Evaluation of a prototype field test for fracture and failure propagation propensity in weak snowpack layers

Researchers and practitioners currently lack a quantitative field test that indicates the propensity of a given slab and weak layer combination to propagate weak layer fracture and failure to an extent that leads to avalanching. We report on the development of such a test and the evaluation of a prototype test method. This test allows researchers to observe propagation and arrest away from the initial weak layer failure by extending the down slope dimension of the small column stability test. Weak layer failure is initiated by cutting the weak layer with a 2 mm thick snow saw either from the upslope or down slope end of the isolated column. Depending on slab and weak layer characteristics, weak layer fracture/failure propagates from the edge of the saw and propagates either to the end of the column, to an indistinct point, or to the vicinity of a fracture through the thickness of the slab. During the winter of 2006, University of Calgary researchers performed over 600 of these tests in the Columbia Mountains of British Columbia, Canada, and the Rocky Mountains in Alberta, Canada. Results of our experiments allow for an evaluation of the relationship between isolated column length and cut length required to initiate propagation in this test. We evaluate the effect of slope angle and cut direction on test results, and propose a standard test column geometry. We relate our results to shear fracture and weak layer collapse theoretical models.