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EXTENDED ABSTRACT
A full scale avalanche dynamics experiment has been car-
ried out forseveral years by the Norwegian Geotechnical
Institute.The experiment site is the Ryggfonn avalanche
path close to NGI's research station in Grasdalen, Western
Norway.
Objective
The Ryggfonn projectis carried out to investigate theforces
on fixed structures from avalanches of different types.In
addition,theeffect ofaretaining damintheavalanche
path is observed.Data from the avalanches are also used
in the development of avalanchedynamics models and for
parametric studies.Today, the project is financedmainly
by Statnett,a division of the Norwegian State Electricity
Board and the Norwegian Geotechnical Institute.
Location
Theexperimentavalanche,Ryggfonn,issituatedin
Grasdalen, Western Norway.The avalanche usually starts
from a north-facing cirqueat around1530 m a.s.l. andruns
down a slightly channelled path into the valley floorbe-
low. The vertical drop from the starting zone to the runout
area is approximately 900 m.
Experimental setup
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A 15 mhigh and 75 m wide retaining dam in the ava-
lanche runout zone. On top of the dam is a 6.5 m high
steel mast that is instrumented with strain gauges and
sometimes an anemometer.
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Threeloadcells,eachwithanareaof0.72m2,are
mountedona4.5mhigh concretestructure situated
230 m up-slope from the dam.
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A 10 m (from 1994: 8.5 m) high tubular steel tower situ-
ated 320 m up-slope from the dam.The tower consists
offoursections,each having a diameterof1335mm
and wall thickness of 15 mm.
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The tower is instrumented with strain gaugesfor meas-
uring shear and moment strains at three sections,me-
chanicalpressureindicatorsforevery0.5manda
geophone fordetecting theavalanches andtriggering
the recording system.
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Geophonesplaced ontheground orsnowsurface 50
and 100mup-slopefromthedam forthe purposeof
determining the velocities of passing avalanches.
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An instrument shelter near therunoutareawith record-
ing equipment.The equipment converts the analogue
signals to digital signals and records them on a digital
tape recorder.
The avalanchesstudied are both natural andartificially
released.In the case of natural avalanches, the recording
systemis started when the signal from a geophone on the
uppermost construction (the steel tower) exceeds a preset
triggering level.
Artificial avalanches are released by detonating up till
five preplaced charges in the staring zone by means of a
radio controlled detonatingsystem.Typically, eachcharge
consists of 75 to 100 kg of dynamite.Artificial avalanches
are usuallyvideo-tapedand photographed withat time-
lapsecamera.Insomecases,filmingisdonefromthe
oppositemountainridge.Avalanche debrisboundaries
are
mappedandthesurfacessurveyedforvolume
calculations.
RESULTS
The volumes of the 27 avalanchesreachingtherunout area
have ranged from between 5000 to 470000 m3.The high-
est velocity recorded is 60m/s and the highest recorded
pressure on the load cells is 540 kPa.With respect to the
effect of dam, theresultsshow that in thecases of wet snow
avalancheswith volumes notexceedingthestoragevolume,
the avalanchesarestopped by thedam. Thestoragevolume
in the runout area upslope from the dam is approximately
70000m3,consequentlymanyavalanches have overrun
the dam.For the dry snow avalanchesthe dam appears to
haveaneffect inreducing theamountofdebrisonthe
leewardside of thedam, but there seemsto be no significant
effect with respect to runout length.Forthe fast moving
powder avalanchesthe dam has hardly any stopping effect
at all.
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