News letter Dam edition

Ingeokring Newsletter
Parameters standard
Circumstances Dip PCS average (8)
Processes
Gravity
Dip PCS high (9)
Dip PCS low (5)
Parameters low
Parameters
standard
1.70 1.44 1.95 1.67
Dyke elastic infill 1.63 1.34 NA NA
Dyke ‘magma’ infill see text see text - -
Dyke no infill 1.27 1.07 - -
Pore pressures riftzone see text see text - -
Future Growth + 200m 1.64 1.40 - -
Future Growth + 400m 1.55 1.32 - -
Future Growth + 600m 1.46 1.23 - -
Future Growth + 800m 1.41 1.19 - -
Future Growth + 1000m 1.33 1.12 - -
Massive flank collapse at La Palma: numerical slope stability
models of the Cumbre Vieja volcano
Janneke van Berlo
In 1999, Steven Ward and Simon Day launched a scientific
article which discussed possible tsunami effects in the USA
as a result of a massive flank collapse of the Cumbre Vieja
volcano at La Palma. One problem of their hypothesis is that
the slope instability has not been investigated in a quantitative
way. The research question therefore aims to partially
solve this problem: “Under what boundary conditions could
the Western flank of the Cumbre Vieja volcano start sliding and
at what recurrent times would such mass movement occur?”
Methodology concentrates on integrally modelling three
model attributes (geometry, material and processes) in a
FEM computer model. The table below shows the modelling
results: calculated Factors of Safety for various worst case
scenarios. ‘PCS’ stands for Post Collapse Sequence: a plane
of weakness below the volcano. Magmastatic dyke pressure
at the back of the possible slide volume with magma infill
and excess pore pressures due to the heating of pore water
resulted in vertical failure mechanisms rather than lateral
flank collapses. This is true if the pore pressures are limited
to the rift zone of the volcano.
In conclusion, two mechanisms, dyke intrusion and future
growth, bring the volcano dangerously close to a failure
situation. However, it is important to note that this only occurs
under the input of very conservative parameters and in
a 2D model situation where friction of the sidewalls of the
landslide mass is not taken into account. Additional mecha-
Calculated Factors of Safety for various scenarios in a most dangerous
volcanic cross section.
nisms, like pore pressures occurring inside the PCS, may
further enhance instability and should be investigated in
further research.
Disregarding the uninvestigated mechanisms in the volcano,
the results indicate that overall flank failure at least
requires a combination of thoroughly weakened rock- and
soil mass and steeper flanks than in the current configuration.
This supplements the conclusion: to reach substantial
growth over the full width of the landslide mass necessary
to trigger failure, a time span in the order of 10 000 years will
be required.
Comparing discontinuity surface roughness derived
from 3D terrestrial laser scan data with traditional fieldbased
methods
Ephrem Kinfe Tesfamariam, ITC
Discontinuity surface roughness estimation is very important
in determining the hydro-mechanical properties of rock
masses. The normal ways of roughness estimation have
been made traditionally by visual and simple instrumental
field observations. These methods may require physical
access to the exposed rock faces. This may expose field personnel
to hazardous situations because the measurements
have to be carried out below steep rock faces, in a quarry or
tunnel. The higher parts of a steep exposed rock face are
often difficult to reach. These traditional methods are also
time consuming and subjective. This thesis describes a new
method by which 3D point cloud data obtained by laser
scanning can be used to model and determine discontinuity
surface roughness in an automated way with high detail,
high accuracy, and no human bias. The laser scan technique
is also much faster and safer than the traditional field-based
methods, since no direct physical access is needed to the
rock face.
In order to determine the surface roughness from the 3D
laser scan point cloud data, two point cloud data sets with
an average spatial resolution of 1 cm and 6 cm are used.
These point data sets are interpolated as a 3D virtual surface
model using a scattered data interpolation technique. To
extract the 3D surface roughness from the 3D virtual surface
model, the mean orientation of circular windows with diameters
of 5, 10, 20 and 40 cm were computed. The largescale
directional roughness is also extracted by cropping
and fitting narrow strips using curve fitting techniques.
The results revealed that laser scan data can be used to
model and estimate rock surface roughness. The extracted
directional large-scale roughness profiles are comparable
Dam edition | Double Issue 2007/2008 | 81