CT test procedures

Thanks for this post.
It is a good reminder of the procedures.
I often get real analytical relating the test results equivalence to real situational circumstances. Clean Q1 shears that are below 20 are simple, but when they get into Q2 or Q3 at 23 I start feeling better about skiing the slopes. Though I never let my guard down anymore. And it's all situational depending on the terrain, weather and so on...

Thanks for this post.
It is a good reminder of the procedures.
I often get real analytical relating the test results equivalence to real situational circumstances. Clean Q1 shears that are below 20 are simple, but when they get into Q2 or Q3 at 23 I start feeling better about skiing the slopes.

My procedure questions are answered, so maybe this is a good time to drift the topic in a new direction (or start a new thread). But Joedabaker you are right, this is a good reminder of the procedures and I like your correlation to real situational circumstances.

Maybe this would be a good thread to hear from others on on what their limits are. "Clean Q1 shears that are below 20 are simple..." I agree. what are others limits here on the no go? Anyones stability test proved to be the right decision in the field (any stability test, not just the CT)? What about those 'moderate' results? Anyone?

This is a great thread.

For Koda, et al., on the "grey area".

With respect to skier-triggering, Q2 is unstable. From The Avalanche Handbook, page 181.

"From the skier-triggering data, only three classes are distinguished, with Q1 and Q2 classes being equally likely for relevance to skier triggering."

***

For RonL:

Collectively, your observations indicate a fair amount of energy in the snowpack and not insignificant risk of delamination. This means that triggering energy is a bit lower than one might like, and that there is energy to drive further structural failure.

Personally, given the same set of observations, I would have rated the stability as Poor. What was your stability rating? Seems like you thought conditions were dicey, and the description in your post sounds like best practise to me: keep acquiring information and making adjustments as needed.

The only thing I might have done differently, is that I would have assigned top priority to the hand shears, because you are always supposed to give precedence to the observation that reveals instability over an observation that does not reveal instability.

On the other hand, surface instabilities are much easier to manage, and they usually don't keep me away from the steeps, though they will keep me away from any terrain where the consequences of an avalanche are very bad.

Koda, thanks for allowing the topic to drift the topic a little.

I wasn't really looking for confirmation of our decision that day. I am just curious about whether one test is more valid or more highly thought of and why it often seems that the two tests give results that seem to vary so much. I could image some one who just did a compression test that day might get confirmation that "hey, it's pretty well bonded, even better than I would have thought" vs someone who just did a shear test and thought to retreat. Maybe it is just my experience but the shear test seems more touchy and could lead to different decisions than just doing compression tests.

I wasn't really looking for confirmation of our decision that day. I am just curious about whether one test is more valid or more highly thought of and why...

Some of this is still in my head from reading the Chapter2.pdf , but its to my understanding that the shovel shear test is best used to only identify the weak layer location in the snowpack. That said I will admit I don't use the shear test that much, and do rely more heavily on the CT test... while I think it is best to be well rounded with all the methods, I tend to favor the CT test mostly due to its repeatability and it does not take too long to perform. Is the CT test more valid... perhaps but the real answer is a it is only one part of the structure/energy/strength triad previously mentioned. I think failing to consider the triad elements can lead to disaster.... Mark Moore said it best: there really are no single tests that will tell you all you need to know.

I should mention if there was one test that is most valid, I have heard the Extended Column test (Propagation Test?) is very accurate in determining energy and can be easily done along side a CT test without much more time. If anyone has a good resource on this procedure I would be grateful if you shared.

back to RonL's question, I would be curious what stability test others here use most often or prefer over other methods, and especially if they feel it is more valid... For me its the CT test.

The following is what I tell my Avy 1 students about instability tests. I would appreciate any feedback as too whether this clarifies or actually complicates the issue.
First, the limitations of instability tests: You are only observing and testing a small column of snow. It is a tiny sample and I have low confidence extrapolating my results to an entire slope. Snow layering and instability changes a lot over terrain, even similar terrain. This is known as Spatial Variability. We observe this when we dig multiple pits only meters apart and find different layers and test results. Instability tests results are only a small part of the puzzle.

Test types:
Shovel Shear and Shovel Tilt are great for identifying weak layers and interfaces. They are qualitative, not quantitative, and give you an insight to the Shear Quality. Do them first, then you know where to watch when you do your Compression Tests.

Compression Test is the most common test done. You observe two characteristics of the weak layer. First, how sensitive is it, that is, how many taps at what force does it take to make the layer fail. Secondly, you observe Shear Quality (Q1,Q2,Q3; Sudden Planar, Sudden Collapse, Resistant Planar, Progressive Compression, Break). Shear quality is a clue to how much energy, or propagation potential, the weak layer has.

Now, I give more weight to the Shear Quality than the number of taps. If I get a fast, clean shear with a lot of energy on tap 30, I'm concerned about that slope. While it may be hard to get the weak layer to fail, it has the potential to propagate once it fails. All it takes is for me to hit the sweet spot and I could trigger a huge slide with serious consequences. Conversely, if I get a Q3 shear within the first few taps, I would interpret that as the weak layer being sensitive but lacking propagation potential. Then you get into less obvious results and interpretation becomes much more a matter of experience and pattern recognition. Can't help you out there. Though Mark's chart is a good start.

The Rutschblock is a larger block, so a larger sample, and the force you are applying correlates more directly to the force you will be putting on a slope. But it takes a long time to perform, and ultimately you only know about the snow within that block.

There is a big hullabaloo about calling these test INstability tests, not stability tests. I used to think this was a matter of semantics, but have come to realize that it is not. The only thing we can PROVE with these tests is that INstability exists in the snowpack. If we perform the tests, and we find instability, then we know it exists. If we find no instability, then we only KNOW that it doesn't exist in the snow we tested. We have not ruled out the possibility of INstability existing in any other snow. Essentially, and INstability test that does not find INstability is a failed test.

There are many more clues that you should rely on more heavily than your Instability Test Results. You have NWAC's discussion, InfoExchange, TAY trip reports, hand shears, probe tests, jump on a switchback, telemetry, current weather observations, etc. By the time you dig a pit, you should have a damn good idea what you expect to find.

If you ever find yourself saying "Let's dig a pit and then decide whether to ski this slope" you should review all the other information you have gathered and make sure you really need that test result to be the deciding factor. The uncertainty inherent in INstability test make them a weak nail on which to hang your hat.