Forces on the Snowpack

An avalanche occurs when the strain that is put on the snow pack exceeds its strength.  In the case of a slab that is supported by a weak layer the Y component of the snows weight exceeds the normal force the weak layer can push back with.  In this case the weak layer fails structurally in compression.  With nothing supporting the slab underneath, it can no longer stay adhered to the slope, it releases as a unit, breaks up into blocks, and slides down the mountain in the form of an avalanche.  In the case of a slab supported by a poor bond it is the frictional force that holds the slab in place.  when the X component of the slabs weight exceeds the frictional force the slab can no longer stay in place and will avalanche.

The forces in the snowpack can be broken into their X and Y components for analysis.

Depending on the type of weak layer the  frictional force or the structeral strength of the weak layer could be responsible for holding the slab in place. Often it is a combination of both.

If ths snowpack is already "stressed out" all it takes is a little help from a human to tip the strength strain balance.

A human triggered avalanche occurs when the snow is at a critical balance between strength and strain.  The bonds supporting the slab are just barely strong enough to hold it in place.  Then a skier or snowmachiner comes along and adds more weight to the snow pack.  Whether it’s the strength of the weak layer or the friction force that is exceeded the snow pack is suddenly applying more strain to its bounds then the bounds can support.  The critical balance between strength and strain has tipped and an avalanche is now in progress.

Slab Avalanche Formation

Snow as a Material

Slab Avalanche Formation

Prevention

Bibliography