WHAT DETERMINES THE STRENGTH OF A SNOW ANCHOR? HOW STRONG DOES A SNOW ANCHOR SYSTEM HAVE TO BE?

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Meagan Atkinson
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1 SUMMARY OF SNOW ANCHOR TESTING AS OF 2015 (HEILMAN) WHAT DETERMINES THE STRENGTH OF A SNOW ANCHOR? 1. Snow strength in compression and shear. 2. The strength, size and stiffness of the buried object. 3. The angle of placement of the buried object. 4. The location of the attachment point on the buried object. 5. The depth of the bottom point of the buried object. HOW STRONG DOES A SNOW ANCHOR SYSTEM HAVE TO BE? 1. Rescue load of 2 kn (450 lbf). 2. Safety factor of 10:1. 3. Anchor system strength of 20 kn (4,500 lbf) kn is 225 lbf. 5. One 20 kn anchor, two 10 kn anchors or three 7 kn anchors would all work as far as strength goes, as long as all of the anchor points are fully equalized. 6. It is realistically impossible to fully equalize multiple anchor points. 7. But, snow anchors are normally not used for lead climbing on 90 degree slopes with potential fall factor 2 loads, they are used for belaying, rappelling, hauling, etc. on moderate to steep angled slopes with much of the load supported by friction. 8. The likely load on a snow anchor is probably substantially less than 2 kn, but there are no studies that have reported on this. HOW DO SNOW ANCHORS FAIL? 1. The equipment can fail. Slings or wire cable can fail in the 8 kn to 15 kn range. A picket can bend in the 7 kn to 16kN range and then be easily pulled through snow. 2. Compression failure of the snow is when an object is pulled through the snow and is compressing the snow in front of the object. This can be a relatively slow and steady type of failure with the anchor being slowly dragged through the snow. 3. Compression failure depends on the compression strength of the snow, the size of the buried object, the stiffness of the buried object and if the load is evenly applied across the buried object (if the object rotates or twists, the load will become uneven) 4. Compression failure occurs in the 1 kn to 9 kn range. 5. Shear failure is when a stress cone is formed around the object. The stress cone spreads out at 45 degree angles from the ends or sides of the buried object (when viewed from top, tension) and up at a 30 degree angle from the buried end or side of the object (when viewed from the side, shear). 6. The failure is normally rapid and explosive, with the anchor and stress cone of snow blowing out of the surrounding snow. 7. Shear failure is in the range of 8 kn to 19 kn.

2 8. The depth of the bottom of the anchor is what controls the size of the stress cone and the strength of the anchor. A horizontal mid-clip anchor has to be buried at 20.5 to have the same stress cone volume as a vertical mid-clip anchor buried to its top. 9. From a few observations, it is likely that the vast majority of the stress cone strength is derived from the shear plane versus the tension plane. 10. In the real world, compression failure of an anchor is much more likely compared to shear failure, given the same snow conditions (1 kn to 9 kn compared to 8 kn to 19 kn). 11. In weak snow, compression failure will be more likely and in strong snow shear failure will be more likely. 12. Assuming a reasonable load for an anchor of 8 kn or more, compression as the failure mode with the lowest failure strength and using a MSR picket mid-clipped with even loading, the compression strength of snow would have to be at the bottom range of pencil hardness or top range of single finger hardness (or 270 kpa). The finger test would assume pushing in the finger as hard as possible and it does not go into the snow. 13. A MSR picket is 2 by 23.5 (5 cm by 60 cm). WHEN SHOULD SNOW BE WORK HARDENED? 1. Not all snow can have its bond strength increased with work hardening. 2. Work hardening depends on the potential for increased bonding and the speed at which bonding occurs. 3. In moist snow, bonding happens very rapidly. In wet snow, there is some initial bonding, but then it actually decreases fairly quickly. In cold snow (less than 23 degrees), bonding is slow and may not occur fast enough to be of any use. 4. A good field test for the ability of snow to bond is the snow ball test. If squeezing hard makes a solid snowball, bonding will be good. If the snowball crumbles (cold snow), work hardening may actually decrease existing bond strength. If the snowball drips, there may be an initial increase in bonding, but it will be a slight increase and then decrease rapidly. DOES ANCHOR MATERIAL OR ORIENTATION MATTER? 1. In knife hard snow, a MSR picket in vertical top clip orientation failed at about 8 kn due to the picket bending almost 90 degrees at the top surface of the snow and then being pulled out. 2. There has been debate about Yates V-shaped pickets and the orientation of the point of the V. The hypothesis is that orienting the point towards the load would create an increase in compression strength of snow as the picket is pulled through the snow. But, if the snow compression strength can be increased, it would be better to just work harden the area in front of the picket. 3. There is a second hypothesis for Yates V-shaped pickets that if the point of the V is faced away from the load, then the picket might rock side to side as the picket is dragged through snow, leading to uneven loading and compression failure. There is no data to support this one way or the other.

