snow deformation

Snow DeformationSnow DeformationSnow DeformationSnow DeformationStress and strain of snowpackStress and strain of snowpackStress and strain of snowpackStress and strain of snowpack

Beginning of a slab Beginning of a slab avalanche.avalanche.

The release was The release was triggered by skis triggered by skis cutting moving near cutting moving near the top of the rounded the top of the rounded ridge seen in the ridge seen in the upper left corner of upper left corner of the picture.the picture.

Beginning of a slab Beginning of a slab avalanche.avalanche.

The release was The release was triggered by skis triggered by skis cutting moving near cutting moving near the top of the rounded the top of the rounded ridge seen in the ridge seen in the upper left corner of upper left corner of the picture.the picture.

Credit: A. Duclos, www.data-avalanche.org

Snow DeformationSnow DeformationSnow DeformationSnow DeformationStress and Strain of SnowpackStress and Strain of Snowpack

Sudden fracturing of the snowpack, which is a clear sign of stress & instability.

Credit: A. Duclos, www.data-avalanche.org

Snow DeformationSnow DeformationSnow DeformationSnow DeformationStress and strain of snowpackStress and strain of snowpack

Deformation of the spx occurs in 3 modes:Deformation of the spx occurs in 3 modes:

CompressionCompression TensionTension ShearShear

Deformation of the spx occurs in 3 modes:Deformation of the spx occurs in 3 modes:

CompressionCompression TensionTension ShearShear

CreepCreep

The alpine snowpack is always The alpine snowpack is always creeping, due to metamorphism creeping, due to metamorphism (90% settlement and 10% (90% settlement and 10% deformation of ice grains) and its deformation of ice grains) and its high porosity.high porosity.

CreepCreep

The alpine snowpack is always The alpine snowpack is always creeping, due to metamorphism creeping, due to metamorphism (90% settlement and 10% (90% settlement and 10% deformation of ice grains) and its deformation of ice grains) and its high porosity.high porosity.

Settlement from rearrangement of ice grains due to weight of layers above

Long-term effect of compressive stress is increase of density & hardness w/r to depth

During densification, snow hardness increases

Hardness is more related to strength than density.

Long-term effect of compressive stress is increase of density & hardness w/r to depth

During densification, snow hardness increases

Hardness is more related to strength than density.

Term Strength (Pa)

Hand test

Graphic

Very low 0 - 103 fist

Low 103 - 104 4 fingers

Medium 104 - 105 1 finger

High 105 - 106 pencil

Very high > 106 Knife blade

1. Simple case: horizontal with constant depth

All deformation is in the vertical direction:

Settlement

1. Simple case: horizontal with constant depth

All deformation is in the vertical direction:

Settlement

Settlement of snow is largely by rearrangement of grains caused by the weight above.

Settlement

Densification and Densification and StrengtheningStrengthening

Settlement

Densification and Densification and StrengtheningStrengthening

Settlement of snow is largely by rearrangement of grains caused by the weight above.

2. Snowpack on inclined slope

Total deformation of snow pack is in the down slope direction

Resolve stress into vector components

2. Snowpack on inclined slope

Total deformation of snow pack is in the down slope direction

Resolve stress into vector components

The stresses that cause deformation in the snowpack

Stress () = force/unit area

ForceForceForce: changes in the state of rest or motion of a

Only a force can cause a stationary object to move or change the motion (direction and velocity) of a moving object.

Force: changes in the state of rest or motion of a body.

Only a force can cause a stationary object to move or change the motion (direction and velocity) of a moving object. Force = mass x acceleration F = ma

Mass = density x volume m = V

Weight is the magnitude of the force of gravity (g) acting upon a mass.

The newton (N) is the basic (SI) unit of force.

Force = mass x acceleration F = ma

Mass = density x volume m = V

Weight is the magnitude of the force of gravity (g) acting upon a mass.

The newton (N) is the basic (SI) unit of force.

