UKH

I am trying to maximise my knowledge of avi tests and will take a course when winter comes around. However, in all the countless readings and videos I've looked at. I can't find anything about the "risk of avalanches" from extended column or compression tests. 

From at least 20 sites, all they do is show you the layering in snow but never about the probability of an avalanche from the layering or how the depth of the first weak layer influences that probability. Whether 10 taps from the wrist or from the elbow or arm is any good. They avoid giving any range. I can understand that avi conditions depend on several interconnected factors e.g. weather, wind, snow, sun, slope... 

I would really appreciate some experiences where you have conducted tests and acted accordingly.

Cheers!

That's beause you cannot discern a hazard level exclusively from a shear test on an isolated block. Avalanche forecasters will describe the stability as one of four possibilities, depending on their observation: Unreactive, Stubborn, Reactive or Touchy.

http://infoexhelp.avalancheassociation.ca/wiki/Sensitivity_to_triggers_defi... 

Bear in mind that there is a great deal of spatial variability in snow packs so what one test exhibits may well be completely different a metre or so away. A shear test in itself has very low value as a safety tool, rather one should be a detective and piece together an assessment from all around: history, forecasts, avalanche forecasts, observations throughout the day etc.

Hadn't seen that table before! I know when I go on my Avalanche refresher's they always show a photo of when the Swiss guy's dug a series of Rutschblock's across a large section of slope and tested each one. the variability in the amount of force it took for each to fail, even from adjacent blocks was amazing!

Rutschblock - the "table" (with a big caveat!) I was taught (full size, ski-length rutschblock, not the smaller compression tests with shovel) was something like this:

1) collapse when digging or cutting or skier getting on the block or just bouncing on the block - DANGER, do not enter at all

2) collapse during shock weighting by the skier (first jump) - VERY POSSIBLE DANGER, avalanches quite possible even by single skier, make precautions in planning and keep distances

3) collapse after multiple shock weighting by the skier - the same, slightly less but nearly the same!

4) collapse after shock weighting (jumping) without skis in boots, etc. - possible but rarer avalanche even by single skier

5) no collapse - "almost" impossible avalanche

The big caveat I mentioned is that obviously you make a rutschblock in a safe location (unless you are crazy), which might be quite different from the tour conditions (even if you do it on the same aspect).

And since many compression tests are not gonna tell you much unless you are a professional and full rutschblocks are bloody time expensive, you should still definitely do them if in doubt, but rely more on prevention and safe planning (former weather, aspects, avalanche forecasts if available, avoiding terrain traps and avoiding heuristic traps - human factor)...

All in all, the rutschblock and other methods are just a tool for when in doubt, safe tour planning (or cancelling) should still come first.

EDIT: Very nicely summarised by AdrianC's reply below mine Esp. the large set of information needed to estimate risk. It's just one of the tools to estimate it, and should not come above the others.

It's a good question to ask and one I've heard quite often.  It doesn't have a simple answer but it certainly opens up a load of interesting stuff.

There's a good reason why websites or books aren't telling you about a connection between column tests or extended column tests and the likelihood of triggering an avalanche; there isn't much of one.

It's because a CT or ECT result is one small, unreliable piece of data in the very large set of information that you'll need in order to have any hope of a reasonably accurate risk estimate.  You need to have an picture of the situation, built up from your knowledge of the existing snowpack and the weather history, together with the terrain on which that snowpack is sitting, before you can start to estimate what's likely to happen if you go and jump on a particular bit of snow.

The place where the CT and ECT (and any other field test) results come into this is in building up your picture of the snowpack when you're out there in it.  As you're travelling around, you find a suitable slope for your test(s) (i.e. safe, representative and undisturbed) and carry them out, noting the results.  Those data points go together with all your other information (observed & reported avalanches, slope testing, pole tests, foot penetration, snow structure, weather observations & history etc.) to help you build up a coherent picture of the kind of avalanche hazard you're dealing with.  That picture should be clear enough to be able to specify how the hazard varies on different pieces of terrain.

As one-off "do I go on this slope or not?" tests they are of very limited value.  They are NOT stability tests and if I had my way we wouldn't call them that.  We'd be better to call them INstability tests because they are a "no-go" test - not a "go" test.  So - if the test indicates instability on that particular slope, well - don't go on it unless the consequences of triggering a slide are trivial.  On the other hand, if the test indicates that the slope is OK and you move onto it then you're betting your life on one unreliable test.

Edit - tried to improve clarity!

A photo from a similar analysis in Canada always sticks in my mind!

A good pit is not a go call. A bad pit is a stop. 

I once had a fairly heated conversation with a guy I was skiing with, he wanted the 3 of us to stop and dig a pit halfway up a slope, in poor vis, with rocky bits beneath us, ( the only thing we agreed on was that we weren't 100% happy we were there).  I refused to stop, and he got in a huff. standing around right in potentially the most dangerous part of the slope postholing through the snowpack, we stood to learn nothing we didnt already know which was  that we had all perhaps been a bit blind and that best to get out of there. So I preferred to keep on and make for the top since we were all further apart that way and the slope angle was soon gonna slacken off, and get to top where perhaps I can figure a safer way down, rather than have us all stop, de skin and then descend the perhaps dodgy slope.

So true - clinging to the mantra of ‘we must do this’ and not just finding a quick way to get the f**k out of Dodge equipped with a reasonable bit of understanding plus few times out in the snow before - and above all a gut feeling that says no.  

And the huff . . .  ?  I heard of people going off in a huff while getting down naving in low vis in winter when they have a difference of opinion.  Low blood sugar ? 

This is by far the best resource I have come across with regards to compression tests:

It is from the Applied Snow and Avalanche Research department of the University of Calgary.

Here is a short intro:

This presentation video for advanced recreationists and practitioners identifies the five types of fracture character and shows the frequency of skier triggering on slopes that exhibited each type of fracture character in compression tests.

vimeo.com/30996756

stick with it, the results are pretty interesting.

In this case no just hubris, ignorance, and idiocy. Same bloke same trip, after the other 2 of us spent breakfast discussing conditions and options and upon presenting our selection to said party, he didnt like it cos it wasn't on the list of ambitions he had written before even in country!  How to get yourself dead 101.