29
Terzaghi Oration
/ Allocution Terzaghi
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 33 presents an illustration to explain the sedimenta-
tion process leading to failure, which supports the hypothesis
that major slides have occurred in the Storegga area on a semi-
regular basis, related to the glacial/interglacial cyclicity.
The bottom illustration in Figure 33 (denoted 1) gives the
last interglacial with deposition of soft marine clays. The middle
illustration (denoted 2) presents the last glacial maximum
(LGM) with the ice at the shelf edge and deposition of glacial
sediments. The top illustration (denoted 3) presents the topogra-
phy after the Storegga slide. Dating (BP, before present) is giv-
en for each illustration. The illustration denoted 3 also shows
two older slide scars that were filled with marine clays. The slip
planes were found in seismically stratified units of hemipelagic
deposits and the thick infill of stratified sediments indicate a late
glacial to early interglacial occurrence of slides (Bryn
et al
2005).
The soft fine-grained hemipelagic deposits were rapidly
loaded by coarser glacial deposits during the short glaciations
period. Excess pore pressures were a destabilizing factor. The
hypothesis of strong earthquake shaking was retained to start
the underwater slide. After the earthquake initiated the move-
ment, the slide continued retrogressively by back-stepping up
the slope where the pore pressures were already high. The mass
movement was further facilitated by the release of support at the
toe.
The stability of the present situation at Ormen Lange was
evaluated by Kvalstad
et al
2005b. The conclusion was that an
extremely strong earthquake would be the only realistic trigger-
ing mechanism for new submarine slides in the area. The annual
probability of third party damage was also investigated and
found to be extremely low (Nadim
et al
2005b). The project
team therefore concluded that developing the Ormen Lange gas
field could be done safely.
Figure 33. Deposition and sliding processes (Bryn
et al
2005).
10.4
Lessons learned
The documentation of the feasibility of pipeline installation
across the Storegga slide would not have been possible without
the integrated inter- and cross-disciplinary study of the devel-
opment, now without the conscious effort to improve knowl-
edge on seafloor morphology, shallow geology, geotechnical
behavior and potential hazards and risks associated with the
area. The interweaving of research and practice, the cooperation
of academia and industry and the integration of the geo-
disciplines were essential for gaining an understanding of the
past slide and providing the possibility to develop the gas field.
11 LANDSLIDE RISK MANAGEMENT
11.1
Landslide prevention in Drammen
In Norway, the hazard ere estimated on the basis of simple theo-
retical evaluations of the potential area that can be involved in a
quick clay slide, in combination with back-calculations of a
number of historical quick clay slides (Aas 1979).
The assessment of the risk associated with slides in sensitive
clays in Norway is a semi-quantitative approach developed for
the Norwegian Water Resources and Energy Directorate (NVE).
Slide areas are classified according to “engineering scores”
based on an evaluation of the topography, geology and local
conditions (to qualify hazard) and an evaluation of the elements
at risk, persons, properties and infrastructure exposed (to qualify
consequence). The risk score to classify the mapped areas into
risk zones is obtained from the relationship
R
S
= H
WS
C
WS
,
where
R
S
is the risk score,
H
WS
is the weighted hazard score and
C
WS
is the weighted consequence score. The risk matrix is di-
vided in five risk classes. Guidelines for the implementation of
the risk matrix are administered by NVE. In practice, the ap-
proach is used to make decisions on required mitigation meas-
ures to reduce the risk. The approach is simple and makes room
for engineering experience and judgment. For detailed regional
planning, slope stability calculations need to be made. The ap-
proach has been described in detail in Gregersen 2005; Lacasse
et al
2003; Lacasse and Nadim 2008; and Kalsnes
et al
2013. A
similar procedure has been developed for sensitive clays in
Québec (Thibault
et al
2008), reflecting the experience with
large retrogressive slides in Québec.
An example of the management of risk based on the above
scores is the preventive actions set in place in Drammen. The
city of Drammen, along the Drammensfjord and the Drammen
River, is built on soft sensitive clay. Stability analyses were
done in an area close to the centre of the city, and indicated that
some areas did not have satisfactory safety against a slope fail-
ure. Based on the results of the stability analyses and the factors
of safety (FS) obtained, the area under study was divided into
three zones, as illustrated in Figure 34:
– Zone I FS satisfactory
– Zone II FS shall not be reduced
– Zone III FS too low, area must be stabilised
Figure 35 illustrates the mitigation done in Zone III: a counter
fill was immediately placed in the river to support the river
bank, and the factor of safety checked again. The counter fill
provided adequate stability. In Zone II, no immediate action
was taken, but a ban was placed on any new structural and
foundation work without first ensuring increased stability. Fig-
ure 36 illustrates required actions:
if an excavation is planned, the clay will have to be stabi-
lised with e.g. anchored sheetpiling or soil stabilisation, for
example lime-cement piles;
if new construction is planned, the engineer needs to check
the effects of the change on the stability down slope: e.g.
adding a floor to a dwelling may cause failure because of
added driving forces; or new piling up slope can cause an
increase in pore pressures and a driving force on the soil
down slope.
With such an approach, focus is set on the need for mitigation
rather than as the risk and potential for failure.
