The Editor,
Journal of Glaciology
Sir,
Reference SommerfeildSommerfeld (1969, p. 451) dismisses Haefeli's contributions with the statement “Haefeli (1942, 1963) speaks of the shear fracture of a weak layer”. Actually, Reference Haefeli and KingeryHaefeli (1963) emphasized the importance of tensile failure in his theory of “stress metamorphosis”. Also, recall his discussion in Reference RochRoch (1966):
“We must take into account that the formation of snow slab avalanches is in its first approach not a problem of shear strength in the sliding surface but of the tensile strength within the fracture surface. The latter stays vertical (perpendicular) to the slope. The opening of the crack and the failure process is progressive. The breaking down of the sliding layer and the overcoming of the shear strengths along the sliding surface is no longer a static but a dynamic process, due to the heavy shock produced by the opening of the crack.”
Although Haefeli and Sommerfeld are in agreement on the importance of tensile failure, Sommerfeld advocates a Griffith criterion and Haefeli a Coulomb-Mohr criterion. A Griffith criterion, based on sound first principles, may be preferable: however, the Coulomb-Mohr criterion is still considered a good approximation for the macroscopic failure of granular materials (Reference Paul and LiebowitzPaul, 1968).
It is noteworthy that the Coulomb-Mohr criterion has been modified to account for “tensile cut-offs” (Reference Paul and LiebowitzPaul, 1968), which makes it quite applicable to the “stress metamorphosis” model.