Interpreting Snowpack Stability Tests

Ronald F. Johnson

Avalanche Specialist

Gallatin National Forest Avalanche Center

Special Note:

This is the original article that was written for an online newsletter for the National Ski Patrol. This article was edited and posted online and was re-edited and published in the Winter Issue 2001 of Ski Patrol Magazine.

Interpreting Snowpack Stability Tests

Ski patrollers commonly use Snowpack stability tests, such as the Rutschblock, Compression “Tap” Test, and Stuffblock. These tests can be useful for determining the “real time” stability of the snowpack as well as help predict the future stability of the snowpack. They will also give you something to do if the skiing is not very good or if you need to look busy. This article will not focus on the specifics of each test (refer to the bibliography for sources of information about each of these tests), but will try to offer solutions for some problems often associated with interpreting the results of any stability test.

The mechanics of performing any of the common stability tests are not difficult. Just ask any recent graduate of a good avalanche course how to execute any of the common tests and they will tell you exactly how to isolate a column of snow and how to go about applying stress to that column of snow. However when asked, based on the results of their just completed battery of stability tests, if they would ski a given slope, they often looked confused and down right frustrated. This frustration stems from the fact that any stability test is good, if the user has the practical experience from which they can interpret the test results and formulate a reasonable assessment of the stability of the snowpack. Frustration caused by interpreting the results of stability tests aren't limited to recent graduates of avalanche courses. Anyone with limited practical experience in using stability tests or experienced professionals dealing with a complex snowpack can be frustrated with determining how a given test result relates to the actual stability of the snowpack. Unfortunately this frustration often leads to the misinterpretation of the test results, which could lead to the wrong assessment of the stability of the snowpack.

Selecting a suitable site to perform tests, misinterpretation of test results and a tendency to place too much emphasis on test results, are common problems associated with stability tests. Because any stability test only measures the stability of a small area on a given slope it is important that the tester realize that the test may not be representative of the snowpack stability on that slope. Selection of a suitable site often requires experience and knowledge of the local terrain and the snow loading characteristics of a given slope. Misinterpretation of test results may lead a tester to believe that a slope is more stable than it actually is. This could be hazardous to their health or their career. On the other hand, a conservative approach based on test results which indicate that the snowpack is less stable than it actually is, though prudent, may have an affect on the enjoyment of clients or guests or even the testers own “fun factor”. Finally, a tester that places too much emphasis on test results and neglects obvious signs of snowpack instability such as recent avalanche activity, and collapsing and cracking of the snowpack, often does so, to justify a desired outcome. An example would be someone who really wants to ski a given slope and even though other signs of snowpack instability are present, uses the outcome of a stability test to make the decision that the slope is safe to ski. This is a classic example of how the “human factor” plays a role in interpreting test results.

Gaining practical experience is key in developing the intuitive skills required to interpret test results. This is a process that continues forever, so you may as well enjoy it. In the meantime, here are some possible solutions to help you better understand the results obtained from stability tests:

Consider using at least two different tests. For example, by using the stuffblock and the rutschblock tests, weaknesses in the snowpack, which one test may miss, may become apparent when the other test is performed. By comparing the results of these tests a better understanding of the snowpack stability may be obtained.

Most stability test results are determined by giving a rating of the relative amount of stress, which is required to get the weak layer in the snowpack to fracture. For example, a Rutschblock score of (3) means that the weak layer in an isolated column of snow fractured when the skier standing on the isolated block flexed their knees. A Rutchblock score of (5) indicates that the skier needed to make two jumps on the isolated block before the weak layer fractured. While this information is useful for determining the strength of the weak layer, it may not provide enough information about the actual stability of the snowpack. In fact, it is the relationship between the weak layer and the overlying slab, which determines the stability of the snowpack. Therefore, instead of focusing entirely on the amount of stress required for shear fracture to occur on a given weak layer, it may be useful to look at the quality of the shear fracture. The quality of the shear fracture may provide an insight as to what the true relationship is between the weak layer and the overlying slab. The quality of the shear fracture can be rated with a subjective rating system of Q1, Q2 and Q3.

Shear Quality Rating System

Q1…..Unusually clean and smooth shear plane, weak layer may noticeably collapse during failure. Slab typically slides easily into the snow pit after the weak layer fractures on slopes steeper than 35 degrees, and sometimes on slopes as gentle as 25 degrees. Sometimes referred to as a “clean shear”.

Q2…..”Average” shear, shear plane appears mostly smooth, but slab doesn’t slide as readily as Q1. Shear plane may have some small irregularities, but not as irregular as Q3. Shear fracture occurs through whole block being tested, and slab may or may not slide into the snow pit.

Q3…..Shear plane is uneven, irregular or rough, shear fracture may occur through the whole block being tested. After weak layer fracture, slab moves little or may not move at all, even on slopes steeper than 35 degrees. Sometimes referred to as a “dirty shear”.

Combining the results of the stability test score, with the quality of the shear fracture may produce a better indication of the snowpack stability. For example, a Rutschblock score of (3) with a shear quality of Q3 may indicate better stability than if the Rutschblock score was (3) with a shear quality of Q1. Of course, there are other factors to be considered such as other signs of instability and depth of fracture, but adding the shear quality seems to help testers better interpret the results of their stability tests.

When trying to determine the overall stability of the snowpack, it is important to remember that stability tests only provide a piece of information which needs to be considered with all other observations of terrain, weather and snowpack characteristics. With this in mind, the following suggestions may prove useful for reducing the confusion and frustration often associated with interpreting the results from any stability test.

· Site selection is important. If you are not sure that your site is representative of the snowpack, then you should take a conservative approach and assume the snowpack is less stable than the stability test indicate.

· Perform at least two different types of stability tests.

· Rate the quality of the shear fracture and combine that with the test score.

· Don’t place too much emphasis on test results; remember to look at the “big picture” so that you don’t miss other obvious signs of snowpack instability such as recent avalanche activity, and/or collapsing or cracking of the snowpack.

So, go forth and dig, cut, jump, drop, pull and kick at the snowpack and think about how these results relate to the actual stability of the snowpack.

Bibliography

Rutschblock:

Fohn, P.M.B. 1987b. The “Rutschblock” as a practical tool for slope stability evaluation. In: Avalanche Formation, Movement, and Effects, IAHS Publ. No. 162, 223-228.

Jamieson, J.B.; C.D. Johnston. 1992. Experience with rutschblocks. Proceeding of the 1992 International Snow Science Workshop, Breckenridge, Colorado, 150-159.

Stuffblock:

Johnson, R.; K.W. Birkeland, 1994. The stuffblock: a simple and effective snow stability test. In: Proceedings of the 1994 International Snow Science Workshop; Snowbird, UT: 518-526.

Birkeland, K.W.; R. Johnson; D. Herzberg. 1996. The stuffblock snow stability test. Tech. Rep. 9623-2836-MTDC. Missoula, MT: 20p.

These papers are available at www.mtavalanche.com

Compression Test:

Jamieson, J.B. 1999. The compression test - after 25 years. The Avalanche Review, Vol. 18, No. 1, Autumn 1999.

Ron Johnson

Gallatin National Forest Avalanche Center

P.O. Box 130

Bozeman, MT 59771

(406) 587-6984

gnfac@avalanche.org