., p
otHitos
f., f
inn
D. & s
iLva
r. (2007) - Three-dimensional discontinuum analysis of structurally controlled failure
mechanisms at the Cadia Hill open pit. Proceedings, Slope Stability 2007, Perth.
s
cavia
c. & c
astELLi
m. (1996) - Analysis of the propagation of natural discontinuities in rock bridges. Proceedings of Eurorock’96,
Turin: 445-451.
s
EmEnza
E. (2001) - La Storia del Vajont raccontata dal geologo che ha scoperto la frana. Tecomproject Editore Multimediale, Ferrara.
s
itar
n. & m
acLaugHLin
m.m. (1997) - Kinematics and discontinuous deformation analysis of landslide movement. Proceedings of
the 2
nd
Panamerican Symposium on Landslides, Rio de Janeiro: 65-73.
s
mitHYman
m. (2007) - Distinct-element modelling of time-dependent deformation in two large rockslides. M.Sc. thesis, University of
British Columbia.
s
mitHYman
m., E
bErHarDt
E. & H
ungr
o. (2009) - Characterization and numerical analysis of intermittent slope displacements and
fatigue in deep-seated fractured crystalline rock slopes. Proc. GeoHalifax 2009, 62
nd
Canadian Geotechnical Conference & 10
th
Joint CGS/IAH-CNC Groundwater Conference, Halifax.
s
pratt
D. & L
amb
m. (2005) - Borehole data interpretation and orientations: Turtle Mountain Monitoring Project. WP15b, University
of Calgary.
s
tEaD
D. & c
oggan
J.s. (2012) - Numerical modeling of rock slope instability. In c
LaguE
J. & s
tEaD
D. (E
Ds
.). Landslides: types,
mechanisms, and modelling. 144-158. Cambridge University Press, New York.
s
tEaD
D., c
oggan
J.s., E
Lmo
D. & Y
an
m. (2007) - Modelling brittle fracture in rock slopes - experience gained and lessons learned.
In: p
otvin
Y. (E
D
.) - Proceedings of the 2007 International Symposium on Rock Slope Stability in Open Pit Mining and Civil
Engineering, Perth. 239-251.
s
tEaD
D., E
bErHarDt
E. & c
oggan
J.s. (2006) - Developments in the characterization of complex rock slope deformation and failure
using numerical modelling techniques. Engineering Geology, 83: 217-235.
s
tEaD
D., J
aboYEDoff
m. & c
oggan
J.s. (2012) - Rock slope characterization and geomechanical modelling. In: E
bErHarDt
et alii
(E
Ds
.). Landslides and engineered slopes: protecting society through improved understanding. 1: 83-100, CRC Press, London.
s
troutH
a. & E
bErHarDt
E. (2009) - Integrated back and forward analysis of rock slope stability and rockslide runout at Afternoon
Creek, Washington. Canadian Geotechnical Journal, 46 (10): 1116-1132.
s
turzEnEggEr
m. (2010) - Multi-scale characterization of rock mass discontinuities and rock slope geometry using terrestrial remote
sensing techniques. Ph.D. thesis, Simon Fraser University.
s
turzEnEggEr
m. & s
tEaD
D. (2012) - The Palliser rockslide, Canadian Rocky Mountains: characterization and modeling of a stepped
failure surface. Geomorphology, 138: 145-161.
s
turzEnEggEr
m., s
tEaD
D. & E
Lmo
D. (2012) - Terrestrial remote sensing-based estimation of mean trace length, trace intensity and
block size/shape. Engineering Geology, 119: 96-111.
s
uLLivan
t.D. (2007) - Hydromechanical Coupling and Pit Slope Movements. In: p
otvin
Y. (E
D
.) - Proceedings of the 2007 International
Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Perth. 3-43.
t
Hompson
a.g. (2011) - Slope instability in jointed rock and design of ground support. Proceedings, Slope Stability 2011, Vancouver.
t
uckEY
z. (2012) - An integrated field mapping-numerical modelling approach to characterising discontinuity persistence and intact
rock bridges in large open pit slopes. M.Sc. thesis, Simon Fraser University.
t
uckEY
z., s
tEaD
D. & E
bErHarDt
E. (2013) - An integrated approach for understanding uncertainty of discontinuity persistence and
intact rock bridges in large open pit slopes. Proceedings Slope Stability 2013, Brisbane.
v
YazmEnskY
a., s
tEaD
D., E
Lmo
D. & m
oss
a. (2010) - Numerical analysis of block caving-induced instability in large open pit slopes:
a finite element/discrete element approach. Rock Mechanics and Rock Engineering, 43: 21-39.
W
ang
c., t
annant
D.D. & L
iLLY
p.a. (2003) - Numerical analysis of the stability of heavily jointed rock slopes using PFC2D.
International Journal of Rock Mechanics & Mining Science, 40 (3): 415-424.
W
EssELoo
J. & D
igHt
p. (2009) - Rock mass damage in hard rock open pit mine slopes. Proceedings Slope Stability 2009, Santiago.
W
YLLiE
D.c. & m
aH
c.W. (2004) - Rock slope engineering. 4
th
ed. Spon Press.
Y
an
m. (2008) - Numerical modelling of brittle fracture and step-path failure: From laboratory to rock slope scale. Ph.D. thesis, Simon
Fraser University.
Statistics