The alpine touring season is in full swing. In keeping with this, the Black Diamond team at the QC Lab, together with mountain guide Mark Smiley, is devoting themselves to the topic of anchoring in the snow. In this article, you will learn more about the subtle art of building, testing and using snow anchors.
If there is neither rock nor solid ice available for abseiling on a high-altitude tour or alpine climbing tour, the only option is to use a snow anchor. In this article, the specialists at the Black Diamond QC Lab show in detail what to look out for.
Snow anchors require some creativity as you are limited to the equipment you carry with you or can find in nature, such as aluminum poles, bags filled with snow, ice axes, ski poles, skis, backpacks, boulders and even tree branches.
Building snow anchors: an important skill for high mountains
The ability to build a snow anchor is a key requirement for climbing technical alpine routes around the world. It's always a little nerve-wracking to trust a rope anchored to something that you wouldn't call as "bombproof" as a typical bolt or ice screw.
And to set up a trustworthy snow anchor, you have to do more than just hook yourself into a buried object and hope for the best. Perhaps the most important thing first:
Snow is a very variable medium and its quality directly affects the strength of your anchor.
There are countless ways to describe snow: powder, crud, mashed potatoes, slush, fresh snow, bulletproof, hardpack, corn, sticky, sulz, wet snow, cardboard snow, old snow, firn, hard snow and many more. This is a complex topic that we won't go into here, but it's important to know that compact, hard, dense and wet snow is ideal for making snow anchors.
The 3 phases of snow anchor construction
Step 1: Build a sturdy anchor
So we headed to the Wasatch Mountains to test a whole range of snow anchor configurations using a pulley system, a portable force transducer, and a lot of muscle power.
These data must be treated with caution, as we are talking about n=1 here. That means ONE data point per anchor configuration at one location, on one side, at one slope angle, with consistent snow quality.
It's basically statistically irrelevant, but it's always kind of cool to break equipment in the name of science.
The test setup was quite simple – bury the test anchor, attach the load cell directly to the anchor sling, attach a static cable to the load cell and set up the pulley system on a tree. The test anchor was then loaded until it failed or until the team could pull no further
We decided to focus on the most common snow anchors, but also tested a few creative configurations for fun.
What you should pay attention to when building a snow anchor
The strongest anchors are those that use an object that maximizes surface contact with the supporting face (front) of the depression. The larger the area the object presses against the snow, the better.
An object that is as stiff as possible and does not bend distributes the load more evenly across the snow when it is loaded.
The object also has to be big enough. Two skis stacked on top of each other are better than one ski. A rolling pin is solid, a pencil is not. You get what I mean.
The object must be buried in dense and compact snow. In compact snow, I usually dig at least 30-50 cm deep, place the object sideways, make a narrow furrow for the anchor loop, close the hole with snow and pack the snow down. Determining the snow quality and knowing how strong the anchor is can be a big challenge even for experienced alpinists, so the bounce test is crucial as the next step.
Mountain guide Mark Smiley offers more information on the construction of different snow anchors in his MTN Sense Mountaineering Courses.
Step 2: Perform bounce test
Before you rely on your self-constructed snow anchor, you should subject it to a strong bounce test. A really good bounce test should generate more force than the anchor has to withstand when rappelling the heaviest person.
The most important thing is to perform the bounce test while the anchor is backed up.
If the anchor fails the bounce test and comes loose from the snow, the backup anchor will keep the team safe. This can be achieved by setting up a second anchor on steep terrain. On slightly sloping terrain, a hip belay in a sitting position or similar may be sufficient.
By using a very static sling and having the heaviest person perform the bounce test, the anchor is subjected to the greatest stress. This involves aggressively throwing your body weight against the anchor 3 or 4 times.
Watch closely for any movement of the snow or buried object. If so, re-set the anchor with a larger object and/or more compact snow. Watch the snow anchor closely as the first climbers descend. When the last person descends, the backup anchor can be removed, provided the main anchor is deemed "strong enough".
After testing the snow anchors in the field, we were curious to see how much load could be generated during a bounce test, so we returned to the QA Lab to take some measurements.
We built an anchor using a UHMWPE sling wrapped around a steel I-beam. We then attached a load cell to this anchor, connected ourselves to it, and began throwing ourselves against the anchor. The results represent the best possible scenario where the highest loads can be generated during a bounce test.
The data collected challenged my previous assumption that bounce testing with a UHMWPE sling would impose extremely high forces on the anchor. This is not actually the case.
You have to throw yourself really hard into the anchor to generate more power than you would get from a long, jerky abseil.
However, the use of a UHMWPE sling produces significantly higher loads than the bounce test when used for abseiling with a rope and ATC
Step 3: Abseil as evenly as possible
Once you have performed the bounce test and are happy with the results, it is now time to abseil down the anchor. When abseiling, slow is steady and steady is safe. A steady abseil reduces the risk of shock loading on the anchor.
If you run the rope quickly through the belay device and then brake abruptly, more than three times your body weight can act on the anchor!
This is extremely worrying when you know how difficult it is to generate equivalent loads during the bounce test. We want to exert as little force as possible on the snow anchors.
A series of approximately 9-meter-long free-hanging abseils were used to measure the amount of force generated at the anchor. The tests used both static and dynamic ropes and, to our surprise, there was not much difference. Perhaps the difference between these two types of rope would be more noticeable on longer abseils.
Before these tests, I thought that even on a steep pitch, the anchor would only have to support my body weight if I abseiled really evenly. However, the analysis of the data shows that even with an extremely even abseil, the anchor must support at least 1,2 times my body weight, or 3,5 times my body weight if I abseiled abruptly and freely from this height.
Building snow anchors: The most important findings
If we assume that an 80 kg person can generate three times their body weight when descending abruptly, we need an anchor that can hold at least 3 kN. And that's without a shock absorber!
Looking at the strength of the snow anchors tested in the field, only 10 of the 16 test configurations would meet this requirement, with two of them being extremely borderline. A properly conducted bounce test would have identified most of these questionable snow anchors.
As a rule of thumb, you can generate 3 to 4 times your body weight during the bounce test with a UHMWPE sling.
An aggressive, jerky abseil can generate more than 3 times body weight, while a steady, free-hanging abseil generates only 1,2 times body weight.
The lesson is that you have to be really aggressive with bounce testing and use as static a sling as possible to generate high enough loads to ensure the anchor is robust. Otherwise, a sudden descent can result in higher loads than those generated by the bounce test.
Conclusion on snow anchor construction
- If you're going to take the time to build a snow anchor, you should do it right. Use a sturdy object with as much contact area as possible on the snow (towards the front of the hole), then fill the hole with snow and compact it around the anchor.
- Assess the snow conditions and continue to compact the snow. If necessary, find another place with better snow quality.
- In dangerous terrain, always use an additional anchor as a backup and do not remove it until the last person has taken their turn and the main anchor has been deemed “strong enough”.
- The bounce test for an anchor should always be performed by the heaviest climber using a UHMWPE sling to create the highest possible test load. Always perform the bounce test as if your life depended on it!
- Abseil as evenly as possible and avoid jerky movements. Slow is even and even is safe.
- An extremely jerky abseiling process can generate higher loads than a bounce test.
- If the snow is too soft to set up a stable snow anchor, consider a secured climb down.
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Credits: Cover picture Black Diamond