Sir,
I am sorry if I have been unfair to Dr Haefeli's pioneering work. However, I don't think it is unfair to imply that Haefeli has been instrumental in inspiring much of the work in the shear failure of snow and particularly of internal layers. The Coulomb-Mohr criterion as proposed by Haefeli presupposes shear failure.
As Perla points out, the Coulomb-Mohr criterion is a good approximation for granular materials. Part of the point of my paper is that it may be more realistic to view snow not as a granular material but as a solid ceramic with the unique property of extremely variable density. This implies that a Griffith criterion would be more applicable.
5 January 1970