6.1.13 Snow stability and the Extended Column Test

The Extended Column Test (ECT) is a relatively new technique for evaluating snowpack stability. While the Stuffblock and other tests identify layers in a snowpack that are likely to initiate a fracture, the ECT identifies layers that are likely to both initiate and propagate a fracture. This helps focus attention on unstable areas, where an avalanche really might occur. To do the test:

Diagram showing preparation for the Extended Column Test used to test snowpack stability

  • Isolate a vertical column 90 cm (35 in) across a slope by 30 cm (12 in) downslope
  • Apply pressure to one end of the column (the “loading” area) and see how much pressure it takes to transmit stress across the column to fracture it. Tap the end of the column up to thirty times: ten times from the wrist, then ten times from the elbow, and finally ten times from the shoulder
  • Note the the number of taps required to initiate a fracture; the number of additional taps required for the fracture to spread across the full column; and the depth of the fracture from the surface
  • Stop the test when the fracture has spread across the entire column or you have given it 30 taps

It’s pretty easy to interpret the results. Fractures typically propagate across the entire column within one or two additional loading steps on unstable slopes. If the fracture propagates across one or more layers or breaks, it’s unlikely that fracture propagation will occur.

There are some limitations to be aware of. The test can overestimate snowpack instability, for example, when a thick hard layer overlies a weak one, or the upper layers of a pack are soft.

Like other tests, the ECT should be done in an area that is representative of the entire slope.

For more information on the ECT, access the paper “The Extended Column Test: A Field Test for Fracture Initiation and Propagation” at http://www.avalanche.org/~nac/NAC/techPages/articles/06_ISSW_Simenhois.pdf.