L2_5

Bonding and failure of bondsAvalanche formation and release from the perspective of bonding and failure of bonds.American Institute for Avalanche Research & EducationLevel II Avalanche Course

Know how avalanche form and release.

Understand common layering scenarios for slab avalanche formation.American Institute for Avalanche Research & EducationLevel II Avalanche CourseLearning OutcomesFor an avalanche to occur, bonds have to break. Snow is a plastic, almost viscous material, and it is possible for failure to occur in the snowpack without an avalanche occurring. Failure is part of the process by which the snow adjusts to changes in load and stress.

Bonding, Failure, and Avalanche ReleaseFailure and fracture are two distinct processes. Failure can occur without fracture. Both processes involve the breaking of bonds. Failure is when there are existing bonds are breaking more slowly than there are new bonds forming; the snowpack adjusts as it heals itself. Failure Existing bonds are breaking more slowly than there are new bonds forming. Problems w/mechanics and materials

Fracture More existing bonds are breaking than there are new bonds forming

Bonding, Failure, and Avalanche ReleaseFailure without fracture is applying pressure to a metal bar: the bar bends but does not break. This is ductile behavior!If we bend the bar far enough, a crack forms and propagates through the material and eventually fracture occurs as the bar breaks along the crack. The terms failure and fracture are generally used interchangeably by avalanche professionals. Poor use of standard terminology.

Bonding, Failure, and Avalanche ReleaseFor a slab avalanches, failure has to be initiated, fracture has to occur, and the fracture has to propagate through the snowpack.In the mountain snowpack, there is always a certain amount of failure occurring. Metamorphism causes the size, shape, and bonds between grains to constantly change and adjust.Slab avalanches require:Initiation of failureFracture

Bonding, Failure, and Avalanche ReleaseIn the mountain snowpack, there is always a certain amount of failure occurring. Gravity, pressure from overlying snow, and incline cause grains to creep, that is, slowly move from one location to another as they try to pack together more closely. Creep = change of rate versus depth of spx!Slab avalanches require:Initiation of failureFracture

In some cases, an entire section of the snowpack glides down the slope.Glide is a slow motion avalanche including the formation of a glide crack which is really a fracture line. In terms of an avalanche starting, failure is initiated by a catastrophic failure that results in snow sliding down the mountainside. Slab avalanches require:Initiation of failureFractureBonding, Failure, and Avalanche Release

Bonding, Failure, and Avalanche ReleaseIf failure reaches a critical point, fracture will result. In some cases, super-weak zones form where fracture is occurring but does not propagate far and/or wide enough to cause an avalanche. When a super-weak zone becomes large enough, a more generalized failure occurs resulting in an avalanche.

Bonding, Failure, and Avalanche ReleaseIf failure reaches a critical point, fracture will result. Fractures propagate by spreading along a layer of snow as bonds between grains break. Fractures also tend to propagate from weak point to weak point in the slab.Weak points:1) shallow areas in the snowpack, 2) thin spots in the slab, and/or 3) places where the integrity of the slab is disturbed (e.g. rocks or trees protruding into or through the pack).

Bonding is a double edged sword. Without bonding, the snow would always be in a state of high instability, it would never strengthen and we would never be able to ski slopes of more than a critical angle.Every time it snowed, the snow would naturally slide off in a loose avalanche or we would always trigger loose avalanches on all slopes above the critical angle (the angle of repose).Bonding, Failure, and Avalanche Release

Bonding is a double edged sword. With bonding steeplopes are relatively stable because the snow gains strength and stays in place on increased inclines. The potential for slab avalanches exists; the slab is cohesive enough to stick to steeper slopes and at the same time cohesive enough to carry propagationBonding, Failure, and Avalanche Release

Bonding, Failure, and Avalanche Release

Bonding, Failure, and Avalanche ReleaseThe bonds in the snowpack are always in a state of balance between strength and stress.

The process of assessing this balance is called snow stability, although it would probably be more accurate to call it snow instability StrengthStressStrength: the bonds that are holding everything together Stress: F/A applied to the bonds that reduce in bond strength

The snow is always unstable: if enough stress is applied, failure and fracture will occur.

