Transcript 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.
Term
Very low
Strength
(Pa)
0 - 103
Hand
test
fist
Low
103 - 104
4 fingers
Medium
104 - 105
1 finger
High
105 - 106
pencil
> 106
Knife
blade
Very high
Ice
Graphic
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:
1)Smooth interface
2)Temperature at the
interface or bottom of spx
at 0°C (need free water)
3)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
Glide
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)
Snow Deformation
Components of shear strength in snow
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 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
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
Slab avalanche
Characteristics
Slope angle
Crown thickness
Slab density
Failure layer density
Slab & bed hardness
Slab & bed temperature
Stratigraphy: slab &
failure layer
Difficult to classify!
Geometry
Continental climates: greater
tendency for slabs fail on weak layers
in old snow
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
Dependent on terrain
Unconfined slopes, open slopes: field data
suggests slab width>downslope length
Confined slopes: length > width
Snow Deformation
Dry Slab Avalanche
Formation
FRACTURE SEQUENCES
Shear stress > shear
strength
Rate of deformation in weak
layer must be fast enough to
inititate fracture
Snow Deformation
Dry Slab Avalanche
Formation
INITIAL FAILURE
Collapse by a interface layer
Shear frx produce tension frx at the crown
Snow Deformation
Dry Slab Avalanche
Formation
INITIAL SHEAR FRACTURE
Crown tension fracture near skier
Snow Deformation
Wet Slab Avalanche
Formation
COMPLEXITY OF WATER FLOW IN WET SNOW
Snow Deformation
Wet Slab Avalanche
Formation
Snow Deformation
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).
Bonding, Failure, and Avalanche Release
Bonding, Failure, and Avalanche Release
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
Triggers:
Natural
Artificial.
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.
Bonding, Failure, and Avalanche Release
For stress to overcome
Slow Loading:
strength, load on the slab
Snowpack adjusts well has to increase or the
strength of the bonds
Rapid Loading:
holding the slab in place
Snowpack adjusts poorlyhas to decrease.
Natural or artificial loads
may be added slowly and
gradually or rapidly.
The bonds in the snowpack
adjust readily to slow
loading.
Bonding, Failure, and Avalanche Release
Slab release:
Shear failure
Tensile failure
Compression failure
Bonding, Failure, and Avalanche Release
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.
Bonding, Failure, and Avalanche Release
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.