THE CANTILEVER BEAM OR "BRIDGEBLOCK" SNOW STRENGTH TEST

Craig Sterbenz*Snow Safety Director, Telluride Ski & Golf Co., Telluride, CO.

ABSTRACT: Weak layers buried within the snowpack are chiefly responsible for slab avalanche formation.These weak layers tend to fail in shear when subjected to internal snowpack stresses. There are myriadsof snow stability tests used to locate and quantify the weakness of these shear layers. Knowledge gainedfrom these shear tests forms the basis for most avalanche forecasting programs. However, therelationship between stress and strength within the snowpack remains complex and important. Stressmust equal or exceed strength for failure and avalanche release. The shear fracture within the weak layermust also precipitate tensile failure of the overlying slab for avalanche release to occur. Because the weaklayers are so important for avalanche formation, little has been done to evaluate the strength of the slablayer. The "Bridgeblock" or cantilever beam test is a simple field test designed to evaluate the tensilestrength of the slab material overlying the bed surface weakness.

KEYWORDS: Avalanche forecasting, avalanche formation, snow mechanics, snow hardness, snowstrength.

1. INTRODUCTION

It is now widely accepted throughout theavalanche community that dry slab avalancherelease begins first, or initiates, with shear failure ina weak layer of the snowpack. Internal snowpackdeformation leads to shear stress, shear failure andsubsequent shear fracture across the bed surface.The shear fracture at the bed surface rapidlyprecipitates additional fractures at the,crown, flanksand stauchwall. The bed surface is, by far, thelargest and most important slab boundary layer toconsider in a study of avalanche releasemechanics. Virtually all snow stability tests aredesigned to find and quantify the relativeweakness of suspected bed surface shear planes.Most avalanche forecasting programs rely heavilyon the knowledge of these buried weak layers.

The continental-radiational snow climate ofthe San Juan Mountains in SouthwesternColorado frequently produces a snowpack that ispredominantly well developed depth hoar. Ameter or more of cohesionless 4 to 6 mm depthhoar is not uncommon. This weak layer is soobvious that traditional snow stability tests arehardly needed to locate or evaluate it. Attempts tofUlly isolate columns of snow frequently fail withcollapse of the entire column. Such a pervasive

*Craig Sterbenz, Snow Safety Supervisor,Telluride Ski &Golf Co.P. O. Box 11155, Telluride, CO. 81435tel: 9707287587; e-mail: sterbie @ rmi .net

and overwhelming weakness makes avalancheforecasting in the San Juans exciting to say theleast. Forecasters visiting from other snowclimates often ask Why it isn't constantlyavalanching. The thickness of the weak layer in theSan Juans occasionally produces interestingavalanche release characteristics. Frequently,early-season "off-piste" skiing there is not evenpossible until a slab or "bridging layer" develops.(Before that it's just "bottom feeding.") Shearfracture of these thick weak layers often results inaudible "woompfing" and visible collapse of thesubstratum. This collapsing substratum isobserved most frequently on flat to gentle terrain,where tension stresses are minimal, and seldomresults in visible fractures of the slab. Occasionally,even on steep terrain, where tension stresses arehigher, this collapse may fail to produce anavalanche fracture or release.

Avalanche release requires the tensilefailure and fracture of the overlying slab material aswell as the shear fracture along the bed surface.The fracture at the crown is a result of tensionfailure. (The collapse of a thick weak layer adds abending moment to the tensile stress on the slababove and may help explain observed crownfractures that are not perpendicular to the bedsurface.) Fractures at the flanks result from bothtensile and shear forces while the stauchwall failsprimarily in shear with the bed surface. If anyoneof these slab boundaries fails to fracture,avalanche release may be partial or not occur at all.Presumably, there is sufficient cohesion or

566

strength in the slab to"bridge" out to anchors orpinning areas and resist tensile failure. Experiencealso indicates that thick strong slabs may spreadout the weight of a skier or new snow load and helpdecrease the stress being added to the weaklayer. It is true that "slabs do not cause avalanches,weak layers do." But, maybe strong slabs can helpprevent avalanches.

2. TESTING SNOWSTRENGTH

Snow data pits can provide goodinformation on snow density, temperature, snowgrain size, type and relative hardness. Some ofthis information is used to infer snow strength.Tensile snow strength within a single layer is simplya measure of cohesion or bonding between thegrains. Shear strength within the snowpack is ameasure of grain cohesion and friction betweenlayers. It can be measured with a shear frame. Lowdensity, large grain snow usually has low strength.Fine grain high density snow is usually stronger.High density large grain snow, like depth hoar, haslittle strength. Density frequently increases withdepth in the snowpack while strength does not.Density and grain size do not seem to be reliablemeasures of snow strength. Hardness, asmeasured by ramsonde or relative resistance topenetration, is a better measure of snow strengththan density. A ram profile may tell us a lot aboutthe strength of the individual layers of thesnowpack. However, it can tell us little about theeffects of strain softening or how several layers ofslab may respond to the stress from shear failurebelow it. It is also an expensive and cumbersometool for use as a quick and simple field test. The

.::'. "Bridgeblock" or cantilever beam test is a quicksimple field test designed to quantify the tensilestrength and resistance to failure of the slabmaterial overlying or "bridging" the bed surfaceweakness.

3.CONDUCTING A BRIDGEBLOCKTEST

First step: Dig a standard snowpit andcollect the usual data.

Second step: Perform standard snowstability tests to locate and evaluate suspect weaklayers.

