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Persistent
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Failure PlanesforSlabAvalanchesinBritishColumbia,Canada
RobertE.Davis1,BruceJamieson2,JillHughes2andColinJohnston2
1 U.S. Army Cold Regions Research and Engineering Laboratory,
72 Lyme Road, Hanover, New Hampshire 03755-1290
2 Dept. of CivilEngineering,University of Calgary, Calgary, Alberta,Canada T2N1N4
Phone 403- 220-7479, Fax 403-282-7026, e-mail: jbjamies@acs.ucalgary.ca
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Keywords:surfacehoar,avalanche,snowmetamorphism,
snow, strength,plane sections,microphotography
ABSTRACT
Torelate shear strength of buried surfacehoar to metamor-
phic changes, prominent layers of surface hoar buried on
7January1995and28December1995intheCariboo
Mountains of British Columbia were sampled forsection
plane analysis, tested with shear frame and photographed
approximately every tendays for two months. Initiallyboth
layers were very unstable and were the failure planes for
many dryslab avalanches.The photographs and section
planes show the metamorphic changesassociated with the
changesin strengthandstabilityduringthewinter. Initially,
well-developedsurfacehoar provides an "umbrella" effect,
effectively preventing subsequent snowfall particles from
contacting(and sintering to)theunderlyinglayer.Fora
strengtheninglayer, preliminary results show that the sur-
face hoar crystals growinglarger bonds to the grainsbelow,
whiletheoverallthicknessofthesurfacehoarlayer
decreases.
INTRODUCTION
In the InteriorRanges ofBritish Columbia,layers of sur-
facehoarcrystalsformthefailureplanes formanyfatal
slab avalanches (Jamieson and Johnston, 1992). Assessing
thestabilityofsuchlayersremainsachallengefor
backcountry recreationists as well as for professional fore-
casters.We employed specialized techniques,as wellas
traditionalmethods,toobservetheevolutionofburied
surfacehoar. Measurements madefrom snow pits provided
the usual estimates of snow density, stratigraphyand grain
properties. Special techniques included rutschblock and
shearframetests,microphotography,and plane section
cuts,which provided comprehensive and detailed meas-
urements of the behaviorof these layers.
METHODS
We made measurements every9-14daysofsurface hoar
layers buried on 7 January 1995 and 28 December 1995 in
the Cariboo Mountains of British Columbia. The study site
was a 30-35[!] east-facing slope at 1600 m in a logged area,
prone to growing large surface hoar.On eachtest day, we
located the buried layer of surface hoar on the side wall of
a snow pit and recorded a snow profile (CAA,1995).As
part ofthesnowprofile,we disaggregated and observed
surface hoarcrystals on a dark-colored metal plate with a
3 mm grid. The shapeandsize of the crystals was recorded
(Colbeckandothers,1990).We alsophotographedthe
disaggregated crystalsonthemetalplateinthesecond
winter.
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We removed the overlying snow towithin 45mm of the
surface hoarlayerand made 7-12shearframe testseach
day (e.g. Sommerfeld, 1984; Jamieson and Johnston, 1995).
We alsosampled the snowforsection plane analysis,
which allows one to measure snow properties at scales not
currently possible with simple tools in a snow pit. An un-
disturbedspecimen consistingof the surfacehoar layer and
thelayersabove andbelowwascollected fromtheside
wallofthepit.The specimenwasisolatedfromthepit
wall and gently placedin a 15 cm long by 10 cm wide by 5
cm deep box.The snow was mechanically stabilized and
sealedusingdimethylphthalateasaporefiller(Perla,
1982),which was subsequently frozen.
In the cold laboratory, we mounted and planed speci-
mensonasledge microtomeandtreated thesurface for
high contrast between the ice grain profiles and the pore
filler.A video camera and frame grabberacquired digital
images ofthe section cutsand grid scales.Image editing
and processing included threshold classification,median
filteringforsmoothingandvariousotherimageediting
steps to produce the closest visual match between ice and
pore classes and each raw digital image.
Wemeasureduser-introducedlinesegmentsand
stereological variables.Line segments allowed us to accu-
mulate precise statistics on the thickness of the buried sur-
face hoar layers, and to precisely measure the dimensions
ofthedisaggregatedcrystals.Stereologicalvariables
included areal and edge densities,and the random inter-
cepts of the ice and pore (Underwood, 1970), which trans-
latetosnowdensity,indices ofgrain andporesize and
surface area (Dozier et al., 1986).We measured these vari-
ablesinthe buried surface hoarlayersand inthe layers
above and below to track changesdue to metamorphism.
RESULTS
Surface hoar layer buried 7 January 1995
On five days between 17 January and 8 March, we tested
thelayerthat wasburied on 7January1995.Specimens
werecollectedand preserved for subsequentsection analy-
sis on each test day.
Figure 1 shows the averageshear strengthfrom the shear
frame tests,plotted along with the 95% confidence inter-
vals. The strength increasedfrom 0.7 kPa on 17 January to
4.7 kPa on 8 March and there was no decrease in strength
since the slight decrease in mean strength apparent on 23
February is not significant.
Therutschblockscored2on17January and6 on27
January. The rutschblock score calculated fromthe shear
frame tests (Jamieson,1995) rose from1.5 on 17 January
to 4.5 on 27 January and remainedbetween 6 and 7 after-
wards (Figure 1).
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