S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

MeasurementsonSkierTriggering
ChristianCamponovoandJürgSchweizer¹

Swiss Federal Institute for Snow and Avalanche Research, CH-7260 Wei ssfluhjoch/Davos, Switzerland

phone: +41 81 417 0222, fax: +41 81 417 0220, e-mail: schweizer@slf.ch

Keywords: snow mechanics, slab avalanche, avalanche for-

mation,dynamic loads

ABSTRACT

The forces induced by a skier (or snowboarder) within the
snowcoverweremeasuredinsituwithloadcellsfor
different snow cover conditions and for different load cell
(weak layer) depths within the snow cover.The different
stepsofdynamicloadingthatareapplieddoinga
rutschblocktestwere studied.Theresults showtheim-
portance of the weak layer depth (the thinner the slab the
easier triggering)andinparticular of thetype of sublayering
of the snow cover,i.e.of the slab properties.Bridging ef-
fects by hard layers are recognized. Skierpenetration has
tobeconsidered. Thestressincreased stronglybutnon-
linearly with rutschblock loading steps.Forcertain snow
conditionsthemeasured impactisofthesameorderof
magnitudeas typicalshearstrength values measured by
pullingshear frames. Thedynamicloads are appliedwithin
fractions of seconds. Provided that deformations are large
enough (dependingon slab thickness and slab properties),
the skier's impact induces a brittle failure within a weak
layeror interface.

INTRODUCTION

About 20 skiers, snowboarders andmountaineers are killed
eachyear inthe Swiss Alps (10 year average). Inmost cases
the victims triggeredthe fatal avalanche themselves. They
representmorethan 90% ofallavalanchevictims.This
portion is typical for most mountainous regions in Europe
and North America. The skierseems to be a very efficient
trigger, despitehis small static load. The skier's impact has
tobeconsideredinstabilityevaluationandavalanche
forecasting.

The skier's load wasintroduced in the evaluation ofthe
stability indexbyFöhn (1987).The numerical modeling
by Schweizer(1993) showed that the layering of the slab
seemstobecrucialforskiertriggering.Schweizeret al.
(1995a,b) studiedthe stressdistribution inthe layeredsnow
coverand in particularthe response of the snow coverin
thecaseofdynamicloading.Sincefieldmeasurements
strongly depend on weather and snow conditions during
thecourseofthe winter,measurements havetobe done
overa coupleyearstomeetdifferentsnow covercondi-
tions. This article represents a summary and update of the
previous results (Schweizer et al., 1995a,b).

METHODS

The skier's impact ismeasured with load cells buried in
the snow cover.The dimension of each of the five identi-
cal load cells is 0.5 x 0.5 m,giving an area of 0.25 m² ,the
thicknessis5cmandthedensityabout400kg/m ³
(Schweizer et al., 1995a).Within the load cell fourcanti-
lever type transducers measure the normal and shear force.
Additionally, temperatureandcellinclinationare recorded.
We presently useadata acquisitionsystemwithascan
frequencyof2kHz,givingatimeresolutionof0.5ms
(Camponovo,1995).
We started the experiments during the winter1993-94
and up to now we performed about 60 experiments.The
measurements were realised in the flat terrain of the study
plot of the Swiss Federal Institute for Snow andAvalanche
Research (SFISAR) at Weissfluhjoch, 2540 ma.s.l., above
Davos,Switzerland. To measure the skier's impact in the
flat terrain is reasonable,because we are primarily inter-
ested in the snow cover response.
The best way toensure realistic measuring conditions
is to put the load cell onto the snow surface just before a

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