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.
Change to full screen on video and turn that volume up...
Dave Gauthier, Bruce Jamieson Wrote a paper, "ON THE SUSTAINABILITY AND ARREST OF WEAK LAYER FRACTURE IN WHUMPFS AND AVALANCHES."
1. INTRODUCTION In early February of 2007, a significant surface hoar layer formed in the Columbia Mountains of British Columbia, Canada. About two weeks later, once this layer was buried by about 0.5 m of new snow, University of Calgary researchers were searching for suitable sites to work on the propagation saw test (PST). Approaching one such site, we triggered a whumpf, which propagated across a creek, spread across some flat terrain, and some distance away released an avalanche from a small slope. We searched the flat area for some indication of how far the whumpf had propagated and found a long vertical ‘perimeter crack’ through the slab. The slab had fractured along a set of old ski tracks across the flats, and we assumed that was where the weak layer fracture arrested. Later, however, we determined that the weak layer fracture had progressed past this break in the slab. We knew the slab fracture must have occurred during or after the whumpf, but when we started thinking about it we wondered how the weak layer fracture could keep propagating after the slab had broken.
Dave Called me after seeing the Facet Blow Out movie Brett and I did earlier this month (utahavalanchecenter.org/facet_blow_out_02082012)
After talking with Dave, I decided to do a little study using three Go Pro cameras to capture collapsing and propagation. Amazing to see just how the column collapsed and propagates a fracture across a weak layer.
Dave explained that in Facet Blow Out movie we had a slab fracture. However, instead of it arresting or stopping the weak layer from propagating passed it ( Like in all the tests he has seen), the weak layer continued pass the slab fracture. He explains that weak layer propagation is a race between the slab fracture and weak layer propagation. Needless to say there is a lot more that we can learn from the PST. He and I are schedule for a conference call later this week and I will have the updated ideas behind all this.... Stay Tuned
A local snowmobiler and his wife set out for an afternoon of low angle riding on March 4th 2012. Planning to stick to mellow terrain they left their avalanche rescue gear in the rig. As they pulled up to Hoyt Peak they met another group they knew and started climbing a steep east facing slope. Tyson was the sixth track on the slope when it broke above him. He tried to throttle off the slab but got thrown from his machine and held onto the bumper, getting completely buried in a slight terrain feature at the toe of the slide.
Please enjoy this remarkable story.
Thanks to all who helped make this video possible.
Utah Avalanche Center