Propagation Saw Test
- the latest of the large column tests (ECT, PST) used as follow up evaluations to the standard compression test. The compression test does a great job in identifying weak layers and suggesting if they are likely to initiate a failure within the snowpack. The limit to the compression test is its inability to indicate whether the failure is likely to propagate into an avalanche
The first large column test or extended column test (ECT) - quick summary of the ECT, I will only go into details for the PST - including a video below from ASARC
- aims to assess both fracture/failure initiation and propagation across an isolated column
- requires an isolated column similar to the standard compression test, but on a larger scale (30cm x 90cm wide) compared to 30cm x 30cm used for the compression test.
- The column is then loaded from one side using the same technique as
the compression test. (loading area in below figure)
- Recent research with the ECT shows that results are most accurate when the weak layer is less than 70cm deep - for weak layers > 70cm deep consider the use of the PST
- Interpreting and Recording Results As:
The key observation is whether or not a fracture occurs across the entire column. If it does fracture across the entire column, this means the weak layer may is capable of propagating a crack across an entire slope thus producing an avalanche. The question is then, how likely are you to initiate this fracture process. Record results in the following manner:
- ECTPV - A fracture occurs across the entire column (propagation) during isolation. A very ustable result.
- ECTP# - A fracture occurs across the entire column (propagation) after some number (#) of taps.
- ECTN# - A fracture initiates but does not occur across the entire column (no propagation) after some number (#) of taps.
- ECTX - No fractures occur in the column after a total of 30 taps. Generally a stable result.
Now: propagation saw test (PST)
- aims to again assess both fracture/failure initiation and propagation across an isolated column, differences being:
- ability to test the tendency of a specific weak layer and slab combination independent of loading/compression or shear force from above for fracture initiation
- in contrast to the ECT the PST can be applied to slab/weak layer combinations ranging from 30cm to >250cm deep and is therefore capable of testing slabs in the skier trigger-able range as well as much deeper slabs whom may become reactive as a result of step down energy
- requires an isolated column 30cm x 100cm upslope, for weak layers > 100cm deep , the column length should be equal to the layer depth
- After identifying the weak layer of interest drag the blunt edge of the saw upslope along the weak layer at 10-20cm/second taking care to keep the blade within the weak layer. Continue until the fracture jumps ahead of the saw
- consider repeating the test if you suspect the saw left the weak layer
- The propagating fracture will either:
- reach the end of the column (END)
- stop at a slab fracture (SF)
- or self-arrest within the layer (ARR)
- Recording Results As:
- PST x/y (ARR, SF, or END) down z on yymmdd
- x = cut length
- y = column length
- z = weak layer depth
- finally the date of test
- PST 25cm/100cm (END) down 85cm on 12/22/2011
- PST interpretation:
- Propagation is only predicted likely if the fracture/failure initiates when the saw is moved 50% of less than the upslope length of the column and continues uninterrupted to the end of the column (Dave Gauthier, Cameron Ross, Bruce Jamieson; How To: The Propagation Saw Test: Journal of Glaciology: Awaiting Publication - 2011)
- All other results indicate propagation is unlikely
- including anytime propagation reaches the end of the column once more than half the column has been cut
- Thus above example PST 25cm/100cm (END) down 85cm on 12/22/2011 indicates high propagation tendency.
- PST results have been shown to indicate a larger number of false STABLE results than other common stability tests, especially among shallow soft slabs (Gauthier et al)
Remember to keep the whole picture in mind when gathering information about snow stability. The above tests are only a piece of the puzzle; forecasts from your local av center, detailed understanding of the recent weather cycles, field observations, combined with stability tests lead to an overall view of the current conditions.
Below is a video from ASARC (Applied Snow and Research Center of Calgary University) on the PST
Propagation saw test (PST) from Bruce Jamieson on Vimeo.
|Column isolation on the left, and saw cutting (using blunt edge) upslope along the weak layer of interest on the right: Photo Courtesy of the American Avalanche Association|