Compression Test

Learn how to: Perform A Compression Test

Compression Test
Dig a trench or pit 60 centimeters (36 inches) wide by one meter (3 feet) deep. Skiers and boarders rarely impact weak layers deeper than about one meter and isolated columns higher than about one meter typically fall over.

First, isolate a column of snow in the pit. To do so, mark the uphill portion of your pit with your shovel blade (A) to determine the flat dimensions of the column.

Next, cut the two sides of the column, being careful that the cuts follow a plumb line. Now you must cut the back of the column in order to totally isolate it.

To do so either cut the back with a snow saw on a pole or trench out one side next to the column to give you enough space to cut the back.

Next cut the back of the column all the way to the floor of your pit so that you have a square column roughly 30 cm by 30 cm (12 inches ) square. Remember, all 4 sides must be cut and must be straight up and down on the plumb.

Now, you test how well the layers are bonded by applying a series of compressive forces to the column. Begin by placing your shovel blade on top of the column and then follow these steps:

Tap the blade 10 times with your fingertips, moving your hand from your wrist.
Tap the shovel blade 10 times moving your hand from your elbow and using only the weight of your forearm.
Hit the shovel blade with your open hand or fist moving from your shoulder and only using the weight of your arm.

D. Layer or slab that is strong and cohesive but poorly bonded to the layer below it.

RESULTS: A weak layer within the column may fracture during any of the steps or there may not be any failure. The following index quantifies the results of your test. The acronyms in parentheses can be used to record results. They are from the Snow, Weather, and Avalanche Guidelines (SWAG) published by the American Avalanche Association and the Forest Service National Avalanche Center.

Very Easy: A weak layer fractures when you isolate the column. (CTV)

Easy: A weak layer fractures during your first 10 taps. (CT1-CT10)

Moderate: A weak layer fractures during your next 10 taps from the elbow (CT11-CT20)

Hard: A weak layer fractures during your taps from your shoulder (CT21-CT30)

No Fracture: Does not fracture (CTN)

SHEAR QUALITY: The fractures or shears defined above are divided into three types:

Quality 1: Very clean, planar, smooth and fast shear surface. The slab slides quickly and easily off the isolated column and into the snow pit and occasionally makes a "popping" noise when the test is conducted on slopes 35 degrees or steeper (and occasionally on slopes as low as 25 degrees). (Q1)

Quality 2: Mostly smooth and planar shear surface but the slab either does not slide off the isolated column or only partially slides off the column. (Q2)

Quality 3: Non-planar, uneven, irregular and rough shear surface. Typically the slab does not move off the isolated column even on slopes steeper than 35 degrees. (Q3)

INTREPRETING RESULTS: Stability tests only test stability in the location the test is performed; consequently, it is important to perform several stability tests in several locations to get a more accurate test sample.

Stability is judged relative to the amount and rate of stress exerted on the snowpack. The more force required to make a layer fail, the better the stability. The location and the quality of the failures is also very important. For example, an "EASY" fracture two feet deep in your pit indicates a much more dangerous situation than a fracture four inches deep.

And a Quality 1 (Q1) shear is typically indicative of much more dangerous conditions than a Q2 shear and certainly a Q3 shear. Be warely of Q1 shears even when they require a moderate or hard force; Q1 shears indicate poor bonding and often very poor stability.

Always be on the outlook for signs of instability; one sign of instability trumps all signs of stability.

Stability tests are only one piece of the puzzle; conduct several of them in different locations and use them in conjunction with other observed clues and Red Flags when you make your assessment and decide whether to ski a slope or not.

Next Section: Rutschblock »