Snow anchors: how strong are they?
Here’s another “tres bien” video from ENSA (“École Nationale de Ski et d'Alpinisme”, or the French National Guide School) where they take engineering tools out into the real world and see how climbing gear and technique really work. (Video link at bottom of page.)
If you haven't seen it yet, be sure and watch this other great video, where they test the usefulness of brake knots in the rope for two person crevasse rescue.
In the mountains near Chamonix, they rigged some pull tests on all manner of snow anchors, some traditional (pickets, snow bollard, ice axes, vertical and horizontal skis), and some unconventional (plastic bags, soda bottle).
Here are some takeaways:
Placing a vertical anchor, like a picket, at a 25° angle leaning away from the direction of pull makes the anchor approximately 40% stronger. (See graphic below.)
Even in a buried plastic bag and a soda bottle held 200+ kg, more than enough to rappel from. (I still think I'd let my friend go first . . .)
From the 2010 International Snow Science Workshop, a paper called “Snow anchors for Belaying and Rescue”, by Don Bogie (New Zealand) and Art Fortini, (USA) is probably the most detailed study on snow anchor strength. It states (pg. 315): "In order to allow some room for error when placing a stake it is recommended that when placing upright mid clips that an angle of 30 degrees back from perpendicular is used.”
So, somewhere in the neighborhood of 30° is probably optimal.
image: screen grab from https://www.youtube.com/watch?v=yYtYZgeUpek
There’s a nice chart near the end that summarizes all of the data. (The fourth column, “strength in daN”, means “dekanewton”, a metric unit of force. It's 0.01 kN, or approximately the same as 1 kg of force.)
image: screen grab from https://www.youtube.com/watch?v=yYtYZgeUpek
Finally, for a very deep dive into snow anchors, here’s a training video for the Mountain Rescue Association featuring a presentation from snow anchor expert Art Fortini.
