QC Lab: The Art and Science of Using Snow Anchors

Photographer: Christian Adam Location: Utah

QC Lab: The Art and Science of Using Snow Anchors

The QC Lab team is joined by IFMGA Mountain Guide Mark Smiley to discuss the subtle art of building, testing, and using snow anchors.

For this installment of the QC Lab, we invited IFMGA Mountain Guide Mark Smiley to help us examine and discuss the art of snow anchors. Mark has guided hundreds of clients on climbing and skiing trips around the world and is the founder of Mountain Sense, an online education platform dedicated to climbing, skiing and mountaineering.

Over the years on various alpine climbing missions, ranging from the Kautz Route on Mt. Rainier to first ascents in China and Peru, I have been faced with the need to rappel, with no solid rock or ice available. In these instances, the only option was to use a snow anchor. Snow anchors take some creativity as you are limited to the equipment you are carrying or can scrounge from nature, such as aluminum pickets, stuff sacks full of snow, ice axes, ski poles, skis, backpacks, rocks, and even tree branches.

Constructing snow anchors is a crucial skill set to have when tackling technical alpine routes around the world. It is always a bit nerve-racking to lean back on a rope anchored to something far less “bomb-proof” than a typical rock or ice anchor, and creating a trustworthy snow anchor requires more than just clipping into a buried object and hoping for the best.

Matt Berry, QA Lab Manager at Black Diamond Equipment, and I joined forces to equip you with some guiding principles for success the next time you need to trust your life to a snow anchor, as well as examine the kinds of forces generated in these kinds of situations.

Matt Berry, QA Lab Manager at Black Diamond Equipment, and I joined forces to equip you with some guiding principles for success the next time you need to trust your life to a snow anchor, as well as examine the kinds of forces generated in these kinds of situations.

STEP 1: BUILD A ROBUST ANCHOR

STEP 2: BOUNCE TEST THE ANCHOR

STEP 3: RAPPEL AS SMOOTHLY AS POSSIBLE

Step 1: Build a Robust Anchor

In the classic unofficial non-thesis sort of way, we headed into the Wasatch Mountains to test a variety of snow anchor configurations using a pulley system, portable load cell, and lots of muscle. This data must be taken with a grain of salt because we’re talking about n=1 here. As in ONE data point per anchor configuration in one location, on one aspect, on one slope angle, with consistent snow quality. It’s basically statistically irrelevant, but it’s always kind of cool to break stuff in the name of science.

The test setup was pretty basic: bury the test anchor object, attach the load cell directly to the anchor sling, connect a static line to the load cell, and rig the pulley system off of a tree. The test anchor was then loaded until failure or until the team couldn’t pull any harder.

We decided to focus on the most common snow anchor configurations but included a few creative ones for fun.

The strongest anchors are those which utilize an object that maximizes surface contact with the load-bearing wall (front) of the pit. The greater the surface area the object has to push against the snow the better. A stiffer object which resists bending will spread the load more evenly over the snow when loaded. The object needs to be beefy. Two skis sandwiched together are better than one. A two-foot-long pine 2x4 is beefy, a Snickers bar is not beefy. You get the idea.

The object must be buried in dense and compact snow. I generally dig at least 12”-20” down (30-50cm) in compact snow, place the object sideways, carve a thin slot for the anchor sling, backfill the hole, and then pack down the snow. Reading snow quality and understanding how strong your anchor object is can be very challenging for even the most experienced alpinists. That is why the next step is critical: bounce testing.

Interested in learning how exactly to construct all these anchors? I dedicate an entire chapter in the MTN Sense Mountaineering Course to the nuts and bolts. Here is a temporarily free preview link.

After testing the snow anchors in the field, we were curious how much load could be generated during a bounce test, so we headed back to the QA Lab to take some measurements. An anchor was built using a UHMWPE sling wrapped around a steel I-beam, a load cell was attached to the anchor, we connected ourselves to the anchor and started going for it. The results represent a best-case scenario in which the highest loads can be generated during a bounce test.

The data we collected challenged my previous assumption that bounce testing with a UHMWPE sling tether applies MASSIVE forces to the anchor. It really doesn’t. One must really wail on the anchor to generate more force than can be potentially generated during a long herky-jerky rappel. Using a UHMWPE sling, however, does generate much higher loads than bounce testing when rigged for rappel with a rope and ATC.

Step 3: Rappel as Smoothly as Possible

Now that you’ve finished bounce testing and are happy with the results, it’s time to commit to the anchor. When rappelling, slow is smooth and smooth is safe. In this case, a smooth rappel reduces the chances of shock loading the anchor. If you let the rope run through the device quickly, and then brake abruptly, this can generate more than 3X your bodyweight on the anchor! That is extremely concerning when you consider the difficulty of generating equivalent loads while bounce testing. We want to apply the least amount of force on snow anchors as possible when rappelling.

A series of 30-foot-long, free-hanging rappels were made to measure how much force is generated at the anchor. A combination of static and dynamic ropes was used during testing and to our surprise there wasn’t a huge difference. It is possible that the difference between these two rope types would become more significant on longer rappels.

Prior to these tests, I thought that even during a steep pitch, if I rappelled really smoothly the anchor would only need to hold my bodyweight. After looking at the data, the results indicate that the anchor needs to hold at least 1.2X my bodyweight during an extremely smooth rappel or 3.5X my bodyweight in a jerky free-hanging rappel at this height.

Conclusions

If we consider an 80 kg (176 lbs) climber is capable of generating 3X their bodyweight on a jerky rappel, we need an anchor that is at least capable of holding roughly 2.5 kN (562 lbs). And that’s with NO BUFFER! Looking back at the strength of the snow anchors that were tested in the field, only 10 of the 16 test configurations would meet this requirement, two of which would be extremely borderline. A proper bounce test would have been capable of identifying most of these sketchy anchors.

Some rules of thumb that we should keep in mind when building any type of anchor:

- You can generate 3 – 4X bodyweight when bounce testing using a UHMWPE tether.

- An aggressive, jerky, rappel can generate more than 3X bodyweight whereas a smooth free-hanging rappel could be as low as 1.2X bodyweight.

The lesson here is that you must be aggressive when bounce testing and use a very static tether to generate loads high enough to properly ensure the robustness of your anchor. Otherwise, a jerky rappel may result in higher loads than you can generate with bounce testing.