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The Rutschblock test gives an indication of the weak layer
strength only if the deformation penetrates deep enough
with enough spatialextent to fail snow in a weaklayer.
Weexpect that in slightly consolidated low density snow
layers (deep ski penetration) the deformation penetration
is poorand the Rutschblock test may not be indicate po-
tential instability. In conclusion, the interpretation of the
Rutschblocktestresultiscomplicatedandoneexpects
highly variable results depending on both, hardness vari-
ations ofthe surface layers and strength variations of the
weak layer. Such highly variable results have been amply
demonstrated(Föhn,1989).Atagivenlocation,the
Rutschblock test isa very goodtest but extrapolation to
other locations is uncertain and might be risky, e.g. if sur-
face hardness conditions change by temperature orwind
packing variations.
If no indication of instability results froma test,one is
left with the question:Have I measured deformation pro-
pensity oris the weak layerstrength such that failure is
unlikely?PreferredlocationsforRutschblocktestsare
thosewithsoft,butwellconsolidatedsurfacelayersto
ensure that deformation has the best chance to penetrate
deeply to give an indication of instability. Unconsolidated
softsnow at the surface will attenuate deformation to re-
duce effective penetration of deformation.
Shovel sheartest
The shovel shear test also has its limitations (forexample
small sample size and more qualitative loading). However,
the shovel sheartest gives a qualitative estimate of weak
layerstrength as well as and indication of the quality of
the shear plane formed. As it does not test the slab prop-
erties it is simpler to interpret, but the information needed
isincomplete.Since theshovelsheartestisessentially
independent of slabproperties one must to use otherin-
formationsuchashardnessvariationstocompletean
evaluation of instability.
Shearframe test
The interpretation of the shear frame test is similar to the
shovelsheartest:itteststruevariationsinweaklayer
strength.Theresults aremore quantitativethanforthe
shovel shear test. Witheither the shear frame or the shovel
shearseveral tests must be done to get consistent results.
When the shearframe measurements are combined with
the normal load to give a non-dimensional stability index
the resultsare independent ofslabproperties including
temperature andhardness. Analysis of instability for skier
triggering should include the slab properties.Again,the
key to understanding snow slab stability is to concentrate
on deformation, rather than stress, and the interpretation
oftheslaband weaklayerpropertiestogetherin acou-
pled mechanical system.
SUMMARY
The effects of snow temperatureson snow slab instability
may be divided intotwocategories:1.immediate influ-
ences which promoteinstabilityunderwarming includ-
ingreduced snowstiffness(orhardness),failuretough-
ness and strength with no time delays;2.delayed effects
such as metamorphismand creep which promotestabil-
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ityunderwarmingwithtimerequiredtoproducethe
changes.
*In order to include the effects of snow temperature in
an analysis of instability,one must seek out informa-
tionaboutsnowhardnessand onemust thinkabout
theprobleminregard todeformationratherthanan
analysis of stresses. Conventional stability evaluation,
for example, calculation of a strength to load ratio will
not contain much of the important information about
temperaturedependence (orsnowhardness) andin-
stability.
*For skier triggering, the most important immediate in-
fluence of snow temperatures is the decrease of hard-
ness ofthe surface layers underwarming.Secondary
effects are decreases of failure toughness and strength
if the weak layeris warmed. Hardness estimates (e.g.
the hand hardness test) implicitly include snow tem-
perature effects.
*The Rutschblock test has the advantage that it implic-
itly includes snow temperatures and slab hardness ef-
fects in an analysis of instability whereas tests like the
shovelsheartestortheshearframe testdonot.The
drawbackis that another source of variation is included
that has to be considered forextrapolation:the test re-
sults depend heavily on the surface layer stiffness. It is
proposed to performRutschblock tests at places with
relatively rathersoft than hard surface layers for com-
parison and extrapolation.
*The advantage of the shovel testand shear frame test
is theycontain direct information about strength and
qualityof weak layerfailures and,therefore,theirin-
terpretation is lesscomplicated than the Rutschblock
test.However,they do not contain the most important
informationaboutsnowtemperaturesandhardness
and,therefore,supplementaryinformationaboutthe
slab properties must be sought. Wepropose thatto com-
plete any stability test, observations of hardness varia-
tions be made (e.g. with a profile), otherwise the infor-
mation gained by the tests is too limited and cannot be
easily be used forextrapolation.
ACKNOWLEDGEMENTS
We aregratefulforthesupportoftheNaturalSciences
and Engineering Research Council of Canada.
REFERENCES
Föhn, P.M.B.1987. Thestabilityindexandvarious triggering
mechanism. IAHS Publication 162, 195-214.
Föhn, P.M.B. 1989. Snow cover stability tests and the aerial vari-
abilityof snow strength. Proceedings International Snow Sci-
enceWorkshop,Whistler,B.C.,Canada,12-15October1988,
262-273.
McClung,D.M.1977.Directsimplesheartestson snowand
their relation toslabavalanche formation. J. Glaciol.,19(81),
101-109.
McClung,D.M. 1995. Theeffect of temperatureon fracture of
dry alpine snow. In: Proc. Int. Symposium: Sciences and moun-
tain - Thecontribution of scientific researchtosnow, iceand
avalanche safety,ANENA, Chamonix, France, May30-June3,
1995, 317-322.
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