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Quick-clay: Geologic origin and geotechnical properties
Foredrag på NTVA-møte i Oslo 23. oktober 201 3
Håkon Heyerdahl, senioringeniør, Norges Geotekniske Institutt (NGI)
Quick clay is a particularly problematic soil type found in Canada. Sweden. Norway. Finland
and Russia. The formation and distribution of quick clay are strongly related to the last
glaciation at the Northem hemisphere. Quaternary marine clay deposits (or clay deposited in
brackish water) have been lifted on-shore by the still on-going isostatic uplift following the last
deglaciation of the ice cap previously covering the Scandinavian peninsula. In Norway,
marine deposits are encountered up to ca. 200 m above sea level in the Oslo and Trondheim
region. The general picture for these regions is that marine deposits and hence potential
quick clay areas coincide with the most densely populated areas, and with the attractive
areas for urban development. In the Stavanger region in South West Norway the uplift is
almost zero, due to smaller thickness of the ice cap in this region.
Quick clay landslides are particularly dangerous due the mechanical properties of the soil
and the potentiallarge extent of the slides. Due to leaching (reduced salt concentration in the
pore water), the clay has a brittle behavior. When loaded to a strain level beyond the peak of
the working curve (i.e. the peak strength), the shear strength of the clay will drop rapidly.
When remolded, the clay will transform into a liquid, with practically no strength. Failure
mechanisms in quick clay slopes may develop as progressive or retrogressive failures
developing quickly over large areas. Quick clay landslides may be triggered by small, local
instabilities caused by human activity as landfills or excavations, or by natura! processes
such as erosional processes and surface sliding, typically in ravines and river slopes.
Relatively small man-made fills at the brink of slopes, and also far behind slepes. are known
to have triggered numerous quick clay slides (e.g. ref. /4/). One example is the Rissa
landslide in 1978 (ref./14/) with a volume of ca. 6 million m
3
,
triggered by a fill of ca. 200m
3
!
The quick clay problem was one important
fador
for establishing NGI (founded formally in
1953, with a precursor from 1950). In October 1953 the disastrous Bekkelaget quick clay
slide in Oslo occurred, killing several persons. The Bekkelaget slide was thoroughly
investigated studied by NGI (ref. /1/). Quick clay studies have been an important activity at
NGI during the 60 following years, both when it comes to geochemical (ref. /3/ and /5/),
geomorphological (ref. /6/) and mechanical aspects of the problem, e.g. (ref. /2/,
141,
/15/).
The gigantic Rissa landslide in 1978 became the starting point of a comprehensive mapping
program for quick clay slide hazard in Norway. The Rissa landslide is presented on web in an
informative documentary movie (ref.
/7/).
NGI lhas been a contributor to mapping of quick
clay hazard zones in all the years to follow, e.g. ref. /8/ and /10/. A semi-quantitative method
has been developed to characterize the hazard level in each mapped danger zone (ref. /131).
Also the potential damage (consequence) of a slide within the danger zone is considered in a
semi-quantitative way. Combining the hazard and the consequence, each zone is placed in a
risk class. The Norwegian system for dealing with quick clay hazards is described in more
detail in ref. /12/.
Jf only a few names are to be mentioned of many NGI-employees involved in quick clay
research and consultancy activities, two names instantly appear; namely Kjell Karlsrud and
Odd Gregersen. These two geotechnicians ha•te been main contributors to the Norwegian
geotechnical community as well as to society within the quick clay field through a period of
ca. 40 years. Many others have contributed, not the least within the field of numerical
calculations.
Since the 1950's there is a tendency of reduced frequency of large quick clay slides (ref. /9/).
This may be attributed to both physical mitigation measures in quick clay areas (erosion
protection and topographical changes of ra'lines for agricultural purposes), and also
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