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| A procedure
used to estimate the stability of the snowpack, often done in
a snowpit. Common tests include the Rutschblock, compression
or tap test, cornice drop, and slope cut |
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Stability
Test:
Any of dozens of different tests to determine the stability
of the snow.
Stability tests might include:
| • Jump on test
slopes (small slopes) to see how they behave |
| • Kick or saw
off a piece of cornice and let it bounce down the slope |
| • Perform a
slope cut |
| • Probe with
your ski pole as you travel |
| • Dig down with
your hand to see how well the snow is bonded to the underlying
layers |
| • Wait for other
people to jump into the slope before you |
| • Dig one or
more snow pits in representative locations |
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Snowpit tests:
Most of the time we can gather enough information about the
snowpack without ever taking out the dreaded shovel. But sometimes
the only way to get good information about deeper weak-layers
is to grease up the elbows and do some honest work for a change.
I personally feel naked unless I dig at least one snowpit in
a representative location to at least get the general picture
of what’s going on in the snowpack.
How
to dig a snowpit:
Contrary to popular belief, snowpits don’t have to take
a lot of time. My philosophy is that if your feet get cold,
you're doing something wrong; I almost never spend more than
10 minutes in a snowpit. Since snow can sometimes vary quite
a bit from place to place, I would much rather dig several quick
pits and average the results than to spend 30 minutes in one
pit documenting every useless detail. We're trying to get a
GENERAL, BIG PICTURE idea of what's going on here. Then move
on to another location. Often I dig the hole without even taking
off my skis or board, but it usually helps to at least take
off the uphill ski or take one foot out of the board binding.
 First,
the shoveling: Get down on one knee when you shovel. Your back
will thank you, and especially if you grew up Catholic, like
me, it somehow feels appropriate to get on your knees when asking
for answers from the unknown. Make the hole wide--about the
width of a ski length. And don't dig a vertical hole, like you're
going to China, shovel out the downhill side so you have room
to work, which actually takes less time in the long run. Just
slide the chunks of snow downhill on your shovel without lifting
it. This only takes a couple minutes if you're on a steep slope
(where you should be) and in soft snow (also where you should
be. If not, move to a different part of the country.)
Then get your tools ready. Get out the snow saw. If you don't
have one, than go buy one. You can get by without one but you
will hate life and hate snowpits and you will quickly quit digging
them. Not a good idea. If you're a skier, get a snow saw that
fits on the end of a ski pole.
After digging the snow pit (which gives you a lot of information
in itself) I like to just dive in and FEEL with my hands. Some
people like to use a little whisk broom and gently brush the
wall, but don't listen to them. You need to FEEL the snowpack.
Run your mittens horizontally across the face of the snowpit
wall and get a nice tactile feel for the different layers. Just
like an eroded rock outcropping, notice how the weak layers
crumble away while the strong layers remain sticking out. Then
stand back and SEE the layers too. Dive in and get your hands
dirty. Remember that this is not just an academic exercise.
This is your life we're talking about here. Just looking and
thinking don't work. Crawl around, shove your arms into the
weak layers. Feel it, see it, chew on it, smell it--live it.
Use as many pathways as possible--BEE the snowpack, as they
say. Sometimes I wish I could shrink down to the size of a gnat
and crawl around through the caverns inside the weak layer.
This is how you know that you have become a complete snow nerd.
Then dust yourself off (if you're not getting snow on you, you're
doing something wrong) and carefully smooth the snowpit wall
in preparation for the various stress tests you will perform.
Make sure it's smooth and vertical. This is very important.
Remember, garbage in--garbage out. But good tests will give
good answers. Whatever tests you do, they must be done exactly
the same each time, so that one can compare one snowpack to
another.
How deep to dig a snowpit:
Since it's difficult for humans to trigger avalanches more than
about 1.5 meters (5 feet) thick, (unless they are triggered
from a shallower spot) I seldom dig snowpits deeper unless I
specifically know there’s a deeper weak-layer that may
cause problems. If you already know that the deep layers have
no worries, then just concentrate on the shallow snow. Each
situation is a little different and in time you will get a feel
for it. But in general, keep your snowpits less than 1.5 to
2 meters deep unless you know of a good reason to go deeper.
Where to dig a snowpit:
Where to dig a snowpit is probably more important than how to
dig one. Choosing a representative location is an art, and art
is difficult to describe.
Dig it on a slope most representative of the slope you are interested
in but without putting yourself in danger. Often you can find
a small representative test-slope--one that won't kill you if
it does slide. Or, you can work your way into progressively
more dangerous terrain. For instance, if a snowpit on safe terrain
gives you a green light, then it gives you the confidence to
dig another one on more dangerous terrain. Green light there?
Then, move onto even more dangerous terrain, and so on. Never
dive into the middle of a dangerous avalanche path without first
gathering lots of additional data about the stability of the
slope.
