Granulometria, segregazione e frammentazione negli accumuli di valanghe di roccia

Abstract

Grain-size, segregation, and fragmentation in rock avalanche deposits ABSTRACT: Much work has been done on characterization and modelling of rock avalanches. Field observations and laboratory tests can be used to characterize rock avalanche accumulations and to find possible controlling factors on their transport and emplacement mechanisms. We collected and analysed data from the accumulation of the 1987 Val Pola rock avalanche (Central Italian Alps). We compare these data with data in the literature concerning rock avalanching, rock fragmentation and comminution. We observed that the Weibull distribution can fit a small part of the entire particle size distribution, with a mean value of the curve shape factor equal to 0.537 ± 0.284. This range is considered typical of multiple comminution, or fragmentation with considerable shearing and continued comminution. Nevertheless, we observe that Weibull and Rosin Rammler distributions do not allow to fit the entire curve and more frequently only the finer tail. The fractal approach allows a more complete fitting. We observed fractal dimension values ranging between 1.3 and 3.2 within the accumulation and with average values of about 2.6-2.7. These values are spread over the entire interval between the theoretical values for the more frequently adopted models to explain rock fragmentation and comminution. This suggests that both texturally immature and mature deposits form the main accumulation and that more than a single comminution process acted during the rock avalanche motion. The spatial description of the grain size distribution within the deposit and the segregation are examined. We finally computed, by different empirical laws, the required fragmentation energy. Key terms: fragmentation, rock avalanche, fractal dimension, Weibull distribution, grainsize Termini chiave: frammentazione, rock avalanche, dimensione frattale, distribuzione di Weibull, granulometria