Though commonly called “stability tests”, these tests should really be called “instability tests”. They are used to search for possible instability in the snowpack. Due to spatial variability, you never want to use a test to tell you the snowpack is stable. Rather, you should use them to tell you the conditions are unstable on a day when you might thing things would be stable. Common tests include the Extended Column Test and the Compression Test, though many other tests also exist.
Digging a snowpit:
Dig your pit quickly in a representative area for your test. Don’t waste time, but also keep your pit wall where you will do your test vertical and smooth.
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) you seldom need to dig snowpits deeper unless you 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. 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 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. 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.
Extended column test:
Extended column tests are becoming the standard stability test for folks in the backcountry. You isolate a block 90 cm wide by 30 cm deep and tap on one side using the same loading taps as the compression test (see below). Look for how many taps it takes to fracture the block. More importantly, note whether the fracture propagated across the entire block or not. Any fractures that propagate across the entire block are a red flag, no matter how hard you have to tap. If you don’t have other information that strongly suggests the snow is stable, avoid slopes with conditions where ECTs are propagating.
Isolate a small column (30 x 30 cm). 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. Easy taps are bad and hard taps are good. However, even with hard taps we strongly urge you to also do an ECT to see if it fully propagates.