The question is: What it will take to tip the scale and cause an avalanche? StrengthStressStableUnstableStressStrengthBonding, Failure, and Avalanche Release

The significant difference between snow and other materials is that most materials (e.g., steel) are fabricated to specific standards, at temperatures far from their melting point, and are highly uniform. The maximum loads are well known, and known safety margins can be calculated and built in. Bonding, Failure, and Avalanche Release

Snow is a material that is not fully understood:

Snow is always within a few degrees of its melting point It is highly variable over time and space Snow is difficult to test in the lab or in a natural setting Natural snow is not created with inherent safety margins.Bonding, Failure, and Avalanche Release

Determining snow stability is largely focused on slab avalanches. We concentrate primarily on slabs because they are more difficult to predict. Slabs

1) Harder to predict2) Weak layers hidden below surface3) Failure propagates Bonding, Failure, and Avalanche Release

Loose snow avalanches involve surface snow.This might include a significant depth, even the entire snowpack, but the bonding problem is visible on the surface.It is relatively easy to assess when loose snow avalanches (especially large, destructive ones) are likely to occur. Slabs

1) Harder to predict2) Weak layers hidden below surface3) Failure propagates Bonding, Failure, and Avalanche Release

Slabs, hide the problem below the surface. Loose snow avalanches are almost always triggered at the start point, whereas slabs can be triggered remotely.In loose snow avalanches, fracture propagates in an obvious fashion: from the start point, down the slope.Slab failure/fracture mechanisms are not well understood.

Slab failure/fracture:

1) There be cohesion between grains; enough that the snow will act as a unit.

2) A strong layer must overlie a weak layer.

3) A trigger must initiate failure.

4) Stress must overcome strength. Bonding, Failure, and Avalanche Release

Slab failures propagate up, down, and across slopes sometimes over significant terrain features and across surprising distances.Slab failure/fracture mechanisms are not well understood.

Slab failure/fracture:

1) There be cohesion between grains; enough that the snow will act as a unit.

2) A strong layer must overlie a weak layer.

3) A trigger must initiate failure.

4) Stress must overcome strength. Bonding, Failure, and Avalanche Release

Bonding, Failure, and Avalanche ReleaseHow cohesive the snow must be to fail as a slab varies considerably and depends on a combination of factors, such as Characteristics of the failure layerTerrain, and Characteristics of the layer(s) that make up the slab. Slab failure/fracture requirements:

CohesionStrong layer over weak layer.Trigger initiates failure.Stress overcomes strength.

Bonding, Failure, and Avalanche ReleaseSoft slabs can be very soft: skis penetrate easily. Hard slabs can be very hard: boots do not penetrate. Soft slabs tend not to carry propagation very far while hard ones may propagate over great distances. Slab failure/fracture requirements:

Cohesion Strong layer over weak layer. Trigger initiates failure. Stress overcomes strength.

Bonding, Failure, and Avalanche ReleaseHow pronounced the strong over weak layering depends on the characteristics of: Failure layer Terrain Characteristics of the layer(s) that make up the slab. Measuring a difference of two hand hardness grades or more is critical. Slab failure/fracture requirements:

Cohesion Strong layer over weak layer. Trigger initiates failure. Stress overcomes strength.

Bonding, Failure, and Avalanche ReleaseThe mechanism that initiates failure (the process of failure and fracture) is referred to as a trigger. The point where failure is initiated is called the trigger point. Natural TriggersArtificial TriggersTrigger point

Bonding, Failure, and Avalanche ReleaseTrigger points can be vague and large.For example, new snow loads a slope over its entire surface and the exact point where failure is initiated is not clear. Or are small and well-defined For example a skier initiates failure at a rock sticking out of the snow surface. Natural TriggersArtificial TriggersTrigger point

Triggers can be natural or artificial. Natural triggers: related to changes in weather or the snowpack, such as, new snow, wind transported snow, temperature, etc. Artificial triggers: related to human activities,: such as skiing, operating machinery, applying explosives, etc. Bonding, Failure, and Avalanche Release

It is important to understand the difference between a start zone.Start: where avalanche are likely to start (we see the fracture line here) and Trigger points: where the failure that causes an avalanche to start is initiated. Trigger points may or may not be in start zones. Triggers:NaturalArtificial. Bonding, Failure, and Avalanche Release