Units of StressUnits of Stress

1 newton = 1 kg meter/sec2 = a unit of force

1 pascal = 1 newton/m2 = a unit of stress

1 newton = 1 kg meter/sec2 = a unit of force

1 pascal = 1 newton/m2 = a unit of stress

1 kPa = 0.145 lb/in2

• 9.81 Pa is the pressure caused by a depth of 1mm of water

1 kPa = 0.145 lb/in2

• 9.81 Pa is the pressure caused by a depth of 1mm of water

Two components stress

1. Normal stress, n and the component that is parallel to the plane, shear stress, s

Normal compressive stresses tend to inhibit sliding along the plane and are considered positive if they are compressive.

Normal tensional stresses tend to separate rocks along the plane and values are considered negative.

2. Shear stresses tend to promote sliding along the plane, labeled positive if its right-lateral shear and negative if its left-lateral shear.

1. Normal stress, n and the component that is parallel to the plane, shear stress, s

2. Shear stresses tend to promote sliding along the plane, labeled positive if its right-lateral shear and negative if its left-lateral shear.

1. Normal stress, n and the component that is parallel to the plane

2. Shear stresses, s , tend to promote sliding along the plane, labeled positive if its right-lateral shear and negative if its left-lateral shear.

1. Normal stress, n and the component that is parallel to the plane

2. Shear stresses, s , tend to promote sliding along the plane, labeled positive if its right-lateral shear and negative if its left-lateral shear.

Stress on a 2-D plane:

Normal stress act perpendicular to the plane Shear stress act along the plane.

Normal and shear stresses are perpendicular to one another

Stress on a 2-D plane:

Normal stress act perpendicular to the plane Shear stress act along the plane.

Normal and shear stresses are perpendicular to one another

Stress on an inclined slope

2 componentsNormal stress & Shear stress

Stress on an inclined slope

2 componentsNormal stress & Shear stress

n = cos2n = cos2

s = sin2s = sin2

STRESSSTRESSVERSUSVERSUS

STRENGTHSTRENGTH

STRESSSTRESSVERSUSVERSUS

STRENGTHSTRENGTH

WHEN STRESS EXCEEDS STRENGTH

FAILURE OCCURS!FAILURE OCCURS!

Shear Stress

Slope angle

Shear Stress

Slope angle

Shear creep deformation depends on the type of snow and the slope angle.

Entire snowpack slips over Entire snowpack slips over the ground or at an the ground or at an interface such as an ice interface such as an ice layer. layer.

Observations show:Observations show:1)1)Smooth interfaceSmooth interface2)2)Temperature at the Temperature at the interface or bottom of spx interface or bottom of spx at 0°C (need free water)at 0°C (need free water)3)3)Slope angle > 15° Slope angle > 15° (roughness of typical (roughness of typical alpine ground cover)alpine ground cover)

Models assume that the water within the spx and at the snow/groundinterface is the critical parameter that determines glide velocity and glide avalanche release.

McClung and Clarke (1987)Clarke and McLung (1999)

Models assume that the water within the spx and at the snow/groundinterface is the critical parameter that determines glide velocity and glide avalanche release.

McClung and Clarke (1987)Clarke and McLung (1999)

Clarke and McClung (1999) emphasize the effect of water on the interface geometry, rather than the effects of varying shear viscosity and viscous Poisson Ratio with varying water content.

Recent studies suggest that the ground showed only minor variation through the winter, while glide rates fluctuated substantially through the winter.

Figure 5. Full-depth glide avalanche trigger mechanisms (source data from Lackinger, 1987 and Clarke and McClung, 1999)

This suggests that the effects of water on partial separation of the snowpack from the glide interface and in filling of irregularities in the ground has a greater affect on glide velocity than varying snow properties.

Figure 5. Full-depth glide avalanche trigger mechanisms (source data from Lackinger, 1987 and Clarke and McClung, 1999)

GlideGlide

Shear failure of alpine snow

Elastic, viscoelastic, and permanent deformationElastic, viscoelastic, and permanent deformation

In general, dry snow can not fracture unless a critical rate is exceeded.

100x or greater than the rate of creep deformation

In general, dry snow can not fracture unless a critical rate is exceeded.

100x or greater than the rate of creep deformation

Ski trigger,snow machine, explosives, etc.

How are high rates produced to cause propagating fractures?

Stress concentrations on asperities.

Fracture mechanics suggest that flaw or crack will increase local stress by ~102

How are high rates produced to cause propagating fractures?

Stress concentrations on asperities.

