Science of Snow: To Shred or Not to Shred?

words by Scott Thumlert

From old school tests like the shovel shear test (SST) to the more modern propagation saw test (PST), backcountry shredders and avalanche professionals have a full quiver of snowpack tests that provide some information about slope stability. But which test should we use and when? Are there conditions where one particular test is more appropriate? Should we even bother digging?

This little article will shed some light on these questions and will point you towards a great video that Mike Conlan (ASARC PhD student) just recently created on the topic. The content here is probably best for folks who have taken an AST 1 or 2 and who know what fracture character is.

Let’s start with a brief intro on two important concepts of avalanche mechanics: propagation and initiation. To get a dry slab avalanche, a failure in a weak layer must initiate (start) and then that failure must propagate (spread out). Then, if the slope is steep enough, the snow slides. We can be initiating failures in weak layers, but if the conditions are not ripe for propagation then no avalanche. Conversely, if the weak layer is primed for wide propagation, but no failure is initiated, then no avalanche. The two are mostly independent, but both necessary for avalanches.

So, how do snowpack tests help? Certain snowpack tests are best for initiation, others are better for propagation and others are good for both. Here are the nuts and bolts:

“I’ve only heard of the compression test and I don’t even remember how to do it!” If you’re thinking this, then here is an amazing video from Mike Conlan that shows the “how to” of each test and gives a little info on each.

The Canadian avalanche industry is a worldwide leader is avalanche training, field research and, you guessed it, snowpack tests. With all our expertise in snowpack tests it may be easy to get a little carried away with them put too much weight into the results. They ultimately are a SINGLE POINT observation in an extremely complex mountain environment. Here I could blab on and on about this point, but Grant Statham, one of brightest and most experienced avalanche heads in Canada, says it best in this two minute video called “When to Give-er”:

For the future, if anyone has any particular questions that they’d like some more info about, feel free to shoot me an idea at: Cheers!