For stress to overcome strength, load on the slab has to increase or the strength of the bonds holding the slab in place has to decrease. Natural or artificial loads may be added slowly and gradually or rapidly. The bonds in the snowpack adjust readily to slow loading.Slow Loading:Snowpack adjusts well.Rapid Loading:Snowpack adjusts poorly. Bonding, Failure, and Avalanche Release

In slow loading, failure occurs but fracture may not: new bonds are forming faster than existing ones are breaking. Under slow load, we see the snowpack adapt to the new stresses, some examples are mushrooms on trees, overhanging cornices, and snow remaining stable on steep inclines. There is no clearly defined boundary between slow or fast loading. Slow Loading:Snowpack adjusts well.Rapid Loading:Snowpack adjusts poorlyPrecipitation rates of less than 3 mm of water equivalent per hour is slow loading. Bonding, Failure, and Avalanche Release

Under rapid loading, the snowpack adjusts less readily and there are existing bonds are more prone to break at a faster rate than new ones are forming. As a result, stress in the snowpack increases at a faster rate than strength builds and it is more likely that fracture will occur. Slow Loading:Snowpack adjusts well.Rapid Loading:Snowpack adjusts poorlyPrecipitation rates of more than 3 mm of water equivalent per hour to be fast loading. Bonding, Failure, and Avalanche Release

Under rapid loading, the snowpack adjusts less readily. There is no clearly defined boundary between slow or fast loading. Triggers such as explosives or a cornice fall are a fast load.Bonding, Failure, and Avalanche ReleaseSlow Loading:Snowpack adjusts well.Rapid Loading:Snowpack adjusts poorlyPrecipitation rates of more than 3 mm of water equivalent per hour to be fast loading.

Changes in bond strength can also occur slowly or rapidly. These changes are related to natural factors, largely weather related.Such as: Metamorphism Temperature fluctuations Changes in solar radiationBonds change rapidly:> 5oC in a three hour period Sudden increase in radiationBonding, Failure, and Avalanche Release

Gradual changes will change bond strength more slowly while more pronounced changes change bond strength more rapidly. Again, there is no clearly defined boundary between a rapid or slow change Bonding, Failure, and Avalanche ReleaseBonds change rapidly:> 5oC in a three hour period Sudden increase in radiation

Temperature change of more than 5C in a three hour period is a rapid change. The sun rising on a steep south-facing slope in spring would have a rapid effect.Bonding, Failure, and Avalanche ReleaseBonds change rapidly:> 5oC in a three hour period Sudden increase in radiation

Metamorphic effects such as rounding or faceting are much slower and take longer to affect bond strength. Whether these kinds of changes are having an effect may be obvious For example, snow becomes wet or slushy, but in many cases, the effects on strength, even of rapid changes, are difficult to assess.Bonding, Failure, and Avalanche ReleaseBonds change rapidly:> 5oC in a three hour period Sudden increase in radiation

Slab release:Shear failureTensile failureCompression failureBonding, Failure, and Avalanche Release

Bonding, Failure, and Avalanche ReleaseOnce stress overcomes strength and the fracture leading to an avalanche occurs, there are at least two and perhaps three events that lead to slab release.

Bonding, Failure, and Avalanche ReleaseShear failure occurs in the layer which lies between the bed surface and the slab (the failure layer). Tensile failure occurs at the fracture line or crown and the flanks. Compression failure may occur in the failure layer and/or at the stauchwall.

Bonding, Failure, and Avalanche ReleaseShear failure Tensile failureCompression failureThe jury is still out on which happens first, followed by others, or is the sequence is always the same, or what is the role of compression in failure, and does it occur sometimes or always.

Bonding, Failure, and Avalanche ReleaseWe observe these failures even when avalanches do not occur. The whumpf or collapse of a slab when we ski onto it is a sign of compressional failure. Cracks that occur in the snow as we ski across it are tensile failures.When we ski across a slope and the slab fails and moves downhill but stops and does not avalanche, shear failure has occurred.

Bonding, Failure, and Avalanche ReleaseThis is a simplified discussion of bonding and failure. While avalanche formation and release is not fully understood, it is worthwhile to have a good general grounding in basic concepts. If nothing else it is important to realize the complexity of the problem before attempting to discuss stability analysis. Our lack of understanding of some of these issues is humbling and helps brings our efforts at decision making into clearer perspective.