S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

TheCompressionTestforSnowStability

BruceJamiesonandColinJohnston

Dept. of CivilEngineering,University of Calgary, Calgary, Alberta

Phone 403-220-7479, Fax 403-282- 7026, e-mail jbjamies@acs.ucalgary.ca

Keywords: snow compression, snow stability, snow strength,

snow physical properties,snow stratigraphy

ABSTRACT

The compression test isa quick stabilitytest.Itinvolves
tapping on a shovel placedon top of a column of snow and
noting thefailures in weaklayers that appearon thesmooth
walls ofthe column.Limited data show a correlation for
compressionscores(numberoftaps)withrutschblock
scoresandwiththefrequencyofskiertriggeringon
avalanche slopes.
The effect of various experimental factors on compres-
sion scores are assessed.Two different designs of shovel
blades (varying in size and shape) did not appear to affect
the compression scores. Increasingthe cross-sectional area
of the columns increased the compression scores.Differ-
ent operators generally obtained a score in the same range
identifiedas easy,easy-moderate, moderate, moderate-hard
and hard.

BACKGROUND

The test method usedfor this study was developedby Parks
Canada wardens in the 1970's although similartests may
havebeen developed elsewhere.Weak layersareidenti-
fied when they fail and become visible on the walls of the
column ofsnow eitherwhile tapping on a shovel placed
on top of the column, or occasionallywhile initiallycutting
the column.

Figure 2 Compression test showing failures in two weak layers

TECHNIQUE

Isolate a 30 cm by 30 cm column of snow deep enough to
expose potential weak layers on the smooth walls of the
column (CAA, 1995, p. 44-45). Rate any failures that occur
while isolating the column as "very easy".Place a shovel
blade on top of the column.Tap with fingertips,moving
hand from wrist and rate any failures as "easy". Tap with
the knucklesfrom elbow andrateany failureas "moderate".
Finally hit the shovel blade with open handor fist and rate
any failures as "hard". In practice, 5-10 easy ormoderate
tapsareappliedbeforeincreasingtheforce.Forthe
purposes of thefollowing fieldstudies,10 taps wereapplied
before increasing the force. For scores from adjacenttests,
variability is typically ±2-3 taps 65% of the time (Jamieson
and Johnston,1995).
Accordingtothe CAAGuidelines(CAA,1995,p.44-
45), if the snow surface slopes, a wedge of snow should be
removedtolevelthe topofcolumn "aftersurface layers
have been tested". Sincewe observed wobbling of very tall
columns (> 1.5 m) whenthe moderate andhard blows were
applied to columns with sloping top surfaces, we decided
to level the top of the column afterthe easy taps from the
wrist.Thisensures thatthemoderateand hardtapsare
directed vertically down the column.

SCORING

The CAA Guidelines give four levels (very easy, easy, mod-
erate orhard)forrating thefailureofweak layersin re-
sponse to tapping with increasing force on the top of the
column.However,somefieldworkersuse"easy-moder-
ate" and "moderate-hard" to describe the transitions. Us-
ing ourpractise of 10 taps perforce level,Table 1 gives a
six-level ratingschemethatincludestheeasy-moderate and
moderate-hardtransitions. Although further experienceis
requiredwiththisscheme,itisusedinseveralofthe
discussions in this paper.

75

Figure 1 Compression test