Snowpits and Stability Tests
Only use snowpack tests to show the snowpack is unstable; do not use them to "prove" the snowpack is stable. Some unstable slopes will have areas with both stable and unstable test results. Therefore, if your snowpack test indicates stability is good, keep looking. If you perform a test that indicates stability is poor, believe it. The more tests you perform the more apt you are to get an accurate assessment.
Interpreting snowpits and stability tests can be confusing; classes and mentorship are needed to learn how to interpret pits and tests correctly.
One sign of instability always trumps all signs of stability.
Why Should I Dig a Pit?
Snow is a highly stratified (layered) in most mountain snowpacks. Avalanche danger is directly related to this stratigraphy. In general, slopes are unstable if weak (less cohesive) snow exists under a stronger (more cohesive) slab of snow.
Where Should I Dig a Pit?
The best place to dig a pit is on a slope representative of the one you want to ride, but with little or no consequence of avalanches. That slope should have the same:
- Snow depth
Do I need to dig on a steep slope?
No. Always start by digging on a slope less than 30 degrees before entering steeper terrain. Recent research has shown that you can conduct snowpack tests on slopes less than 30 degrees and find similar results as you would on steeper slopes (as long as the slope is representative and the snowpack is reasonably consistent). If you want to learn more about slope angles and stability tests, read this article or this article.
Avoid digging in an area that is wind scoured, has old ski or board tracks, or excessive vegetation.
What Am I Looking For?
The Red Flags in your pit are:
- More cohesive or harder, stronger snow over less cohesive or softer, weaker snow.
- Very weak, sugary snow lower in the snowpack.
- Crack propagation in a weak layer of snow more than a couple of inches deep (refer to extended column test)
Extended Column Test
Extended Column Tests (ECTs) give information about both the initiation and propagation of a crack in a weak layer. It tests an isolated column that measures 90 cm across the slope and 30 cm up the slope. Essentially, ECTs gauge how much force it takes to create a "mini-avalanche" on a "mini-slope."
How to isolate the column
There are several ways to isolate a 90cm by 30cm column.
- Place a probe or ski pole vertically in each of the two uphill corners of the column. Place a piece of knotted cord around the two poles and pull both ends of the cord in a sawing motion.
- Cut the two sides of the column with a snow saw. Then attach the saw to a ski pole or other extension and cut the back with a vertical sawing motion.
How to conduct an ECT
1. Isolate a column 90 cm by 30 cm as discussed above. It should be isolated down to the weak layer of interest, or 100-120 cm deep with straight, clean, and smooth sides.
2. Place your shovel blade on one end of the isolated column.
3. Apply a series of compressive forces to the shovel blade just as you would do for a compression test: 10 taps from the wrist, then 10 taps from the elbow, then 10 taps from the shoulder
4. Record the results and pay close attention when a fracture occurs across the entire column. Fracture across the entire column is the propagation of a crack through the weak layer, a red flag.
The key observation is whether or not a fracture occurs across the entire column. If it does fracture across the entire column, this means the weak layer may be capable of propagating a crack across an entire slope and produce an avalanche. Record results in the following manner:
ECTPV - A fracture occurs across the entire column (propagation) during isolation. A very unstable result.
ECTP# - A fracture occurs across the entire column (propagation) after some number (#) of taps.
ECTN# - A fracture initiates but does not occur across the entire column (no propagation) after some number (#) of taps.
ECTX - No fractures occur in the column after a total of 30 taps. Generally a stable result.
Always be on the outlook for signs of instability; one sign of instability trumps all signs of stability.