Fracture mechanics suggest that flaw or crack will increase local stress by ~102

Strain softening

Resistance to deformation decreases after peak strains.

Shear bands or slip surfaces form during deformation.

Combine the strain softening with natural flaws will concentrate shear deformation

Strain softening

Resistance to deformation decreases after peak strains.

Shear bands or slip surfaces form during deformation.

Combine the strain softening with natural flaws will concentrate shear deformation

Shear failure of alpine snow deformed at 0.1mm/min.

Snow DeformationSnow DeformationSnow DeformationSnow Deformation

Components of shear strength in snowComponents of shear strength in snow

Shear failure depends on:

Density Hardness Temperature Rate of deformation Quality of bonding to adjacent layers

Components of Shear Strength Cohesion Friction

Shear failure depends on:

Density Hardness Temperature Rate of deformation Quality of bonding to adjacent layers

Strength property that determines avalanche type is cohesion.

Loose snow avalanche = lack of cohesionSlab avalanches = cohesion that forms blocks

Cohesion:Bond strengthShape of snow crystalsDensity of bonds (bonds/unit volume)

Loose snow avalanche = lack of cohesion

Low Cohesion:Cold temperaturesSnow falling w/ windless conditionsLow density snow

Controlling factor in slab avalanches

Friction:Snow textureWater contentWeight of snow layers above (increase normal stress)

Snow DeformationSnow DeformationSnow DeformationSnow DeformationComponents of shear strength in snowComponents of shear strength in snow

Shear failure under different normal pressure. Cohesive strength at STP is 3kPa.

Shear failure in snowShear failure in snow

Shear Strength Density Grain size Temperature Overburden

Fracture Line Studies

Strength highest in fine-grained snow with rounded grains.

Snow is stiffer and stronger as it gets colder.

Compressive forces (normal stress) on a weak layer increases to friction component of strength.

Loose snow avalancheLoose snow avalanche

Little cohesionNear surface initiationLittle cohesionNear surface initiation

Free water in snow, subsurface now entrained if wet

Little cohesionNear surface initiationLittle cohesionNear surface initiation

Angle of repose for dry snowAngle of repose for dry snow

Snow DeformationSnow Deformation

Slab avalancheSlab avalanche

Cohesive layer overliesthinner, weak layerCohesive layer overliesthinner, weak layer

CharacteristicsSlope angleCrown thicknessSlab densityFailure layer densitySlab & bed hardnessSlab & bed temperatureStratigraphy of slab and failure layerGeometry

CharacteristicsSlope angleCrown thicknessSlab densityFailure layer densitySlab & bed hardnessSlab & bed temperatureStratigraphy: slab &

failure layerGeometry

Hardness of slabs range from very low (fist) to high (pencil).

Soft slab = v. low or low hardness

Hard slab = medium or harder

Difficult to classify!Continental climates: greater tendency for slabs fail on weak layers in old snow

failure layerGeometry Dependent on terrain

Unconfined slopes, open slopes: field data suggests slab width>downslope lengthConfined slopes: length > width

Dry Slab Avalanche FormationDry Slab Avalanche Formation

FRACTURE SEQUENCES

Shear stress > shear strength

Rate of deformation in weak layer must be fast enough to inititate fracture

INITIAL FAILURE

Collapse by a interface layerShear frx produce tension frx at the crown

INITIAL SHEAR FRACTURE

Crown tension fracture near skier

Wet Slab Avalanche FormationWet Slab Avalanche Formation

COMPLEXITY OF WATER FLOW IN WET SNOW

Wet Slab Avalanche FormationWet Slab Avalanche Formation

LUBRICATION MECHANISM FOR GLIDING ON ICE LAYER

Bonding, Failure, and Avalanche Release

If 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).

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.

It is important to understand the difference between a start zone and trigger point.

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:

Natural

Artificial.

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

Slab release:

Shear failure

Tensile failure

Compression failure

What happens first…, followed by…, or is the sequence is always the same, or what is the role of compression in failure, and does it occur sometimes or always.

Slab release:

Shear failure

Tensile failure

Compression failure

We observe rapid snow failure 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.

snow deformation

normal stress

shear stresses

unit of stress

ss normal compressive

normal tensional stresses

strain of snowpackcreeplong

strain of snowpackbeginning

strain of snowpackcreep1

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