Joined: Sun Oct 18, 2009 3:18 pm Posts: 471 Location: New Castle, Colorado
What are your thoughts about PST and the deep instability in this year's snow pack?
Propagation Saw Test (PST) 1. 30cm cross slope and 100cm upslope. If weak layer is deeper than 100cm, the length should be equal to the layer depth. 2. Isolate the column at the front and one side by digging in the snow, by cord cut the back and remaining side. All walls should be vertical and cut. 3. Identify the weak layer, then insert the blunt edge of the saw at the front of the column. 4. The propagating fracture will either reach the end of the column (End) or Stop at a slab fracture (SF), or self arrest within the weak layer. 5. Record results by measuring how far you cut into the weak layer to the length of the column. Example 30cm/100cm 6. Propagation is predicted to be likely when the fracture propagates to the end of the column and is less than half the length of the column.
Joined: Thu Oct 29, 2009 3:22 pm Posts: 664 Location: Durango, CO
Seems like a massive amount of work. Also, what does it actually tell you? It may just spook people out more then it should. I haven't ready any studies on it yet. I'll keep researching, but as of now, I'm going to hold off.
I'm a fan of tests that simulate rider load on the snow. Those results can vary alot even within the area of one pit, saw prop I find gives consistent results within a pit area, but generally indicates more instability then other tests if done back to back. Can be a good thing - but I just don't really like the mechanics of the test and how they relate to a failure from rider load. In my opinion - many tests are always better then one - and I'd rather dig 15 compression tests or ECTs then 8 saw prop.
Joined: Fri Feb 18, 2005 8:56 pm Posts: 467 Location: Meyers, CA
All tests have their strengths and weaknesses.
The PST is really just for examining how easy/difficult it is to propagate a known weak layer. If you haven't identified a weak layer, the PST doesn't do you much good. It also doesn't really give info on fracture initiation. It's awesome for tracking persistent weak layers over time and getting an idea if the weak layer is gaining strength or remaining weak. The good dudes at the Sierra Avalanche Center have been using them to track our deep slab instability here in Tahoe.
Unlike virtually all the compression tests (except the deep tap), the PST can be done on deeply buried weak layers (more than 100-120cms down). Obviously it will take a while to isolate the column if it's buried 7 feet down, but if the weak layer is shallow, you can bust out several PSTs quickly.