Strength of the slab?

[sfotex]

Here's a situation I encountered last weekend, and wanted to see what the collective thought:

Background:
Weather has been warmer a few days before (35~40 in the day, freezing at night) , the day we were out it was cool (below freezing and cloudy with a brisk wind). Skinning up a south facing slope, 30 degrees, light / medium tree cover. Snow surface was very very firm, with an ice glaze on top in spots. Dug a pit and the top 12" was rock hard packed dense snow. Could barely get the ski edges to bite when skiing. Under the 12" of slab was a couple of inches of faceted crystal, then a nice bed surface. We got a clean shear but compression test showed good strength. We mulled it over and felt that as long as the temprature remained cool the slab had enough strength to support us, and that the trees, while not that dense, were a good enough anchor with the strength of the slab. We also felt that if the slab had been thinner, or if it had been a warmer day this would probably be a bad slope to be on.

Thoughts?

[Toby]

If the compression test was good (like 25+ taps) and the snow was knife density, then I would read that as a green light. 12" is pretty thick so I'd imagine the slab would bond well to itself and support your weight well too. As the weather warmed, it might've broken down the slab bonds a bit.

[greg]

its strange that you got different results under those conditions from a shear and a CT.

in my oppinion you were save. you would probably have been saver if there weren't any trees. it gets dangerous as soon as you start breaking in.

[Toby]

its strange that you got different results under those conditions from a shear and a CT.

I thought about that too. I imagine that the slab was strong enough to support a fair bit of compression, but that the shear happened when he'd cut all the way (or almost all the way) through the slab and hence broke all cohesion? Dunno. I'm not a big fan of shears, although I'll do them for fun. Conventional wisdom says shears are good for finding hidden layers (like thin SH), but hard to quantify vs. CTs. I use them the same way - identify bed surfaces but I'm not experienced enough to compare how they release. I like the CT way better, especially the 5 classification system that U. Calgary promotes.

[sfotex]

We were getting a shear where the slab met the facets. It was probably more the slab falling out when we cut the back vs. a real 'shear' with a pop.

I wrote this thread b/c this situation could easily describe a red light scenerio, slab on facets, but the pack appeared pretty bomber. But, we were depending on the cohesion and strength of the slab to keep us safe, which Tremper seems to frown upon....

[Meadow Skipper]

A super-firm slab with anchors on a cool day would have had me thinking it would probably be okay to ski. Since the shear test didn't pop until it hit the facet layer, that would indicate to me that the slab was pretty cohesive. I'd say it was a good call.

[wra]

Reliable shear tests of spring snow are questionable at best. Melt freeze reacts in a different manner from dry snow. Weather history and current conditions have more emphasis for me in spring. Several days of marginal or no refreeze will sometimes saturate underlying snow layers, even though the surface crust has refrozen.
Digging to see how deep the freeze and how wet underlying snow layers, ain't a bad idea, but I've not had good luck shear testing, even wet faceted layering.
The elusive corn slab can be a problem after a marginal freeze, predictable using other observations, like air temperature, depth of surface crust, rapid warming, aspect etc.
Cloudy skies, cool air temperatures and a foot deep "rock solid" dense layer would have me, whippet in hand, more worried about taking a slider, than avalanche.

[AltaPowderDaze]

Cloudy skies, cool air temperatures and a foot deep "rock solid" dense layer would have me, whippet in hand, more worried about taking a slider, than avalanche.

this guy is BSing you. he don't need no stinking whippet!