Weak Layer Considerations

including the interesting topic of how the snowpack weak layers can stabilize and then subsequently be re-activated by additional loading or warming to the point where triggering is once again possible. I think understanding that process or those processes is crucial in the skiing game.

If you read the research by Colbeck, McClung, Herwijnen, Heierli and others ( including McClung's extensive work on his UBC homepage ):

* At a fundamental level, avalanche formation is underpinned by a type of material failure called delamination.
* There are two models of why delamination occurs: McClung's shear fracture model and the anti-crack model proposed by Herwijnen et al.
* In any composite visco-elastic material with few flaws, the material is mostly stable across time and space during the application of dynamic force.
* Macroscopic flaws such as buried surface hoar create relatively large airspaces in the snowpack.
* From a composite materials perspective, these airspaces are flaws, which may or may not exceed the critical size required for catastrophic failure.
* For any flaw exceeding the critical size, failure is induced by applying force at a critical rate.
* Confusing the issue even more, failure of the material itself may or may not lead to catastrophic failure.

It's helpful to think of persistent weaknesses as persistent imperfections in the material. Sometimes these imperfections heal and sometimes they don't. Sometimes people trigger large avalanches, sometimes they don't. Klassen concludes that most people, including very experienced ski guides, are not very good at managing avalanche problems presented by deep imperfections. I tend to share his conclusion.

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Do you think this clarification is helpful?

The persistence of flaws isn't particularly interesting or suprising or mysterious.

Buried surface hoar resists strength gains from overburden pressure and favourable metamorphic regimes because of its shape and its anisotropic strength characteristics. In addition, such weaknesses spend a lot of the winter being 'refrigerated' inside the snowpack.

For any flaw that exceeds the critical size, avalanche formation ( catastrophic failure ) is possible provided very simple energy requirements are met.

* en.wikipedia.org/wiki/Delamination
* en.wikipedia.org/wiki/Composite_material ( See Plywood )

If you read the research by Colbeck, McClung, Herwijnen, Heierli and others ( including McClung's extensive work on his UBC homepage ):

* Macroscopic flaws such as buried surface hoar create relatively large airspaces in the snowpack.
* From a composite materials perspective, these airspaces are flaws, which may or may not exceed the critical size required for catastrophic failure.
* For any flaw exceeding the critical size, failure is induced by applying force at a critical rate.

Do you think this clarification is helpful?

I think these three points have the most relevance for the skier who probably doesn't spend too much time thinking about "materials". The key is that there is a low bond density such that the structure of weak layers can easily be broken down. Essentially that is what these five sentences say.

As a skier I probably don't exactly care through which process the cracking begins and propagates. That is a worthwhile subject for a physicist who deals in materials.

By the way I don't think that the problem of dealing with weak layers is limited to guides. Evaluating a snowpack with known weak layers is tricky at best. All of my close calls since I had a modicum of experience have been with significant weak layers. In several cases I triggered slides at my feet, including remotely when I stopped because I didn't like the shape of the terrain. I have one friend who had a heli-operation in a dry part of the Purcells and described the snowpack as a "House of Cards". He left that operation because he considered it too risky.

This is a video discussing the concepts: vimeo.com/29201289

I think the video is very good and is something backcountry skiers on this website should look at in understanding the process of transmitting energy to the snowpack and also as regards snow testing methodology.

The bulb digram is also very good and I hope lots of people take the time to look at it.

A hoar Example: See Picture. This slide was triggered by a friend while guiding,  many years ago(this is why guides go first). Three weeks prior to this slide I had seen large crystal surface hoar growth that was confined to a certain elevation band (may have been due to stratus cloud, don’t remember).

Surface hoar crystals at specific elevations ( aka the bathtub ring ) are fairly common. Bruce Jamieson wrote a paper on this, but I can't find it.

I called my friend and we discussed the implication of these large crystals. I remember him telling me that they can become a persistent weak layer and would tend to resist overburden pressure. These crystals became buried for several weeks and persisted in this area (my speculation, fit profile, limited other likely explanation at time)  which is a protected, open to the sky, North aspect at around 5,800 feet elevation.

Yes, the persistent forms resist strength gains from overburden pressure because they have anisotropic strength characteristics. ( Weaker in shear than compression. ) This means that the pore size doesn't decrease very readily, except when the pack delaminates right before an avalanche. For more discussion, see above.

QUOTE BY COOKIEMONSTER; “If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre“

I thought this number was 2 meters? What about 2 or more skiers, skinning or skiing the same slope at the same time, same place? And then of course you have to consider this number invalid with new snow loading such as 1 skier+ depth and density of new snow.

Page 230 of The Avalanche Handbook ( 3rd Edition ): "However, when buried persistent layers are deeper than about 1 m, instances of skier triggering become rare, while snowmobiles may still trigger avalanches..."

Thanks for the kind words. Everyone here makes great contributions to the discussion.

From Cookie:

“If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre“

That is the number that is bandied about. It is thought that deeper slabs are triggered from areas where the weak layer is more shallowly buried. But I can say definitively that I've triggered a 4' slab which I described elsewhere and which I believe had as a weak layer a faceted suncrust. I've also remotely triggered a pair (nearly simultaneously) of 3 to 3-1/2' slabs on surface hoar. In both cases if I was in a location where the weak layer was buried less deeply when I triggered it, I didn't know it at the time.

I think it is a probability thing where the number of slides that can be triggered is something like three standard deviations from the norm at 3'. But there are undoubtedly outliers that are rarer that can be triggered at greater depths.