Snow Deformation

Snow Deformation Stress and strain of snowpack Beginning of a slab avalanche. The release was triggered by skis cutting moving near the top of the rounded ridge seen in the upper left corner of the picture. Credit: A. Duclos, www.data-avalanche.org

Snow Deformation Stress 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 Deformation Stress and strain of snowpack • Deformation of the spx occurs in 3 modes: • Compression • Tension • Shear

Snow Deformation Stress and strain of snowpack Creep The alpine snowpack is always creeping, due to metamorphism (90% settlement and 10% deformation of ice grains) and its high porosity. Settlement from rearrangement of ice grains due to weight of layers above

Snow Deformation Stress and strain of snowpack Creep 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.

Snow Deformation Stress and strain of snowpack Creep 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.

Snow Deformation Stress and strain of snowpack Settlement Densification and Strengthening Settlement of snow is largely by rearrangement of grains caused by the weight above.

Snow Deformation Stress and strain of snowpack Creep 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 (s) = force/unit area s = F/A

Force 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 =rV r = 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 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

Two components stress 1. Normal stress, sn and the component that is parallel to the plane, shear stress, ss 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.

Two components stress 1. Normal stress, sn and the component that is parallel to the plane 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, ss, 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 an inclined slope 2 components Normal stress & Shear stress sn = s cos2q ss = s sin2q

STRESS VERSUS STRENGTH WHEN STRESS EXCEEDS STRENGTH FAILURE OCCURS!

Snow Deformation Stress and strain of snowpack Shear Stress & Slope angle Shear creep deformation depends on the type of snow and the slope angle.

Snow Deformation Stress and strain of snowpack Glide Entire snowpack slips over the ground or at an interface such as an ice layer.

Snow Deformation Stress and strain of snowpack • Glide • Observations show: • Smooth interface • Temperature at the interface or bottom of spx at 0°C (need free water) • Slope angle > 15° (roughness of typical alpine ground cover)

Snow Deformation Stress and strain of snowpack Glide Models assume that the water within the spx and at the snow/ground interface is the critical parameter that determines glide velocity and glide avalanche release. McClung and Clarke (1987) Clarke and McLung (1999)

Snow Deformation Stress and strain of snowpack Glide 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.

Snow Deformation Stress and strain of snowpack Glide 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)

Snow Deformation Stress and strain of snowpack Glide 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)

Snow Deformation Stress and strain of snowpack Glide

Snow Deformation Stress and strain of snowpack Shear failure of alpine snow ss Elastic, viscoelastic, and permanent deformation

Snow Deformation Stress and strain of snowpack 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.

Snow Deformation Stress and strain of snowpack 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

Snow Deformation Stress and strain of snowpack 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 Deformation Components 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

Snow Deformation Components of shear strength in snow • Components of Shear Strength • Cohesion • Friction Strength property that determines avalanche type is cohesion. • Loose snow avalanche = lack of cohesion • Slab avalanches = cohesion that forms blocks • Cohesion: • Bond strength • Shape of snow crystals • Density of bonds (bonds/unit volume)

Snow Deformation Components of shear strength in snow • Components of Shear Strength • Cohesion • Friction • Loose snow avalanche = lack of cohesion • Low Cohesion: • Cold temperatures • Snow falling w/ windless conditions • Low density snow

Snow Deformation Components of shear strength in snow • Components of Shear Strength • Cohesion • Friction • Controlling factor in slab avalanches • Friction: • Snow texture • Water content • Weight of snow layers above (increase normal stress)

Components of shear strength in snow Snow Deformation • Components of Shear Strength • Cohesion • Friction Shear failure under different normal pressure. Cohesive strength at STP is 3kPa.

Snow Deformation Shear failure in snow • Shear Strength • Density • Grain size • Temperature • Overburden Fracture Line Studies

Snow Deformation Shear failure in snow • Shear Strength • Density • Grain size • Temperature • Overburden Strength highest in fine-grained snow with rounded grains.

Snow Deformation Shear failure in snow • Shear Strength • Density • Grain size • Temperature • Overburden Snow is stiffer and stronger as it gets colder.

Snow Deformation Shear failure in snow

Snow Deformation Shear failure in snow • Shear Strength • Density • Grain size • Temperature • Overburden Compressive forces (normal stress) on a weak layer increases to friction component of strength.

Snow Deformation Loose snow avalanche Little cohesion Near surface initiation Free water in snow, subsurface now entrained if wet

Snow Deformation Loose snow avalanche Little cohesion Near surface initiation

Snow Deformation Loose snow avalanche Angle of repose for dry snow

Snow Deformation Slab avalanche Cohesive layer overlies thinner, weak layer

Snow Deformation Slab avalanche • Characteristics • Slope angle • Crown thickness • Slab density • Failure layer density • Slab & bed hardness • Slab & bed temperature • Stratigraphy of slab and failure layer • Geometry

Snow Deformation Slab avalanche • Characteristics • Slope angle • Crown thickness • Slab density • Failure layer density • Slab & bed hardness • Slab & bed temperature • Stratigraphy: slab & failure layer • Geometry

Snow Deformation Slab avalanche • Characteristics • Slope angle • Crown thickness • Slab density • Failure layer density • Slab & bed hardness • Slab & bed temperature • Stratigraphy: slab & failure layer • Geometry Hardness of slabs range from very low (fist) to high (pencil). Soft slab = v. low or low hardness Hard slab = medium or harder

Snow Deformation Slab avalanche • Characteristics • Slope angle • Crown thickness • Slab density • Failure layer density • Slab & bed hardness • Slab & bed temperature • Stratigraphy: slab & failure layer • Geometry

Snow Deformation