Don't dig it along ridgelines where the wind has affected the
snow--a common mistake. Although sometimes the crown face of
an avalanche may break right up to the ridge, the place where
we most often trigger avalanches is 100 or more feet (30 meters)
down off the ridge. Avoid thick trees because conditions are
often quite different than on open slopes. Avoid compression
zones and tension zones. Avoid places where people have compacted
the snow.
Bottom line:
LOOK FOR NEUTRAL, OPEN AREAS AT MID SLOPE WITHOUT WIND EFFECTS.
Hot tip:
Use an avalanche probe to find a representative place with average
depth. Poking around with a probe can save a lot of time digging
in stupid places, like on top of a rock or tree or where a previous
party had their lunch.
Many cagey avalanche professionals dig their snowpits just above
a tree so they can grab it if the slope does slide. Better yet,
tie a belay rope onto that tree and dig below the tree. People
don’t tend to ski, snowboard or snowmobile just below
trees. I almost always carry a lightweight belay rope and use
it on regular basis. Most important, dig lots of snowpits in
lots of different areas because the snow can vary quite a bit
from place to place. Look for the pattern of instability.
Shear Quality:
What we call “shear quality” is possibly even more
important than the results of compression or Rutschblock tests.
Shear quality tells you how much elastic energy is stored in
the snowpack. You also hear the terms “primed” or
“stretched rubber band” or “sensitive”
to describe the same thing that shear quality tells you. It’s
difficult to describe what a high quality shear looks and feels
like but you will know it when you see it. It pops out like
it’s spring loaded and it does so on a clean, planar fracture.
It gets your attention. Rate the shear as a quality 1, 2 or
3:
| Shear Characteristics |
Shear Rating |
| Breaks on a clean
and smooth like it’s spring loaded |
Q1 |
| Breaks on a smooth
plane but more stubborn, not like it’s spring
loaded |
Q2 |
| Breaks on a rough,
broken plane |
Q3 |
|
For almost all of these snowpit tests you need to be on a slope
of at least 30 degrees in steepness. The optimum steepness is
38 degrees since that's the most dangerous slope steepness for
slab avalanches. Finally, use a snow saw, which makes all these
test go much faster, but you can get by without one in a pinch.
The times listed for these snowpit tests don't include the time
of digging the hole. Most snowpits in reasonably soft snow,
with a good shovel and on a steep slope take only a couple minutes.
For very hard snow it may take twice that time. So you can add
a couple minutes to the times listed for digging a hole.
Compression Test:
 I
love this test (and its cousin, the stuff block test).
Besides the Rutschblock test, it's about the only one
I do anymore. It's quick, easy to interpret and works
for most kinds of weak layers. Start by isolating a
column about the same size as the blade of your shovel,
in other words, about one foot by one foot (30 x 30
cm). Be sure to completely isolate the column. Then
take the blade of the shovel and lay it flat on top.
Finally start tapping progressively harder on the shovel
blade until the column fails. Start with ten taps by
articulating from your wrist, then ten more taps by
articulating from your elbow, then ten more from your
shoulder using the full weight of your arm. Don’t
push your arm into the snow, but let it fall with its
own weight. In this way, the test is somewhat quantifiable.
In other words it doesn't depend on “feel”
or the opinion of the tester, but it has a reproducible
number which is more or less same for most people and
can easily be communicated to others. For instance,
it failed on an easy tap from the elbow, or it failed
on a moderate tap from the elbow or perhaps a hard tap
from the shoulder. Since snow stability is dependent
on the size of the trigger required to make it fail,
this test is especially easy to interpret. Of course,
if you have an unusually light arm or an unusually heavy
one, you need to take that into account.
| Results
- Compression Test: |
What
it means |
| Breaks when
articulating from the wrist |
Red light |
| Breaks when
articulating from the elbow |
Yellow
light |
| Breaks when
articulating from the shoulder |
Green
light |
|
Stuffblock Test
Karl Birkeland and Ron Johnson of the Gallatin National
Forest Avalanche Center in Bozeman, Montana have developed
what they call the "stuff-block test", which
I think is a great test. Take a stuff sack, fill it with
10 pounds of snow (weigh it with a lightweight fishing
scale), then, place the shovel blade on top of the column
and drop the stuff sack onto the column from progressively
greater heights until it fails. In other words, it is
similar to the compression test but more quantifiable
than using the varying weight of people's arms.
| Results
- Stuffblock Test: |
What
it means |
| Breaks with
drop from 20 cm or less |
Red light |
| Breaks with
drop from 20-40 cm |
Yellow
light |
| Breaks with
drop from over 40 cm |
Green
light |
| Advantages: |
Disadvantages |
| • |
Quick |
| • |
Easy to interpret |
| • |
Works for any type of weak-layer,
especially effective with faceted snow |
| • |
Works well for non-skiers
since you don’t need skis |
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| • |
Small sample size.
