Vertical Mid-clip Snowstakes

Snowstakes are most commonly used in the heavily glaciated New Zealand mountains to abseil or belay over crevasses and bergschrunds. In the event of a crevasse fall, they are also used for building rescue anchors. They are occasionally used for pitching moderate angled snow slopes but climbers need to be aware that the potential forces when leading can be at the upper limit or exceed the strength of standard snow anchors. It is therefore important to understand likely loads, be able to recognise the factors that affect snow strength, and be able to choose the best solution from a variety of snow protection options.

Snow Anchors

Unlike rock protection, the strength of snow can vary to a large degree and can change on an hourly and daily basis. Snow can be hard, almost ice, through to soft cold powder or warm wet snow. The only way to develop an awareness of snow anchor strength is to practice building and testing them in different snow conditions over and over again. Any snow anchor relies on experience and judgement. As they are often used singularly, when they fail, they do so catastrophically. If in any doubt, it is possible to combine multiple anchors to ensure the anchor system is fit for purpose.

In firm snow conditions, or when the snow is moist and can be compressed harder, the strongest possible use of a snowstake is the vertical mid-clip orientation. This requires a sling or cable attachment to the middle of the stake.

Vertical Mid-clip Attachments

Testing by Aspiring Safety, has provided guidance on suitable mid-clip attachments. Their research suggests that improvising an attachment on the Aspiring Safety V-stakes with a clove or girth hitch sling is not ideal due to the deformation of the stake under load. Also, having tape or cord go over sharp metal edges is not recommended due to the possibility of the material being cut. A permanent snowstake attachment material must therefore resist abrasion.

During testing, a 4mm galvanised steel cable came out as the strongest attachment with 5mm Aspiring Dyneema cord the next strongest. Stitched Aspiring 5mm Dyneema cord withstood forces over 8kN before the stake started deforming and no abrasion occurred. Dyneema chord is much lighter than cable so a reasonable weight compromise for loads anticipated during usual snow anchor application and a bit easier to manage.

Thanks to Aspiring Safety for fabricating our snowstakes. We prefer not having top reinforcements due to weight and find that with hammering, they can cause the V to split down the middle rather than just deforming. This gives the stake a little more lifespan.

Placing Snowstakes

For a vertical mid-clip, the stake should be placed at least 25^o back from perpendicular to the surface (aiming for 30^o gives a small margin).

For a V-section snowstake, the open part of the V points in the direction of load. For a T-section snowstake, the web of the ‘T’ points in the direction of the anticipated load.

The channel for the attachment should be cut as narrow as possible with an ice axe pick or a snow saw, and deep enough so the attachement doesn’t inadvertently pull upwards on the stake. Also try not to disrupt the snow in front of the stake unless the snow in front of the stake can be compacted to increase strength.

Importantly, the research from Don Bogie suggests that a vertical mid-clip gives more certainty of a strong anchor than a top clip does. Even if 10 cm or so of a mid-clip stake is left sticking out of the snow it still gives a far stronger anchor than a top-clip in the same snow.

When the snow is moist and compressible or can be squeezed into a snowball, the stake should be placed and the snow compacted under the attachment. The hole can then be backfilled with more snow and compressed again. Hands work best for a more even compaction.

The wire cable, Dyneema chord, and double-length slings are 120cm long, twice the length of a typical snow stake. This is useful because if the top of the snow stake and the end of the wire/sling is flush with the surface of the snow and the wire/sling is not kinked, the vertical mid-clip snowstake will be positioned at the correct angle back from perpendicular.

For all snowstake anchors, test them before relying on them. The best method is putting the shaft of an ice axe through the focal point and giving a sharp tug on the attachment. For low-stretch attachments such as steel cables or Dyneema, this will put a high force on the stake, make sure it is well seated, and give a good indication of it’s strength.