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Approximate Solutions for the Interruption of Creep and Glide by Avalanche Defenses

  • R. J. Evans (a1) and J. A. Langdon (a1)

Abstract

This paper presents a numerical calculation procedure for the stress and deformation analysis of a snow mass which is creeping and gliding on an inclined surface and which may, in addition, be restrained by rigid obstacles. The analysis is based on the assumption that the velocity profile at any position in the snow-pack is linear, that is, that sections initially plane remain plane during deformation. Such motion has been observed to occur in the neutral zone and is certainly true at a rigid barrier; accordingly, it appears to be reasonable to restrict the velocity field to be plane everywhere. A constitutive law for steady creep is used which is consistent with the observed neutral zone motion. The solution procedure is illustrated by its application to some problems of avalanche defense barriers. The cases investigated include: continuous barriers, cylindrical barriers, and barriers of finite width. Barrier forces and the extent of back-pressure zones are given.

Résumé

Solutions approximatives pour le probléme de l’interruption du fluage et du glissement par les outrages de défense orate l’avalanche. Get article presente un programme de calcul numérique pour l’analyse des efforts at des déformations d’une masse de neige en train de fluer et de glisser sur une surface inclinée at qui pent, de surcroit, être freinée par des obstacles fixes. L’analyse est basté sur l’hypothése que le profil des vitesses à quelque point que ce soit dans le manteau neigeux est linéaire, c’est-à-dire que les sections initialement planes restent planes durant la deformation. On a observé des mouvements de ce type dans les zones neutres et c’est certainement vrai contre un obstacle rigide; c’est pourquoi, it semble raisonnable de supposer que le champ des vitesses est plan partout. Une loi pour le fluage constant est utiliseé qui est cohérente avec le mouvement observé dans la zone neutre. Le programme solution est illustré par son application à quelques problémes drouvrages pare-avalanches; les cas traités comprennent: des ouvrages continus, des obstacles cylindriques, et des ouvrages de largeur finie. Les valeurs des contraintes sur les ouvrages et l’étendue de la zone de compression en amont sont données.

Zusammenfassung

Näherungslösungen für die Hemmung von Kriechen and Gleiten durch Lawinenverbauungen. Diese Arbeit enthält ein Verfahren zur rechnerischen Analyse der Spannung und Verformung von Schneemassen, die auf einem geneigten Hang kriechen und gleiten und ausserdem durch ein starres Hindernis gehemmt werden mögen. Die Analyse beruht auf der Annahme, dass das Geschwindigkeitsprofil an jeder Stelle des Schneepackens linear ist, d.h. dass urspünglich ebene Querschnitte während der Deformation eben bleiben. Das Auftreten einer derartigen Bewegung wurde in der neutralen Zone beobachtet; sie herrscht sicherlich vor einem starren Hindernis. Dementsprechend erscheint es angebracht, das Geschwindigkeitsfeld überall als eben anzunehmen. Grundlegend wird ein Gesetz fur stetiges Kriechen benutzt, das mit der beobachteten Bewegung in der neutralen Zone konsistent 1st. Das Lösungsverfahren wird in seiner Anwendung auf einige Probleme der Lawinenverbauung erläutert; die Untersuchung erstreckt sich den Fall langgestreckter und zylindrischer Hindernisse sowie von Hindernissen mit bregrenzter Ausdehnung. Die Hemmkräfte und das Ausmass der Rückstauzonen werden angegeben.

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References

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Bader,, H., and others .1939. Der Schnee und seine Metamorphose, Bader,, von H. Haefeli,, R. Bucher,, E. Neher,, J. Eckel,, O. Thams,, G. Niggli., P. Beiträge zur Geologic der Schweiz. Geotechnische Serie. Hydrologic Lief. 3. [English translation: U.S. Snow, Ice and Permafrost Research Establishment. Translation 14, 1954.]
Brown,, C. B., and Evans,, R. J. [1975.] Effect of glide and creep on rigid obstacles. [Union Gésique et Geophysique Internationale. Association Internationale des Sciences Hydrologiques. Commission des Neiges et Glacesd Symposium. Mécanique de la neige. Acies du colloque de Grindelwald, avril 1974 p. 40714 . (IAHS-AISH Publication No. 114.)
Brown,, C. B., and others .1972. Slab avalanching and the state of stress in fallen snow, [by] Brown,, C. B. Evans, R. J. and LaChapelle., E. R. Journal of Geophysical Research Vol. 77 No. 24. P. 457080 .
Brown,, C. B., and others .1973. Incorporation of glide and creep measurements into snow slab mechanics, by Brown,, C. B. Evans, R. J. and McClung., D. U.S. Dept. of Agriculture. Forest Service. General Technical Report RM-3, p. 713 .
Langdon,, J. A. Unpublished. Interruption of the creep and glide of snow by avalanche defenses. [M.S. thesis, University of Washington, 1975.]
McClung,, D. M. Unpublished. Avalanche defense mechanics. [Ph.D. thesis, University of Washington, 1973.]
Salm,, B. 1977. Snow forces. Journal of Glaciology, Vol. 19, No. 81, p. 67100 .

Approximate Solutions for the Interruption of Creep and Glide by Avalanche Defenses

  • R. J. Evans (a1) and J. A. Langdon (a1)

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