Triggering of dry snow slab avalanches: stress versus fracture mechanical approach B.M. Chiaia, P. Cornetti ⁎ , B. Frigo Department of Structural and Geotechnical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi,24, 10129 Torino, Italy Received 16 March 2007; accepted 15 August 2007 Abstract This paper analyses the conditions for triggering of dry snow slab avalanches. As suggested by several Authors, we assume as a basic mechanism for avalanche triggering the mode II fracture of the weak layer lying beneath the stiff snow slab, i.e. we assume the presence of super-weak zones in the basal layer. By means of a linear elastic analysis, the shear stresses in the weak layer as well as the strain energy release rate caused by an increment of the super-weak zone are evaluated. Hence we introduce a stress failure criterion as well as an energy one. It is shown that the latter criterion can be seen as an extension of the criterion firstly proposed by McClung [McClung, D.M., 1979. Shear fracture precipitated by strain softening as a mechanism of dry slab avalanche release. Journal of Geophysical Research, 84(B7), 3519-3526.] for dry snow slab avalanche release. Finally we couple the two criteria, showing that the weak layer can fail only in a min–max band of thickness. © 2007 Elsevier B.V. All rights reserved. Keywords: Dry snow slab avalanches; Super-weak zones; Fracture mechanics; Shear lag 1. Introduction Snow stratigraphy studies at the fracture line of dry slab avalanches often indicates a situation that consists of a harder thick layer overlying a thin weak layer. This means that most of shear deformation concentrates in a narrow band. After the pioneering paper by McClung (1979), most of the researchers dealing with the triggering of dry snow slab avalanches have assumed as the basic failure mechanism the formation of a shear band at a stress concentration in the weak layer (named super-weak zone). A slow strain softening at the tip of the band follows, until a critical length is reached; then the basal crack starts propagating under highly unstable mode II conditions. Although this mechanism is commonly assumed in the literature, it should be noted that no direct proof is available. Furthermore, recently, other approaches like collapse theory (Heierli and Zaiser, in press) have been proposed. In the present paper, however, we will follow the scheme proposed by McClung (1979). Once the critical size of the defect in the weak layer is reached, the triggering mechanism of dry slab avalanches is sequentially represented by the following steps: (i) fracture along the weak layer on the bed surface (mode II), (ii) fracture at the crown (mode I), (iii) fracture at the flanks (mode II and mode III) and (iv) fracture at the stauchwall (crushing). The paper analyses the propaga- tion of the first one. Assuming the presence of super-weak zones in the snowpack, this paper introduces a new criterion for dry slab avalanche triggering induced by mode II collapse of Available online at www.sciencedirect.com Cold Regions Science and Technology 53 (2008) 170 – 178 www.elsevier.com/locate/coldregions ⁎ Corresponding author. Tel.: +39 011 5644901; fax: +39 011 5644899. E-mail address: pietro.cornetti@polito.it (P. Cornetti). 0165-232X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2007.08.003