Using 2 m Extended Column Tests to assess slope stability

Abstract Extended Column Tests (ECTs) are used to assess crack initiation and propagation. Previous research shows that tests 90 cm in length may propagate, suggesting instability, while tests 2 m in length may not propagate, suggesting stability, for identical snowpacks. A practical question is: are 90 cm ECTs optimal for assessing stability? To test the added value of 2 m ECTs for stability evaluation, we collected data on 220 ECTs, with 136 side-by-side standard length ECTPs (full propagation indicating instability) followed by 2 m ECTs. We only performed 2 m ECTs after a standard ECT propagated because we assumed 2 m ECTs would not propagate if standard length tests did not. These tests were preceded by an a priori stability assessment. Our results show imbalances for both tests. The ECT had a similar probability of detection (0.88–0.92, POD), i.e. the ability to detect unstable conditions, as in previous studies, but a much lower probability of null events (0.54–0.75, PON), i.e. the ability to detect stable conditions, with variation due to the binary classification of “Fair” stability as stable or unstable. Adding a 2 m test after an ECTP result lowered the POD (0.49–0.58), but substantially raised the PON (0.88–0.98) of the combined tests. The proportion of tests in agreement, i.e. ECTP and 2 m ECTP, increases with decreasing stability. We conclude that an ECTP followed by a 2 m ECTP is a clear red flag, indicating instability. The interpretation of an ECTP followed by a 2 m ECTN/X (no propagation) is not clear. Though this result suggests stability, a high potential for a false stable result means we cannot recommend the 2 m ECT for binary stability assessments.