3 4. Pickets and attachment systems, in general, should be rated to at least one third of the potential load, given the assumption that there will be three pickets used to create an anchor system. 5. Yates pickets are rated to about 8 kn before the attachment hole will fail. The slings that CMRU uses are rated to 22 kn and the carabiners to 24 kn. I could not get information for MSR pickets, but from some testing, they seem to be in the 8 kn range. 6. In strong snow, the strength of the picket is not as important since the snow will support the picket. In weak snow, a picket that is not stiff enough can bend and fail at loads of 7 kn. DOES PLACMENT OF SNOW ANCHORS MATTER? 1. Top clipped pickets are almost always weaker than mid-clipped pickets. But, in knife hard snow where a picket has to be hammered in, cutting a slot for the attachment sling can be very difficult. In those cases, top clipped pickets have been tested to hold in the range of 6 kn to 12 kn. 2. In strong snow, the depth of the bottom of a vertical picket determines the size of a stress cone. As a picket is leaned back, the depth to the bottom of the picket decreases, which decreases the size of the stress cone and the strength of the anchor. 3. In weaker snow where compression is the dominant style of failure, a vertical picket is more likely to pop out of the snow. The angle of the picket as it is pulled through the snow will influence if the picket moves deeper into the snow or moves up and out of the snow. 4. Given an attachment sling that is at least twice the length of the picket and weak snow that can t be work hardened, leaning the picket back 30 degrees from perpendicular to the snow surface should help the picket stay in the snow pack as it is pulled through the snow. ISSUES TO CONSIDER WHEN CREATING MULTI-POINT ANCHOR SYSTEMS 1. Assuming a MSR vertical mid-clipped picket and strong snow, the theoretical size of the stress cone would extend 42 towards the load from the picket and the front edge of the stress cone (towards the load) would be 84 long. 2. Given the theoretical size of the stress cone described above, using 4 long slings from each of three pickets and a 30 long piece of cord would give a design with 30 degree interior angles and no interference with stress cones. 3. If compression failure may be an issue, set up individual anchor points so that they will not be dragged through where another anchor was located. SO, WHAT ABOUT THE POOR OLD HORIZONTAL MID-CLIP ANCHOR? 1. In strong snow, the horizontal mid-clip anchor can be a very strong anchor. The slot dug for the picket should be as deep as the length of the picket. This normally means that a much larger area of snow would have to be work hardened compared to a vertical mid-clip anchor.

4 2. In weak snow that can t be work hardened, horizontal mid-clip anchors could be the only option. Disturb the snow in front of the picket as little as possible and dig a narrow trench for the attachment sling. LESSONS LEARNED 1. Just like any other rule in mountain rescue, everything is situation dependent. If any of the five major components of snow anchor strength (snow strength, anchor size, anchor burial angle, anchor attachment point, anchor burial depth) change, the rule will not apply. 2. The lower end of snow anchor strength in many situations, other than compression failure in cold, weak snow, is in the 7 to 8 kn range. So, in most snow situations in our region, a snow anchor system should usually have three anchor points. This also creates a redundant anchor system. 3. It is the depth of the lowest point of an anchor that determines the size of a stress cone in strong snow. A horizontal anchor has to be buried at about 21 to create the same size stress cone as a vertical anchor of the same size. 4. Mid-clipped anchors distribute force more evenly compared to top-clipped anchors. 5. Given number 3 and 4 above, a mid-clipped vertical anchor is faster to put in, requires less area for work hardening and takes advantage of snow layering compared to mid-clipped horizontal anchors. 6. Work harden snow only when squeezing hard creates a solid snowball with no dripping. 7. Use vertical top-clipped anchors when the snow is knife hard and it is almost impossible to dig a trench for a sling. This type of anchor has a lower strength of about 6 kn. 8. In strong snow, a vertical anchor should be perpendicular to the snow surface. 9. In weak snow, lean a vertical anchor back 30 degrees from perpendicular to the snow surface. 10. An attachment sling should be at least twice the length of a vertical anchor. 11. Given the Unit s picket length, theoretical stress cone size is 42 towards the load with a base 84 long. 12. Theoretically, to create a three picket anchor that does not interfere with the adjoining stress cones, place two pickets 7' from each other. In the middle of the two pickets, place a third picket 3.5 above the other two pickets. Use 4 long slings from each picket and dig a narrow trench so that the end of the sling is just emerging from the trench. Attach all three slings with a single 30 piece of 8mm cord and secure all the loops in a power point, which should end up about 6 feet from the bottom two pickets. 13. In weak snow that can t be work hardened, use horizontal mid-clipped pickets and do not disturb the snow down slope of the pickets.

6 LITERATURE Bogie, D Snow Anchors. Christchurch, New Zealand. Bogie, D and Fortini, A Snow anchors for belaying and rescue. International Snow Science Workshop. Corvallis Mountain Rescue Unit Snow anchor testing. Santiam Pass, OR Corvallis Mountain Rescue Unit Snow anchor testing. Santiam Pass, OR Fortini, A On the use of pickets and flukes as snow anchors. International Technical Rescue Symposium, Denver, CO. Fortini, A Predicting failure modes of snow anchors. International Technical Rescue Symposium, Denver, CO. Fortini, A Failure modes of snow anchors during drop testing. International Technical Rescue Symposium, Denver, CO. Fortini, A. and Morales, J Failure modes of snow anchors. International Technical Rescue Symposium, Denver, CO.

Predicting Failure Modes of Snow Anchors

Predicting Failure Modes of Snow Anchors ART FORTINI Sierra Madre Search & Rescue November 2005 International Technical Rescue Symposium Disclaimer The information contained in this document is based on