Fragmentation in the Val Pola rock avalanche, Italian Alps

TLDR

The study analyzes grain‑size data from the 1987 Val Pola rock avalanche, applying Weibull and fractal distributions to characterize the fragmentation and comminution processes within the deposit. Weibull fitting reveals a mean shape factor of 0.54 ± 0.28, indicating multiple comminution, while fractal dimensions averaging 2.6–2.7 span theoretical values for plane‑of‑weakness, pillar‑of‑strength, and constrained comminution models, implying both mature and immature deposits, multiple fragmentation mechanisms, and that 1–30 % of the avalanche energy was expended in fragmentation.

Abstract

Grain size data from the deposit of the 1987 Val Pola rock avalanche (central Italian Alps) are compared with data concerning rock avalanching, rock fragmentation, and comminution. The Weibull distribution fits a small part of the entire particle‐size distribution of debris samples, with a mean value of the curve shape factor of 0.54 ± 0.28. This is typical of multiple comminution, or fragmentation with much shearing. A fractal distribution fits over a greater size range. Computed fractal dimensions range between 1.3 and 3.2 within the deposit, with average values of about 2.6–2.7. These values cover the range between the theoretical values of the plane‐of‐weakness model (1.97) and the pillar‐of‐strength model (2.84) and are close to the theoretical value for the constrained comminution model (2.58). These suggest that both texturally mature and immature deposits are present and that more than a single comminution process acted during the rock avalanche motion. Variation of the grain size distribution within the deposit and grain size segregation show as trends in the fractal dimension and arise from variation in the fragmentation process. A variety of different physical and empirical laws suggest that 1–30% of the energy expended in the rock avalanche was consumed in fragmentation.