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Key Words:AvalancheProtection,SupportingStructures,
Hazard Maps
ABSTRACT
First the principlesof supporting structuresare explained.
Two case studies ofwinterobservations in areas control-
ledbysupportingstructuresarepresented.Thetaskof
supporting structuresto preventavalanchesor to limit snow
motiontoaharmlessmagnitudeisinvestigated byfield
studies. Examples of the interaction betweenmoving snow
and the structures are given.Then the main criterias for
safety analysis are described andrecommendationsfor the
use of supporting structures are given.
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PRINCIPLES OF SUPPORTING STRUCTURES
The principle, calculation and design of supporting struc-
tures areexplained in the"Swiss Guidelines for avalanche
control in the starting zone" (1990). All insights and expe-
riencesaccumulatedover several decadesare summarized
there.TheSwissGuidelines(1990)arethebaseforall
applications of supporting structures in Switzerland.
The task of supporting structures is to prevent large ava-
lanches orat least to limit snow motions - they cannot be
completelyeliminated-toaharmlessmagnitude.Fully
developedavalanches cannot bestoppedbysupporting
structures. The first problem isto produce an overall in-
crease inthe stabilityoftheslopingsnowpackbyaddi-
tional compressive stresses and reduced shear stresses in
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the weak layer due to the wall. The second problem con-
sistsin limiting the size of the snowmasses which have
been set in motion and in retarding and catchingthem.
Slopes from 30º to 50º are generally considered to be in
therange that justifiesconstructions.Theprimaryloca-
tion for supporting structures is below the highest fracture
linethatisobservedorisexpected.Thecontinuous
arrangement ofstructuresin lineswithlengths between
20m and 50 mispreferred. The height ofa structure is
decisive forthe avalanche safety during situationsofin-
tense snow accumulation and forthe design of the struc-
tures. In Switzerland the vertical height of structure must
correspond at least tothe extreme snow depth with a re-
turnperiodof100years.Thestudyofthesnowdepth
distribution overthe project area is very important.Typi-
cal structureheights used in the Swiss Alps are 3 m, 3.5 m
and 4 m. Inthe Swiss Guidelines(1990) the distancesalong
theslopebetweenlinesofstructuresaredetermined
according tofollowingcriteria's:The structureshave to
withstand the maximumstatic and dynamical snow pres-
sures.Furthermorethey havetoreduce the velocitiesof
small avalanchesby the roughnessof the supporting plane.
Todaysteel snow bridges and flexible snow net systems
are most commonly used. The snow pressure loads are up
to 100 kN/m.The foundations are made with micropiles
and anchors.The costs forsupporting structures are high:
one hectare costs about 1 million Sfr.The costs of an ava-
lanche control project are typically shared in the follow-
ingway:FederalGovernment60-70%,Canton15-20%,
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