From failure to fracture: concurrent field measurements of weak layer strength and fracture toughness.

Dry-snow slab avalanches are governed by two distinct but equally significant mechanical properties of weak snow layers: strength, which controls failure initiation, and fracture toughness, which governs whether an initial failure evolves into a self-sustaining crack. On inclined slopes, crack propagation occurs under mixed-mode loading, making avalanche release a combined multiaxial strength–fracture problem. A physically meaningful description of slab avalanche release requires both properties to be quantified together within the same weak layers.In recent years, substantial progress has been made in characterizing these properties: multiaxial strength has been investigated extensively through laboratory experiments and numerical modeling, while mixed-mode fracture toughness has increasingly been measured directly in the field. However, these advances have largely remained disconnected. To date, strength and fracture toughness have not been measured simultaneously for the same weak snow layers, resulting in an incomplete mechanical description of the material's failure behavior.Here, we therefore present recent experimental advances that enable direct field measurements of both failure and fracture properties of weak layers. We present a unique field-based dataset comprising multiaxial strength and mixed-mode fracture toughness measurements from two layers of buried surface hoar. Measuring both properties within the same layers allows us to construct both failure and fracture envelopes, providing a unified mechanical description of avalanche release. While both envelopes exhibit an elliptical shape, our results reveal contrasting behavior: weak layer strength is higher in compression than in shear, whereas fracture toughness is higher in shear than in compression. Relating these envelopes to weak layer microstructure, we qualitatively investigate how microstructural characteristics control failure initiation and crack propagation. Jointly constraining strength and fracture behavior with field data provides critical input for process-based avalanche release models and represents a significant step toward more physically consistent and reliable avalanche forecasting.