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| Titel |
An analytical approach for the Propagation Saw Test |
| VerfasserIn |
Lorenzo Benedetti, Jan-Thomas Fischer, Johan Gaume |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123451
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| Publikation (Nr.) |
 EGU/EGU2016-2704.pdf |
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| Zusammenfassung |
| The Propagation Saw Test (PST) [1, 2] is an experimental in-situ technique that has been
introduced to assess crack propagation propensity in weak snowpack layers buried
below cohesive snow slabs. This test attracted the interest of a large number of
practitioners, being relatively easy to perform and providing useful insights for the
evaluation of snow instability. The PST procedure requires isolating a snow column of
30 centimeters of width and -at least-1 meter in the downslope direction. Then,
once the stratigraphy is known (e.g. from a manual snow profile), a saw is used
to cut a weak layer which could fail, potentially leading to the release of a slab
avalanche. If the length of the saw cut reaches the so-called critical crack length, the
onset of crack propagation occurs. Furthermore, depending on snow properties, the
crack in the weak layer can initiate the fracture and detachment of the overlying
slab.
Statistical studies over a large set of field data confirmed the relevance of the PST,
highlighting the positive correlation between test results and the likelihood of avalanche
release [3]. Recent works provided key information on the conditions for the onset of
crack propagation [4] and on the evolution of slab displacement during the test
[5]. In addition, experimental studies [6] and simplified models [7] focused on the
qualitative description of snowpack properties leading to different failure types,
namely full propagation or fracture arrest (with or without slab fracture). However,
beside current numerical studies utilizing discrete elements methods [8], only little
attention has been devoted to a detailed analytical description of the PST able to give a
comprehensive mechanical framework of the sequence of processes involved in the
test.
Consequently, this work aims to give a quantitative tool for an exhaustive interpretation of
the PST, stressing the attention on important parameters that influence the test outcomes.
First, starting from a pure mechanical point of view, a broad phenomenology of the main
failure types of the PST is outlined. Then, the Euler-Bernoulli beam theory is applied to the
test setup, allowing an easy description of the snowpack stress state in the quasi-static regime.
We assume an elastic-perfectly brittle model as constitutive law for the snow slab. Besides,
considering the weak layer as a rigid bed of crystals with an a priori inclination, a local
instability problem is formulated in order to take into account the combined effect
of compressive and shear loading. As a result, the onset of slab and weak layer
fracture is described in terms of cut length, slab dimensions and the main mechanical
parameters. A condition on the possible propagation of the crack is proposed as
well.
References
[1] C. Sigrist and J. Schweizer, “Critical energy release rates of weak snowpack layers
determined in field experiments”, Geophysical Research Letters, Volume 34, L03502,
2007.
[2] D. Gauthier and B. Jamieson, “Evaluation of a prototype field test for fracture and
failure propagation propensity in weak snowpack layers”. Cold Regions Science and
Technology, Volume 51, Issue 2, Pages 87-97, 2008.
[3] R. Simenhois and K.W. Birkeland. “The extended column test: Test effectiveness,
spatial variability, and comparison with the propagation saw test.” Cold Regions Science and
Technology, Volume 59, Issue 23, Pages 210-216, 2009.
[4] J. Heierli, P. Gumbsch, M. Zaiser, “Anticrack Nucleation as Triggering
Mecchanism for Snow Slab Avalanches”, Science, Volume 321, Pages 240-243,
2008.
[5] A. van Herwijnen, J. Schweizer, J. Heierli, “Measurement of the deformation field
associated with fracture propagation in weak snowpack layers”, Journal of Geophysical
Research, Volume 115, F03042, 2010.
[6] K. W. Birkeland, A. van Herwijnen, E. Knoff, M. Staples, E. Bair, R. Simenhois, “The
role of slabs and weak layers in fracture arrest”, Proceedings of the International Snow
Science Workshop, Banff, 2014.
[7] J. Schweizer, B. Reuter, A. van Herwijnen, B. Jamieson, “On how the tensile strength
of the slab affects crack propagation propensity”, Proceedings of the International Snow
Science Workshop, Banff, 2014.
[8] J. Gaume, A. van Herwijnen, G. Chambon, K. W. Birkeland, J. Schweizer. “Modeling
of crack propagation in weak snowpack layers using the discrete element method”, The
Cryosphere, Volume 9, Pages 1915-1932, 2015. |
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