I n s t r u m e n t s

a n d

M e t h o d s

ImprovementsbyMeasuringShearStrengthofWeakLayers

Paul Föhn and Christian Camponovo

Swiss Federal Institute for Snow and Avalanche Research

7260 Weissfluhjoch, Davos, Switzerland

E-mail: foehn@slf.ch / camponov@slf.ch

Keywords:Snowslab,weaklayers,strainrate,shear

strength,measuring procedures

ABSTRACT

Concurrentmeasurements in situ of the shear strengthand
strain rate of weak layers orinterfaces have not been ex-
ecutedup tonow.Inordertouncovertherelationship
between thetwo parameters andto clarify if the usual load-
ing times (0.1 to 3 seconds) were adequateto produce brit-
tle fractures parallel acceleration and deformation meas-
urements were performed. Both additional measurements
show that the critical strain rate limit for brittle fractures is
always exceeded, thus the dynamic loading ofa moving
skieriswellreproduced bysuchshearframe measure-
ments.Themeasuringprocedures,theenvironmental
conditionsandthemeasured shearstrengthvaluesare
presentedas well as some error sources. Finally the practi-
cal consequencesofsuch measurements are displayed by
somethereofcalculatedrelationships betweenstrength,
stress and stability.

INTRODUCTION

Slab avalanches are most often released in weak layers or
interfaces. In order to explain a posteriori such releases or
toforecasta slabsituation,itiscommonpractice toap-
proximateamechanical stabilityindexusingvaluesof
shearstrength and overburden stresses of the slab layers
measured inthefield(ande.g.ofanadditionalskier).
However because the strain rate of these strength values is
notmeasuredconcurrently,thereissomedoubt,ifthe
dynamic ofthese measurements isrepresentative forthe
brittle behaviour whichgenerally is attributed to skier-trig-
geringof slabs. In order to clarify the type of fracturemode
forourfieldmeasurements,wedecided tocombinethe
strength measurements with acceleration and deformation

measurements.

EXPERIMENTAL APPARATUS

Theinstrumentalset-upusedinourfieldcampaigns is
shown in Fig. 1.
Theusedshearframe isthesocalled"Swiss"shear
frame. This stainless steel frame has six cross-members, is
sharpened at the loweredges and has an area of 0.05 m2
(0.2 x 0.25 m).The total weight with mounted accelerom-
eter is 0.85 kg.
The appliedshear forcewasmeasured withanelectronic
forcegauge attached tothe framewithtwosteelhooks.
The gaugerange is ± 490 N with a precision of ± 1 N.The
stress is obtained,dividing the force by the frame area.
In orderto obtain the strain rate and the strain during
the experiment, we have mounted an accelerometeronto
the shear frame. Integrating the measured acceleration we
are in a state to calculate the displacement velocity of the
frame andthestrainrate,scalingthe velocitywith theframe
length.Integratingthevelocitywecancalculatethe
displacement of the frame and the strain (Fig. 2).For one
third of the measurements we have mounted on the shear
frame also a sensor, which measures directlythe displace-
ment. Wedid not use it for every experiment, becausea lot
of time was required to fix the sensor in the snow cover or
undergroundcarefully.Theanalog outputsofthethree
sensors were wiredwith cables to a signal conditioner and
finallytoalaptopcomputer,wherewerecordedthe
measureddata. Thecomparison betweenthemeasured and
the calculated(from acceleration) displacementshow afair
agreement (Fig. 3).
The shear force, the frameacceleration and, when used,
the displacement length were continuously recorded with

Fig. 2. An exam ple of measured force, acceleration and of calculated
strain rate and displacement .

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