You need to do several tests for consistent
results |
| • |
Doesn't work on flat slopes |
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Rutschblock Test:
The Rutschblock test (pronounced ROOTCH block) and it's
cousins the Ski Block and the Rutschkiel test (pronounced
ROOTCH-kyle) have rapidly become the standard snowpit
test of choice for avalanche professional who do a lot
of snowpits. The main advantage is that they work with
a larger sample size, which tends to smooth out any local
variations in the snow. Second, the test is quantifiable
and very easy to interpret. Finally, it duplicates the
kind of shock to the snowpack when a skier crosses the
slope.
First, on a slope of at least 30 degrees, completely isolate
a block of snow about a ski length across, and a ski pole
length up the slope (2 meters wide by 1.5 meters upslope).
Remember that you have to cut out the back for it to be
a Rutschblock test. IF YOU DON’T CUT OUT THE BACK
IT’S NOT A RUTSCHBLOCK. YOU NEED TO CALL IT SOMETHING
ELSE--NAME IT AFTER YOUR DOG OR SOMETHING--SO YOU DON’T
CONFUSE PEOPLE.
If you use a snow saw which mounts on the end of a ski
pole you can cut the block in under a minute. With two
people working together with snow saws the job takes about
30 seconds. You can also insert two probe poles at the
upper corners and run a parachute cord around the outsides
of the probes. Two people can grab each end of the cord
and saw out the block. It goes pretty quickly but you
need two people. You can also use the tail of a ski to
saw out the block but it takes longer. Finally, you can
shovel out the block, but this takes a very long time
especially in hard snow. I think a snow saw mounted on
the end of a ski pole works best. It doesn’t require
two people, it’s very quick and lightweight and
you can saw cornices with it--definitely standard equipment
for anyone venturing into avalanche terrain.
Next, simply step onto the block while wearing your skis
or snowboard and jump progressively harder until the block
fails. Most people rank the test on a scale of one through
seven.
| Results
- Rutschblock
Test: |
What
it means |
| Fails while
isolating the block |
1 |
Red light |
| Fails while
stepping onto the block |
2 |
Red light |
| Fails with
an easy weighting |
3 |
Red light |
| Fails with
one easy jump |
4 |
Yellow
light |
| Fails with
one hard jump |
5 |
Yellow
light |
| Fails with
several hard jumps |
6 |
Green
light |
| Doesn't
fail |
7 |
Green
light |
| Advantages: |
Disadvantages |
| •
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Large sample size
makes test more reliable |
| •
|
Duplicates what
happens with a skier on the slope |
| •
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Easy to interpret |
| •
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Quantifiable |
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| •
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Takes
more time but with the proper snow saw not
much more time |
| •
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Climbers and snowmobilers
have to do it on foot, which skews the results. |
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Shovel
Shear Test:
 The
good-old shovel shear test has been taught in most every
avalanche class since the dawn of time but unfortunately,
it is often the only test taught in many classes. Even
the inventors of the shovel shear test agree that it may
be a good test for finding and identifying weak layers,
but it's not a very good test for determining the stability
of the snowpack because of: (1) the small sample size,
(2) difficulty in interpreting the results and (3) the
subjective nature of the test. Nevertheless, I still do
the test because first, it's a good test for finding surface
hoar, and second, it's one of the only tests that work
well on a flat slope, for instance, when you're in camp
or having lunch. Compression tests can also locate surface
hoar but they tend to destroy it more easily.
First, make vertical cuts with the snow saw in the snowpit
wall about the same width as your shovel. Then cut behind
the column with the snow saw---NOT THE WHOLE COLUMN (a
common mistake) but only about a foot or two down. Then,
insert the shovel behind the column and pull. Don't lever
on the shovel, but pull straight out. Then cut another
foot or two down, and pull again, and so on until you
reach the bottom of the column. Pay attention only to
the smooth, straight shears that pop out easily, and rank
the shears as easy, moderate, hard, and so on. Turn each
block upside down to see what weak-layer was involved.
In my opinion, the test is hard to interpret because you
are removing the overlying snow before you test each layer.
Therefore, you find that the deeper weak-layers tend to
be stronger because they must support the load of overlying
snow. It's difficult to take this factor into account.
In my experience, most beginners using the shovel shear
test tend to consistently over rate the danger. Finally,
because of the small sample size, you need to do many
tests to get a true feel for the stability of the snow.
| Advantages: |
Disadvantages |
| •
|
Works on a flat
slope. |
| •
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One of the only
tests which finds and identifies surface hoar. |
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Works for non-skiers |
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Hard
to interpret - requires a skilled observer |
| •
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Subjective |
| • |
Small sample size
(must do many tests for consistent results) |
| • |
Snow seems less
stable than it actually is |
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| Additional Terms: |
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