Stability:
The snowpack struggles through its life in a constant state
of conflict between opposing forces—the strength of the
snowpack and the stress on the snowpack. We can use a simplified
analogy of a furniture mover trying to carry different kinds
of household appliances. The furniture mover represents all
the forces that hold a slab in place (crystalline bonds, friction
and anchors). The household appliances represent all the forces
that try to make the weak layer fracture (the weight on top
of the weak layer and the shear stress on the weak layer). In
very stable conditions, a big, burly furniture mover carries
an empty cardboard box. In less stable conditions, the burly
furniture mover carries a microwave oven. When things are very
unstable, the Mr. burly is trying to carry a refrigerator. In
other words, we have the same strength, but different stress.
Also notice that we can tweak the equation from the other side.
The burly guy can carry the refrigerator or a wimpy little guy
could try to carry the refrigerator (same stress, different
strength). One way works but the other way doesn’t. When
we do stability analysis, our job is to figure out where we
sit in the conflict between stress and strength.
The bad news is that we're not only dealing with hundreds of
different combinations between slabs and weak layers, but we’re
dealing with something that's invisible. The good news is that
most of the time the invisible becomes visible through observations
and tests, but only as long as you know how to ask the questions
and listen for the answers. In this chapter, we'll explore a
number of simple observations and tests that can reveal snow
stability.
Judging snow stability is a lot more like playing the Wheel
of Fortune than using any rules of thumb or equations. In other
words, you never, ever get all the pieces of the puzzle in front
of you at one time--there's always missing pieces that you have
to fill in with your imagination. The more knowledge and skill
you have, the easier the game becomes. In other words, stability
evaluation means INTEGRATING lots of different pieces of information--putting
the pieces of the puzzle together in an organized way. It's
a very serious mistake to latch onto just one test or observation
and base your whole opinion on that one test--kind of like deciding
to get married on the first date. Bad mistake. No, you need
to shop around, do some homework, put them through a rigorous
series of stress tests, travel for a month in a third world
country, meet the in-laws, remodel the kitchen together. Then
you'll have a lot better idea whether to invest your life in
this person--or, in this case, invest your life in the slope
you are about to cross.
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