January 2001 - Tech Tip Supplement
Shovel Shear Test Variations
Important: This is not meant to replace hands-on
instruction, nor to replace the need for hands-on practice.
There are many reasons for the large variations that can be
possible in snow strength and stability tests. The shear test
is especially hard to quantify and is generally not used in
the recording of formal data except as a note on the edge of
a profile. (Compression tests and Rutschblocks are usually recorded
as more formal test results, and while they are more consistent
than shovel shears they are still not without their shortcomings.)
These reasons for variation fall under two broad categories
- snowpack variations and people variations (i.e. implementation
Here is a graph showing the frequency of various scores for
tests done on different layers. This graph should thoroughly
debunk the idea that a shovel shear harder that "easy"
means a slope will not slide. (Original source of graph is unknown.)
A couple years ago I was instructing a one-day awareness seminar in
Washington and one of the students/organizers had taken a Level I avalanche
class. He claimed that his instructor had told them that he had never
seen an avalanche occur at a time when his shovel shear test results
were not easy or very easy.
This instructor was a person who spends a great deal of time in a maritime
snowpack, and perhaps that had been his experience. But it has always
bothered me that anyone would make such a blanket statement to students
in a class. Most of the best instructors I know choose not to emphasize
stability testing much at all in a basic class. Many have decided over
time to essentially eliminate it. I still believe in introducing stability
tests since they are referred to so often, including in public bulletins.
However, I try to emphasize to the greatest extent possible that localized
testing can be misleading and that it needs to be put into a broad context
with everything else known.
The conditions which are so widespread this winter make it clear why
stability testing can be misleading. Slopes have supported many tracks,
and they probably would have tested as stable in many areas. Then these
same slopes have slid (tracks and all) when somebody hit a weak spot.
The graph above should thoroughly debunk the idea that
a shovel shear harder that "easy" means a slope will not slide.
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