I n s t r u m e n t s

a n d

M e t h o d s

by Schweizer (1993,1995).From Fig.11 we may also see
that weak layers deeperthan 1 mbelow the surface will
only be slightly influencedby a skier under normal condi-
tions.This result yields also the hint to practitioners that
trenchesfor Rutschblocks or profiles may often be limited
to this depth.
Finally Fig. 12 shows the approximate relationship be-
tween the skierstabilityindex S' (orSS,Jamieson,1995)
and the measured shearstrength by various snow condi-
tions.Theshearstrengthisobviouslyaveryimportant
parameter forthe stability.Shear strength values smaller
then 500 Pa are definitely insufficient (S' <= 1.0) whatever
theexact shearstresswillbe,valuessmallerthen 1kea
probably also, becauseof thesafetymarginof S' >= 1.5, which
has to be taken into account when analyzingsafety aspects
(Föhn,1987,Jamieson and Johnston,1993).

CONCLUSIONS

ACKNOWLEDGEMENTS

For their expertise and field work at various times, we are
gratefultoCharlesFier z,GeorgesKrüsi,Christof
Oberschmied.Thanks to André Roth forthe photos.
Many thanks also to many unnamed part-time students,
who helped in the field and to Stefan Felix,Marco Gaia,
UrsLiebing,ChristophMartyandKasparPlattner,who
helped compile the data of the previous field campaigns.

REFERENCES

Föhn, P.M.B. 1987. The stabilityindexand various triggering
mechanisms.IAHS Publication 162,195 - 211.

Föhn, P.M.B. 1993. Characteristics of weak snow layers or in-
terfaces. Proceedings of the International Snow Science Work-
shop in Breckenridge, Colorado, October 4-8, 1992. ISSW '92
Committee, c/o Colorado Avalanche Information Centre, 10230
Smith Road,Denver,Colorado,80239 USA,171-175.

Fukuzawa, T. and H. Narita. 1993. An experimental study on
the mechanical behaviourof a depthhoarlayerundershear
stress:preliminaryreportProceedingsoftheInternational
Snow Science Workshop in Breckenridge, Colorado, October
4-8,1992.ISSW '92Committee,c/oColoradoAvalanche In-
formation Centre, 10230 Smith Road, Denver, Colorado, 80239
USA, 160-170.

Jamieson, J.B. 1995. Avalanche prediction for persistent snow
slabs. Ph.D. Thesis, University of Calgary, Dept. of Civil Engi-
neering,Calgary,Alberta, Canada,258 pp.

Jamieson,J.B. and C.D.Johnston.1993. Shearframe stability
parametersforlargescaleavalancheforecasting.Annalsof
Glaciology18, 268-273.

Jamieson,J.B.andC.D.Johnston.1995.Monitoringashear
frame stabilityindexandskier-triggered slabavalanches in-
volving persistent snowpackweaknesses.Proceedings of the
International Snow ScienceWorkshop at Snowbird,Interna-
tional Snow Science Workshop 1994, P.O. Box 49, Snowbird,
Utah 84092, USA, 14-21.

Johnson, J.B. 1995. Quasi-static and dynamic mechanical prop-
erties of snow. In: Future directions in sno w and ice research,
workshop on the Future Directions in Snow and Ice Research,
Bozeman, Montana, p. 4/5 (unpublished).

McClung, D.M. and P.A. Schaerer. 1993. The Avalanche Hand-
book.The Mountaineers, Seattle,271pp.

Narita, H. 1983. An experimental study on tensile fracture of
snow.Contributions fromtheInstituteof LowTemperature
Science,Series A,No. 32, 1-37.

Schweizer,J.1993. The influenceof the layered characterof
thesnowcoveronthetriggeringofslabavalanches.Ann.
Glaciol., 18. 193-198.

Schweizer,J.,Camponovo,Ch.,Fierz,Ch.and Föhn,P.M.B.
1995.Skiertriggeredslabavalanche release- some practical
implications.In:Proc.Int.Symposium:Sciences and moun-
tain - The contribution of scientific research tosnow, ice and
avalanche safety, ANENA, Chamonix, France, May 30-June 3,
1995,309-316.

Fig. 12. Snowpack stability index (S') of asnowpack loaded by a skier
vs. shear strength measured over many winters by var ious conditions.
Weak layer shear strength smaller than 1000 Pa indicate more or less
instable conditions if a safety margin of S' = 0.5 is included (S' <= 1.5
means triggering of slabs probable).

Byuseofanadequateinstrumentation(digitalforce
gauge,accelerometer,displacement-meter) it is possible
to determine by shearframe measurements not only the
shearstrength,butalsotheshearstrainandstrainrate
independently. Thus after all also the strain rate depend-
ence ofshearing thin weaklayerscouldbe determined
in situ on representative small slopes.This isimportant
becausesnowsamplescontainingweaklayersmay
rarelybetransportedintocoldlabsformeasurements
withoutrupturing.
The results clarify the following points:

*The "fast" pull range(0.1 <= dt <= 3 s) yields a brittle frac-
turing witha strain rate of 10^-2to 10 ^-1per second,which
is well beyond the critical limit forbrittle fractures 10^-
4_{per second.}

*Dry weak layers show by shearing a more or less linear
elasticbehaviour inthelimited strainrangeexperienced.

*It seems that bysuch measurements also the fast frac-
turing process of a skieris well approximated.

*Depending on the individual pulling action, the shear
strength values,measured up to now without record-

133