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
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.
LOOK FOR NEUTRAL, OPEN AREAS AT MID SLOPE WITHOUT WIND EFFECTS.
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.
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|
Simple Snowpit Tests:
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.
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|
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|
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|
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.