Third step: Measure and mark 1 meter inwidth across the top of the snowpit wall. (SeeFigure 1.) From each end of this mark; againmeasure and mark 1 meter back on the snow

567

surface behind the pit wall. (Similar to preparationfor a rutschblock test.) From each end of the firstmark; mark plumb lines on the pit wall to just belowthe weakest layer (or other weak layer of interest).

Figure 1. Pit wall with layout and partially isolatedslab.

Fourth Step: Create an artificial bedsurface fracture by excavating one square meter ofweak layer from between the plumb lines using aski pole or pole &snowsaw. Cut horizontallyacross the face of the snowpit wall and cut parallelto the slope a meter back from the pit wall surface.If the slab above the cut fractures during theexcavation it gets a score of zero. If it survivesexcavation it gets a score of 1.

Fifth step: Cantilever the suspended slabalong the flank by first cutting 1/2 meter deep intothe slab along the plumb line on either side andfollOWing the 1 meter marks laid out on the snowsurface earlier. If it doesn't fail, it gets a score of 2.Next make another 1/2 meter deep cut along theother plumb line. If it doesn't fail, it gets a score of3. Half of tbe beam is now cut on both flanks andcantilevered out 1/2 meter. Next; extend theplumb side cuts, one at a time, the second half

meter back until the slab is now cantilevered a fullmeter, like a diving board. (See Figure 2.)Each cutgetting scores of 4 and 5 respectively.

Sixth step: If the "bridge" layer survivesthe excavation and successive flank cuts (score of5) the next step is to load the cantilever usingsome of the stuff block, shovel tap or loadedcolumn techniques of standard stability tests untilthe cantilevered slab fails in tension at thecrown...providing scores of 6 or more.

.Figure 2. Fully isolated and cantilevered beam.

4. METHODOLOGY AND OBSERVATIONS

Several seasons were spent conductingthese tests in the San Juans. Initially, the test wasconducted by cutting the flanks back 1 meter oneach side and then measuring how far the weaklayer needed to be excavated before the slabwould fail. This provided good information but ledto operational difficulties when tools and workerswere invariably buried by the falling beams. Thetests were conducted primarily, but notexclusively, on relatively flat study plot areas inorder to isolate possible slope angle factors. Thetest seems easiest to conduct (and possibly more

germane?) in a faceted layer at least several cmthick. Most, if not all, of the crowns resulting fromthe test were not perpendicular to the snowpacklayers. This seemed to suggest that both tensileand bending stress were being applied to the slab.

Over the course of the 1997-1998season, a low density snowfall from earlyDecember developed into a very weak layer offaceted grains at about 50 cm from the ground.(See Snow Cover Profile 2). This weak layerpersisted throughout the winter. It was the failureplane for most of the deep slab avalanches duringthe remainder of the season. This was the layer ofinterest excavated during the bridgeblock testsconducted at the Gunroom Study Plot. The testwas very useful for tracking increases anddecreases in slab strength above this layer.Cantilever beam scores were low, but increasingthrough late February. (See Snow Cover Profiles 3and 4). Heavy snowfall in late February and earlyMarch produced an avalanche cycle releasing inthe December layer. Deep new snow and warmertemperatures qUickly improved snow strength andby mid-March cantilever beam scores were high.New snow added 30 cm to the snowpack betweenMarch 15 and March 19. (See Snow Cover Profiles5 and 6) This new snow load decreased cantileverbeam scores and resulted in another deep slabcycle with numerous releases in the sameDecember weak layer. Warm temperatures andstrengthening outpaced heavy new snowfall frommid-March through early April. Beam test scoresagain increased and deep slab activity ended.

6. TEST RESULT INTERPRETATION

Scores of a to 1 were very common,especially early season, in the San Juans. Thesescores suggested very little slab strength butseldom produced much in the way of slabavalanche activity, even with new snow events.Loose cohesionless sugar sluffs or isolated deeppockets of soft slab with little horizontal fracturepropagation characterized the avalanche activity.

Scores of 2 to 3 seemed to indicatemoderate slab strength. However, new snowloading resulted in an observed increase in thefrequency and magnitude of deep slab avalancheactivity. Most of these were medium size soft slabswith moderate horizontal fracture propagation.

Scores of 4 to 5 were seldom recordeduntil late season in the San Juans. These scoresappeared to suggest strong slab material. A

568

/

decrease in the frequency of slab avalanches wasobserved. However, large storm events continuedto produce deep slab avalanches. These werefrequently observed as class 4 or 5 hard slabs withcrowns that would propagate long distances.

Scores of 6 and higher were rare in midwinter in the San Juans. These high scores wereonly found during mid to late spring following meltfreeze cycles and the development of multipledense layers. Scores of 6 and greater indicatedconsiderable snow strength. No deep slabavalanches were observed during periods withthese high scores. However, scores wereobserved to lower again with new snow load.

6. CONCLUSIONS

After several seasons of conducting thebridgeblock test and observing related avalancheactivity several conclusions may be drawn. Thetest is repeatable and quantifiable on a relativescale. Tensile snow strength of the slab can bemeasured with such a test. Snow strength of theslab does have some relationship with observedavalanche activity. Difficulty in preciseinterpretation of test results continues to obscurethe nature of this relationship. High test scores didnot appear to preclude future avalanche activity.Interestingly, bridgeblock test scores did seem tocorrelate well with avalanche size. Perhaps thistest is better suited as an indicator of the type andsize of avalanche activity that might be expectedthan it is as a forecasting tool for avalancheoccurrence.

569

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