Experience with Shear Frames

Abstract

ABSTRACT. The shear frame is a simple in situ device for indexing the s hear strength of thin weak layers. The index is sensitive to shear-frame geom etry, rate-of-pull , and shear-frame ma ss. It is time-consuming to carefu ll y align the device on the Gleitschicht (shear failure plane) in a slab avalanche zone. The ratio shea r fra me index/ shear stress of the Gleitschicht has a high va rian ce, a nd may not be a fundamental mea s ure of slab avalanche sta bility. Corrections for th e normal stress on the Gleitschicht reduce th e vari ance only s lightl y. Despite these limitations, the shear frame is a useful tool for gat hering statistica l data on strength distributions and anisotropies of the Gleitschicht until a more fundamental technique is developed . RESUME. Experiences avec ja uges a cisaillemellt. La jauge a cisaillement es t un simple appareil in situ pour indexer la resistance au cisaillement de niveaux faibles et minces. L'indice est sen sible a la geometrie de la jauge, a cisaillement, a I'impulsion qu'on lui donne, et a la masse de la jauge. C'est une manoeuvre tres longue que de pl acer avec soin I'appareil sur le " gleitschicht" (pl a n de rupture au cisaillement) d ans une zone d 'avala nch e de plaque. Le rapport indice de cisaillement/effort de cisaill ement sur le "gleitschicht" a uhe forte va ri abi lite et ne peut constituer une mesure fondamentale de la stabilite des avalanches de pl aq ues. Les correcti o n s pour lenir compte de I'effort normal s'exercrant s ur le " gleitschicht" ne red uit que peu cette variabilite. En depit d e ces limiles. la jauge a cisaillement est un outil utile po ur rassembler des donn ees statistiques s ur la distribution des effort s etles anisotropies du "gleitschicht" jusqu 'a ce qu 'on ait mi s a u point un e methode plus rationnelle.ZUSAMMENFASSUNG. Erfahrungen mil Scher-Rahmen. Der Scher-Rahmen ist eine ei nfac he Vorrichtung zur in silu -Feststellung der Scherfestigkeit diinner und schwacher Schichten . Di e Anzeige reagiert auf die Geometrie des Rahmens, die Zugrate und die Rahmenmasse. Die sorgfalti ge Einpassung des Gerates in die Gleitschicht einer Lawinenzone erfordert viel Zeit. Das Verhaltnis zwisc hen Scherrahmen-Anzeige zur Scherspannung der Gleitschicht ist stark veranderli ch und diirfle kein Grundmass fiir die Stabilitat von Schneebrettern sein . Ko rrektu ren wegen Normalspannung in der Gleitschicht verringern die Schwankungen nur wenig. Trotz dieser Einsc hrankungen ist der Scher-Rahm en ei n brauchbares Mittel zur Gewinnung statistischer O aten iiber die Festigkeitsverteilung und die Anisotropien in der Gleitschicht, bis eine tiefergreife nde Technik entwickelt wird .

Linking the effective thermal conductivity of snow to its shear strength and density

“…This observation is similar to that of Perla and Beck [1983], who observed that higher pull rates on a shear frame lowered shear strength values by 25%. Perla and Beck [1983] also noted that shear strength measurements were affected by the normal load applied to the frame. The only way to improve accuracy has been to increase the number of measurements while using a rotation speed as constant as possible, about 1 s for a 180°rotation, which we consider fast.…”

Determination of failure envelope for faceted snow through numerical simulations

“…The observation of increase in the shear strength with applied compressive pressure is in agreement with different studies (Gaume et al, 2014;Perla and Beck, 1983;Zeidler and Jamieson, 2006). In all these studies the shear strength of snow is found to be increasing up to a certain value of applied compressive pressure.…”

“…FCsf (Chandel et al 2014b , 10 Feb 2010 e. FCsf (Chandel et al 2014b , 19 Feb 2010 f. Cohesive Snow (Perla and Beck, 1983) g.…”

“…The initiation of slab avalanche release is caused by the combined compressive and shear loading and due to this reason researchers started to explore the effect of combined loading on snow and avalanches. Perla and Beck (1983) and Zeidler and Jamieson (2006) conducted combined compressive and shear loading experiments and observed an increase in the shear strength with normal compressive load for different snow types. Nakamura et al (2010) investigated the behavior of rounded polycrystals snow and measured the shear strength for a varying compressive load and reported a linear increase in shear strength with compressive pressure.…”

Formation of refrozen snowpack layers and their role in slab avalanche release

“…In the first 24 hours some 0.5 mm facets formed where the dry snow lay on the freezing wet layer. Also, the shear strength measured with a shear frame [see, e.g., Perla and Beck , 1983; Sommerfeld , 1984] dropped 50%. This decrease was attributed partly to a shear stress concentration resulting from the freezing and stiffening of the wet layer and partly to the formation of facets at the interface.…”