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 |
| |
|
|
| • |
Small sample size. You
need to do several tests for consistent results |
| • |
Doesn't work on flat slopes |
|
|
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 |
| • |
Large sample size makes
test more reliable |
| • |
Duplicates what happens
with a skier on the slope |
| • |
Easy to interpret |
| • |
Quantifiable |
|
| • |
Takes more
time but with the proper snow saw not much more
time |
| • |
Climbers and snowmobilers
have to do it on foot, which skews the results. |
|
|
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. |
| • |
One of the only tests
which finds and identifies surface hoar. |
| • |
Works for non-skiers |
|
| • |
Hard to
interpret - requires a skilled observer |
| • |
Subjective |
| • |
Small sample size (must
do many tests for consistent results) |
| • |
Snow seems less stable
than it actually is |
|
|
|
