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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.

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Presentation on theme: "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."— Presentation transcript:

1 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. 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,

2 Snow Deformation Stress and Strain of Snowpack Sudden fracturing of the snowpack, which is a clear sign of stress & instability. Credit: A. Duclos,

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

4 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. Creep Settlement from rearrangement of ice grains due to weight of layers above

5 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. 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. TermStrength (Pa) Hand test Graphic Very low fist Low fingers Medium finger High pencil Very high> 10 6 Knife blade Ice

6 Snow Deformation Stress and strain of snowpack Creep 1. Simple case: horizontal with constant depth All deformation is in the vertical direction: Settlement 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.

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

8 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 Creep 2. Snowpack on inclined slope Total deformation of snow pack is in the down slope direction Resolve stress into vector components

9 The stresses that cause deformation in the snowpack Stress (  ) = force/unit area  = F/A Stress (  ) = force/unit area  = F/A

10 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: 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  = 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  = 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.

11 Units of Stress 1 newton = 1 kg meter/sec 2 = a unit of force 1 pascal = 1 newton/m 2 = a unit of stress 1 newton = 1 kg meter/sec 2 = a unit of force 1 pascal = 1 newton/m 2 = a unit of stress 1 kPa = lb/in Pa is the pressure caused by a depth of 1mm of water 1 kPa = lb/in Pa is the pressure caused by a depth of 1mm of water

12 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. 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.

13 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. 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.

14 Two components stress 1. Normal stress,  n 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,  s, 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,  n 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,  s, tend to promote sliding along the plane, labeled positive if its right-lateral shear and negative if its left-lateral shear.

15 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

16 Components of stress  Three normal stresses  Components parallel shear stresses  Reference system x, y, z Components of stress  Three normal stresses  Components parallel shear stresses  Reference system x, y, z

17 State of Stress  If the 3 principal stresses are equal in magnitude = isotropic stress  If the principal stress are unequal in magnitude = anisotropic stress  The greatest stress is called  1  The intermediate stress,  2 and minimum stress is called  3  If the 3 principal stresses are equal in magnitude = isotropic stress  If the principal stress are unequal in magnitude = anisotropic stress  The greatest stress is called  1  The intermediate stress,  2 and minimum stress is called  3  1 >  2 >  3 What is it called if all three principal stresses are equal?  1 =  2 =  3

18 Stress on an inclined slope 2 components Normal stress & Shear stress Stress on an inclined slope 2 components Normal stress & Shear stress  n =  cos 2   s =  sin 2 

19 STRESSVERSUSSTRENGTHSTRESSVERSUSSTRENGTH WHEN STRESS EXCEEDS STRENGTH WHEN STRESS EXCEEDS STRENGTH FAILURE OCCURS!

20 Squeeze a block of snow between two planks of wood AB, trace of fracture plane that makes an angle  with    The 2-D case is simple, since      (atmospheric pressure) Squeeze a block of snow between two planks of wood AB, trace of fracture plane that makes an angle  with    The 2-D case is simple, since      (atmospheric pressure) Snow

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

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

23 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) 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)

24 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) 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)

25 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. 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.

26 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. 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)

27 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. 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)

28 Snow Deformation Stress and strain of snowpack Glide

29 Snow Deformation Stress and strain of snowpack Glide

30 Snow Deformation Stress and strain of snowpack Glide

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

32 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 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.

33 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 ~10 2 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 ~10 2

34 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 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.

35 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 Shear failure depends on:  Density  Hardness  Temperature  Rate of deformation  Quality of bonding to adjacent layers Components of Shear Strength  Cohesion  Friction

36 Snow Deformation Components of shear strength in snow Components of Shear Strength  Cohesion  Friction 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)

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

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

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

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

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

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

43 Snow Deformation Shear failure in snow

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

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

46 Snow Deformation Loose snow avalanche Little cohesion Near surface initiation Little cohesion Near surface initiation

47 Snow Deformation Loose snow avalanche Angle of repose for dry snow

48 Snow Deformation Slab avalanche Cohesive layer overlies thinner, weak layer Cohesive layer overlies thinner, weak layer

49 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy of slab and failure layer  Geometry

50 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy of slab and failure layer  Geometry

51 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy: slab & failure layer  Geometry

52 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy: slab & failure layer  Geometry

53 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 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

54 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy: slab & failure layer  Geometry

55 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 Characteristics  Slope angle  Crown thickness  Slab density  Failure layer density  Slab & bed hardness  Slab & bed temperature  Stratigraphy: slab & failure layer  Geometry Difficult to classify! Continental climates: greater tendency for slabs fail on weak layers in old snow

56 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 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

57 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

58 Snow Deformation Dry Slab Avalanche Formation INITIAL FAILURE Collapse by a interface layer Shear frx produce tension frx at the crown

59 Snow Deformation Dry Slab Avalanche Formation INITIAL SHEAR FRACTURE Crown tension fracture near skier

60 Snow Deformation Wet Slab Avalanche Formation COMPLEXITY OF WATER FLOW IN WET SNOW

61 Snow Deformation Wet Slab Avalanche Formation LUBRICATION MECHANISM FOR GLIDING ON ICE LAYER Snow Deformation

62 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).

63 Bonding, Failure, and Avalanche Release

64 The 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 StrengthStress Strength: the bonds that are holding everything together Stress: F/A applied to the bonds that reduce in bond strength

65 Slab avalanche 1) Harder to predict 2) Weak layers hidden below surface 3) Failure propagates Bonding, Failure, and Avalanche Release 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.

66 Bonding, Failure, and Avalanche Release How 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.

67 Bonding, Failure, and Avalanche Release The 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 Triggers Artificial Triggers Trigger point

68 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

69 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. Bonding, Failure, and Avalanche Release

70 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

71 Slab release: Shear failure Tensile failure Compression failure Bonding, Failure, and Avalanche Release

72 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 Bonding, Failure, and Avalanche Release

73 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. Bonding, Failure, and Avalanche Release


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