2141
Technical Committee 208 /
Comité technique 208
complexities of cohesive internal forces. Initial results indicate
the importance of particle size on the flow velocity of the
granular mixture. Results are intended to supplement numerical
work that is on-going.
Choi et al.
present a paper on a series of flume tests that
examine the influence of baffle row number on reduction of
debris frontal velocity. The average frontal velocity of the
debris flow downstream of the array of baffles was measured.
Results show one row to be ineffective in reducing the debris
frontal velocity, however, two to three rows of baffles exhibit
notable frontal velocity reduction.
Law et al.
present a related paper to that of
Choi et al
. on the
dynamic interaction between a granular surge flow and baffles.
In contrast however, Law et al. use the three-dimensional
discrete element method with the granular flow medium
modeled as spherical discrete elements and the baffles as rigid
square objects. The location, velocity and forces acting on the
individual elements during the impact and interaction process
are presented. Assessment of the numerical results indicates that
a single row of baffles is effective in reducing the kinetic energy
and discharge of the granular flow. Interestingly, this actually
runs counter to the findings of
Choi et al.
One possibility for
this may be a difference in the relative size of largest particles to
the baffle spacing used in the experiments by
Law et al
. and
Choi et al
. although data on this are not available from
Choi et
al.
Figure 7. Buscarnera and Whittle. MIT-S1 constitutive model
predictions for a sand in undrained shear at different initial void ratios.
Buscarnera and Whittle
present a framework for evaluating
the triggering of flow slides in infinite slopes using the
anisotropic MIT-S1 model. The selected soil model is able to
simulate realistic transitions in the contractive/dilative response
of sands and enables the prediction of the shear perturbations
able to induce instability (Figure 7), as well as the location of
potentially unstable zones within the soil mass. In order to show
the capabilities of the proposed approach, the methodology has
been applied to the well-known case of slope failures in the
Nerlerk berm: results suggest that static liquefaction is likely to
have contributed to the failures. The analyses illustrate a unified
methodology that combines the theory of material stability, the
critical state framework for sands and data from in situ tests.
The authors state that the methodology offers a simple,
consistent and complete geomechanical framework for
interpreting and predicting the triggering of flow slides that can
be easily applied to other similar engineering cases.
Thakur et al.
propose a new approach to assess the potential
for flow slides via a new laboratory test procedure referred to as
the quickness test. The test focuses on remoulded behaviour of
sensitive clays in terms of a numerical value referred as
quickness (Q). Based on their study, it is found that a Q < 15 %
or remoulded undrained shear strength c
ur
> 1 kPa seems to be
the threshold limit above which the extent of retrogression of a
landslide is limited to the initial slide. This criterion has been
supported using data available from several Norwegian
landslides in sensitive clays.
Winter et al.
present fragility curves as a means of describing
the vulnerability of elements at risk to impact by a debris flow.
The specific category of element considered in the paper is a
road. Expert engineering judgment was used to develop
preliminary fragility curves, from a compilation of responses to
a questionnaire. Analysis and interpretation of the collected data
is made with reference to two events, in an attempt to relate
event volume to the probability of a state of damage.
4 EARTHQUAKE INDUCED SLOPE INSTABILITY
Consideration of seismic slope stability occurs in several
papers, two of which are considered here and five of which are
more rationally addressed within the context of multi-hazard
considerations in the section that follows this one.
Strenk and Wartmen
present the findings of a sensitivity
analysis conducted on an idealized slope, to examine
uncertainty in seismic deformations established using the
Newmark rigid-block method, given variability in ground
motion. The effect was found to be most pronounced at low
acceleration ratios, and little influence of frequency content and
waveform was observed. The findings suggest that use of
spectral matching to create variability in both intensity and
frequency may be “of limited utility”.
Paçi et al.
presents a study on the stability of a new road
located in a very disturbed tectonic zone where the rock quality
is extremely poor. Active surface slides are very unstable due to
water ingress especially during seismic events. Geotechnical
parameters of the intact flysch / clay and limestone rock are
determined from laboratory tests and empirical methods, while
for residual parameters, a back-analysis is carried out using
Plaxis 2D software. Hence, slope stability problems resulting
from seismic loading are examined in two stages. The first
model determines a critical sliding plane (if any) under an
artificial accelerogram that takes account of near-source effects
and then the second uses residual parameters for the soil
strength mobilized within the sliding plane. The second model
is used to define and design the engineering measures (walls,
piles, anchored wires, nails, etc.). This two-step process, is a
pragmatic approach for designing mitigation measures for
prefailed slopes, and may also be used for pore-pressure
induced instabilities (e.g. Tatarniuk and Bowman, 2012).
5 MULTI-HAZARD INSTABILITY OF SLOPES
Nadim and Liu
present on the use of Bayesian networks to
inform reasoning when there is uncertainty and reliance on
expert judgment, such as is done in many branches of civil
engineering (for example, avalanche risk assessment, dam risk
analysis etc.). They built a network for consideration of
earthquake-triggered landslides using the open-source
MATLAB package BNT, and used it to conduct a sensitivity
analysis. The method is believed to have merit in deciding
where to take no action, versus installing a warning system, else
opting for active countermeasures.
Chang and Huang
present an interesting case study on the
influence of the Chi-Chi earthquake in 1999 on a road above
slope that had been stable. Although no obvious damages to the
road were observed at the time of shaking, during Typhoon Nari
in 2001, several parallel cracks developed along the road
alignment – suggestive of underlying instability. Remedial work
