Article By Anna
Anchors. They are the critical part of any vertical system; subject to human error and in some cases environmental factors. In rock climbing, bolts secured in rock provide peace of mind. In many vertical situations, climbers or a SAR team may encounter have burly trees to set anchors on. Trees are inherently strong in our minds-after all, as kids we climbed them; the trees didn't tremble under our weight in the slightest. For ice climbers, there are screws and V-threads. Mentally grasping and trusting a V-thread is a little more out there than anchoring off a tree. However, we can set our minds at ease knowing that 4 inches of lake ice will easily hold a human and 8 will hold a car. But snow anchors? That light fluffy white stuff supporting our life safety? That can be a little hard to truly believe.
So, How Strong are Snow Anchors?
I had the pleasure of spending a weekend putting snow anchors to the test. Our personal goal was to fail the actual physical anchor itself. Unfortunately, snow conditions made meeting that goal difficult. We had a fun time with the failures anyway!
Test Subject: 36 inch pickets and downhill skis.
Anchor Orientation: Horizontally buried
Force application: a 9:1 (up to 8 kN) and then a 27:1 (above 8kN) pulley system
Measure: A CMC Enforcer Load Cell Kit attached the the master point of the anchor recording maximum load.
Snow Conditions: Snow pack was shallow and consisted of difficult to compact rounds (Fist strength). To simulate a stronger snow pack, we also placed anchors behind a snowberm created by plow trucks (4 finger to 1 finger strength).
Results of Snow Anchor Failures
All results have been rounded down to the nearest kN.
3 snow pickets buried in about 12" of rounds, solidly compacted by foot failed at approximately 2 kN. Failure was in the snow pack.
2 skis buried in about 12" of rounds, solidly compacted by foot failed at approximately 3 kN. Failure was in the snow pack.
2 skis buried in a pre-compacted and somewhat hardened snow berm about 24" high failed at approximately 10 kN. Failure was in the skis.
2 snow pickets on either side of one pair of skies buried in a pre-compacted and somewhat hardened snow berm about 24" high failed at approximately 15 kN. Failure was in the snow pack.
Snow anchors made from 3 pickets can provide enough strength to hold an average weight person with some gear with a 2:1 safety factor even in weak and shallow snow conditions. Using the increased surface area of two skis can increase that to 3:1. Beware, the limiting factor in this system is the snow pack strength. These tests were conducted using rounds naturally at fist hardness. For the anchors, significant effort was put into packing down the snow prior to testing.
For stronger snow packs (4F to 1F strength), the strength of the anchors was significantly increased. In one test, one of the ski anchors fully failed before the snow pack.
For the three point anchors, the middle anchor point failed first. It is suspected that this anchor took the most force due to the anchor cable being doubled up and thus stretching less.
Our goal for a vertical rescue situation with a patient would be to create a system with a theoretical 10:1 safety factor using a main belay and a backup or a two tensioned rope system for redundancy and safety. These systems and safety factors were not used for simplicity of testing and recording in this demonstration.
I should also mention that all gear used was out of service and will not be used for any life safety in the future.